Ep. 31: Hypothermia Basics

In this episode, Dr. Patrick Fink reviews accidental hypothermia for wilderness providers, building on Ep. 27 Thermoregulation, explaining physiologic changes as core temperature falls  and why accurate core temperatures are difficult to obtain in the field. 

We learn the modified Swiss staging system (0–4) based on signs and symptoms and outlines field management: 

- 0 cold stress (shelter, dry/insulate, warm calories; avoid alcohol), 

- 1 mild hypothermia (ataxia/fine motor impairment; passive plus active external rewarming with a hypothermia wrap and heat near axilla/groin; frequent reassessment), 

- 2 moderate hypothermia (confusion/lethargy; active external rewarming, evaluate hypoglycemia, gentle handling and no exertion to avoid afterdrop, cardiac monitoring, expedite evacuation), 

- 3 severe hypothermia (unresponsive with vital signs; airway/ventilation support, extremely gentle transport)

- 4 hypothermic cardiac arrest (60-second pulse check, CPR, limited shocks under 30°C, withhold epinephrine/amiodarone under 30°C, consider intermittent CPR per protocol, and note potassium’s role in termination decisions). 

We also briefly contrasts slow non-extracorporeal warming with ECMO/cardiopulmonary bypass, previewing a future episode on advanced care and the HOPE score

Links

Episode 27: Thermoregulation

WMS 2019 Accidental Hypothermia Guideline

AHA Hypothermia Guidelines

European Resuscitation Council Hypothermia Algorithm

Chapters:

00:00 Welcome and Overview
00:48 Hypothermia Basics
01:12 Cooling Physiology
04:06 Severe Effects and Arrhythmias
06:38 Field Temp Challenges
07:41 Swiss Staging Intro
08:35 Stage 0 Cold Stress
11:40 Stage 1 Mild Hypothermia
16:04 Stage 2 Moderate Hypothermia
20:44 Afterdrop and Handling
22:54 Stage 3 Severe Hypothermia
27:00 Stage 4 Cardiac Arrest
27:57 CPR Shocks and Drugs
37:01 Advanced Rewarming ECMO
43:26 Key Takeaways Summary
46:57 Wrap Up and Support

As always, thanks for listening to Wilderness Medicine Updates, hosted by Patrick Fink MD FAWM.

Connect with us by email at wildernessmedicineupdates@gmail.com.

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Transcript

Patrick Fink 0:00

Hello and welcome back to Wilderness Medicine. Updates the show for providers at the edges. I'm your host, Dr. Patrick Fink. Today building on the foundation we laid in episode 27 on thermo regulation. We're gonna deep dive into hypothermia. We'll talk about what happens when the body gets dangerously cold, how to recognize the stages of hypothermia using the simple Swiss field grading system, and what you can do in the wilderness to treat someone who's too cold at the end. We'll touch briefly on advanced treatments in a hospital setting. But we're largely gonna reserve that for a second episode on hypothermia to come prior to next winter, which will focus on advanced treatments, extracorporeal warming, the hope score and more.

0:46

Let's begin with Okay.

