1 00:00:00,040 --> 00:00:04,320 So picture this, it is 3:00 in the morning. 2 00:00:04,560 --> 00:00:07,280 Oh, the classic ICU hour. Right, The ventilator is just 3 00:00:07,280 --> 00:00:10,200 alarming incessantly. You've got a patient whose BO2 4 00:00:10,200 --> 00:00:15,400 is dropping rapidly like through the 70s, and the arterial blood 5 00:00:15,400 --> 00:00:18,400 gas you just pulled makes absolutely no logical sense. 6 00:00:18,400 --> 00:00:19,520 Yeah. In the clinical world, that's 7 00:00:19,520 --> 00:00:22,920 just a standard Tuesday night. Exactly, but you know in your MD 8 00:00:22,920 --> 00:00:27,240 anesthesiology Viva voce this is the exact moment the examiner 9 00:00:27,240 --> 00:00:30,240 decides your fate. Do you freeze or do you 10 00:00:30,240 --> 00:00:33,320 completely deconstruct the Physiology and like, defend your 11 00:00:33,320 --> 00:00:35,320 ventilator strategy right there under pressure? 12 00:00:35,320 --> 00:00:38,400 And that's exactly why we do not accept surface level answers 13 00:00:38,640 --> 00:00:41,760 from postgraduate candidates. I mean, when you step into that 14 00:00:41,760 --> 00:00:44,920 hall or when you stand at the foot of that ICU bed, you are 15 00:00:44,920 --> 00:00:47,400 the ultimate guardian of tissue oxygenation. 16 00:00:47,440 --> 00:00:49,560 Right. So today, for everyone 17 00:00:49,560 --> 00:00:52,440 listening, we are not just memorizing textbook definitions, 18 00:00:52,440 --> 00:00:55,000 you are stepping into the examiner's mind with us. 19 00:00:55,240 --> 00:00:58,440 We're going to dissect respiratory failure today, not 20 00:00:58,440 --> 00:01:03,640 as a static disease, but as a dynamic, evolving physiological 21 00:01:03,640 --> 00:01:06,120 collapse. If you can explain the why and 22 00:01:06,120 --> 00:01:09,480 the how behind a falling PO2, you won't just pass, you'll 23 00:01:09,480 --> 00:01:11,560 command the room. I mean, if you can trace that 24 00:01:11,560 --> 00:01:14,000 oxygen molecule from the ventilator circuit all the way 25 00:01:14,000 --> 00:01:16,440 down to the mitochondria, that's the gold standard. 26 00:01:16,920 --> 00:01:19,680 So let's start at the foundation, but let's push past 27 00:01:19,680 --> 00:01:21,800 the basic Med school stuff. Absolutely. 28 00:01:22,040 --> 00:01:25,040 Respiratory failure is strictly defined as the inability of the 29 00:01:25,040 --> 00:01:27,960 respiratory system to perform adequate intrapulmonary gas 30 00:01:27,960 --> 00:01:30,440 exchange. But the numbers attached to this 31 00:01:30,440 --> 00:01:35,400 like a PO2 of less than 60mm of mercury on room air and a PECO 2 32 00:01:35,400 --> 00:01:39,040 greater than 49, they always felt, I don't know, somewhat 33 00:01:39,040 --> 00:01:40,960 arbitrary to me when I was a junior resident. 34 00:01:40,960 --> 00:01:42,360 Well, they're anything but arbitrary. 35 00:01:42,400 --> 00:01:42,840 Right. Why? 36 00:01:43,080 --> 00:01:45,800 It's a 60 for the oxygen, Why not 55? 37 00:01:45,800 --> 00:01:48,440 This is exactly where a Viva examiner is going to start 38 00:01:48,440 --> 00:01:51,000 probing you. The number 60 is chosen 39 00:01:51,000 --> 00:01:53,480 specifically because of the sigmoidal shape of the 40 00:01:53,480 --> 00:01:55,240 oxyhemoglobin dissociation curve. 41 00:01:55,240 --> 00:01:59,360 The Cliff, Exactly the Cliff at APO 2 of 60. 42 00:01:59,800 --> 00:02:02,760 You are standing precisely on the precipice of the steep 43 00:02:02,760 --> 00:02:06,560 portion of that curve. So if a patient's PO2 drops from 44 00:02:06,560 --> 00:02:10,600 a perfectly normal 100 down to 60, their hemoglobin saturation 45 00:02:10,919 --> 00:02:15,320 really only falls from, say, 98% down to about 90%. 46 00:02:15,320 --> 00:02:17,000 So they're still compensating pretty well there. 47 00:02:17,080 --> 00:02:19,600 Right. But once you drop from 60 to 50, 48 00:02:20,080 --> 00:02:22,400 you fall off the Cliff. The saturation just plummets 49 00:02:22,400 --> 00:02:25,320 into the 70s or worse. The examiner wants to see that 50 00:02:25,320 --> 00:02:28,800 you understand that 60 is the point of imminent catastrophic 51 00:02:28,800 --> 00:02:30,920 failure. OK, that makes perfect sense. 52 00:02:31,280 --> 00:02:35,160 And what about the Pad 2 threshold, the 49mm of mercury? 53 00:02:35,200 --> 00:02:37,520 That's the point at which the carbon dioxide accumulation 54 00:02:37,520 --> 00:02:40,160 definitively overcomes the body's normal buffering 55 00:02:40,160 --> 00:02:43,280 capacity, assuming, you know there's no primary metabolic 56 00:02:43,280 --> 00:02:46,040 alkalosis driving it, right? But more importantly for 57 00:02:46,040 --> 00:02:49,920 anesthesia, you must articulate why these thresholds matter to 58 00:02:49,920 --> 00:02:52,120 us uniquely. I mean, every single time you 59 00:02:52,120 --> 00:02:55,560 induce general anesthesia, you are actively pushing the patient 60 00:02:55,560 --> 00:02:58,320 toward respiratory failure. Because we're abolishing airway 61 00:02:58,320 --> 00:03:00,080 tone and depressing the central drive. 62 00:03:00,320 --> 00:03:02,840 Yes. And we are drastically reducing 63 00:03:02,960 --> 00:03:05,480 their functional residual capacity or FRC. 64 00:03:05,880 --> 00:03:08,360 Let's hover on FRC for a second, because the relationship between 65 00:03:08,360 --> 00:03:12,160 FRC and closing capacity is just an absolute goldmine for written 66 00:03:12,160 --> 00:03:13,600 answers. It really is. 67 00:03:13,800 --> 00:03:17,200 When we paralyze a patient, you know, the diaphragm relaxes, it 68 00:03:17,200 --> 00:03:20,080 gets pushed cephalid by the abdominal contents of the FRC 69 00:03:20,080 --> 00:03:25,160 plummets and if that FRC falls below the closing capacity, the 70 00:03:25,160 --> 00:03:28,880 small Airways in the dependent regions of the lung just they 71 00:03:28,880 --> 00:03:31,400 collapse during normal tidal breathing. 72 00:03:31,640 --> 00:03:34,520 They do, and blood continues to flow past those collapsed 73 00:03:34,520 --> 00:03:36,840 alveoli, which it's a functional shunt. 74 00:03:37,480 --> 00:03:41,280 And that hysiological cascade is exactly why the traditional 75 00:03:41,480 --> 00:03:45,120 binary classification, you know, just Tye hyoxemic and Tye 2 76 00:03:45,120 --> 00:03:48,000 hyercagnic, it's no longer sufficient for a modern exam. 77 00:03:48,040 --> 00:03:49,680 Right. The examiners want more now. 78 00:03:49,680 --> 00:03:52,160 They expect you to utilize the expanded classification system. 79 00:03:52,360 --> 00:03:55,040 You absolutely have to bring type 3 and type 5 into your 80 00:03:55,040 --> 00:03:56,800 written essays and your Viva discussions. 81 00:03:57,040 --> 00:03:59,600 It proves you have a comprehensive understanding of 82 00:03:59,600 --> 00:04:02,120 perioperative medicine. OK, so let's map out that 83 00:04:02,120 --> 00:04:03,880 expanded system for the listener. 84 00:04:04,120 --> 00:04:07,200 We have a patient crashing over that oxygenation Cliff. 85 00:04:07,800 --> 00:04:11,040 If it's a pure oxygenation failure, the classic type I 86 00:04:11,040 --> 00:04:15,320 we're defining it by APO 2 below 60 with a normal or even low 87 00:04:15,400 --> 00:04:16,320 PO2. Correct. 88 00:04:16,320 --> 00:04:18,560 So type I is primarily a parent chemical issue. 89 00:04:18,839 --> 00:04:20,440 The lung tissue itself. Exactly. 90 00:04:20,480 --> 00:04:23,720 The alveoli, the interstitium, the capillary interface, that's 91 00:04:23,720 --> 00:04:25,680 what's diseased. And this is acute and life 92 00:04:25,680 --> 00:04:28,400 threatening because, well, the brain and the heart simply 93 00:04:28,400 --> 00:04:30,600 cannot survive prolonged hypoxemia. 94 00:04:30,640 --> 00:04:32,840 Right. And then Type 2 conversely is 95 00:04:32,840 --> 00:04:36,080 hypercapnic respiratory failure. So that's ventilatory 96 00:04:36,080 --> 00:04:38,960 insufficiency. Yes, the PO2 is still below 60, 97 00:04:39,400 --> 00:04:42,480 but the hallmark here is a PO2 greater than 45 or 50 98 00:04:42,600 --> 00:04:44,560 accompanied by a respiratory acidosis. 99 00:04:44,720 --> 00:04:47,360 This isn't the lung tissue, this is a failure of the pump. 100 00:04:47,360 --> 00:04:49,840 So the central nervous system drive, the neuromuscular 101 00:04:49,840 --> 00:04:52,640 junction, the chest wall mechanics precisely. 102 00:04:52,680 --> 00:04:55,480 But then we get to type 3, which is honestly arguably our 103 00:04:55,480 --> 00:04:58,240 specific domain as anaesthesiologists, the 104 00:04:58,240 --> 00:05:01,000 perioperative or atelectatic respiratory failure. 105 00:05:01,160 --> 00:05:05,360 We are essentially talking about the iatrogenic consequences of 106 00:05:05,360 --> 00:05:07,240 our interventions, right? And surgical trauma. 107 00:05:07,360 --> 00:05:11,720 Indeed, we are. Type 3 relates specifically to 108 00:05:11,720 --> 00:05:14,520 post operative atelectasis and altered lung mechanics. 109 00:05:15,080 --> 00:05:17,480 I mean, think about it, When a patient undergoes major 110 00:05:17,480 --> 00:05:20,720 abdominal surgery, they experience significant pain. 111 00:05:20,800 --> 00:05:22,240 Don't they splint their diaphragm? 112 00:05:22,280 --> 00:05:24,440 Exactly. They simply will not take deep 113 00:05:24,440 --> 00:05:26,560 breaths. Combine that with the residual 114 00:05:26,560 --> 00:05:31,240 effects of volatile anesthetics, neuromuscular blockade, opioid 115 00:05:31,240 --> 00:05:33,800 induced respiratory depression. And just being supine. 116 00:05:34,040 --> 00:05:37,240 Right supine positioning, their FRC just collapses. 