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Severe Acute Respiratory Syndrome in the Obstetric ...
Severe Acute Respiratory Syndrome in the Obstetric Patient: Etiologies, Respiratory Support, and Why the Obstetric Patient Is Unique
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All right. I haven't figured out where my glasses should be, so this might be a little bit tricky. So thank you all for coming. No, thank you. Definitely not. So it's a small and modest topic that Frank asked me to cover. And so we're going to do what we can with it. All right. So I want to say that if you are an intensivist, you're worried when your patient comes to the ICU in some kind of respiratory distress because you're thinking, okay, can I do the same things for her I would do for anybody else? Is this dangerous for the fetus? If you're an obstetrician and your patient is in the ICU, you're doing this, and then you're thinking, well, maybe I should just get her delivered and that should just be the end of it and they will stop calling me. You can't even really do that unless you have an ICU. But both of you were wondering things like where do I need her oxygen to be, okay, because it's in some textbook somewhere that you need to have it above a given number. But I want to put it to you that these are probably not the things you should be worrying about. They're probably the things you should be worrying about are why exactly pregnant women are so vulnerable and why we never test therapeutics on pregnant women or interventions on pregnant women to know if they actually work. All right, so when I originally did this lecture, I did 100 years of respiratory pandemics and then Frank told me I had 15 minutes. So we're going to do the 20-minute, the 20-year version, okay. Hands up if you were old enough to remember SARS-1. Okay, most of you. All right, so that was the first major coronavirus pandemic, right? It wasn't huge. There were, you know, a few thousand cases, mostly in Asia and then in Canada. H1N1, right? Oh, yeah, H1N1. That was, I don't know, a billion cases maybe, yeah. That was pretty much a swine flu with an assortment of avian influenza viruses swapped in. MERS, most of us will not have had any experience with MERS, but that was coronavirus number two. It wasn't as common. It didn't really get out of the Middle East. And then here we are in the tail end, I hope, of the third major coronavirus pandemic, which as of this morning was 670 million cases worldwide, probably more since then. So if you have a look at case fatality rates, okay, they are anywhere from, you know, 1% to 30%, except look at the case fatality rates in pregnancy because they are considerably higher, right? And a thing I want you to think about is why that might actually be. We've known about influenza for a long time, right? Influenza is uniquely dangerous for pregnant women. Thinking back to H1N1, pregnant women are 1% of the U.S. population, and we're 5% of the deaths from influenza. And that's been true as long as we've known about influenza. For your archival pleasure, I pulled up JAMA 1918, and you see that Cook County in Chicago, they had a 33% case fatality rate, except among their pregnant women, those were over 50. So influenza we've known is dangerous. For COVID, okay, it's a busy slide, so bear with me. For COVID, all the outcomes are worse, okay, if you are pregnant and have COVID than if you're not. So for example, ICU admissions, 3% generally, but, you know, 12% if you had COVID, critical illness, ventilation, ECMO. You don't see the difference in maternal deaths because maternal deaths are not denominated in percents, that is in per 100. They're denominated in per 100,000. I ran these numbers. This is equivalent to a maternal mortality ratio of 155 per 100,000, which is, let me just put that in perspective for you. In 2020, the U.S. maternal mortality ratio was 25. So what is that, sixfold? Same thing, this is outside of the U.S. This is a bunch of other institutions, a bunch of other nations. Again, looking at the COVID positive and the COVID negative, five times the risk of being admitted to ICU, almost twice the risk of being delivered preterm. Most of the neonatal morbidity relates to premature delivery. A bump in preeclampsia and a 20-fold increase in maternal death. So, again, we see COVID is a bad actor, although not as lethal perhaps as influenza. This one here, this is an ongoing project. This is a World Health Organization, and they update this a couple of times a year. Last time was in 2022, spring or summer 2022. So there's two different comparison groups here. So here are pregnant women with COVID-19, okay? Here's the comparison group that are non-pregnant women, so roughly 15 to 50 with COVID-19. This comparison group, pregnant women with COVID-19, and women who are pregnant without COVID-19, okay? So two different comparison groups. Compared to the non-pregnant group with COVID, pregnancy made it more likely people would be admitted to ICU, more likely they'd need a vent, more likely they'd need ECMO, and so on. Compared to, hard to know about all-cause mortality here. It looks like it's 50% increased, but that, as you see, that confidence interval crosses one, so maybe not so much. Looking at pregnant women without COVID-19, six times the rate of death, right? United Kingdom maintains an obstetric surveillance system for new and rare and emerging disorders. And I'm going to show you some data from here. So what they did was look at four different epochs for COVID-19. So this is the original strain. This is the alpha strain. This is delta. This is omicron, okay? So this is whatever was the prevalent virus circulating at that time in the UK. Here is when Moderna and Pfizer were commonly available and were recommended for pregnant women. So this is along about April 2021. So have a look, okay? So wild type, 24% risk of moderate infection or severe infection versus 36 with alpha versus 43 with delta. Okay? Respiratory support got worse as the virus progressed. 