Patrick Fink 0:48

Accidental hypothermia starts when the body's core temperature drops by just a few degrees below. Its normal 37 degrees Celsius baseline with hypothermia technically beginning at 35 degrees Celsius. Let's begin by marching through the signs and symptoms that are present in a cooling person to give a sense for the spectrum that we see between mild cold stress and severe hypothermia. To hearken back to episode 27, let me remind you that our bodies aren't passive in the face of cold stress. When thermoreceptors in the skin and deeper tissues detect a drop in temperature, they send signals to the hypothalamus. In the brainstem, the brain responds by increasing heat production through shivering, by releasing more thyroid and adrenal hormone. By constricting blood vessels in the skin and by prompting behavioral changes like seeking shelter and putting on extra clothing, shivering can boost heat production several fold, but it also uses up energy stores pretty quickly, and shivering generally persists through stages of mild hypothermia. And it can generate enough heat to maintain a core temperature above 32 degrees Celsius. However, once shivering stops, heat production falls dramatically, and the risk of rapid cooling increases during early cold exposure. The body ramps up its metabolism, blood pressure, and muscle tone rise when the body reaches a temperature of around 36 degrees at 35 degrees Celsius, shivering reaches its peak. And most patients at this stage of mild hypothermia are often still clearheaded, although they will be complaining about being cold as core temperature dips further, however, cognitive function begins to falter, and around 34 degrees Celsius is when we start to see potentially some slurred speech, clumsiness altered or poor decision making. When core temperature reaches 33 degrees, that clumsiness becomes more pronounced and breathing may initially first speed up, but then ultimately starts to slow down and the kidneys begin producing more urine as peripheral vasoconstriction causes a shift of fluid volume towards the core. The subtle altered mental status, such as slurred speech, poor decision making, clumsiness marks the transition towards a more dangerous stage of hypothermia. If cooling continues, once we reach the low thirties Celsius. Things really hit the fan because it's at this point that cellular mechanisms start to be impaired by the cold. So energy production within the cells and heat production by the body becomes impaired, and energy production drops accordingly. Consciousness tends to fade at this level by 32 degrees Celsius. People often drift into a stupor. They become. Significantly less responsive and oxygen consumption falls by about a quarter. As metabolism slows, essentially they're being refrigerated. Shivering will stop around 31 degrees Celsius. So at this point, this patient no longer produces a significant amount of heat for themselves. So any warming for this patient around 31 degrees Celsius or below has to come from outside the body. At around 31 degrees, the heart's electrical system also becomes sluggish. Heart rate begins to slow, and the conduction intervals on an EKG lengthen, and you may see an extra notch on that EKG called a J wave. When the temperature drops below about 32 degrees, progressing into the realm of truly severe hypothermia. As the core temperature drops into the high twenties, the risk of cardiac arrhythmia rises sharply. Cardiac output at this stage falls to about half of normal, and that cold heart becomes extremely sensitive to any jostling or rough handling as the core temperature moves below 30 degrees Celsius. This is now a severe hypothermia. At 30 degrees C, there's a good chance of developing atrial fibrillation or other irregular heart rhythms, and the body at this stage becomes euthermic, which is a terrible term. That means the temperature of the body may match the temperature of the environment. At this stage, insulin also stops working effectively. So cells which had impaired metabolism now can no longer take up glucose from the bloodstream. Mental status deteriorates and around 29 degrees Celsius, there's a marked decline in consciousness, pulse and breathing slow. Significantly. Pupils dilate, and if the patient exhibits any preserved consciousness, this is the stage at which bizarre behavior like paradoxical undressing may occur by 28 degrees. Oxygen consumption is half of normal, and both blood pressure and respiratory rate tend to be dangerously low. Because the heart is significantly irritable at these temperatures, even a bump to a patient who is this cold can send them into dangerous rhythms like ventricular fibrillation. Meanwhile, neurologic and psychiatric changes progress to absent reflexes, and if the patient has any level of responsiveness at all, they may have very pronounced confusion or active hallucination. Muscles at this stage can stiffen, which sometimes mimics rigor mortis and patients at 28 degrees Celsius or below usually appear unresponsive. And because of their significantly suppressed metabolic cardiac and respiratory function, they can appear to be in cardiac arrest. And this is why in these patients who may be subject to profound hypothermia, we spend an abnormally long time checking for a pulse or for respirations before starting CPR. Now, as you might've picked up, as we went through this cascade of pathology, many of these changes are marked by changes in core temperature. The problem for field management is that measurement of core temperature in the field, particularly in moderate to severe hypothermia, is gonna be challenging at best. Take a listen back to episode 27 for more discussion of how we can measure temperature. But suffice it to say that rectal or esophageal temperature measurement is really what we need in the severely hypothermic patient as other means