117 00:05:37,240 --> 00:05:39,400 Secretions pool the Airways close. 118 00:05:40,000 --> 00:05:42,440 Examiners want to see you classify this distinctly because 119 00:05:42,440 --> 00:05:45,040 the management is entirely different from a classic Type I 120 00:05:45,040 --> 00:05:46,840 pneumonia. Yeah, you're looking at early 121 00:05:46,840 --> 00:05:50,320 mobilization, regional anesthesia for pain control, 122 00:05:50,320 --> 00:05:54,560 incentive spirometry, maybe some CPAP, not antibiotics and 123 00:05:54,560 --> 00:05:55,600 proning. Exactly. 124 00:05:55,600 --> 00:05:57,120 OK. What about type 5? 125 00:05:57,120 --> 00:05:59,400 Because I feel like this is the one that really trips candidates 126 00:05:59,400 --> 00:06:01,360 up. Shock related respiratory 127 00:06:01,360 --> 00:06:02,640 failure. That's a tricky 1. 128 00:06:02,840 --> 00:06:06,080 If a patient is in profound hemorrhagic or cardiogenic 129 00:06:06,080 --> 00:06:09,320 shock, their primary problem is hemodynamic collapse. 130 00:06:09,520 --> 00:06:11,440 So why? Why on earth do we classify this 131 00:06:11,440 --> 00:06:13,240 under a respiratory failure umbrella? 132 00:06:13,440 --> 00:06:17,000 That is a phenomenal analytical question. 133 00:06:17,480 --> 00:06:19,680 Let's look at the hemodynamics of the respiratory muscles 134 00:06:19,680 --> 00:06:22,480 themselves. In a healthy resting individual, 135 00:06:22,800 --> 00:06:26,160 the work of breathing consumes merely 2 to 3% of the total 136 00:06:26,160 --> 00:06:28,240 cardiac output. It's negligible. 137 00:06:28,880 --> 00:06:31,840 But in a state of profound shock, where there is systemic 138 00:06:31,840 --> 00:06:35,280 hypoperfusion and severe metabolic acidosis, the 139 00:06:35,280 --> 00:06:39,320 respiratory rate skyrockets. The body is desperately 140 00:06:39,320 --> 00:06:42,640 attempting to blow off CO2 to compensate for the lactic 141 00:06:42,640 --> 00:06:45,080 acidosis. So the diaphragm and intercostal 142 00:06:45,080 --> 00:06:47,560 muscles are just working furiously. 143 00:06:47,560 --> 00:06:50,760 Furiously, and in this state, the work of breathing can 144 00:06:50,760 --> 00:06:53,440 consume up to 50% of total oxygen delivery. 145 00:06:53,480 --> 00:06:55,680 Wait 50% just for breathing? Yes. 146 00:06:55,720 --> 00:06:58,400 So the respiratory muscles are actively stealing blood flow 147 00:06:58,400 --> 00:07:01,080 from the splash, neck, bed, the kidneys, the brain, just to keep 148 00:07:01,080 --> 00:07:02,520 the chest wall moving. Exactly. 149 00:07:03,000 --> 00:07:06,000 The cardiovascular system is already failing and it's simply 150 00:07:06,000 --> 00:07:08,920 cannot supply enough oxygen to these vigorously working 151 00:07:08,920 --> 00:07:11,040 muscles. Eventually those muscles 152 00:07:11,040 --> 00:07:14,440 experience ischemia, day fatigue, day fatigue, and you 153 00:07:14,440 --> 00:07:18,920 get sudden respiratory arrest. When we intubate a patient in 154 00:07:18,920 --> 00:07:22,200 severe shock, we are not necessarily doing it just 155 00:07:22,200 --> 00:07:25,600 because their PO2 is low. We're doing it to take over the 156 00:07:25,600 --> 00:07:26,880 work of breathing. Wow. 157 00:07:26,880 --> 00:07:29,880 So by paralyzing and mechanically ventilating them, 158 00:07:30,040 --> 00:07:32,880 we instantly liberate that massive percentage of cardiac 159 00:07:32,880 --> 00:07:34,960 output and give it back to the vital organs. 160 00:07:34,960 --> 00:07:38,320 Precisely, if you explain that hemodynamic redistribution in 161 00:07:38,320 --> 00:07:42,320 your exam, you prove that you understand the integrated path 162 00:07:42,320 --> 00:07:44,720 of Physiology of a critically I'll patient. 163 00:07:44,920 --> 00:07:48,160 Man, that is the kind of insight that turns a failing grade into 164 00:07:48,160 --> 00:07:50,320 a distinction. All right, so we know the broad 165 00:07:50,320 --> 00:07:52,200 classifications. Let's drill down into the 166 00:07:52,200 --> 00:07:54,080 microscopic breakdown. Let's do it. 167 00:07:54,240 --> 00:07:57,160 If the respiratory muscles are intact but oxygenation is still 168 00:07:57,160 --> 00:08:00,000 failing, what is breaking down within the lung itself? 169 00:08:00,240 --> 00:08:03,040 We need to thoroughly dissect the five mechanisms of 170 00:08:03,040 --> 00:08:05,800 hypoxemia. This is exam gold. 171 00:08:05,800 --> 00:08:07,480 It's non negotiable for the exam. 172 00:08:07,640 --> 00:08:10,160 You have to be able to list, define and physiologically 173 00:08:10,160 --> 00:08:12,680 explain all 5. The 1st and overwhelmingly the 174 00:08:12,680 --> 00:08:15,720 most common is ventilation perfusion mismatch or VQ 175 00:08:15,720 --> 00:08:17,760 mismatch. This occurs when alveoli are 176 00:08:17,760 --> 00:08:21,320 under ventilated relative to their perfusion, so you get a 177 00:08:21,320 --> 00:08:24,720 low VQ ratio. And to really understand VQ 178 00:08:24,720 --> 00:08:27,560 mismatch at a postgraduate level, we have to bring up 179 00:08:27,560 --> 00:08:30,000 West's zones of the lung, right? Absolutely. 180 00:08:30,320 --> 00:08:33,440 Because gravity dictates both ventilation and perfusion, so 181 00:08:33,440 --> 00:08:36,400 they're never perfectly matched. Even in a totally healthy 182 00:08:36,400 --> 00:08:40,480 person, perfusion is heavily gravity dependent, so it pools 183 00:08:40,480 --> 00:08:43,159 in the lung bases. Yes, and ventilation also 184 00:08:43,159 --> 00:08:45,960 increases at the bases, but just not to the same degree as 185 00:08:45,960 --> 00:08:46,960 perfusion. Right. 186 00:08:46,960 --> 00:08:50,520 So the base of the lung naturally has a lower VQ ratio 187 00:08:50,520 --> 00:08:52,120 than the apex. Precisely. 188 00:08:52,360 --> 00:08:55,160 And any pathological state that exacerbates this normal 189 00:08:55,160 --> 00:08:58,440 inequality causes hypoxemia. I mean, take it Asthma 190 00:08:58,440 --> 00:09:01,280 exacerbation. Intense bronchoconstriction 191 00:09:01,280 --> 00:09:04,560 limits ventilation to certain lung units, but blood continues 192 00:09:04,560 --> 00:09:07,240 to flow past them. So the blood leaving those units 193 00:09:07,240 --> 00:09:09,360 has a super low oxygen content. Right. 194 00:09:09,520 --> 00:09:12,080 And when that mixes with the well oxygenated blood from the 195 00:09:12,080 --> 00:09:15,360 healthy units, the overall arterial PO2 drops. 196 00:09:15,400 --> 00:09:18,160 But the body has a defense mechanism against this, doesn't 197 00:09:18,160 --> 00:09:19,320 it? Hypoxic pulmonary 198 00:09:19,320 --> 00:09:23,640 vasoconstriction or HPV. Yes, and you must mention HPV in 199 00:09:23,640 --> 00:09:25,000 your answers. It's critical. 200 00:09:25,600 --> 00:09:28,800 When an alveosis becomes hypoxic, the mitochondria in the 201 00:09:28,800 --> 00:09:33,360 adjacent pulmonary smooth muscle cells actually sense that drop 202 00:09:33,360 --> 00:09:35,480 in oxygen. And that triggers a cascade. 203 00:09:35,880 --> 00:09:38,040 It does. It changes potassium channel 204 00:09:38,040 --> 00:09:41,920 activity leading to cellular depolarization, calcium influx, 205 00:09:42,200 --> 00:09:44,080 and intense local vesoconstriction. 206 00:09:44,640 --> 00:09:47,520 The body intelligently shunts blood away from the poorly 207 00:09:47,520 --> 00:09:50,600 ventilated alveolus toward the better ventilated areas. 208 00:09:50,800 --> 00:09:53,120 Trying to fix its own VQ mismatch? 209 00:09:53,120 --> 00:09:54,600 Exactly. But what happens when that 210 00:09:54,600 --> 00:09:57,280 mismatch is so severe that it just completely eliminates 211 00:09:57,280 --> 00:10:00,000 ventilation? That brings us to mechanism #2 212 00:10:00,480 --> 00:10:04,560 shunt, specifically right to left shunt, where the VQ ratio 213 00:10:04,560 --> 00:10:07,880 equals absolute 0. A shunt occurs when mixed venous 214 00:10:07,880 --> 00:10:10,360 blood from the right side of the heart passes to the left side 215 00:10:10,640 --> 00:10:12,880 without ever participating in gas exchange. 216 00:10:13,120 --> 00:10:15,240 So it completely bypasses the system. 217 00:10:15,360 --> 00:10:17,080 Right. This could be intracardiac like 218 00:10:17,080 --> 00:10:20,880 a VSD or tetralogy of phthalate, or more commonly for us it could 219 00:10:20,880 --> 00:10:23,560 be inter pulmonary. Like when alveoli are totally 220 00:10:23,560 --> 00:10:26,760 filled with fluid in ARDS or completely collapsed and severe 221 00:10:26,760 --> 00:10:29,240 atelectasis. Let me push you on a critical 222 00:10:29,240 --> 00:10:31,960 clinical scenario regarding shunt, because this comes up a 223 00:10:31,960 --> 00:10:34,800 lot. If a true shunt means blood is 224 00:10:34,800 --> 00:10:38,880 completely bypassing ventilated alveoli, then walking up to the 225 00:10:38,880 --> 00:10:43,400 ventilator at 3:00 AM and cranking the FIO 2 up to 100%, 226 00:10:44,160 --> 00:10:45,800 that's not just unhelpful, right? 227 00:10:46,040 --> 00:10:50,200 It's potentially causing oxygen toxicity without actually fixing 228 00:10:50,200 --> 00:10:53,240 the profound hypoxia. That right there is the ultimate 229 00:10:53,240 --> 00:10:55,680 test of a candidate's understanding of oxygen 230 00:10:55,680 --> 00:10:58,680 transport. What's the physiological reason 231 00:10:58,680 --> 00:11:02,240 for that failure to respond? Remember the oxygen delivery 232 00:11:02,240 --> 00:11:04,400 equation. The vast majority of oxygen is 233 00:11:04,400 --> 00:11:07,200 bound to hemoglobin. Only a tiny tiny fraction is 234 00:11:07,200 --> 00:11:10,160 dissolved in the plasma. In a severe shunt, the blood 235 00:11:10,160 --> 00:11:13,400 perfusing the colast alveoli leaves completely deoxygenated. 