Chapters Transcript High-Altitude Physiology: Flying with a Ventilator Course: Advances in Home Mechanical Ventilation: From the Iron to Artificial Lung So I'll be talking about it, I, I think a topic that um maybe a lot of you haven't really had much experience with, honestly, and uh it's uh really rare, I think, to see, and we're lucky enough to have a, a lab here at Bellevue that allows us to actually do some of the tests that I'll be talking about going forward. So we'll start off with a case. Um, this is a 66-year-old patient that came to our clinic. He has severe kyphoscoliosis and, uh, a somewhat reduced baseline FEC of 1.45 L, and he's already established on non, uh, nighttime non-invasive ventilation. He comes to our office and he says that he has plans to fly to Costa Rica, and so he comes with the following questions that I'm sure, uh, some of you have also encountered from some of your patients. So, one, is it safe for me to fly and how do I know? Two, do I need to bring my device with me? 3, can I use my device on the plane even? Uh, do I need to use my device any differently, potentially? And do I need a special letter from my doctor? So, the goal of this talk is gonna be to answer all of these questions and hopefully some more as well for some of you. Um, I'll start by talking about some of the basics of high altitude physiology and some of it will, uh, probably have a little bit of overlap with some of the strong foundations that have been built by some of our previous speakers. Like Bernardo, thank you. Um, we'll talk about, uh, the implications of the physiologic implications of high altitude travel for patients with neuromuscular disease. Uh, I'll talk about some of the general principles of what's called high altitude simulation testing or hypoxic challenge testing. We'll review some of the societal recommendations, mainly from the British Thoracic Society, which are kind of the leading ones regarding high altitude travel for this patient population. Um, I'll talk a little bit about some of the barriers, uh, that these patients have for high altitude travel and honestly travel in general, because these are some of the things that, uh, other than knowing how to interpret hypoxic challenge testing are probably some of the most important to deal with. Um, and my goal by the end of the talk is gonna be to provide what I think is, uh, a fairly practical framework for preparing some of these patients, uh, as they travel to or via high altitudes. Um, I will also welcome, uh, the opinions and, uh, expertise of some of our senior faculty who have been doing this for much longer than I have. So, uh, let's start off with some basic definitions and facts. What is high altitude? There isn't really a truly agreed upon definition, but within the literature and, uh, for the most part colloquially, it's about 8000 ft or so. And this is the altitude at which negative effects of decreased barometric pressure can begin to manifest for our European colleagues, that's at about 2500 m. The CDC formally, or not formally, but they kind of define it as 8000 ft, but um, Some of these definitions are subject to change, and some of the numbers might be a little bit bigli in the coming years. Uh, like I said, travel via, uh, and to high altitudes is common. Um, according to the global aviation data, about 3.3 billion people traveled by air in 2022. This is a 42% increase compared to the previous year. And in 2019, as many as 375 million people traveled internationally for mountain tourism. Uh, why do I mention travel via high altitudes? Uh, that's mainly because, uh, commercial airliners, their cabins are, uh, are pressurized to an altitude of about 8000 ft. Um, which is in line with the definition I had mentioned, um, and obviously travel to high altitudes is common as well. So we'll talk about a concept called hyperbaric hypoxia, which is uh what happens as everyone ascends to altitude. This is uh just a graph showing what is the uh expected uh inhaled partial pressure of oxygen for a given altitude of some of the uh very populous, uh, very popular tourist destinations including Mont Blanc, La Paz, Mexico City. I put in a couple more over here. So Bogota, Colombia, Aspen. And then importantly, this is actually uh the altitude or the simulated altitude of commercial airliners. In order to kind of understand some of the underlying physiology, we need to get back to the alveolar gas equation, which some of us probably haven't seen since medical school. Um, but this essentially describes what the expected alveolar partial pressure of oxygen would be, uh, for a given barometric pressure, for a given fraction of inspired oxygen. here. I don't know how to use a pointer, but, OK. Um, and for a given, uh, partial pressure of carbon dioxide in the alveoli as well. So, the PIO2 is one of the things to kind of focus on uh throughout the talk, and that is the uh partial pressure of