may be inaccurate. Things like a temporal thermometer are gonna be off because of profound peripheral vasoconstriction and peripheral cooling. Those esophageal and rectal temperatures are, it's fair to say, difficult to obtain, particularly serially in an austere setting. There are means of doing so in more advanced treatment cases where you may have an esophageal temperature probe, where you may have a rectal temperature probe, which can be left in place. But in both of these cases, there are, you know, attendant issues with exposure to the patient. Accessing those areas, being able to reevaluate those areas, airway control, et cetera. So thankfully, for field recognition and treatment of hypothermia, there is another classification system which does not rely on knowing the patient's core body temperature. And that system is called the modified Swiss system. It relies on signs and symptoms rather than temperature to recognize hypothermia severity and to dictate treatment. We will work our way through this system now and use it to discuss treatment of hypothermia at each stage. The stage is run from zero to four, where zero is simple cold stress, and four is hypothermic cardiac arrest. As we discuss each stage, I'll explain the clinical signs and symptoms associated with that stage and outline the field management which is appropriate for that patient. As we go along, we'll touch on some common misconceptions and some unique physiology issues like after Drop I. In this stage, which is associated with a core body temperature over 35 degrees, these patients are simply cold stressed. This is a relatively healthy, normal patient who is having to work to warm themselves in the setting of a cold environment. These patients have normal mental status. They may be shivering, they might exhibit mild tachycardia or mild tachypnea depending on how significant the stress is. The symptoms that these patients will report are cold discomfort, possibly mild numbness in the hands or feet, and associated mild clumsiness because of that treatment for cold stress is gonna be variable depending on your situation. Obviously if you can remove someone from a cold environment, that is the easiest way to address cold stress. So we want to remove them from the cold, the wind, the wet and dry, and insulate any cold parts of them. But these people also have a functioning metabolic system and a functioning muscular system, so they can be provided warm, delicious calories, warm sweet liquids with easy to process glucose, and they can participate in activity such as movement to generate heat if the environment is safe for that. Cold stress is going to be remarkably common, and in many situations, patients or even rescuers frankly, are going to be subject to cold stress for hours, on hours, on hours, potentially through an operation or throughout an evacuation. The most important. Issue here is to continue to support the body's ability to maintain its own warmth. So environmentally speaking, we're trying to reduce exposure to wet clothing, wind shelter appropriately, but also making sure that the body remains appropriately fueled with hydration and easy to process calories to avoid progression to more significant stages of cold injury. A quick note here on a common misconception, alcohol is not helpful in this setting. You might be familiar with the laur called Jagermeister, which my understanding is that in German, this means hunting man because hunters. Allegedly used Le occurs like these to keep themselves warm while they were out on long, long days stocking deer in the black forest of Germany. Alcohol does produce a sensation of warming because it results in flushing of the skin. But if you recall from episode 27. One of the most significant means of thermo regulation is our ability to vasoconstrict at the skin and move our blood away from that cold stimulus and towards the core so that we do not cool as quickly. So while alcohol may make someone feel warmer in the short term, it will actually speed cooling of the core. So in the cold, stressed person, a couple shots of Jagermeister is not an appropriate treatment. Stage one in the Swiss system corresponds to a core temperature of about 32 to 35 degrees Celsius. This is mild hypothermia. What we see in these patients are patients who are awake, they're coherent, they exhibit significant shivering. They may be pale and ataxia may start to emerge, so they may have difficulty maneuvering or performing tasks due to kind of clumsiness and lack of coordination. The symptoms that they report are feeling cold, and this may be associated with more significant numbness or pain in their extremities from that cooling, and then they're gonna have impaired fine motor control. This motor control issue is practically the difference between cold stressed and mild hypothermia in the Swiss system. You can test this in the field because a patient who can't reliably do simple tasks like zipping a jacket, tying a knot, operating buckles, or a patient who has a notably unsteady gait, should be treated as stage one mild hypothermia. Even though they're awake and conversing with you, the treatment here includes both passive and active rewarming. So passive rewarming tactics are those removing cold stressors, removing wet clothing, and placing insulating layers around the patient so that their own heat loss is slowed and they can capture more of their own heat production. But active external rewarming implies adding heat packs or other warm things around the patient to help warm them directly. In most cases, this looks like a hypothermia wrap. And the typical hypothermia wrap worth a quick Google is essentially a pair of vapor barriers with a sleeping bag in between. So you might, wrap the patient in a space blanket, stick'em inside a sleeping bag, wrap them in a tarp and that's gonna help insulate around