236 00:11:13,400 --> 00:11:14,880 If there's no air. There right? 237 00:11:15,160 --> 00:11:17,880 Meanwhile the blood perfusing the healthy alveoli is already 238 00:11:17,880 --> 00:11:21,080 100% saturated. Even at room air or moderate Fio 239 00:11:21,080 --> 00:11:24,760 2, you physically cannot put more than 4 molecules of oxygen 240 00:11:24,960 --> 00:11:27,480 on a single hemoglobin molecule. Right, it's maxed out. 241 00:11:27,680 --> 00:11:32,520 So by increasing the Fio 2 to 100%, you were only slightly 242 00:11:32,520 --> 00:11:36,080 increasing the dissolved oxygen in the plasma of those healthy 243 00:11:36,080 --> 00:11:40,520 units when that tiny amount of extra dissolved oxygen mixes 244 00:11:40,600 --> 00:11:44,560 with the massive volume of completely deoxygenated shunted 245 00:11:44,560 --> 00:11:45,720 blood. It does nothing. 246 00:11:45,720 --> 00:11:47,720 The PO2 barely moves. Exactly. 247 00:11:47,840 --> 00:11:50,120 Oxygen is essentially useless for a true shunt. 248 00:11:50,880 --> 00:11:53,680 So what's the actual physiological intervention then? 249 00:11:53,800 --> 00:11:57,000 You must apply positive and expiratory pressure or PEEP. 250 00:11:57,520 --> 00:12:00,400 PEEP physically splints the collapsed Airways open. 251 00:12:00,880 --> 00:12:03,600 It forces alveolar fluid back into the interstitium and 252 00:12:03,600 --> 00:12:06,360 recruits those collapsed units back into the functional lung 253 00:12:06,360 --> 00:12:09,880 volume. Converting A0 VQ ratio back into 254 00:12:10,000 --> 00:12:12,120 a viable ventilated unit Precisely. 255 00:12:12,120 --> 00:12:14,720 OK, so we have VQ mismatch and shunt. 256 00:12:14,880 --> 00:12:17,200 What is the third mechanism of hypoxemia? 257 00:12:17,200 --> 00:12:20,120 Diffusion impairment. This relates directly to fixed 258 00:12:20,120 --> 00:12:23,200 law of diffusion, which states that the rate of gas transfer is 259 00:12:23,200 --> 00:12:25,520 directly proportional to the surface area and pressure 260 00:12:25,520 --> 00:12:28,240 gradient and inversely proportional to the thickness of 261 00:12:28,240 --> 00:12:30,280 the membrane. Right, so conditions like 262 00:12:30,280 --> 00:12:33,280 pulmonary fibrosis or interstitial lung disease, where 263 00:12:33,280 --> 00:12:36,000 the membrane gets severely thickened with scar tissue. 264 00:12:36,280 --> 00:12:39,440 And the classic Viva trap here involves exercise, doesn't it? 265 00:12:39,840 --> 00:12:42,480 Like a patient with early pulmonary fibrosis might have a 266 00:12:42,480 --> 00:12:46,120 normal PO2 sitting on the exam table, but the moment they walk 267 00:12:46,120 --> 00:12:48,720 down the hall they become profoundly desaturated. 268 00:12:49,160 --> 00:12:51,600 Exactly, and you need to explain the timing. 269 00:12:52,040 --> 00:12:56,440 At rest, a red blood cell spends about .75 seconds traversing the 270 00:12:56,440 --> 00:12:59,160 pulmonary capillary. Even with a thickened membrane, 271 00:12:59,480 --> 00:13:01,800 oxygen usually has enough time to equilibrate. 272 00:13:02,360 --> 00:13:06,000 But during exercise, cardiac output massively increases. 273 00:13:06,000 --> 00:13:08,640 Right, which drives the blood through the capillary much 274 00:13:08,640 --> 00:13:10,680 faster. The transit time might drop to 275 00:13:10,680 --> 00:13:14,200 .25 seconds. The oxygen simply cannot diffuse 276 00:13:14,200 --> 00:13:17,200 across that thick fibrotic barrier fast enough before the 277 00:13:17,200 --> 00:13:19,560 red blood cell is just swept away into the systemic 278 00:13:19,560 --> 00:13:21,080 circulation. That makes perfect sense. 279 00:13:21,440 --> 00:13:23,800 OK, 4th mechanism. Hypoventilation. 280 00:13:24,000 --> 00:13:26,480 This is a global reduction in minute ventilation. 281 00:13:26,760 --> 00:13:29,160 We'll explore this deeply when we look at the alveolar gas 282 00:13:29,160 --> 00:13:31,760 equation, but essentially, if you aren't bringing fresh air 283 00:13:31,760 --> 00:13:35,080 into the lungs, the partial pressure of carbon dioxide in 284 00:13:35,080 --> 00:13:38,480 the alveolus rises steadily as metabolism continues. 285 00:13:38,640 --> 00:13:41,680 And because the total pressure in the alveolus is fixed by 286 00:13:41,680 --> 00:13:46,280 atmospheric pressure, that rising CO2 physically displaces 287 00:13:46,280 --> 00:13:48,960 the oxygen molecules. Leading to hypoxemia, Yes. 288 00:13:49,160 --> 00:13:51,440 Got it. And the 5th and final mechanism. 289 00:13:51,440 --> 00:13:55,480 Reduced inspired oxygen tension or a low Fio 2. 290 00:13:56,400 --> 00:13:59,440 The textbook example is ascending to high altitude where 291 00:13:59,440 --> 00:14:02,440 the barometric pressure drops, thus dropping the partial 292 00:14:02,440 --> 00:14:05,360 pressure of oxygen. But in the realm of anesthesia, 293 00:14:05,360 --> 00:14:06,960 we need to talk about equipment, right? 294 00:14:07,000 --> 00:14:08,960 Absolutely. You must mention the risk of a 295 00:14:08,960 --> 00:14:13,080 hypoxic gas mixture due to a pipeline crossover and empty 296 00:14:13,080 --> 00:14:15,360 oxygen cylinder or a flow meter failure. 297 00:14:15,360 --> 00:14:17,120 Great. So let's apply these mechanisms 298 00:14:17,120 --> 00:14:20,560 to real clinical pathologies. How does the examiner want us to 299 00:14:20,560 --> 00:14:23,840 categorize the causes of Type I and Type 2 failure? 300 00:14:23,840 --> 00:14:26,720 For type I, the parenchymal failure, you're looking at 301 00:14:26,720 --> 00:14:28,960 diseases that fill or collapse the alveoli. 302 00:14:29,400 --> 00:14:32,280 Pneumonia fills them with pus and inflammatory exudate. 303 00:14:32,800 --> 00:14:35,600 Cardiogenic pulmonary edema fills them with transsudative 304 00:14:35,600 --> 00:14:38,320 fluid due to elevated hydrostatic pressure from a 305 00:14:38,320 --> 00:14:41,440 failing left ventricle. And ARDS fills them with protein 306 00:14:41,440 --> 00:14:43,840 rich fluid because of overwhelming capillary leak. 307 00:14:43,880 --> 00:14:46,440 Let's talk about pulmonary embolism because this is just a 308 00:14:46,440 --> 00:14:48,400 fascinating pathological evolution. 309 00:14:48,560 --> 00:14:52,600 If the examiner gives you a scenario day three post op from 310 00:14:52,600 --> 00:14:56,720 major orthopedic surgery, sudden hypoxemia and tachycardia, we 311 00:14:56,720 --> 00:14:58,200 suspect APE. Classy. 312 00:14:58,240 --> 00:15:00,680 But wait. APE physically blocks blood flow 313 00:15:00,680 --> 00:15:03,600 to a segment of the lung that creates ventilation without 314 00:15:03,600 --> 00:15:07,880 perfusion, which is alveolar Dead Space or an infinitely high 315 00:15:07,880 --> 00:15:10,720 VQ ratio. And Dead Space impairs CO2 316 00:15:10,720 --> 00:15:12,800 clearance leading to hypercapnia. 317 00:15:13,040 --> 00:15:16,840 So why is the hallmark of an acute PE profound hypoxemia? 318 00:15:17,000 --> 00:15:20,280 That is the exact physiological paradox the examiner wants you 319 00:15:20,280 --> 00:15:22,200 to solve out loud. You're correct, the initial 320 00:15:22,200 --> 00:15:25,760 insult is pure Dead Space, but within minutes a complex 321 00:15:25,760 --> 00:15:27,880 secondary cascade begins. The platelets. 322 00:15:27,880 --> 00:15:29,560 Yes, the platelets. Adhering to the embolus and 323 00:15:29,560 --> 00:15:31,920 degranulate, They release potent vasoactive and 324 00:15:31,920 --> 00:15:35,040 bronchoconstrictive mediators like serotonin and thromboxane 325 00:15:35,040 --> 00:15:37,360 A2. This causes intense localized 326 00:15:37,360 --> 00:15:40,640 bronchospasm in the surrounding uninfarcted lung tissue. 327 00:15:40,640 --> 00:15:42,320 Wow. And the lack of pulmonary blood 328 00:15:42,320 --> 00:15:44,200 flow starves the type 2 pneumocytes too. 329 00:15:44,200 --> 00:15:46,520 Right, exactly. They cease producing surfactant. 330 00:15:46,720 --> 00:15:51,000 And without surfactant, alveolar surface tension skyrockets and 331 00:15:51,000 --> 00:15:52,720 the alveoli collapse. Precisely. 332 00:15:52,760 --> 00:15:55,040 So now you have severe adilectosis and 333 00:15:55,040 --> 00:15:57,880 bronchoconstriction in the lung units surrounding the embolus. 334 00:15:58,200 --> 00:16:01,080 As blood flow is diverted away from the blocked artery, it's 335 00:16:01,080 --> 00:16:04,720 forced to perfuse these now collapsed, unventilated alveoli. 336 00:16:04,960 --> 00:16:08,600 So the initial high VQ Dead Space has rapidly evolved into a 337 00:16:08,600 --> 00:16:13,200 low VQ functional shunt. Yes, that secondary shunting is 338 00:16:13,200 --> 00:16:17,520 the primary driver of the severe refractory hypoxemia seen in a 339 00:16:17,520 --> 00:16:20,240 massive PE. That is a brilliant breakdown. 340 00:16:20,360 --> 00:16:23,160 Now let's look at the pump failure, the causes of type 2 341 00:16:23,160 --> 00:16:25,680 respiratory failure. For your exam structure, always 342 00:16:25,680 --> 00:16:29,040 break type 2 down anatomically. Start at the brain and work your 343 00:16:29,040 --> 00:16:31,760 way down. OK, so first CNS depression. 