inhaled oxygen that is a consequence of the barometric pressure and the FIO2 and taking into account the partial pressure of water vapor as well. So, uh, this is an example of what, uh, we all encounter at sea level, assuming that you have a stable or normal PCO2 and a respiratory quotient of about 0.8, which is based off of a Western diet. And so, uh, the alveolar partial pressure is about 100 millimeters of mercury. But what happens when somebody goes to La Paz, for example, uh, assuming that PCO2 remains unchanged, you would see that, uh, for a barometric pressure of about 470 at that location. You would see a corresponding partial pressure of oxygen of 38 millimeters of mercury, which, as you can imagine, is not compatible with life. And so we'll go on to kind of talk about how travelers don't drop dead once they get off the plane. It's important to also review the oxygen dissociation curve briefly. I'm sure a lot of you have seen this. So, uh, this describes the relationship between hemoglobin saturation for a given partial pressure of oxygen. Uh, some of the things to kind of note, so in the equation I mentioned earlier, the example, this would be the expected oxygen saturation for somebody at La Paz who maintains a CO2 of about 40, so you would see a SA of about 70%. Another important cutoff here is, is this one right around here, which occurs at around 55 to 60 millimeters of mercury. Which is the point at which we say patients essentially quote unquote fall off the curve, meaning that if the partial pressure of oxygen decreases any more, then you see a very rapid drop in resultant oxygen saturation. Luckily, the body has a protective uh and an adaptive response to maintain normoxia, and this is called a hypoxic ventilatory response to acute hyperbaric hypoxia. And this essentially occurs within minutes to seconds of exposure to hypoxia at altitude. And it's, it's driven primarily by uh peripheral chemoreceptors that are located in the aortic and the carotid bodies as well. And the response varies significantly between individuals and, and it, it's certainly uh hindered in patients with neuromuscular disease or any other restrictive disorders as well. Um, Taking into account the oxygen hemoglobin curve I mentioned earlier, that response essentially increases significantly once PO2 drops to about 50 to 60 in an attempt to maintain normaloxia. And so this is well represented on this graph here from West High altitude physiology. So you see, As uh PAO2 slowly starts to decrease, you see generally a relative maintenance of, of, uh, ventilation that at some point kicks up exponentially once, uh, once that PAO2 drops to 50 to 60 or so. We know that from physiologic studies, this augmentation of ventilation is driven, uh, at least in healthy subjects, mainly by an increase in, in alveolar ventilation or an augmentation of tidal volumes, which we'll go over a little bit. And we know that as patients start to ventilate more and more, they blow off CO2, and that results in a, in a mitigation of this hypoxic ventilatory response from the resultant respiratory alkalosis, and this is well delineated by the alveolar ventilation equation which describes the relationship between alveolar, or in other words, arterial CO2 content, um, alveolar ventilation, and CO2 production. So, um, this is somebody who, let's say, goes to La Paz. And this would be what they expected CO2 would be if they didn't have any hypoxic ventilatory response. But lucky for uh most most people, uh, they have an intact ventilatory drive, and so that CO2 is able to be blown down to 20. And so what you see is a maintenance of normoxia at altitude. Um, this hypoxic ventilatory response lasts for minutes to hours to days, and as it gets further along, Uh, there's this, uh, integrated adaptive response called an acclimatization to altitude, um, and there's multiple mechanisms for this response, uh, which includes increases in carotid body sensitivity to hypoxia, alterations in various transcription factors as well. Um. But what you'll note is that as somebody spends more and more time at altitude, there is a progressive increase in ventilation, which here is denoted by, uh, I guess indirectly by the PACO2. But ultimately, the way that the process works is there's an increase in alveolar ventilation. PACO2 drops. In order to maintain acid-based homeostasis, there's an increase in renal bicarbonate excretion. There's a resultant decrease in blood and therefore cerebrospinal fluid, uh, bicarbonate. And what you get is something that actually looks like this. So these, uh, this is also a graph from uh West Physiology where there are two lines plotted. The upper line is, uh, is a plot of data taken from subjects who were exposed to a decreasing, uh, barometric pressures, and it shows essentially what their alveolar PCO2, or in other words, their ventilatory response is as that barometric pressure or PO2 essentially