21%, 28%, 33%. ICU admission, 8% versus 12 versus 16. Okay? So every novel COVID variant that comes along hit the pregnant population hard. There is, as it happens, a way to fix some of that. They looked at omicron specifically, outcomes by vaccination. So you see here that moderate to severe disease was much more common in women not vaccinated compared to women who had one dose compared to women who had two doses compared to those few who had three doses. And again, you can see the same thing, ICU admission. So vaccination, the more vaccines you got, the more vaccine and booster you got, the less the chance that your pregnant self got severe disease or admitted to ICU. So we do have some strategies available, and we have those strategies for influenza, too. And I can't tell you how many times I see a patient and inquire about her vaccination status and she tells me no. All right, so pregnant women are uniquely vulnerable to pandemic respiratory viruses. And then the question is why? So, I mean, you're probably all acquainted with a bunch of physiology that's different in pregnancy. But two things I want to bring to your attention that we maybe are not taking into consideration would be the immunology of pregnancy and our own medical practices. I promised you two physiology slides. Brace. Okay. So cardiovascular changes. So pregnancy is a state of expanded plasma volume and decreased peripheral resistance. So cardiac output is up. SBR or peripheral resistance is low. Plasma volume is expanded. All of these things are necessary to accommodate the metabolically active fetus and placenta. Respiratory changes. This is, again, you know this. So minute ventilation is increased. FRC is decreased. Oxygen demand is up by about 20% because, again, the fetus and placenta are metabolically active. They're not just sitting in the middle of the belly being fed. They are active participants in the metabolic process. So in addition to that, the immunology is a little bit different. So the fetus is a foreign organism, right? Half its genes are somebody else's. So pregnancy isn't immunodeficient per se, but it has to be immunotolerant where you don't hang on to the pregnancy. So it's incorrect to say it's immunodeficient because B cells are actually ramped up. Antibody levels are higher, but cell-mediated immunity is decreased. So there is a vulnerability to parasitic and viral infections. And this is particularly interesting, but I won't inflict it upon you because time-wise that would be eating into Dr. Reed's time. There is a change in balance between T helper 1 and T helper 2 cells so that you have increased the immunotolerant T cells and decreased the ones that tend to fight viral infections. Okay, so here's audience participation question. Ready? All right. What do you think? What's this? Nobody? Is this normal? Abnormal? What? Oh, okay. So it's normal, right? So this is a normal pregnancy blood gas, yeah? So it's a compensated metabolic alkalosis, okay? And as part of the compensation, you lose a buffer, which also means that you are then vulnerable to other types of acidemia. You can't buffer properly. So oxygen transport, I'm going to talk about this. I'm sorry, but we have to do it, all right? It's not this. It's not, you know, mom, like, fire hoses baby, okay? It's more like this, all right? So two things to think about. One, it matters which way maternal and fetal blood flow go, okay? So the countercurrent exchange where maternal blood flow is one direction and fetal is another, that's a very efficient mechanism for getting things from higher to lower concentrations. And if you're a horse, you do this well. The concurrent exchange, maternal and fetal blood flow in the same direction. So this is a much less efficient process, okay? Sheep do this. But it takes a lot longer. The process is a lot slower. It's so slow in the sheep that you actually get to a sort of a venous equilibration by the end. We don't really need to talk about cross-current exchange because to my knowledge, there's only one mammal that has this. Humans are sometimes thought to have a concurrent exchange, but sometimes thought to have a pool exchange, okay? All right, I know. It's overwhelming, right? The reason I put it up is to show you the different ways you can have a placenta, okay? This is the horse, the pig, the sheep, okay? There are six layers, right, between the fetal and maternal blood, okay? It's an epithelial chorial placenta. Humans, though, have a hemochorial placenta, which is to say that the placenta has actually eroded into the maternal endothelium and replaced maternal endothelium in the uterine vessels with a vasononreactive trophoblast shell, okay? So this is useful for a lot of reasons, but one thing it does is it means you've got to have all of these steps to get oxygen from here, on the maternal side, down to there, okay, on the fetal side. Okay, I'm going to show you arterial blood gas number two. Thoughts? Sorry? Abnormal. Anybody else? Nope, this is perfectly normal. Absolutely, that's a perfectly normal umbilical venous blood gas. This is where they live. Now, I will say it's a little bit of a trick question, okay, because the umbilical vein is actually more highly oxygenated than the umbilical artery, but still, it's not that much of a difference. Here's the umbilical artery, which I just showed you. Here's the umbilical vein, okay? You've still only got a PO2 of 30, right? So when we talk about where we're worried about maternal oxygenation, consider that there's already this huge demarcation between maternal and fetal PO2s, and the reason that fetuses have oxygen at all is not so much