344 00:16:31,880 --> 00:16:34,760 Yes, this includes drug overdoses like opioids, 345 00:16:34,760 --> 00:16:37,840 benzodiazepines, or barbiturates that suppress the medullary 346 00:16:37,840 --> 00:16:40,280 respiratory centers. It also includes brain stem 347 00:16:40,280 --> 00:16:43,360 strokes or profound hypothyroidism leading to Myxima 348 00:16:43,360 --> 00:16:45,440 coma. Second would be the wiring, the 349 00:16:45,440 --> 00:16:48,320 neuromuscular disorders. Right conditions like Guillain 350 00:16:48,320 --> 00:16:52,280 Barre syndrome, myasthenia gravis, ALS, or a high cervical 351 00:16:52,280 --> 00:16:54,480 cord lesion. Take Guillain Barre for example. 352 00:16:54,720 --> 00:16:58,000 You have an acute ascending demyelinating polyneuropathy. 353 00:16:58,080 --> 00:17:00,920 As the demyelination reaches the phrenic nerves and intracostal 354 00:17:00,920 --> 00:17:02,800 nerves, the motor units just fail. 355 00:17:02,800 --> 00:17:05,440 They do. The patient's vital capacity 356 00:17:05,440 --> 00:17:08,880 progressively drops. They physically cannot generate 357 00:17:08,880 --> 00:17:12,000 the negative intrathoracic pressure required to pull air 358 00:17:12,000 --> 00:17:15,160 into the lungs. Alveolar hypoventilation ensues 359 00:17:15,319 --> 00:17:18,720 and CO2 begins to climb. 3rd the physical structure itself. 360 00:17:18,839 --> 00:17:22,160 Chest wall and plural disorders. Yes, severe chyphoscoliosis 361 00:17:22,160 --> 00:17:23,920 mechanically restricts expansion. 362 00:17:24,119 --> 00:17:27,319 Morbid obesity causes massive chest wall loading, which 363 00:17:27,319 --> 00:17:31,520 drastically reduces compliance, or a massive pleural effusion or 364 00:17:31,520 --> 00:17:34,440 tension pneumothorax that physically compresses the lung 365 00:17:34,440 --> 00:17:36,720 tissue. And finally, airway obstruction. 366 00:17:37,160 --> 00:17:41,120 COPD is the most common chronic cause here, but severe acute 367 00:17:41,120 --> 00:17:44,440 asthma exacerbations can also lead to type 2 failure when the 368 00:17:44,440 --> 00:17:46,960 patient finally fatigues after hours of breathing against 369 00:17:46,960 --> 00:17:50,080 immense airway resistance. OK, we know the definitions, the 370 00:17:50,080 --> 00:17:53,480 classifications, the mechanisms, and the causes, but how do we 371 00:17:53,480 --> 00:17:56,840 actually prove which mechanism is at play in the clinical 372 00:17:56,840 --> 00:17:58,640 setting? We need to transition to 373 00:17:58,640 --> 00:18:01,400 arterial blood gas interpretation because reading 374 00:18:01,400 --> 00:18:05,640 an ABG in an OSCE under a strict time limit is terrifying. 375 00:18:05,720 --> 00:18:08,600 It can be the math gets jumbled, the numbers blur together. 376 00:18:08,640 --> 00:18:10,840 We need a rigid, bulletproof system. 377 00:18:10,840 --> 00:18:14,120 You must approach every ABG like a mathematical proof. 378 00:18:14,320 --> 00:18:17,560 Step one, look at the pay O2. Is the patient hypoxemic? 379 00:18:17,880 --> 00:18:22,040 Step 2 look at the pay O2 is it low, normal or elevated? 380 00:18:22,560 --> 00:18:25,440 This immediately differentiates type I from type 2. 381 00:18:25,760 --> 00:18:29,560 Step three, look at the pH. Acidemia, alkalimia or normal? 382 00:18:29,840 --> 00:18:32,800 Exactly and Step 4 assess compensation. 383 00:18:32,800 --> 00:18:35,720 This is where you differentiate an acute respiratory acidosis 384 00:18:36,040 --> 00:18:38,360 from a chronic 1. Let's demystify the math of 385 00:18:38,360 --> 00:18:40,560 compensation. How much should the bicarbonate 386 00:18:40,560 --> 00:18:42,400 rise in response to a climbing CO2? 387 00:18:42,760 --> 00:18:46,360 You must memorize the one for 10 and four for 10 rules in acute 388 00:18:46,360 --> 00:18:49,320 respiratory acidosis, say an acute opioid overdose. 389 00:18:49,720 --> 00:18:51,680 The kidneys haven't had time to compensate. 390 00:18:52,160 --> 00:18:54,080 The initial buffering is purely cellular. 391 00:18:54,240 --> 00:18:58,400 So for every 10mm of mercury rise in pack O2 above the normal 392 00:18:58,400 --> 00:19:01,760 baseline of 40, the serum bicarbonate will passively 393 00:19:01,760 --> 00:19:03,680 increase by 1 mill equivalent per liter. 394 00:19:03,960 --> 00:19:07,680 Correct, but if it's a chronic condition like severe COPD, the. 395 00:19:07,760 --> 00:19:10,160 Kidneys have had days or weeks to adapt. 396 00:19:10,320 --> 00:19:12,200 Right. They up regulate the retention 397 00:19:12,200 --> 00:19:15,040 of bicarbonate and the excretion of hydrogen ions. 398 00:19:15,440 --> 00:19:17,600 In this state. For every 10 milliliter of 399 00:19:17,600 --> 00:19:20,920 mercury rise in pack O2 above 40, the bicarbonate will 400 00:19:20,920 --> 00:19:23,120 increase by 4 mill equivalents per liter. 401 00:19:23,480 --> 00:19:26,760 OK, so if a patient has a baseline pack O2 of 70, which is 402 00:19:26,760 --> 00:19:30,080 30 above normal, their bicarbonate should be elevated 403 00:19:30,080 --> 00:19:34,160 by roughly 12, putting them at a baseline bicarb of around 36. 404 00:19:34,160 --> 00:19:36,880 Exactly. And if their bicarb is 36 and 405 00:19:36,880 --> 00:19:39,920 their pH is near normal, you know this is a chronic 406 00:19:39,920 --> 00:19:42,720 compensated state. You absolutely must not rapidly 407 00:19:42,720 --> 00:19:46,320 ventilate their CO2 down to 40 or you'll cause a profound life 408 00:19:46,320 --> 00:19:50,040 threatening metabolic alkalosis. That's such a crucial point, but 409 00:19:50,040 --> 00:19:53,640 the absolute Holy Grail of ABG interpretation in an exam 410 00:19:53,640 --> 00:19:57,320 setting is the alveolar arterial or a a ingredient if you fail to 411 00:19:57,320 --> 00:19:59,160 calculate this in Aviva. You will likely fail the 412 00:19:59,160 --> 00:20:00,040 station. Period. 413 00:20:00,240 --> 00:20:02,640 Let's focus heavily on this. I've always struggled with the 414 00:20:02,640 --> 00:20:05,240 abstract nature of the alveolar gas equation. 415 00:20:05,240 --> 00:20:07,080 It's just a string of letters and numbers. 416 00:20:07,440 --> 00:20:10,920 Can we conceptualize this? Like, let's think of the 417 00:20:10,920 --> 00:20:14,680 alveolus not as a balloon, but as a rigid box with a fixed 418 00:20:14,680 --> 00:20:16,280 volume. I like that. 419 00:20:16,600 --> 00:20:19,720 That's an excellent framework. At a given atmospheric pressure 420 00:20:19,720 --> 00:20:22,920 at sea level, that box can only hold a certain absolute number 421 00:20:22,920 --> 00:20:24,200 of gas molecules. Right. 422 00:20:24,200 --> 00:20:27,960 The pressure inside that rigid box is determined by atmospheric 423 00:20:27,960 --> 00:20:29,880 pressure. Now the majority of the 424 00:20:29,880 --> 00:20:32,720 molecules in the box are nitrogen, which is inert and 425 00:20:32,720 --> 00:20:34,360 doesn't participate in gas exchange. 426 00:20:35,040 --> 00:20:37,280 Water vapor takes up a fixed amount of pressure too. 427 00:20:37,400 --> 00:20:40,200 So what is leftover? Is the space available for just 428 00:20:40,320 --> 00:20:42,880 oxygen and carbon dioxide? Precisely so if a patient 429 00:20:42,880 --> 00:20:45,800 develops Guillain Barre and stops ventilating, the metabolic 430 00:20:45,800 --> 00:20:49,920 production of CO2 continues. CO2 molecules begin flooding 431 00:20:49,920 --> 00:20:53,120 into that rigid box. Because the box cannot expand 432 00:20:53,200 --> 00:20:56,640 and the pressure must remain constant, those accumulating CO2 433 00:20:56,640 --> 00:20:59,640 molecules physically displace and push the oxygen molecules 434 00:20:59,640 --> 00:21:00,880 out of the alveolus. Wow. 435 00:21:00,880 --> 00:21:02,400 OK. So the partial pressure of 436 00:21:02,400 --> 00:21:06,480 oxygen in the alveolus capital PAO 2 must fall as the Paco 2 437 00:21:06,480 --> 00:21:08,800 rises? That physical displacement is 438 00:21:08,800 --> 00:21:12,120 represented mathematically by the alveolar gas equation. 439 00:21:12,960 --> 00:21:14,440 Listeners write this down carefully. 440 00:21:15,080 --> 00:21:19,120 PAO 2 equals the Fio 2 multiplied by the atmospheric 441 00:21:19,120 --> 00:21:21,320 pressure minus the vapor pressure of water. 442 00:21:21,320 --> 00:21:24,960 OK, from that entire product you subtract the pay KO2 divided by 443 00:21:24,960 --> 00:21:27,160 the respiratory quotient, which is typically .8. 444 00:21:27,320 --> 00:21:29,880 Let's do the math live for a specific clinical case so they 445 00:21:29,880 --> 00:21:32,800 can hear it in action. Patient is on room air which is 446 00:21:32,800 --> 00:21:36,560 21% or .21 Fio 2. We're at sea level so 447 00:21:36,560 --> 00:21:39,960 atmospheric pressure of 760. Water vapor is 47. 448 00:21:40,280 --> 00:21:44,920 So 760 -, 47 is 713. Multiply that by .21 and you get 449 00:21:44,920 --> 00:21:48,520 a approximately 150. 150 This is the partial pressure of oxygen 450 00:21:48,520 --> 00:21:51,040 in the conducting Airways before gas exchange begins. 451 00:21:51,040 --> 00:21:52,560 Correct. Now we must subtract the 452 00:21:52,560 --> 00:21:55,680 alveolar CO2. Since alveolar CO2 rapidly 453 00:21:55,680 --> 00:21:59,360 equilibrates with arterial CO2. We use the Peco 2 from our blood 454 00:21:59,360 --> 00:22:02,720 gas divided by .8. OK, let's say our exam candidate 455 00:22:02,720 --> 00:22:05,080 is handed an ABG. The patient is breathing room 456 00:22:05,080 --> 00:22:07,280 air. Their PO2 is 50 and their PECO 2 457 00:22:07,280 --> 00:22:09,200 is 70. The immediate reaction of a 458 00:22:09,200 --> 00:22:12,200 panic student is to look at the low PO2 and say, oh, the patient 459 00:22:12,200 --> 00:22:13,680 has lung disease, maybe pneumonia. 460 00:22:13,800 --> 00:22:16,560 Which is immediately incorrect. Let's run your numbers through 461 00:22:16,560 --> 00:22:19,480 the rigid box. We start with 150, then we 462 00:22:19,480 --> 00:22:25,240 subtract the result of 70 / .8. 