drops. The lower line is, uh, is a line that's taken from data from patients who are already acclimatized to altitude, and what you see is that there's a much earlier and much more pronounced augmentation of ventilation in response, in response to the same barometric pressure or hypoxic environment. So, uh, I'm gonna kind of skip through the slide a little bit because, you know, we're all here for the same reason. And uh I think we all know a lot about neuromuscular disease, but just to briefly outline, we know it's, uh, a collection of a heterogeneous collection of acquired or inherited disorders. They have variable rates of progression and severity, and they're heterogeneous in their manifestations within the same disease and between patients, obviously. Um, and they can affect any or multiple parts of, of any of this neuromuscular access. I've included here, uh, this little addition which, as you can imagine, didn't make it into netters, is, uh, the, um, the thoracic cage. And so for the purposes of this talk, we also include patients with thoracic abnormalities such as severe kyphoscoliosis. Um, they carry a very high rate of morbidity and mortality, and, uh, the, the main pulmonary manifestation, as we know, is chronic hypercapnic respiratory failure, and this leads to significant morbidity and mortality in these patients. What's also important is that there's a severe impairment in bodily functions and mainly in mobility. And so the reason why this is important is because this results in an increase in negative perception that these patients have of their environmental, physical, and psychosocial aspects of their quality of life. And so it's, it's incumbent upon us as providers to preserve that quality of life as best as we can. And this includes preparing these patients for travel wherever they might go. So, uh, chronic hypercapnic respiratory failure, we know it's characterized by a low alveolar ventilation, in other words, a high dead space fraction. So, uh, going back to medical school as well, this is a, a, a very simplified schematic of an alveolus, uh, with the red here representing the dead space, or, uh, which is the part of the alveolus or the respiratory circuit that does not participate in gas exchange, and the alveolus down low where it does. This is a mathematical representation of what alveolar volume is, and we often talk about dead space fraction within this patient population, which is the ratio of, of dead space to the total uh to the tidal volume itself. And a normal dead space fraction is about 0.3 or so. And this is just uh reminding you of the alveolar ventilation equation as well. So, um, What we see in patients with neuromuscular disease is there's this pattern of what we call rapid shallow breathing, where there's a high dead space fraction as you see, and not really much reaching that alveolus. And so you can imagine that this is a big problem for people who are, for these patients who are going to be going to altitude where a hypoxic ventilatory response is necessary in order to keep them alive, basically. We also know that as chronic alveolar hyperventilation occurs, especially if it's uncorrected or poorly managed, for example, there's an increase in bicarbonate, and this can also further suppress the ventilatory drive. So What happens in healthy patients that ascend to altitude and you see an augmentation of their tidal volumes, and this is an effective way of essentially decreasing CO2. As we know, our patients cannot do that, and so their response looks a little bit something like that. This is probably the, the highlight of my medical education. This is probably the coolest thing I've ever done, and Um, And so, like I said, they, the, the HVR in these patients is, is reduced because they can't effectively augment their alveolar ventilation. They require, uh, or they rely upon an increase in their respiratory rate, which, uh, as Bernardo mentioned, requires a lot of extra energy expenditure. Um, And I also mentioned the further blunting by the increase in bicarbonate. And so this is kind of a, a, a mathematical comparison of two patients. So this is somebody, let's say, who has an intact hypoxic ventilatory response and you see that normoxia is maintained. And this is probably something a little bit more, uh, more close to some of the patients we see who have a suboptimal hypoxic ventilatory response. They might be able to blow down their CO2 a little bit, but you see that, uh, they're still quite hypoxic. And this is where those two patients would ultimately fall on the oxygen hemoglobin dissociation curve. Obviously, kind of taking into account that the curve itself will shift accordingly in response to changes in pH and PCO2 and temperature, as we all know. So what are we gonna do about this? And I, I hope that this is kind of the part of the talk where you'll, you'll get something practical out of it to take back to