their body. You are gonna remove any wet or nasty clothing ahead of time, and then any heat that you can add into that situation, whether it's another human in with them, risk being cooling, that human or you know, hot packs, hot water bottles near the axil, near the groin where they can help to warm the core temperature, warm the blood is gonna help to increase that patient's core temperature. They can be given warm fluids if they can reliably swallow, but understand that you are never going to raise someone's core temperature significantly by giving them warm things to drink. If you think about how much heat is in a liter of water versus the entire rest of the body, the amount of warm water that we would have to add to that patient to affect their core temperature at all is. Ridiculously large. This patient, the patient would end up looking like an edematous balloon. The point really there is that you don't want to do anything that's gonna cool the patient additionally. So if you're gonna give them something to drink. At least make sure it's, you know, a normal body temperature or warmer so that it is not contributing to the cooling of the patient. If these people are still alert and able to swallow, then giving them warm caloric beverages like hot chocolate is great because it's gonna contribute to their metabolism as well. Our key points here are to reduce heat loss and add heat from the outside to these patients, it may be useful to have them lying flat, keep them horizontal to encourage blood flow and to be able to more easily manage them in terms of packing heat around the axilla and groin. But these patients are also appropriate to. Evacuate by walking if they are able to do so, and if you can prevent additional cooling. The problem is that they're often ataxic enough that they're kind of stumbles and so they, they can't reliably hike out on uneven surfaces in the cold. And so now we're talking about wrapping them up in a burrito, trying to keep them super warm when they can't exert themselves. We need to reassess these patients in mild hypothermia very frequently to ensure that they are at least remaining the same. Ideally improving, but are not progressing to moderate hypothermia Stage two in the Swiss system corresponds to a core temperature of about 32 to 28 degrees Celsius. The hallmark symptom of progression to moderate hypothermia is confusion, decreasing level of consciousness, confusion, lethargy, marks the transition from mild to moderate hypothermia. In these patients, they may have reduced or absent shivering. There are case reports all over the place about people shivering down to 28 degrees Celsius or not shivering at 35. So it's not a reliable sign, but at this point, shivering stops being quite so effective. And when we look at the patient's vital signs, we start to see potentially bradycardia. Hyperventilation and there's more pronounced ataxia than there is for mild hypothermia. The patient becomes a less reliable reporter of symptoms at this stage because of confusion. They may exhibit apathy in that they don't seem to care about the fact that they're becoming dangerously cold. They may deny that they're cold. They may become nauseated or even mildly combative. These are patients who can no longer participate in their warming in any way, so they need active external rewarming. As we touched on earlier, as we talked about going down through the levels of cooling, once shivering stops or is decreased or impaired and cellular metabolism starts to be impacted, this patient's ability to create heat for themselves has become grossly impaired. And unless we intervene on that, they will continue to spiral down until they reach the temperature of the environment. So these patients need to be in that hypothermia wrap with active external rewarming. So that's again, hot packs near the groin, near the axilla. Heat blankets if those are available, if you have enough people, enough shelter and you can do it safely. Putting other warm bodies in that burrito wrap with the patient to contribute their own body heat can be an excellent source of heat. These patients should be evaluated for hypoglycemia, and if that is present, then we want to try to replete glucose so that whatever cellular metabolism persists can still contribute to that patient's warming. If we're successful in starting to warm this patient and their cellular metabolism kicks back on, we want there to be sugar for those cells to burn so it can, they can produce energy to induce, shivering to start. Contributing to their own warming. However, these patients, because they're altered, are unlikely to be able to swallow. So now we're talking about needing IV access. We're talking about needing to be able to give D 50, D 10, you know, glucose containing fluids. And those fluids. You know, D 50 is like a tiny amount of fluid, but if you're having to give D 10, which is a more reasonable amount of fluid, then ideally you want to warm those IV fluids. Again, not because putting warm IV fluids into a patient is gonna warm them significantly, but because we don't want to cool them further. Same thing goes for if you're using an advanced airway or having to ventilate this patient, warmed humidified oxygen, if available, use that so you know you're at least not cooling them down through insensible losses, through the breath. Most rescue outfits, most SAR teams are not gonna be able to provide warmed humidified oxygen. That's more of a ICU level tool. If you can place these patients on a continuous cardiac monitor, that's something you should do because they are at risk of arrhythmia now. Okay. These patients definitively need to be evacuated from the field. There's two reasons for that, that impaired consciousness is a significant risk to the patient, and they definitely could no longer participate in their own warming. But because as