70 / .8 is 87.5. 463 00:22:25,240 --> 00:22:30,080 Right, so 150 -, 87.5 leaves us with a PA O2, the oxygen 464 00:22:30,080 --> 00:22:33,880 actually sitting in the alveolus of 62.5mm of mercury. 465 00:22:33,920 --> 00:22:38,280 So the box only contains 62.5mm of mercury of oxygen, purely 466 00:22:38,280 --> 00:22:40,360 because the CO2 took up all the space. 467 00:22:40,360 --> 00:22:42,560 Exactly. Now we calculate the gradient. 468 00:22:42,720 --> 00:22:46,720 The albeolar oxygen is 62.5. The ABG tells us the arterial 469 00:22:46,720 --> 00:22:50,880 oxygen is 50. We subtract the 2. 62.5 - 50 is 470 00:22:50,880 --> 00:22:55,000 an A a gradient of 12.5. An A gradient of 12.5 on room 471 00:22:55,000 --> 00:22:57,480 air is entirely normal. A normal gradient generally 472 00:22:57,480 --> 00:23:00,120 falls between 5 and 15, increasing slightly with age. 473 00:23:00,560 --> 00:23:02,720 What this normal Grady definitively proves is that 474 00:23:02,720 --> 00:23:04,600 there is absolutely no barrier to diffusion. 475 00:23:04,600 --> 00:23:07,200 There is no shunt. There is no VQ mismatch. 476 00:23:07,240 --> 00:23:09,760 The lung parent commit is pristine, the oxygen transferred 477 00:23:09,760 --> 00:23:11,480 perfectly from the alveolus to the blood. 478 00:23:11,480 --> 00:23:13,920 The only reason the arterial oxygen is low is because the 479 00:23:13,920 --> 00:23:16,480 alveolar oxygen was displaced by hypoventilation. 480 00:23:16,600 --> 00:23:18,760 That is the ultimate Viva trap right there. 481 00:23:18,960 --> 00:23:22,720 If you see a Peyo 2 of 50 and a Peyco 2 of 70, you must 482 00:23:22,720 --> 00:23:25,840 calculate the A gradient before you blurt out a diagnosis. 483 00:23:26,400 --> 00:23:28,280 If it's normal, the lungs are fine. 484 00:23:28,360 --> 00:23:32,360 You need to investigate the CNS, the spinal cord, or the 485 00:23:32,360 --> 00:23:34,240 neuromuscular junction. Exactly. 486 00:23:34,320 --> 00:23:37,480 And if you calculate it and the gradient is wide in say 40 or 487 00:23:37,480 --> 00:23:40,720 50, then you know you have concurrent parenchymal disease. 488 00:23:40,960 --> 00:23:44,120 That single calculation is the difference between passing and 489 00:23:44,120 --> 00:23:46,080 failing. The mathematics dictates the 490 00:23:46,080 --> 00:23:48,640 clinical management. Now let's move away from the 491 00:23:48,640 --> 00:23:50,240 paper and look at the patient in the bed. 492 00:23:50,880 --> 00:23:53,360 We need to discuss clinical features and diagnosis. 493 00:23:53,400 --> 00:23:56,600 We're always taught to look for cyanosis as the cardinal sign of 494 00:23:56,600 --> 00:23:59,400 hypoxia, but is it actually reliable? 495 00:23:59,760 --> 00:24:03,160 It is notoriously unreliable depending on its critical error. 496 00:24:03,480 --> 00:24:05,960 Central cyanosis, the bluish discoloration of the tongue and 497 00:24:05,960 --> 00:24:08,600 mucous membranes, does not depend on the percentage of 498 00:24:08,600 --> 00:24:10,920 oxygen saturation. What does it depend on? 499 00:24:10,920 --> 00:24:13,680 It depends on the absolute concentration of deoxygenated 500 00:24:13,680 --> 00:24:17,680 hemoglobin in the capillaries. Specifically, you need about 5g 501 00:24:17,680 --> 00:24:21,280 per deciliter of deoxygenated hemoglobin to visually perceive 502 00:24:21,280 --> 00:24:23,480 cyanosis. So if a patient has severe 503 00:24:23,480 --> 00:24:25,840 anemia, let's say they had a massive intraoperative 504 00:24:25,840 --> 00:24:28,880 hemorrhage and their total hemoglobin is now only 6. 505 00:24:29,000 --> 00:24:33,200 If their total hemoglobin is 6, they could have an Spo 2 of 70%, 506 00:24:33,280 --> 00:24:36,160 meaning they are profoundly hypoxic and suffering cellular 507 00:24:36,160 --> 00:24:40,480 death, but they will only have roughly 1.8g of deoxygenated 508 00:24:40,480 --> 00:24:42,440 hemoglobin. So they won't look blue at all. 509 00:24:42,600 --> 00:24:45,520 They will look pale, perhaps ashen, but they will never turn 510 00:24:45,520 --> 00:24:48,640 cyanotic because they simply do not possess enough hemoglobin to 511 00:24:48,640 --> 00:24:52,800 cross that 5g visual threshold. Conversely, a patient with 512 00:24:52,800 --> 00:24:56,200 severe polycythemia and a hemoglobin of 20 might look 513 00:24:56,200 --> 00:24:58,880 completely blue while maintaining a perfectly adequate 514 00:24:58,880 --> 00:25:02,120 oxygen delivery. Wow, so you must rely on your 515 00:25:02,120 --> 00:25:05,000 ABG and your clinical signs of sympathetic activation. 516 00:25:05,400 --> 00:25:08,040 Tachycardia, tachypnia, agitation, confusion. 517 00:25:08,040 --> 00:25:09,360 Absolutely. What about the signs of 518 00:25:09,360 --> 00:25:11,520 hypercapnia? Carbon dioxide is a potent 519 00:25:11,520 --> 00:25:14,160 peripheral vasodilator and a CNS depressant. 520 00:25:14,400 --> 00:25:17,280 Clinically, you'll see warm flushed extremities, a bounding 521 00:25:17,280 --> 00:25:21,400 pulse, tremors, asterixis or flapping tremors, headache, and 522 00:25:21,400 --> 00:25:23,520 progressively deepening drowsiness or coma. 523 00:25:23,600 --> 00:25:26,640 The vasodilation aspect is a massive trap in neuroanesesis, 524 00:25:26,680 --> 00:25:28,280 right? It is paramount. 525 00:25:28,760 --> 00:25:31,160 CO2 doesn't just dilate peripheral vessels. 526 00:25:31,400 --> 00:25:34,360 It is the primary regulator of cerebral blood flow. 527 00:25:34,920 --> 00:25:39,200 For every 1mm of mercury rise in Paco 2, cerebral blood flow 528 00:25:39,200 --> 00:25:42,840 increases by approximately 1 to 2 milliliters per 100 grams of 529 00:25:42,840 --> 00:25:45,600 brain tissue per minute. So if you have a patient with a 530 00:25:45,600 --> 00:25:49,720 traumatic brain injury who goes into type 2 respiratory failure. 531 00:25:49,720 --> 00:25:53,800 That rising CO2 will massively engorge the cerebral vasculature 532 00:25:54,120 --> 00:25:57,120 in a fixed cranial vault. That increased blood volume 533 00:25:57,120 --> 00:26:00,520 causes a spike in intracranial pressure, potentially leading to 534 00:26:00,520 --> 00:26:03,800 brain herniation and death. This is why strict Paco 2 535 00:26:03,800 --> 00:26:07,400 control, often targeting low normal levels, is mandatory in 536 00:26:07,400 --> 00:26:10,560 neuroanesthesia. OK, aside from the ABG, what 537 00:26:10,560 --> 00:26:13,240 other investigations are examiners looking for? 538 00:26:13,240 --> 00:26:16,160 The chest X-ray is fundamental primarily to differentiate the 539 00:26:16,160 --> 00:26:20,000 causes of acute pulmonary edema. If the X-ray shows cardiomegaly, 540 00:26:20,000 --> 00:26:22,640 curly B lines indicating intracellular fluid and pleural 541 00:26:22,640 --> 00:26:25,560 effusions, you're likely dealing with cardiogenic failure. 542 00:26:25,760 --> 00:26:28,880 But if the heart size is normal and you see diffuse bilateral 543 00:26:28,880 --> 00:26:32,640 patchy alveolar infiltrates the classic whiteout lung, you 544 00:26:32,640 --> 00:26:36,040 suspect non cardiogenic edema, specifically ARDS. 545 00:26:36,120 --> 00:26:38,560 And what role do pulmonary function tests play? 546 00:26:38,840 --> 00:26:40,720 Because we don't usually do them at 3:00 AM. 547 00:26:40,760 --> 00:26:44,720 True, we don't, but for chronic causes of respiratory failure, 548 00:26:44,720 --> 00:26:47,880 KFT's are vital for differentiating obstructive from 549 00:26:47,880 --> 00:26:51,120 restrictive diseases. In obstructive disease like 550 00:26:51,280 --> 00:26:57,320 COPD, the hallmark is a low FEV 1 FVC ratio, typically less than 551 00:26:57,320 --> 00:26:59,360 .7. And in restrictive disease like 552 00:26:59,360 --> 00:27:02,440 pulmonary fibrosis. The total lung capacity and FEC 553 00:27:02,440 --> 00:27:06,360 are low, but the FEV one FEC ratio remains normal or even 554 00:27:06,360 --> 00:27:09,600 high because the elastic recoil is preserved or increased. 555 00:27:10,080 --> 00:27:12,360 You should be prepared to draw the flow volume loops on the 556 00:27:12,360 --> 00:27:14,360 whiteboard too. Right, an obstructive loop will 557 00:27:14,360 --> 00:27:18,000 show a classic scooped out or coved expiratory phase due to 558 00:27:18,000 --> 00:27:19,920 dynamic airway collapse. Exactly. 559 00:27:19,960 --> 00:27:22,520 You must be able to draw that. All right, we have classified 560 00:27:22,520 --> 00:27:24,920 it, mapped the path of Physiology, calculated the 561 00:27:24,920 --> 00:27:26,480 gradient and reviewed the imaging. 562 00:27:26,840 --> 00:27:29,880 Now we must treat the patient. Management is absolutely 563 00:27:29,880 --> 00:27:31,440 essential. Where do we begin? 564 00:27:31,720 --> 00:27:34,200 You never ever skipped the basics in an exam. 565 00:27:34,600 --> 00:27:38,160 You start with the rigid ABC framework airway assessed 566 00:27:38,160 --> 00:27:40,640 patency. If the patient has an altered 567 00:27:40,640 --> 00:27:44,560 sensorium and absent gag reflex where is unable to clear copious 568 00:27:44,560 --> 00:27:47,400 secretions, you secure the airway immediately with 569 00:27:47,400 --> 00:27:50,920 endotracheal intubation. Breathing, optimize oxygenation 570 00:27:50,920 --> 00:27:53,720 and assess the need for mechanical ventilatory support. 571 00:27:53,800 --> 00:27:57,600 Right and circulation. Establish large bore IV access, 572 00:27:57,960 --> 00:28:01,440 commence fluid resuscitation if indicated, and manage shock with 573 00:28:01,440 --> 00:28:04,680 vasopressors or inner tropes to ensure that once you oxygenate 574 00:28:04,680 --> 00:28:06,800 the blood, it actually gets to the tissues. 