your clinics. Um, we have something called hypoxic challenge testing, as it's often referred to in the literature, um, also referred to as hypox uh, high altitude simulation test, uh, which is what we call it at our lab here at Bellevue. And the, the purpose of this test is to essentially reproduce at sea level, uh, the low partial pressures of oxygen that passengers face when they ascend to altitude, whether that's uh via commercial airliner or if they travel to places like Machu Picchu, La Paz, etc. Uh, I'm gonna keep this down low here for your reference, uh, so you can kind of, uh, follow along. And so there's two types of tests. There's the gold standard test, which is hyperbaric hypoxic challenge testing, which essentially utilizes a sea-level, uh, barometric chamber to essentially decrease the barometric pressure accordingly and simulate what happens at altitude. Um, and so this is an example of what that would look like. So they would drop the pressure to about 556 millimeters of mercury to replicate the cabin environment. And then you see uh that that is the result in inspired oxygen. Obviously, while this is the gold standard, it's not particularly practical and it's not really found in many institutions, mainly it's located on military bases. Um, So, what do we do here? We do something called normobaric hypoxic challenge testing, which essentially instead of decreasing the barometric pressure, we maintain it at sea level, but we decrease the fraction of inspired oxygen accordingly to essentially simulate that hyperbaric environment. And so we have patients that come in and there's different types of modalities of this test specifically, so you can uh One of the things that we do is we have patients, uh, essentially breathe, uh, this hypoxic mixture that's in this Douglas bag over here, and so you see that the outcome is the same, that the uh inspired oxygen is ultimately about 107 millimeters of mercury. Um, so what are the current recommendations regarding high altitude travel for these patients? Unfortunately, they are, uh, they are scant and they're kind of vague, and, and mainly that's because there hasn't really been a lot, uh, uh, there haven't been that many studies, uh, uh, that have kind of looked at this patient population. And the ones that have been done are mainly retrospective based off of very small patient cohorts and presented as abstracts, um, with the exception of one that I'll mention. Um. The main society that's really put out any sort of recommendation has been the British Thoracic Society, um, and other societies like the Aerospace Medical Association have followed suit, ultimately kind of, uh, citing their recommendations. And so this is kind of a summary of, of, of what they recommend. So they say that hypoxic challenge testing is recommended for all adult patients with a forced vital capacity of less than 1 L. That comes from this prospective trial that was done by Mstri in I believe 2009. It's like one of the only prospective studies that really looked at this patient population. Um, and so they found that, uh, that a force vial capacity of less than 1 L was, was more predictive than resting oxygen saturation, um, of hypoxic challenge test desaturation. This kind of changed the dogma of testing. They also say that you should do it for those believed to be at particular risk and so that this is kind of the biggest problem with these recommendations and it's really no fault of theirs because the data is so limited. Um, they don't really classically or, or really clearly define what, what it means to be a particular risk and so I'll, I'll do my best to try to kind of highlight some of the things that I, uh, that I personally, uh, believe based off of the, the data that I've looked at, um, and like I said, I also welcome, uh, the opinions of our, uh, of our senior colleagues as well. For people who can't, uh, undergo spirometry reliably or those who can't do hypoxic challenge testing, like, like I said, I imagine it's probably not available at many institutions, maybe not at your institutions. Um, a six-minute walk test or any other, uh, exercise equivalent, uh, is, um, is considered a, a, a good alternative potentially. Uh, they also recommend that patients take daytime flights where possible, and this obviously comes from the fact that neuromuscular disease patients have an increased incidence of sleep disordered breathing and therefore nocturnal hypoxia. And they also talk about further planning, uh, and how it's required. Um, and so I will outline what that planning should ultimately look like. And this is what you do with hypoxic challenge test results. So patients who experience a desaturation to less than 85%, uh, the recommendations say that uh they should be placed on in-flight oxygen. Um, you can also perform a titration during testing. It might be a little bit difficult, uh, but I think we've had that similar issue at