dependent on maternal oxygenation as on uteroplacental perfusion and the fact that fetal hemoglobin binds oxygen differently, okay? So what should our oxygenation goals be in pregnancy? Well, yeah. I see this all the time in textbooks, okay? This is sheep data. This is not human data. There isn't any human data. There isn't any experimental human data, and I've just showed you that the sheep placenta is really different from the human placenta. This isn't the same thing at all. Occasionally, you'll see like somebody who bravely takes a stand and says, okay, you can have the maternal PO2 to 60 as long as the SpO2 is above 90. Well, that's good, but there isn't any data for that either. I can show you experimental human data where women were given an FiO2 of 0.1. I can show you case series where maternal SpO2s were in the mid-80s. I am not going to, trust me, I'm not going to make you look at this except to show you that in Kansas City, here at the top, in Kansas City where you're, I don't know, 300 meters above sea level, here's your maternal PO2. Here's your umbilical venous PO2, okay? In Cusco or La Paz or Lhasa, okay, where you are 3,600 meters, here's your maternal PO2. Here's your fetal PO2. It's not much different, okay? You maintain fetal oxygenation not because of this as much as the other, as much as blood flow and fetal hemoglobin, right? So quickly, actually, they did this in Finland. They took 10 healthy women, healthy term pregnancies. They gave them a hypoxic gas mixture. They saw no difference in the fetal heart rate tracing. They showed no changes in umbilical artery Doppler flow or the middle cerebral artery Doppler. It's short term, but still, right? Hypercapnia, though, I wonder about hypercapnia. CO2 is also transferred across the placenta, but it has to be transferred across a concentration gradient, which is to say if maternal PCO2 is too high, you can't transfer efficiently and you develop fetal acidemia. So we do know experimentally maternal hypercapnia decreases fetal heart rate variability on the fetal heart rate tracing. I don't know if it's clinically significant, but it is an effect. Okay, fine. So plant, can you just cut to the chase? What should we do here? All right. So if you need to intubate, it's going to be more difficult. You need an experienced intubator and a plan for failure. You need a smaller endotracheal tube. You need to have a backup plan. You need to pre-oxygenate, right, because not only is oxygen demand higher, but the time from apnea to desaturation is so much shorter. Where should you set the vent? Well, I don't know. Set the vent wherever you would set the vent for somebody who isn't pregnant, okay, because you don't have any data to tell you otherwise. Your total thoracic compliance is decreased because the weight of the chest wall is increased in pregnancy, but your lung compliance shouldn't be any different. FIO2 target, I don't know, but I'm not worried about keeping, you know, her sat over 95%. I do worry a little about permissive hypercapnia, and the way I decide how much is too much is the fetal heart rate tracing, okay, because that will show me fetal acidemia. So, and if you are an intensivist and you don't read these, it's fine. Somebody does. Just make sure your patient is hooked up. Role of delivery, well, you know, delivery doesn't actually make a difference to the internal outcome. We used to think maybe it improved respiratory parameters. It might make your daily care easier, but you can prone pregnant people. Who are we kidding? If you're going to contemplate delivery, you've got to know that you're delivering a preterm baby because it's often going to be a preterm baby in a place that can take care of them. Recent paper from Vasquez in Argentina, no difference in ventilatory parameters before and after delivery. And so the final thing I want to leave you with is a plea to incorporate pregnant women into research studies because in critical care we exclude pregnant women from research studies. So we don't actually know if things are different. And until we do, we ought to treat mothers with the same respect that we would treat anybody else. Thank you.
Video Summary
The speaker discusses the vulnerability of pregnant women to respiratory viruses, particularly focusing on the COVID-19 pandemic. Pregnant women have higher case fatality rates and are more likely to experience severe illness and complications from COVID-19 compared to non-pregnant individuals. The speaker emphasizes the need for vaccination to reduce the risk of severe disease in pregnant women. They also highlight physiological changes that make pregnant women more susceptible to respiratory viruses, including cardiovascular and respiratory changes, as well as alterations in the immune system. The speaker explains the unique placental structure and its role in oxygen exchange between the mother and fetus. They emphasize the lack of concrete guidelines for maternal oxygenation during pregnancy and the need for more research and inclusion of pregnant women in studies. The speaker concludes by calling for the equal treatment of pregnant women in critical care and research.
Asset Subtitle
Pulmonary, Obstetrics, 2023
Asset Caption
Type: one-hour concurrent | The Obstetric Patient and Respiratory Failure: Lessons Learned From COVID-19 and SARS Avian Flu (SessionID 1228152)
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Pulmonary
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Obstetrics
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Acute Respiratory Distress Syndrome ARDS
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Obstetrics
Year
2023
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pregnant women
respiratory viruses
COVID-19 pandemic
vaccination
severe disease
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