we talked about, when when you get closer to that 32 degree Celsius, you stop shivering, you stop being able to warm yourself. You have to be warmed externally, and the best way to do that is not in the field. The best way to do that is in the hospital, and so we should expedite evacuation if that is feasible and safe. When we do so, we want to handle that patient very gently because this is the stage at which cardiac arrhythmia starts to become possible just from jostling or, or blows to the heart. So if you need to move the patient, do it smoothly, do it gently. Try not to bump or jostle their stretcher. We don't want the patient to move and exert themselves. The reason for that is the phenomenon called After drop. Like we talked about in episode 27, when you are exposed to cold, one of the most profound responses that you have to help. Warm yourself and preserve your cold core temperature is to vasoconstrict your periphery so the blood vessels in your skin out in your appendages, out in your legs are gonna constrict and shunt blood away from the cold stimulus and towards the core. To a patient who is in mild or worse hypothermia, who's exhibiting that decreased or altered level of consciousness, has already undergone those significant peripheral vasoconstriction changes. And there may be a very significant difference in the temperature of their tissue, in their extremities versus in their core. So they could have a core body temperature of 32 to 28 degrees, but out in their periphery, their periphery could be 24 degrees Celsius, could be significantly colder. So the risk is if you get that patient up and you make that patient move around, you increase their cardiac output and you drive blood flow into those extremities, those extremities can act like a heat sink For the core, that core blood, which has had some heat preserved, is then circulated out to the periphery where it's cold, like it's going through a, a heat pump. And it returns to the core even colder than before, and you can send that patient into severe hypothermia or hypothermic, cardiac arrest. There are numerous documented cases of situations like cold water immersion where someone is stuck up to their neck in an icy lake in cold water, and they're in moderate to severe hypothermia. Rescuers arrive and pull them from the water. And they get up and try to walk, and they go into hypothermic cardiac arrest because their core temperature immediately drops to an untenable level. So in moderate hypothermia, we need to be careful and minimize movement or exertion from the patient to WA to reduce the risk of after drop, during handling and rewarming. These patients have a core body temperature of less than 28 degrees Celsius. They exhibit all the signs of severe hypothermia, and what differentiates them from stage four is that they still have some vital signs. The difference between severe and moderate hypothermia is that the patient with severe hypothermia is unresponsive. They're no longer confused in ataxic and apathetic and trying to fight with you. No, they're unconscious and unresponsive. That's what defines severe hypothermia in the field. What we may observe is that in this unconscious patient, they still have pulse, they still have breathing, but both of these things may be very slow. They may have a marked bradycardia. They're likely to be hypotensive, and they may be in some form of arrhythmia. They will not report any symptoms to you because they're unresponsive. These patients need advanced treatment and transport to hospital as soon as possible. They need everything that we've done for that moderate hypothermia patient with consideration of more advanced warming measures if those can be delivered. The same management is going to pertain to teams who don't have, really advanced care or access to an ambulance or a helicopter. These patients need to be handled extremely gently. They need to be packaged with external rewarming and limiting additional cooling, and they need to be transported to advanced rewarming capabilities in hospital as soon as possible. These folks are more likely to require management of the airway, provision of ventilation. So they may require a supraglottic airway or they may require intubation to facilitate ventilation. If they have a respiratory rate of four, that's not enough. Even though their metabolic rate is very low, we want to try to push them back more towards normal physiology. So we're going to ventilate those patients. We are not going to pace them. We are not going to try to increase their heart rate because that heart is very fragile. The heart rate will improve gradually as we warm that patient. So we want active external rewarming. Again, if available. Make sure your IV fluids are warm, humidified, heated oxygen, if that's available to go to hospital. If you are uncertain about whether or not a patient is in severe hypothermia versus hypothermic cardiac arrest, then the key point is to check the pulse and breathing for longer than usual up to 60 seconds instead of a normal ten second pulse check. Because that patient's heart rate might be six 10, could be one to two beats per ten second period. They may be breathing one to two times per minute spontaneously because of the fragility of the cold heart. We want to, again, avoid unnecessary movement, and we want to avoid declaring these patients dead, so we don't, we don't want to think that someone who is very, very cold and has very, very few vital signs is beyond resuscitation. So this is where the maximum of nobody is dead until they are warm and dead comes into play. So someone who is severely cold should not be considered, you know, beyond retrieval until they are warmed to a relatively normal temperature. They should be taken to hospital, even if it seems like they're, they're just irre recoverable. They've been cold for a really long time