our lab because you have to essentially break the seal. Um, but depending on what type of modality you're using, you can actually titrate oxygen accordingly and find out what some of these needs really are. Uh, I will talk a little bit later about, uh, potentially using NIV, uh, to reverse the hypoxic effects as well. So things that should be done in the clinic even before you refer somebody to um Uh, hypoxic challenge testing, you need to understand kind of the barriers that these patients have to high altitude travel. So, mobility is an obvious one. A lot of these patients are wheelchair-bound or slow-moving. Um, they don't, might not have the motor function to essentially, uh, manage their own device, whether that's a portable oxygen concentrator or a non-invasive, uh, ventilatory device as well. Costs and logistics is kind of the biggest one that's, uh, that's faced not only by patients, but providers alike. Um, costs mainly because a lot of airlines actually require patients to provide their own oxygen when they do go fly. Um, some airlines will, will offer in-flight oxygen for their patients, but not everybody. And so that's why it's important to actually understand what airlines somebody might be flying with. Um. Not only that, the patients also have to provide their own batteries for their devices, whether that's their NIV device or their portable oxygen concentrator, and that, that battery life should last at least 150% of the total flight time. That's what most airlines recommend. Um, but the logistics is kind of the other really big portion too, and that is probably the one where a lot of providers face a lot of challenge because they might not actually know any of this. And so that's ultimately the purpose of this talk. Some patients will also require an extra NIV device as well, which uh lends itself to even a higher cost burden. And like I mentioned, there's a probably generally a lack of hypoxic challenge testing. And so, uh, even just getting somebody to the test might be very difficult even from a pure mobility or logistics issue as well. And then provider expertise is the other big barrier. Uh, this is an example of just some of the forms that, uh, some of the airlines require patients to fill out, uh, in, in order to kind of say that somebody needs a portable oxygen concentrator. So these are kind of the more, uh, more simple forms, I would say. So there's American Airlines on the left and then Delta over here on the right, they've partnered with this company called Oxygen 2 Go, where passengers just need to essentially fill out this form and then they can rent a portable oxygen concentrator through the company as well. But then forms can be pretty, uh, pretty burdensome. So this is, uh, this is what Lufthansa recommends or requires. So it can definitely take a lot of time and it can be, uh, um, pretty daunting if you've never done it. Uh, so how do we actually prepare these patients? So it all starts in the clinic. So timing is probably the biggest thing. And, and what I'll say about this is that, uh, you know, as soon as you have a patient with neuromuscular disease in your clinic, one of the earlier conversations you should have is you need to let me know if you're gonna be flying, and you need to let me know way ahead in advance, um, because ultimately getting these people to where they wanna go and preserving that quality of life should be, uh, should be of utmost importance for everybody. Um, but other than that, obviously, the, the simple question of when are you leaving? Give me a timeline for when, you know, uh, for when certain tasks need to be completed, and, uh, when are you actually flying. Where is your patient flying? Are there gonna be layovers or stops? This will help kind of inform the duration of battery life that they might need, uh, as well as what their final destination is. And importantly, you need to know, are you traveling to a high altitude location as well? What is gonna be the duration of the trip, the total trip time, time at each stop, the transit time, this kind of all lends itself to understanding how much battery they might need, um, as well as just extra support if they need uh somebody to come with them. Most importantly, you need to know, are you gonna be flying or not? Obviously, what airline, uh, for the purposes of filling out those, uh, daunting forms, are there gonna be any layovers? And then overnight versus daytime flights again, uh, just because you're, uh, in case you end up encountering, um, the issue of, uh, sleep apnea for some of your patients. And then access is another big thing. So access to communication with you, uh, with family as well, uh, access to a hospital, access to electricity, and this all kind of lends itself to, uh, making sure that you and your patient are able to uh develop what is essentially an emergency action plan in case things go south. Another really big one is understanding