575 00:28:07,400 --> 00:28:09,600 Let's talk about oxygen therapy under breathing. 576 00:28:09,920 --> 00:28:12,760 The junior reflex is often to just crank the flow meter up to 577 00:28:12,760 --> 00:28:15,440 15 liters through a non rebreather mask for everyone who 578 00:28:15,440 --> 00:28:17,360 looks breathless. But that's dangerous, isn't it? 579 00:28:17,360 --> 00:28:19,800 Saying that in Aviva will immediately cost you marks. 580 00:28:19,960 --> 00:28:22,680 Oxygen is a drug. It has specific indications, 581 00:28:22,680 --> 00:28:25,640 toxicities and targets. In a patient with no chronic 582 00:28:25,640 --> 00:28:29,120 lung disease presenting with acute hypoxemia, your target SO2 583 00:28:29,120 --> 00:28:31,880 is generally 92 to 98%. But in a patient with chronic 584 00:28:32,040 --> 00:28:35,080 COPD who is a known CO2 retainer, your target is 585 00:28:35,080 --> 00:28:38,880 strictly 88 to 92%. We need to explain exactly why 586 00:28:38,880 --> 00:28:43,640 oxygen can kill a COPD retainer. The old dogma was simply that 587 00:28:43,640 --> 00:28:47,840 their brain is so used to high CO2 that their respiratory Dr. 588 00:28:47,840 --> 00:28:51,320 switches entirely to a hypoxic Dr. and if you give them oxygen 589 00:28:51,320 --> 00:28:54,280 they just stop breathing. But modern Physiology tells us 590 00:28:54,280 --> 00:28:55,720 it's much more complex than that. 591 00:28:55,880 --> 00:28:59,320 The hypoxic Dr. theory is a gross oversimplification. 592 00:28:59,920 --> 00:29:03,840 Giving high flow oxygen to a COPD patient exacerbates 593 00:29:03,840 --> 00:29:07,440 hypercapnia through 2 distinct physiological mechanisms that 594 00:29:07,440 --> 00:29:10,360 you must articulate. The first is the Haldane effect. 595 00:29:10,440 --> 00:29:13,360 Let's break that down. Deoxygenated hemoglobin has a 596 00:29:13,360 --> 00:29:15,520 high affinity for binding carbon dioxide. 597 00:29:16,000 --> 00:29:18,600 When you flood the blood with oxygen, the hemoglobin becomes 598 00:29:18,600 --> 00:29:21,800 fully saturated with oxygen. This oxygenated hemoglobin 599 00:29:21,800 --> 00:29:24,440 physically changes its confirmation and drastically 600 00:29:24,440 --> 00:29:28,120 lowers its affinity for CO2. Forcing massive amounts of CO2 601 00:29:28,120 --> 00:29:30,800 to be dumped from the red blood cells back into the plasma, 602 00:29:30,800 --> 00:29:33,080 sharply spiking the PECO 2. Exactly. 603 00:29:33,080 --> 00:29:35,840 That's the chemical mechanism. And the hemodynamic mechanism. 604 00:29:36,080 --> 00:29:38,880 The second mechanism involves the reversal of hypoxic 605 00:29:38,880 --> 00:29:42,520 pulmonary vasoconstriction. Remember, the COPD patient has 606 00:29:42,520 --> 00:29:45,560 chronic poorly ventilated emphysemaidis boule. 607 00:29:46,240 --> 00:29:49,160 Their body has intelligently constricted the blood vessels 608 00:29:49,160 --> 00:29:52,720 around those useless Boulet to shunt blood to better areas. 609 00:29:52,920 --> 00:29:53,960 Right. So when you give them high 610 00:29:53,960 --> 00:29:57,160 concentrations of inspired oxygen, that oxygen diffuses 611 00:29:57,160 --> 00:29:59,520 into those poorly ventilated spaces. 612 00:29:59,880 --> 00:30:03,680 The local hypoxia is temporarily reversed and those constricted 613 00:30:03,680 --> 00:30:05,920 pulmonary vessels suddenly dilate. 614 00:30:06,320 --> 00:30:09,320 Now massive amounts of blood flow are rushing to lung units 615 00:30:09,320 --> 00:30:11,200 that have terrible ventilation mechanics. 616 00:30:11,200 --> 00:30:14,760 So you've massively worsened the global VQ mismatch and vastly 617 00:30:14,760 --> 00:30:16,920 increased the alveolar Dead Space. 618 00:30:16,920 --> 00:30:19,200 Leading to a further dangerous climb in pack O2. 619 00:30:19,360 --> 00:30:20,960 That is a master class explanation. 620 00:30:20,960 --> 00:30:24,760 Now, what devices are we using to deliver oxygen precisely? 621 00:30:24,880 --> 00:30:27,560 Ventry masks are excellent for these patients because they 622 00:30:27,560 --> 00:30:31,280 utilize the Bernoulli principle to entrain room air at a fixed 623 00:30:31,280 --> 00:30:34,880 ratio, delivering a highly controlled, precise FIO 2 say 624 00:30:35,080 --> 00:30:38,720 exactly 24% or 28%. And we also rely heavily on high 625 00:30:38,720 --> 00:30:41,240 flow nasal cannula HFNC. Yes. 626 00:30:41,440 --> 00:30:45,720 HFNC provides heated, humidified oxygen at flow rates up to 60 627 00:30:45,720 --> 00:30:48,480 liters per per minute, which flushes out the anatomic Dead 628 00:30:48,480 --> 00:30:51,880 Space in the knees or pharynx. Importantly, the high flow 629 00:30:51,880 --> 00:30:55,600 generates A mild C pap effect of roughly 3 to 5 centimeters of 630 00:30:55,600 --> 00:30:58,920 water, which helps splint the upper Airways open and recruit 631 00:30:58,920 --> 00:31:01,320 alveoli. But if HF and C isn't enough, we 632 00:31:01,320 --> 00:31:04,920 move up the escalation ladder to non invasive ventilation NIV. 633 00:31:05,280 --> 00:31:09,400 Right NIV, specifically by PAPI, which provides both inspiratory 634 00:31:09,400 --> 00:31:12,000 and expert ORY. Positive airway pressure is 635 00:31:12,000 --> 00:31:14,360 strongly indicated for two main pathologies. 636 00:31:14,720 --> 00:31:18,760 First, acute exacerbations of COPD complicated by hypercapnic 637 00:31:18,760 --> 00:31:22,040 acidosis. Generally a pH between 7.25 and 638 00:31:22,040 --> 00:31:24,640 7.35. And 2nd acute cardiogenic 639 00:31:24,640 --> 00:31:26,760 pulmonary edema. Yes, for the positive pressure 640 00:31:26,760 --> 00:31:29,480 literally pushes the hydrostatic fluid back out of the alveoli 641 00:31:29,600 --> 00:31:31,440 and decreases left ventricular preload. 642 00:31:31,480 --> 00:31:33,120 The advantages of NIV are obvious. 643 00:31:33,120 --> 00:31:35,760 You avoid the morbidity of an endotracheal tube ventilator, 644 00:31:35,760 --> 00:31:38,200 associated pneumonia and the need for deep sedation. 645 00:31:38,680 --> 00:31:41,640 But when must we say no to NIV and proceed straight to 646 00:31:41,640 --> 00:31:44,200 intubation? Absolute contraindications 647 00:31:44,360 --> 00:31:48,080 include a patient in a coma or with an altered sensorium who 648 00:31:48,080 --> 00:31:51,760 cannot protect their airway, hemodynamic instability or 649 00:31:51,760 --> 00:31:56,120 severe shock, facial trauma or burns, precluding a mask seal, 650 00:31:56,600 --> 00:31:59,200 and a high risk of aspiration such as active hemonesis. 651 00:31:59,200 --> 00:32:02,640 So if those are present, or if the patient's pH drops below 652 00:32:02,640 --> 00:32:07,240 7.25 despite NIV, you must proceed to invasive mechanical 653 00:32:07,240 --> 00:32:08,840 ventilation. You have no other choice. 654 00:32:08,840 --> 00:32:11,360 OK, the patient is intubated. They have severe ARDS. 655 00:32:12,000 --> 00:32:15,760 What are the high yield examiner approved ventilator settings? 656 00:32:15,960 --> 00:32:18,760 The examiner wants to hear you systematically describe lung 657 00:32:18,760 --> 00:32:21,760 protective ventilation specifically based on the ARDS 658 00:32:21,760 --> 00:32:24,240 NET protocol. You must drill these specific 659 00:32:24,240 --> 00:32:26,080 parameters into your memory first. 660 00:32:26,080 --> 00:32:29,480 You utilize low tidal volumes, specifically targeting 6 661 00:32:29,480 --> 00:32:32,360 milliliters per kilogram of predicted body weight, not 662 00:32:32,360 --> 00:32:34,480 actual body weight. Let's emphasize that why 663 00:32:34,480 --> 00:32:36,920 predicted body weight. Because your lungs do not grow 664 00:32:36,920 --> 00:32:40,720 larger when you gain adipose tissue, a 150 kilogram man of 665 00:32:40,720 --> 00:32:44,000 average height has the exact same lung capacity as a 70 666 00:32:44,000 --> 00:32:47,080 kilogram man of the same height. Right, so if you calculate 6 667 00:32:47,080 --> 00:32:49,840 millilograms based on actual body weight for the obese 668 00:32:49,840 --> 00:32:52,600 patient, you'll deliver a massive destructive tidal 669 00:32:52,600 --> 00:32:54,920 volume. Exactly, we strictly use low 670 00:32:54,920 --> 00:32:58,200 tidal volumes to prevent volume trauma, the physical over 671 00:32:58,200 --> 00:33:01,480 distension and tearing of the fragile alveolar epithelium. 672 00:33:01,680 --> 00:33:03,760 The second parameter is pressure. 673 00:33:03,760 --> 00:33:06,960 We must maintain a plateau pressure, the static pressure in 674 00:33:06,960 --> 00:33:10,360 the alveoli at the end of inspiration of less than 30 675 00:33:10,360 --> 00:33:13,040 centimeters of water. This prevents Barrow trauma 676 00:33:13,040 --> 00:33:16,760 which can cause macroscopic alveolar rupture leading to 677 00:33:16,760 --> 00:33:18,720 pneumothorax or new immediate acidism. 678 00:33:19,080 --> 00:33:22,880 And the third parameter is PEEP. We've mentioned it before, but 679 00:33:22,880 --> 00:33:25,120 we need to discuss the hemodynamic trade-offs. 680 00:33:25,520 --> 00:33:29,120 We crank up the PEEP to open the lung, recruit collapsed alveoli 681 00:33:29,120 --> 00:33:30,840 and overcome the right to left shunt. 682 00:33:31,320 --> 00:33:33,800 But the heart is sitting right in the middle of those lungs. 