unless they're frozen solid, which is generally a criterion for. Determining death in the field unless they're frozen solid or they have some other clearly life-ending injury. The very, very cold patient who appears dead or nearly dead needs to be warm before we make that determination. So these patients generally have a core temperature of less than 24 degrees Celsius. As you might surmise from the title of stage four, they have no pulse and they're not breathing. They have no signs of life, and that's really what differentiates them from severe hypothermia. The management of a patient who is in cardiac arrest in the setting of accidental hypothermia is slightly different. Okay. Significantly different from our usual management of cardiac arrest, both the UK Resuscitation Council, as well as the American Heart Association. Provide some guidance as to how to manage these patients, but there is not a standalone algorithm provided by either of these organizations. In the show notes, you're gonna see a link to both the clinical practice guideline from the Wilderness Medical Society, as well as their algorithm for the treatment of accidental hypothermia What the Wilderness Medicine Society Practice Guideline recommends is that if there are no signs of life after checking for pulse respirations for up to 60 seconds that CPR should be initiated, and those should be chest compressions at a standard rate. Where this starts to differ from usual cardiac arrest care is how much we defibrillate and how often we give drugs. Warming remains an issue, right? We want to try to warm these patients. It has to be external rewarming because their core temperature is gonna be, you know, 30 degrees Celsius or below. So they're not producing any of their own heat. So we still want to try to warm these patients. When it comes to defibrillation, if, if you see ventricular fibrillation or ventricular tachycardia on your monitor, or if your a ED advises a shock, they recommend that you attempt one round of defibrillation. However, if the patient's core body temperature is below 30 degrees Celsius. Depending on whether you're looking at the Wilderness Medicine Society guideline or the American Heart Association, you might cap the number of shocks that you give at either one or three. Wilderness Medicine Society says only shock once if you're under 30 degrees C. The a HA says up to three shocks. Both of them say that you can withhold additional shocks until. The patient has been warmed a few degrees Celsius or until their body temperature is greater than 30 degrees Celsius. And the reason for this is that the, the genesis of that ventricular fibrillation is a heart that is irritable due to cold, and it may be. That you're just not gonna be able to kick that V-fib until the patient is actually warmed. So delivering those shocks is just delivering additional insults to a heart that is not going to recover and kick back into a normal rhythm just by virtue of you adding some electricity. So for a patient who is in hypothermic cardiac arrest, we're gonna shock at most three times at least once. If you see V-fib or vtac or if your a ED is advising a shock, but then after that we're gonna withhold additional shocks until the patient is actually starting to warm. If you can't measure core body temperature, then you're gonna withhold those shocks until you're in a setting where that can be measured, which is generally gonna be in hospital, maybe in some kind of ambulance or or flight rendezvous. The other change that's a significant change from the standard algorithm is medications. And our two main medications in cardiac arrest currently in the year 2026. This is always changing. Fewer, fewer and fewer medications are effective. Those two main medications are epinephrine and amiodarone, and usually epinephrine is our first line drug for someone who's in cardiac arrest. And then we add amiodarone. Both the Wilderness Medical Society and the American Heart Association recommend that you do not give any vasoactive drugs, any epinephrine or amiodarone to a patient whose core body temperature is less than 30 degrees Celsius. The reason for that is that in patients who are that cold, those drugs dwell in the circulation for a really long time and their presence. May be counterproductive. The epinephrine may be pro arrhythmic. The amiodarone itself can be pro arrhythmic in the setting of a irritable heart, and those things are gonna stick around. So both guidelines recommend withholding those drugs until the patient's core body temperature raises to above 30 degrees Celsius. If you're actually succeeding at rewarming this patient in the field, and you have a means of measuring that core body temperature. Once you get above 30, but you're still below 35 degrees C, so 30 to 35 degrees C, we extend the interval between episodes of epinephrine. We may give it every 10 minutes, every six to 10 minutes rather than every three to five. So it's half as often, and that's because of that increased dwell time in a hypothermic patient, if you see. Arrhythmia ventricular fibrillation, ventricular tachycardia in that range, 30 to 35 degrees C. You may give one round of amiodarone. You may give one dose of amiodarone, and then see if you get any results from that, but I wouldn't give any additional, antiarrhythmic until that patient is essentially fully warmed. Finally, intermittent CPR is a consideration. What is intermittent CPR intermittent CPR is saying, okay, we can do two minutes of compressions every 10 minutes, and in between we can work on moving this patient. This is something which is case reportable. It is not well supported in guidelines and not well supported by research. But if you have a patient who is profoundly cold, and the only way that you're gonna get them out of the field is if you can occasionally pause CPR to move them. People have survived using this intermittent CPR idea being you're refrigerated, your metabolic rate is zilch. We're gonna stop every 10 minutes. Say and give two minutes of quality CPR to, you know, move some oxygen, pump the blood around, and then continue. Could be two minutes every five. This is, this needs to be driven by your local protocol because. What people consider acceptable is gonna be pretty widely variable here, and it's gonna be very practice setting dependent. The most advanced groups here may be able to measure a core body temperature and may be able to measure a potassium level in the field. Potassium is the linchpin of in-hospital decisions around advanced measures for hypothermic cardiac arrest. Potassium levels that are greater than 12. I don't even know the units. It's probably milliequivalents per deciliter. 