your patient's prior experience with flying. And so that, you know, in, in cases where things are a little bit gray or kind of not clear, knowing whether or not your patient, assuming they have stable disease, has flown, let's say within the last 3 or 2 months, might actually make you feel a little bit more comfortable in saying, OK, you know what, you're gonna be good to go. Um, but obviously that needs to be taken into account, uh, within the context of their, of their disease stability and rate of progression. So, uh, how do we prepare these patients so going forward? You wanna make sure there's no contraindications. Uh, you, you can look at some more of these in the actual, uh, recommendations in the BTS. Um, but one of the big ones, obviously, making sure they don't have a pneumothorax, for example, you can get a chest X-ray about a week afterwards to document resolution, making sure there's no active hemoptysis, and making sure they don't have any severe C-level hypoxia, which Generally, the cutoff I would say is anything about 4 L of nasal can a requirement of 4 L of nasal cannula or more might be concerning, mainly because that's kind of the limit of portable oxygen concentrators and their ability to provide continuous flow. It's also the maximum flow rate that I believe commercial airliners can can provide patients in case things go south. Understanding their comorbidities as well is very important because some of these comorbidities will have their own, uh, uh, will have their own recommendations or needs in terms of preparing for high altitude travel. So understanding whether somebody already has an indication for either hypoxic challenge testing or in-flight supplemental oxygen is really important. And then disease stability is also important to note because if you know that somebody is having rapid progression of their disease or recently came out of a hospitalization, it might not be the best time to fly. So taking that into account is, is crucial. And then the other one is obviously disease severity, not only in terms of their oxygen requirements, but what are their NIV requirements? Are they quote-unquote maxed out on their vent? Is there even room to augment their ventilation if needed? Um, is their disease controllable? Is your patient amenable to, you know, higher respiratory pressures or not, even if their disease is, you know, is still, um, relatively stable. So all things to consider. Some of the practical things you can do as well, kind of in preparation for all of these, making sure that your patient has updated pulmonary function tests. I don't have a, you know, a clear cutoff for this, but again, it needs to be taken into, uh, into consideration, uh, within the context of their disease. So, uh, if they don't have PFTs within the last year, make sure you get, uh, you get them before they go. It's kind of part of our protocol when we do hypoxic challenge testing here. Um, but you can also consider doing it sooner if, let's say they had a recent exacerbation, uh, a couple of months ago. Uh, obviously, a hypoxic challenge test if indicated, making sure that you have a baseline or at least an updated, uh, measure of their ventilation. And so that can either include, uh, either arterial CO2 levels, transcutaneous CO2 levels, or at least a baseline or updated serum bicarbonate level as well. And then any other relevant imaging or testing that, uh, it kind of, uh, within the context of their other comorbidities like pneumothorax. So this is kind of the, the uh The bones of this talk, I would say this is an algorithm that I've, uh, developed, and, uh, I hope that it kind of, uh, provides some sort of practical framework for how to go about assessing these patients. So, let's say you have somebody with neuromuscular disease and they say that they wanna, uh, travel somewhere by plane. So, uh, some of the first questions you need to ask are any, are any of the following present. So clearly, are there any contraindications? Um, is their disease severe or difficult to control? Is it rapidly progressive or unstable? And so if any of those are true, Um, you need to worry about trying to reverse some of those contraindications and doing what you can to stabilize their disease, optimize it with whatever interventions are necessary. If that's not possible, unfortunately, high altitude travel is not advised at that time, but the important thing for all of us to remember is that we always have to reassess and go back and do what we can to essentially reverse the things that we can and try to get these people to where they need to go. So if they don't have any of those, uh, any of those criteria, uh, then look at the following. Essentially, these are all measures of, uh, their ventilatory capacity, I would say. So, um, assuming that your patient has all the following, if they're not requiring any non-invasive ventilation, if their, uh, measures of carbon