683 00:33:33,800 --> 00:33:35,560 It's not a free lunch. Far from it. 684 00:33:35,560 --> 00:33:38,840 This is a critical concept that bridges pulmonary Physiology 685 00:33:38,960 --> 00:33:40,960 with cardiovascular intensive care. 686 00:33:41,440 --> 00:33:45,240 PEEP is inherently unnatural. Humans breathe via negative 687 00:33:45,240 --> 00:33:48,440 intrathoracic pressure, which sucks blood back into the right 688 00:33:48,440 --> 00:33:50,520 atrium. When we apply significant PEEP, 689 00:33:50,520 --> 00:33:52,840 we're transmitting positive pressure throughout the thoracic 690 00:33:52,840 --> 00:33:53,840 cavity. Right. 691 00:33:54,040 --> 00:33:56,720 And this positive pressure compresses the superior and 692 00:33:56,720 --> 00:34:00,440 inferior vena cava, drastically impairing venous return or 693 00:34:00,440 --> 00:34:03,520 preload to the right heart. And if the right heart doesn't 694 00:34:03,520 --> 00:34:06,600 fill, it can't pump. Furthermore, high PEEP 695 00:34:06,600 --> 00:34:09,560 compresses the delicate pulmonary capillaries running 696 00:34:09,560 --> 00:34:13,199 between the distended alveoli. This increases pulmonary 697 00:34:13,199 --> 00:34:16,480 vascular resistance, which is essentially an increase in right 698 00:34:16,480 --> 00:34:19,320 ventricular afterload. So the right ventricle is 699 00:34:19,320 --> 00:34:22,560 receiving less blood and it has to pump harder to push it 700 00:34:22,560 --> 00:34:24,880 through the lungs. Consequently, right ventricular 701 00:34:24,880 --> 00:34:28,360 stroke volume drops, which means left ventricular filling drops, 702 00:34:28,600 --> 00:34:31,480 causing a severe reduction in cardiac output and systemic 703 00:34:31,480 --> 00:34:33,719 blood pressure. So let's tie this back to the 704 00:34:33,719 --> 00:34:36,239 ultimate goal, tissue oxygenation. 705 00:34:36,239 --> 00:34:38,520 What does this mean for the patient mathematically? 706 00:34:38,920 --> 00:34:41,239 We return to the oxygen delivery equation. 707 00:34:42,000 --> 00:34:45,719 Oxygen delivery DO 2 is the product of cardiac output 708 00:34:45,719 --> 00:34:48,239 multiplied by arterial oxygen content. 709 00:34:48,719 --> 00:34:51,360 The oxygen content is mostly determined by hemoglobin 710 00:34:51,360 --> 00:34:54,360 concentration and saturation. So if you increase the PEEP from 711 00:34:54,360 --> 00:34:58,080 10 to 18, you might recruit alveoli, resolve the shunt and 712 00:34:58,080 --> 00:35:01,400 see the PO2 and saturation on the monitor rise beautifully. 713 00:35:01,600 --> 00:35:03,960 But if that high PEEP simultaneously drops your 714 00:35:03,960 --> 00:35:07,440 cardiac output by 40% because of impaired venous return, the 715 00:35:07,440 --> 00:35:11,040 total product of the equation, the actual molecules of oxygen 716 00:35:11,040 --> 00:35:13,840 arriving at the brain and kidneys, might plummet. 717 00:35:14,240 --> 00:35:18,080 You literally sacrificed cardiac output for a prettier blood gas 718 00:35:18,080 --> 00:35:20,760 and the patient still dies of cellular hypoxia. 719 00:35:20,760 --> 00:35:22,360 Exactly. The art of mechanical 720 00:35:22,360 --> 00:35:26,640 ventilation is titrating PEEP to find the optimal point where 721 00:35:26,640 --> 00:35:29,760 alveolar recruitment is maximized without severely 722 00:35:29,760 --> 00:35:33,240 compromising cardiac output. That complex hemodynamic 723 00:35:33,240 --> 00:35:36,480 interplay is exactly what a senior examiner is waiting to 724 00:35:36,480 --> 00:35:38,920 hear. This transitions perfectly into 725 00:35:38,920 --> 00:35:41,800 Part H complications and ICU care. 726 00:35:42,040 --> 00:35:44,880 We've touched on Baritron and volley trauma under the umbrella 727 00:35:44,880 --> 00:35:48,120 of ventilator induced lung injury or vilai, but what are 728 00:35:48,120 --> 00:35:50,760 the other components? Aside from bear trauma, pressure 729 00:35:50,760 --> 00:35:54,800 damage and volume trauma, volume tearing, you must define atelect 730 00:35:54,800 --> 00:35:57,080 trauma. This is the sheer stress injury 731 00:35:57,080 --> 00:36:00,720 caused by the repetitive opening and closing of unstable fluid 732 00:36:00,720 --> 00:36:03,040 heavy alveoli with every single breath. 733 00:36:03,080 --> 00:36:05,600 It's like constantly bending a paper clip until it snaps. 734 00:36:05,840 --> 00:36:10,720 Exactly adequate P prevents this by keeping the alveoli splinted 735 00:36:10,720 --> 00:36:12,880 open throughout the respiratory cycle. 736 00:36:13,440 --> 00:36:17,040 Finally, there is bio trauma. Bio trauma is fascinating. 737 00:36:17,040 --> 00:36:19,680 It's where physical physics turns into systemic biology. 738 00:36:19,680 --> 00:36:22,040 Precisely, it is mechanotransduction. 739 00:36:22,360 --> 00:36:24,720 The physical stretching and tearing of the alveolar 740 00:36:24,720 --> 00:36:28,840 epithelium trigger a massive release of pro inflammatory 741 00:36:28,840 --> 00:36:33,040 cytokines like interleukin 6 and TNF alpha into this systemic 742 00:36:33,040 --> 00:36:35,240 circulation. So you're literally mechanically 743 00:36:35,240 --> 00:36:38,840 generating a state of Systemic Inflammatory response syndrome 744 00:36:38,840 --> 00:36:41,000 or SIRS. The lung trauma becomes a 745 00:36:41,000 --> 00:36:44,080 systemic sepsis mimic leading to multi organ dysfunction 746 00:36:44,080 --> 00:36:46,080 syndrome. Other complications include 747 00:36:46,080 --> 00:36:48,680 ventilator associated pneumonia, which carries a high mortality, 748 00:36:49,040 --> 00:36:52,640 and oxygen toxicity where prolonged exposure to 100% IO 2 749 00:36:52,640 --> 00:36:54,920 generates reactive oxygen species that destroy the 750 00:36:54,920 --> 00:36:57,600 pulmonary endothelium. So how are we monitoring these 751 00:36:57,680 --> 00:36:59,440 intubated patients to prevent this? 752 00:36:59,600 --> 00:37:02,920 Meticulous continuous monitoring Arterial lines are mandatory for 753 00:37:02,920 --> 00:37:05,600 continuous blood pressure assessment, especially given the 754 00:37:05,600 --> 00:37:08,840 hemodynamic effects of PEEP we just discussed, and for frequent 755 00:37:08,840 --> 00:37:11,040 ADG sampling. We also rely heavily on 756 00:37:11,040 --> 00:37:15,160 continuous capnography or ET. Because a sudden precipitous 757 00:37:15,160 --> 00:37:19,480 drop in ETCO 2 could immediately indicate a massive pulmonary 758 00:37:19,480 --> 00:37:22,240 embolism or a disconnect in the ventilator circuit. 759 00:37:22,320 --> 00:37:26,360 Exactly, while a gradual rise indicates increasing Dead Space 760 00:37:26,360 --> 00:37:29,320 or worsening hypoventilation. Let's look at the end game. 761 00:37:29,760 --> 00:37:31,920 Getting them off the ventilator weaning. 762 00:37:32,520 --> 00:37:35,320 You assess readiness with spontaneous breathing trials, 763 00:37:35,840 --> 00:37:39,000 but examiners love to test the cardiovascular impact of 764 00:37:39,000 --> 00:37:41,200 weaning, don't they? It's kind of the inverse of the 765 00:37:41,200 --> 00:37:43,520 PP problem. It is the exact inverse. 766 00:37:43,520 --> 00:37:45,840 We call it weaning induced pulmonary edema. 767 00:37:46,120 --> 00:37:49,080 Think about the hemodynamics. Patient's been on positive 768 00:37:49,080 --> 00:37:52,280 pressure ventilation for a week. The positive pressure has been 769 00:37:52,320 --> 00:37:55,240 suppressing their venous return and artificially assisting their 770 00:37:55,240 --> 00:37:58,520 left ventricle by reducing its transmural pressure gradient, 771 00:37:58,760 --> 00:38:00,920 essentially reducing LV after load. 772 00:38:01,120 --> 00:38:04,000 Now we suddenly turn off the positive pressure and make them 773 00:38:04,000 --> 00:38:06,800 breathe on their own through an endotracheal tube which acts 774 00:38:06,800 --> 00:38:08,320 like a narrow straw. Right. 775 00:38:08,560 --> 00:38:11,440 To pull air through that narrow resistance, the patient must 776 00:38:11,440 --> 00:38:14,360 generate a massive negative intrathoracic pressure. 777 00:38:14,680 --> 00:38:17,960 This sudden violent vacuum effect has two catastrophic 778 00:38:17,960 --> 00:38:19,680 consequences. Oh wow. 779 00:38:19,760 --> 00:38:22,840 First, it sucks a massive volume of venous blood back into the 780 00:38:22,840 --> 00:38:25,160 right heart, flooding the pulmonary circulation. 781 00:38:25,160 --> 00:38:27,760 Exactly. Second, the extreme negative 782 00:38:27,760 --> 00:38:31,120 pressure surrounding the heart vastly increases the after load 783 00:38:31,120 --> 00:38:34,560 on the left ventricle. It is like the left ventricle is 784 00:38:34,560 --> 00:38:36,480 trying to pump blood out of a black hole. 785 00:38:36,520 --> 00:38:39,400 The sudden volume overload combined with immense after load 786 00:38:39,400 --> 00:38:41,720 can cause acute left ventricular failure. 787 00:38:41,720 --> 00:38:45,960 Yes, the fluid backs up and they develop flash pulmonary edema. 788 00:38:46,760 --> 00:38:49,120 If they aren't failing to breathe, their heart is failing 789 00:38:49,120 --> 00:38:50,360 from the sudden work of breathing. 790 00:38:50,400 --> 00:38:52,840 That is an incredible physiological narrative. 791 00:38:52,840 --> 00:38:56,040 Let's pull everything together into Part 1 exam integration. 792 00:38:56,160 --> 00:38:58,400 We need to build a mental comparison table that our 793 00:38:58,400 --> 00:39:01,000 listeners can write down for Type I versus Type 2. 794 00:39:01,080 --> 00:39:04,080 Listeners visualize a table with type I in the first column and 795 00:39:04,080 --> 00:39:08,360 type 2 in the 2nd row. One pathophysiology, type I is 796 00:39:08,360 --> 00:39:12,000 fundamentally AVQ mismatch or a right to left shunt. 