12 US units means it's a sign of profound cellular death, and those patients do not survive even if they're put on ECMO and have advanced rewarming. So in the field, your organization, your group may have a field termination protocol for cardiac arrest. And that's gonna depend on having a profoundly low core body temperature. And then if that potassium is above a certain level, you may decide not to prolong CPR and extricate those patients from the field. And you can spare your rescuers that prolonged extrication and prolonged resuscitation if you don't have that available to you. And it is safe for you to evacuate the patient, then CPR should continue as you evacuate that patient. And this is a great situation if you have access to mechanical CPR, like a Lucas device or an auto pulse, because that allows you to apply continuous compressions, stop intermittently to ventilate the patient and move that patient out of the field. So to summarize, for treatment of cardiac arrest due to severe hypothermia, the big differences here are long pulse. Check check for 60 seconds to make sure there's no signs of life. When you start CPR, we're gonna try to do continuous compressions if available. But if you can't do continuous compressions, you might do five minutes on, five minutes off, two minutes every five minutes. You can follow your protocols. For intermittent CPR, if that's the only feasible option. If there's arrhythmia, we're only gonna defibrillate once, maybe up to three times for a core body temperature of less than 30. If you can't measure the core body temperature, then shock one to three times while that patient remains unresponsive. But don't go beyond three. Generally speaking, if you can't measure the core body temperature, withhold any vasoactive drugs, epinephrine or amiodarone, if you have the ability to. Measure core body temperature, hold those drugs until the core body temperature is 30 to 35 degrees Celsius. And then I only think one round of amiodarone is worthwhile in this setting, and epinephrine dosing intervals should be doubled. Once the patient is above 35 degrees Celsius, you return to normal. A CLS intervals for drugs, shocks, et cetera. And finally, if you have the capability to measure potassium in the field, you can potentially have a field termination protocol that allows you to avoid resuscitating people who are not gonna be successfully resuscitated in the field. But that's a, it's a rare case, and that's something we'll discuss more when we get to advanced management of hypothermic cardiac arrest. When we talk more about. In hospital management and the hope score I mentioned earlier in this episode, I'm planning a second episode. Next fall to talk about advanced resuscitation of hypothermic cardiac arrest. And when I say advanced, what I mean is in a critical care setting, whether that's a critical care ambulance in a hospital, in an ICU, in an emergency department. Places where we have access to advanced tools. In advanced procedural capability. And usually we're talking about where a physician is able to perform advanced procedures to try to resuscitate these patients. For right now, I just wanna give you a sense of what is available and what does and doesn't work. Generally speaking, when these patients arrive to an advanced care center, our goal is to start what's called extracorporeal rewarming, and the two main options there are either to put the patient on cardiopulmonary bypass that's going to an operating room and putting them on a heart lung machine there like they were gonna get surgery on their heart, or we're gonna try to put them on ecmo. Extra corporeal membrane oxygenation. Very popular during COVID. This is an external heart lung machine, which again, blood comes out, goes through a pump and a warmer and returns to the body. The reason that we do this is because this is by far the most effective way to warm someone's core body temperature. We're just directly warming the blood, putting it back into them. It's exactly how my heat pump heats my house. Coolant comes out, it gets warmed up, it gets put back in. Same thing happens for the body warming the patient from the outside, like with a heat blanket with hot packs, even submerging them in. Warm water. This is very, very slow and it's not very effective, and the reason for that is that peripheral vasoconstriction, so the the blood is simply not out near the periphery. Your best hope with those tactics is that you're maybe gonna get one, maybe two degrees Celsius change in core body temperature every one to two hours. So if we're talking about someone who's in profound hypothermia, maybe 24 degrees Celsius, or potentially even lower. Now we're looking at if that's your only way to raise that patient's core body temperature. We're looking at a resuscitation that could be six hours before you're in a range where you can consider terminating care on that patient. In contrast, you might get five degrees Celsius core body temperature change on ECMO or on cardiopulmonary bypass. So you can actually