dioxide, whether transcutaneously or by arterial blood gas analysis are within normal limits, as well as their serum bicarbonate. Um, if they don't have any significant restriction on their updated PFTs, so I put here the FEC of over 1 L, and that's really based off of the, off of the formal recommendations and BTS. There have been more recent data that have come out in an abstract that was actually presented at um GIVD. Where they actually found that patients who had a force vital capacity of less than 2.7 L probably should get hypoxic challenge testing. And so this is kind of one of the things that we're looking at as part of our research and identifying whether there's any other predictive pulmonary function test parameters that would kind of allow us to know who actually does need hypoxic challenge testing. But for all intents and purposes, these are kind of the cutoffs, and that 60% predicted cutoff is based off of a, off of a review that was done in, in chests a couple of years ago. Um, making sure your patient is not requiring supplemental 02. And if they don't have any other comorbidities that require hypoxic challenge testing or in-flight oxygen, then they should be OK to travel with a couple of caveats that I'll talk about in a couple of minutes. But if they meet any of those criteria, then you assess all the things that I mentioned earlier, timing, location, duration, how they're getting there, access to uh communication and their previous experience with flying, and then if you can refer them for hypoxic challenge testing. If their SPO2 is 85% or greater, then they should be OK to travel. Again, with a couple of caveats. There are gonna be some patients who experience some quote-unquote borderline desaturations, and that's where you have to uh um get a little bit clever in terms of your interventions or at least your emergency action plan. If patients experience a DSAT to less than 85%, then you need to understand how correctable is their hypoxia, whether that's utilizing supplemental oxygen or non-invasive ventilation, or even sometimes both, which has recently been shown to be possible, essentially. Um. In terms of deciding between any of these modalities, this is where kind of understanding your patient, where they're going and how long they're going for is kind of important. If this is somebody who is otherwise relatively low risk with stable disease, you might get away with just giving them a portable oxygen concentrator for their flight, so long as the, uh, so long as the, um, the flow requirement is, is 4 L per minute or less. But if it's somebody who already has NIV and you might want to spare them the cost, you can just oftentimes just place them on their NIV. With one caveat, which is that, uh, you know, a lot of centers, probably most centers won't actually have the ability to titrate NIV during hypoxic challenge testing. There were a couple of studies done from, uh, some of our colleagues in Portugal that actually showed that they could modify hypoxic challenge testing to include non-invasive ventilation. And so, and that is probably what I imagine going to be the future of this test for this patient population, um, as it should be because that's where you're kind of focusing on the actual underlying issue. Um, And so, if you can't correct their hypoxia with any of those modalities, then again, you go back to the starting line. But if they are correctable, then you want to insure all of these things, so making sure that their device is FAA approved, as most of them probably are at this point, making sure they have enough battery life for their devices, making sure the appropriate documentation is completed and that you have an appropriate emergency plan. You also want to encourage daytime flights. This is also for people who you felt were OK to travel, um, especially if anyone has sleep disorder breathing, um, then you might want them to bring their NIV device with them. Uh, for some of those quote-unquote borderline patients, what we've done is, uh, we've recommended they just bring a pulse oximeter with them and a portable oxygen concentrator and say, you know, measure your oxygen during flight, and if you deset, then turn on your POC or use your non-invasive ventilate, uh, your NIV device. Um, and then for patients and who, uh, you know, who do have sleep disorder breathing, for example, you might want to consider multiple short flights, uh, to prevent sleeping during flight time as opposed to just doing one long haul flight. And like I said, HCT is not readily accessible to everybody. And so, in those patients, you want to try to address what those barriers are. Uh, you know, if your institution doesn't have one, and the closest institution that does have one is like hundreds of miles away, that, that might just not happen. But, um, but if the barrier is transportation or cost and trying to find any way to facilitate, uh, getting them