797 00:39:12,320 --> 00:39:15,040 Type 2 is fundamentally alveolar hypoventilation. 798 00:39:15,120 --> 00:39:19,760 Row 2 ABG findings Type I shows a pay O2 less than 60 with a 799 00:39:19,760 --> 00:39:23,240 normal or low pack O2. Type 2 shows a pay O2 less than 800 00:39:23,240 --> 00:39:26,720 60 but with a pack O2 greater than 45 and respiratory 801 00:39:26,720 --> 00:39:29,840 acidosis. Row 3 causes Type 1 is caused by 802 00:39:29,840 --> 00:39:33,320 alveolar filling or collapse, AR, DS, pneumonia, pulmonary 803 00:39:33,320 --> 00:39:35,520 edema. Type 2 is caused by pump 804 00:39:35,520 --> 00:39:39,120 failure, COPD, neuromuscular blockade aid, opioid overdose. 805 00:39:39,360 --> 00:39:44,000 And row 4 management focus. Type I requires PEEP to recruit 806 00:39:44,000 --> 00:39:46,560 alveoli and carefully titrated Fio 2. 807 00:39:46,880 --> 00:39:50,280 Type 2 requires ventilatory support, either NIV or invasive 808 00:39:50,280 --> 00:39:53,040 ventilation to physically increase minute ventilation and 809 00:39:53,040 --> 00:39:55,400 clear the CO2. Exactly what are the specific 810 00:39:55,400 --> 00:39:57,440 Viva traps that trip up candidates? 811 00:39:57,560 --> 00:40:00,120 The most common mistakes include failing to calculate the A 812 00:40:00,120 --> 00:40:03,920 gradient before diagnosing a mixed ABG, confusing the causes 813 00:40:03,920 --> 00:40:07,520 of Dead Space versus shunt. And as we discussed, believing 814 00:40:07,520 --> 00:40:10,800 that administering 100% oxygen will cure a severe true shunt, 815 00:40:11,040 --> 00:40:13,960 examiners will also explicitly ask you to define permissive 816 00:40:13,960 --> 00:40:16,160 hypercapnia in the context of ARD's. 817 00:40:16,200 --> 00:40:18,800 How should they phrase that definition to secure full marks? 818 00:40:19,080 --> 00:40:23,200 The examiner approved answer is permissive Hypocapnia is an 819 00:40:23,200 --> 00:40:26,600 intentional lung protective strategy where elevated packet 2 820 00:40:26,600 --> 00:40:29,720 is tolerated in order to minimize tidal volumes and keep 821 00:40:29,720 --> 00:40:32,320 plateau pressures below 30 centimeters of water. 822 00:40:32,880 --> 00:40:35,600 The goal is to prevent ventilator induced lung injury, 823 00:40:35,800 --> 00:40:39,680 provide that the arterial pH does not fall below 7.20 and 824 00:40:39,680 --> 00:40:42,520 there are no contraindications such as elevated intracranial 825 00:40:42,520 --> 00:40:44,080 pressure. Boom, perfect. 826 00:40:44,360 --> 00:40:48,200 And regarding visual aids in a written exam, a good diagram is 827 00:40:48,200 --> 00:40:51,280 worth an entire page of text. You must be able to confidently 828 00:40:51,280 --> 00:40:54,640 draw 3 diagrams. First the VQ mismatch spectrum. 829 00:40:54,960 --> 00:40:58,400 Draw a horizontal line. On the far left, show Dead Space 830 00:40:58,400 --> 00:41:01,240 where VQ equals Infinity because perfusion is 0. 831 00:41:01,840 --> 00:41:05,240 In the middle, show normal matching where VQ is roughly .8. 832 00:41:05,440 --> 00:41:08,520 On the far right show shunt where VQ equals 0 because 833 00:41:08,520 --> 00:41:10,840 ventilation is 0. Second, draw the oxygen 834 00:41:10,840 --> 00:41:14,000 dissociation curve, explicitly detailing the bore effect and 835 00:41:14,000 --> 00:41:16,960 the metabolic factors like temperature 2-3 DPZ P and pH 836 00:41:16,960 --> 00:41:19,400 that shift it left or right. And third, provide a clean 837 00:41:19,400 --> 00:41:21,720 branching flow chart for your ABG interpretation. 838 00:41:21,840 --> 00:41:24,760 Let's do a rapid fire summary of the absolute high yield take 839 00:41:24,760 --> 00:41:27,080 homes. Number one, define failure 840 00:41:27,080 --> 00:41:31,400 rigorously. PAO 2 under 60, PICO 2 / 49. 841 00:41:31,520 --> 00:41:35,040 Know why those numbers matter on the dissociation curve #2 842 00:41:35,520 --> 00:41:39,400 classify completely Use types before particularly emphasizing 843 00:41:39,400 --> 00:41:42,160 the FRC collapse in perioperative type 3 and the 844 00:41:42,160 --> 00:41:45,240 massive hemodynamic theft by the respiratory muscles in shock 845 00:41:45,240 --> 00:41:48,440 related type 4. #3 Understand the five mechanisms of 846 00:41:48,440 --> 00:41:51,960 hypoxemia, specifically how PE evolves from Dead Space to shunt 847 00:41:51,960 --> 00:41:56,800 due to lack of surfactant and bronchoconstriction #4 Always, 848 00:41:56,920 --> 00:42:00,040 always calculate the A gradient using the rigid box of the 849 00:42:00,040 --> 00:42:03,200 alveolar gas equation to isolate peranchymal disease from 850 00:42:03,200 --> 00:42:07,320 hypoventilation. #5 Execute lung protective ventilation, 6 Miller 851 00:42:07,320 --> 00:42:10,280 kilogram predicted body weight, plateau pressure under 30, and 852 00:42:10,280 --> 00:42:13,440 titrated PEEP and #6 master the hemodynamics. 853 00:42:13,520 --> 00:42:16,440 Articulate how PEEP drops cardiac output and how sudden 854 00:42:16,440 --> 00:42:19,360 negative pressure during weaning triggers acute left ventricular 855 00:42:19,360 --> 00:42:21,360 failure. If a candidate reaches the end 856 00:42:21,360 --> 00:42:24,080 of a 20 mark written essay on this topic, they need a strong 857 00:42:24,080 --> 00:42:26,000 closing statement. Could you dictate a model 858 00:42:26,320 --> 00:42:27,840 conclusion paragraph? Absolutely. 859 00:42:28,200 --> 00:42:31,640 In conclusion, respiratory failure represents a critical 860 00:42:31,640 --> 00:42:33,960 collapse of oxygenation, ventilation, or both, 861 00:42:34,160 --> 00:42:36,680 fundamentally disrupting tissue oxygen delivery. 862 00:42:37,160 --> 00:42:39,840 Management mandates a rapid systematic approach centered on 863 00:42:39,840 --> 00:42:43,160 robust ABG interpretation to identify the underlying 864 00:42:43,160 --> 00:42:46,040 physiological mechanism, whether it be intrapulmonary shunt, 865 00:42:46,360 --> 00:42:49,880 severe VQ mismatch, or alveolar hypoventilation. 866 00:42:50,440 --> 00:42:52,960 Definitive therapy requires A delicate balancing act, 867 00:42:53,320 --> 00:42:56,520 restoring adequate gas exchange via targeted oxygenation and 868 00:42:56,520 --> 00:42:59,840 lung protective mechanical ventilation, while vigilantly 869 00:42:59,840 --> 00:43:03,080 mitigating the severe hemodynamic compromises and bio 870 00:43:03,080 --> 00:43:05,480 trauma inherent to positive pressure support. 871 00:43:05,560 --> 00:43:07,720 That is authoritative. If you write that, you have 872 00:43:07,720 --> 00:43:09,240 proved you belong in the specialty. 873 00:43:09,240 --> 00:43:11,720 Before we conclude, I want to leave you with one final 874 00:43:11,720 --> 00:43:13,920 advanced physiological concept to ponder. 875 00:43:14,360 --> 00:43:17,360 We've said this entire session rigorously discepting macro 876 00:43:17,360 --> 00:43:21,280 circulatory parameters. We have balanced PO2PO2, cardiac 877 00:43:21,280 --> 00:43:23,160 output and oxygen delivery equations. 878 00:43:23,520 --> 00:43:26,120 But remember, the ultimate frontier in modern intensive 879 00:43:26,120 --> 00:43:28,920 care is the microcirculation. All right, the capillaries. 880 00:43:29,120 --> 00:43:31,960 Yes, you can perfectly recruit the lungs. 881 00:43:32,200 --> 00:43:36,320 You can achieve A flawless ABG, you can utilize inner tropes to 882 00:43:36,320 --> 00:43:38,920 generate a completely normal cardiac output and blood 883 00:43:38,920 --> 00:43:41,760 pressure. But in severe sepsis the 884 00:43:41,760 --> 00:43:45,120 microcirculation fails. Endothelial swelling and 885 00:43:45,120 --> 00:43:48,880 microthrombie physically shunt blood flow past the capillary 886 00:43:48,880 --> 00:43:52,040 beds directly into the venules, bypassing the mitochondria 887 00:43:52,040 --> 00:43:54,080 completely. So the oxygen is right there, 888 00:43:54,080 --> 00:43:55,640 but it can't cross. Exactly. 889 00:43:55,840 --> 00:43:58,520 Furthermore, the mitochondria themselves become poisoned and 890 00:43:58,520 --> 00:44:01,520 unable to utilize whatever oxygen does arrive, a state 891 00:44:01,520 --> 00:44:05,000 known as cytopathic dysoxia. The cells starve in a sea of 892 00:44:05,000 --> 00:44:06,920 plenty. The next time you were standing 893 00:44:06,920 --> 00:44:10,000 at the bedside of a septic patient staring at a perfect ABG 894 00:44:10,000 --> 00:44:12,960 and a normal blood pressure, but the lactate continues to rise 895 00:44:12,960 --> 00:44:16,120 and the patient is still dying, ask yourself what is happening 896 00:44:16,120 --> 00:44:18,000 at the microscopic mitochondrial level. 897 00:44:18,000 --> 00:44:20,640 And realizing how much we still have to control at that 898 00:44:20,640 --> 00:44:23,240 microscopic level is exactly what keeps the specialty so 899 00:44:23,240 --> 00:44:25,920 humbling and so fascinating. Thank you for an incredibly 900 00:44:25,920 --> 00:44:29,680 rigorous, exhausting, but ultimately exam crushing master 901 00:44:29,680 --> 00:44:30,680 class. It was my pleasure. 902 00:44:30,760 --> 00:44:33,760 And to you listening, remember that while this deep dive equips 903 00:44:33,760 --> 00:44:37,680 you to dominate your exams, the true final goal is saving the 904 00:44:37,680 --> 00:44:40,440 physiological life of the patient in that ICU bed. 905 00:44:40,800 --> 00:44:43,760 The diagnostic landscape might be muddy, but your physiological 906 00:44:43,760 --> 00:44:46,920 reasoning is now crystal clear. Keep studying the mechanisms, 907 00:44:46,920 --> 00:44:49,160 keep questioning the numbers, and keep diving deep.