39:34

get a patient

Patrick Fink 39:35

within an hour maybe an hour and a half. To a core body temperature where you might expect to be able to resuscitate them. There are all manner of tactics that have been tried to warm the core body temperature without ECMO and without bypass, and those usually involve circulating warm fluids into a body cavity. So we're talking about putting in chest tubes into the lung spaces, filling the hemithorax with warm water and sucking it back out. Basically ravaging it in and out. Same thing goes with a Foley catheter instilling warm water into the bladder or even into the peritoneal space. So putting a tube into the abdomen and trying to put warm water in around the guts. I've done this once. I did this during COVID. We had a subversion patient who was stuck in a cold river for a long time till his head finally went underwater and they popped up and he was in cardiac arrest and he was brought to us. And because of the concerns about COVID, we didn't have our normal resources available to us. Normally we would, we would've been able to put this patient on ecmo, but everyone was all worried about. Going to the operating room, sharing COVID, we were just remained in the emergency department and we tried all of these tactics to warm him. So he was intubated and ventilated with heated humidified oxygen. He got chest tubes in both sides of his chest with lavage of warm fluids. He had a Foley catheter in place. We're ravaging his bladder with warm fluids. He had a heat blanket as well as a bear hugger, which is a, it's just blowing warm air on him. And we spent. An ungodly amount of time in that room. I was probably in there for three, four hours. Everyone in there was just sweating to death'cause the room was so warm and over the course of two, three hours, his core body temperature changed about two degrees. It was just impossible. He went from 22 to 24 degrees Celsius. It was not going anywhere. And we eventually were able to. Obtain a blood sample and get a potassium level that was just too high, and so we called it, but that was my personal experience with all of these tactics. The literature supports it that you're not going to get a reasonable change in core body temperature by performing a number of procedures that pose risk to your caregivers. Needle sticks, you know, body fluid exposures. It's just not worth it. So these patients need to go to hospital settings where there is ECMO or where there is cardiopulmonary bypass. And if your nearest hospital center, your nearest referral center, doesn't have those things, it's worth considering speaking with them about how to manage these patients, whether they should bypass that hospital and go somewhere several hours away, or whether they have a relationship with an advanced center. That has an ECMO retrieval team where that they can send a team to that outside hospital while resuscitation is ongoing, to put them on ECMO and warm them or take them back to their care center. So these are all the things that you want to figure out for your. Search and rescue group, your ski patrol, you know, whatever, whatever. Even your, if you're working in a critical access hospital, you wanna sort these things out before you actually receive one of these patients, because otherwise you're looking down the barrel of a very prolonged resuscitation before guidelines will allow you to either terminate or you would expect to see any improvement. We will talk more about all this stuff next fall and really dive into the HOPE score, which is the prognostic score that helps us determine who is appropriate for advanced rewarming tactics. But for now, I just wanted to give you a sense of how we manage these patients I had like to give you a quick run through a summary of what we know about recognizing and treating hypothermia just so we can nail all this home. So in general, we're gonna use that modified Swiss system, which relies on signs and symptoms to diagnose the stage of the patient's hypothermia that goes from one to four. And the terms that we use for those are cold, stressed, mild hypothermia. Moderate hypothermia, severe hypothermia and hypothermic cardiac arrest. So the cold stressed patient is alert, their shivering, but they're functionally able to care for themselves. And those people just need to have reduced heat loss and increased heat production. Feed them, get them up, move them around, get them out of the cold. The mildly hypothermic patient is now struggling to care for themselves. They may have ataxia or mild confusion. They're, they're not able to reliably perform simple tasks. They're the ones who need your help. We need to protect them from further cooling by insulating them. We need to get them into shelter and get their wet clothing off. If we can measure their temperature, fantastic. But if they're showing ataxia the difficulty with simple tasks, we should start passively rewarming them. We need to give them calories. We need to give them some warm, sugary fluids and protect them from additional heat loss. And if you have the ability, you can actively rewarm these patients. The moderately hypothermic patient is starting to show some confusion. That's kind of the, the tipping point for me. And these folks need external heat. They need you to add heat because their their metabolism, their shivering may be impaired. These folks are the ones who need the heat packs. They need the full burrito. And if you have the capabilities, we're gonna start getting more advanced with our monitoring. You want to get an IV in them. They may not be able to eat or drink safely. You may need to give IV glucose if you do need to give fluids or what have you to support the patients blood pressure, those fluids should be warmed rather than cold. And we generally, we don't want to move these patients. They with no standing, no walking. They are relying on us now. The severely hypothermic patient is the one who's showing altered mental status. They're unresponsive. These folks are gonna start needing advanced life support measures like Supraglottic Airways, potentially assistance with ventilation. We need to be very careful with them. They need all of the rewarming tactics, and they need to go to an advanced rewarming center as soon as possible. Finally, those folks who are in cardiac arrest, we're gonna provide prolonged cardiac arrest resuscitation until that patient's warm. That can be intermittent. CPR, if that's the only option available to you. We're gonna use fewer shocks, less drugs, nothing below 30 degrees on the drugs. And if you have the capability to measure a potassium, you may be able to save yourself a really long resuscitation. If not, these patients need to go to a hospital that either has. Extra corporeal rewarming available or has a relationship with an ECMO retrieval team so that they can be warmed before they're declared dead. Because to cap this episode off, you're not dead until you're warm and dead. That's it for this episode of Wilderness Medicine Updates. I hope that this basic walk through the fundamentals of hypothermia is useful to you if you need, if you're a visual person, I really recommend that you look at the Wilderness medicine clinical practice guidelines. It. They have a really nice. Kind of flow chart of the care of these patients. They have a visual for how to assess the cold patient, and then they have a really nice graphic that shows you how to make that hypothermia wrap the burrito if you've never encountered that before. There's a lot of variation in, in how folks can do this many. There's a lot of right ways to do this. There aren't terribly many wrong ways, but if you need a visual, this is a great place to look and it's freely available. Down in the show notes. If you have any questions that came up, any comments or other topics that you want to hear, please reach out to me at Wilderness Medicine updates@gmail.com. I have really been enjoying the discussions I've been having with some of our listeners, the back and forth, the folks who have benefited from recent episodes and been able to apply those tactics, those mindsets. To their care of patients in the field. That's really rewarding to me and it makes me want to get more of this content out to you guys. So please do reach out if there's anything of interest to you, and I will try to get on those special guests, those experts, or as with this one, sit down and do some reading and try to bring you some good inform. If you wanna support the show, the best way that you can do that right now is just open up your little app, your iTunes, your Spotify, and give the show five stars. Throw a comment in there that drives it up in the ratings and exposes it to more people. Also, word of mouth is incredibly powerful. So if you're part of a search and rescue ski patrol organization, you volunteer with the sheriff's office, you work at a hospital, you're a medical student, you're a nurse, you're a doctor, share it with a colleague who you think might enjoy it. Because again, the more people that this reaches, the better and uh, the more the reach of the show. This is a passion project of mine that I'm bringing to you because I want it to be valuable, and the more people it reaches, the more valuable it is. So thank you for your help sharing the show. Until next time, this is your host, Dr. Patrick Fink. Stay fit, stay focused, and have fun.