to testing is, is really important. But otherwise, you can consider a 6-minute walk test. Obviously, that can be a huge barrier for this patient population who have mobility issues. Um. There are, uh, you can also do a shuttle walk test, and there was one study that looked at kind of a stepping exercise as well as a potential, uh, prognostic marker of, of, uh, of desaturation during HCT. But certainly other tests are needed for this population. And then, ultimately, you know, if, if travel is really important for your patient and they don't have access to HCT then that kind of, uh, ends up leading you into this, you know, important but obviously difficult risk-benefit discussion. Um, and that will, that is kind of where I guess the art of medicine really kicks in and you have to take into account what is your patient's disease, uh, severity? Is it stable? Is it rapidly progressive, um, and weighing all those factors. So, This was actually our patient. We ended up referring him to uh hypoxic challenge testing, and these are some of the graphs of the data that we ended up collecting during testing. So what you'll see is, you know, despite the fact that he had a forced vital capacity of over 1 L, he still experienced a decline in his, um, in his oxygen saturation. You'll see that his, his uh CO2 certainly fluctuated throughout, as it has respiratory rate and title volume, but ultimately, Um, he was not able to blow down his CO2 accordingly. You see actually early on over here, there's this rapid increase in respiratory rate and a drop in his tidal volume, which corresponds with that rapid shallow breathing that I had mentioned earlier. So, what do we do for him? We actually placed him on his home NIV settings and his hypoxia resolved, and we do all the things that I mentioned. So, uh, we notify that he does need to use his NIV throughout his flight. Um, we confirm that his device is in line with FAA regulations for in-flight use. We give him a prescription for additional batteries in order to, uh, last 150% of his total flight time. We ask him to notify his airline ahead of time of the need to use NIV in flight. Uh, we fill out the appropriate airline-specific forms, and we confirm that he'll have access to the internet during travel in case he needs to reach us. And we recommend that he also bring a pulse oximeter with him just in case. Uh, and then we, uh, uh, we brought him into the clinic to establish an emergency, uh, NIV settings to be used in case of hypoxia. So some of the take-home points, you know, neuromuscular disease patients uh are at an increased risk of developing hypoxia with high altitude travel. However, it is still possible despite this risk, and it's important for us to remember that, and we should do everything that we can to facilitate it. Uh, Obviously these patients face many barriers to high altitude travel, but with the appropriate interventions we can overcome them, especially early preparation for travel is going to be key in doing so. Hypoxia challenge testing allows us to identify the patients that are more likely to experience hypoxia at altitude. Um, and I, I guess my personal takeaway, and I kind of, again, welcome some debate is, uh, that neuromuscular disease patients with otherwise very mild or stable disease and essentially no evidence of any ventilatory impairment yet, um, are likely safe for high altitude travel, obviously with some caveats, and, and that's kind of your own expertise and knowledge of, of the patient. High altitude travel is not necessarily contraindicated in patients with severe neuromuscular disease as long as we can uh provide them with the necessary interventions, whether that's NIV or oxygen. And we have to continually reassess patients in whom high altitude travel is not advisable or contraindicated and ultimately with the purpose of maintaining a quality of life for them. Uh, these are my references, and thank you all for listening. OK. Thanks, Steve, for an amazing uh overview of hypoxic uh challenge testing and travel, and any of, anyone who's actually been in the field for a while, I think this is what we really take joy out of really getting people to be able to do things that a lot of people might have thought. Um, was impossible, but then also making them do it safely because we've had several patients really, I think, crash and burn after a flight because no one was thinking about, you know, really the underlying physiology. So, for any of the pulmonary folks out there or general medicine, like the alveolar gas equation is one thing that you really wanna stick in your head, memorize, understand its applications, um, because that's how you're gonna be thinking about patients with different physiologies. Uh, so with that, actually, we're obviously running late, but I think this is all really rich discussion, so. Published March 14, 2025 Created by
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