What is Hypothermia Anyway?

Part 1 of a 2 part series. (Part two is here.)

We all understand that hypothermia occurs when the patients core temperature drops secondary to exposure to cold. But, clinically, what is hypothermia really? What happens to the body as it cools down and struggles to maintain its core temperature? Can we predict a patient’s relative core temperature based on our physical findings? Is treatment really as simple as bringing them in and warming them up?

Let’s review the basics of the hypothermia progression and explore how to treat these sometimes challenging patient presentations. Put your mittens on, this is a chilly ride.

When we consider the typical hypothermia victim we often envision the lost hiker stumbling through the snowy woods looking for home. Perhaps our victim stumbles through a freezing stream, the sun begins to set and hypothermia sets in.

No doubt, our lost hiker is at risk for hypothermia, but we should consider that anything that decreases the body’s ability to produce heat or increases the body’s propensity to loose heat, can lead to eventual hypothermia. The prepared hiker may be less at risk for hypothermia than an urban soccer player who gets caught in an unexpected rain storm or an elderly nursing home patient left too long on an outdoor patio in the wind or an intoxicated transient patient who didn’t bundle up well enough before nightfall.

Hypothermia occurs more often in urban settings that rural settings and you’re just as likely to encounter it in Detroit, Michigan as you are in Missoula, Montana.

Recall that there are five ways for the body to lose heat:

Conduction: Touching cooler objects transfers body heat.

Convection: Air blowing across a warm body carries heat away.

Radiation: Heat radiates off the body into the air.

Evaporation: Sweat and moisture drying on te skin carries heat away from the body.

Respiration: Air gathers warmth in the respiratory tract and we exhale it.

Other factors both external and internal can speed the body’s willingness to sacrifice heat. Water conducts heat away from the body 25 times faster than air. Underlying medical conditions can also affect the body’s compensatory function. Any medical condition that hinders heart function, metabolic function, vasoconstriction or shivering will affect the patients ability to maintain core warmth. 

Being elderly, young, sick, drunk, male or high all make you more likely to succumb to hypothermia.

So now that we know how the body looses heat and what things might accelerate it, lets take a walk through a typical plummeting core temperature. The following numbers represent degrees Fahrenheit.

103.0-96.4 The normal core temperature range. Remember that body heat fluctuates quite a bit during a typical day. Exercise might raise core temperature from a standard 98.6 to temperatures as high as 103.0. Body temperature may also fall as low as 96.4 at rest. Temperatures in our limbs tend to stay 1-2 degrees lower than our core.

95.0 Mild hypothermia. Shivering begins.

93.2 Amnesia and minor errors in judgment begin. An individual may become overly emotional or easily irritated.

91.4 Major motor coordination begins to fade.

90.0 Moderate hypothermia. More significant changes in mentation including confusion and stupor.

87.8 Shivering stops. One this vital heat generation mechanism disappears core temperature will drop more rapidly.

86.0 Irregular heart rhythms emerge. Atrial fibrillation is the most common hypothermia arrhythmia.

85.2 Loss of consciousness more likely. Pupils Dilate.

82.4 Spontaneous ventricular fibrillation possible.

80. 6 Severe Hypothermia. Inability to move voluntarily.

78.8 Unresponsiveness to pain.

75.2 Significant hypotension sets in. Pulses become difficult to discern.

71.6 Ventricular fibrillation likely.

56.7 Coldest successful adult resuscitation.

57.6 Coldest pediatric resuscitation.

A good rule-of-thumb assessment for the hypothermia patient is to lay your hand on the patients abdomen. If he abdomen feels cool to the touch, generalized hypothermia is likely.

With your basic assessment and a rough idea of how the body devolves as hypothermia progresses, you can take a fairly accurate guess at the patients core temperature before you even hit the power button on the thermometer.

Looking at the progression of symptoms above, one of the real challenges of treating severe hypothermia becomes apparent. As the body metabolically shuts down in an effort to save itself, many of the symptoms of moderate and sever hypothermia mimic death and our aggressive interventions can easily do more harm that good.

We’ll talk moe about that next time.

Now it’s your turn: Have you treated hypothermia in the field? What were the symptoms like? What challenges did you encounter?

Want to learn more stuff?

 Responding to Hypothermia

What Is Ketosis Anyway?

What Is Muscular Dystrophy Anyway?

What Is The Duty To Act?

What Is Epinephrine?

 

 

 

Comments

  1. Could you cite some of your sources? I’ve never seen a breakdown beyond mild, moderate, and severe hypothermia before and not all of it matches up with what I’ve been taught in WFR and articles such as http://www.aafp.org/afp/2004/1215/p2325.html (which isn’t terribly well-cited) and NEJM 331:1756-1760 (unfortunately not freely available).

    Also, I’m not sure what you meant that limbs tend to be 1-2 degrees cooler than core — did you mean blood temperature deep in the thigh, or did you mean somewhere more peripheral? This totally unrelated study ( http://www.springerlink.com/content/bh54458q70244616/ ) mentioned average hand temperature being about 87°F. This other article ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1752199/ ) lists differences between core and peripheral more conservatively at 3-8°F in “moderate” environments.

    I’m seeing the coldest resuscitations-to-survival as 57.6°F (rectal) for a toddler and 56.7°F for an adult, according to sources cited by the AAFP article above.

  2. Thanks for the article.

    I don’t know that I agree with your introductory characterization of hypothermia as something that happens “secondary to exposure to cold.” I mean, of course for a patient to become cold, their environment has to be colder than normal body temperature, but as you later indicate, anything that causes someone to have diminished heat-generation capacity can cause it. Patients can become hypothermic laying directly on the ground (conductive) when it’s eighty degrees out.

    Julio.

  3. Steve Whitehead says:

    @Reader Thanks for the links. They’re great. I use about seven different EMT and paramedic textbooks when I’m writing depending on which cover the subject at hand best. The exact temperatures listed above come from a table in EMT Prehospital Care, 4th Edition Authors: Mark C. Henry & Edward R. Stapleton
    ISBN: 9780323055475. I thought that text did a great job covering hypothermia in detail.

    Regarding the AAFP article, I don’t see anything that contradicts the information provided by the text I cited. Your article lumps symptoms into a more broad range of temperatures. But I don’t see any symptoms on my list that fall into different temperature ranges in your article. In fact. I’m surprised at how well the two pieces match up.

    What symptoms do you feel aren’t “matching-up?”

    Also keep in mind that moderate and severe hypothermia research is limited because you can’t really freeze people and record their symptoms. So we’re all going off of very little real research. The most comprehensive research numbers come from the Nazi experiments on prisoners pre-Nuremberg and there’s some debate over whether those figures should be used at all because the research itself was unspeakably cruel. Differences from one source to another may be explained by whether or not the chose to include WWII era research. Some do, some don’t.

    The figures for lowest successful resuscitation come from a power point that was about six years old. I think your numbers are better and I’ll change the article to reflect that data.

    Thanks again for your contribution.

  4. Steve Whitehead says:

    @Julio You’re right Julio. If the ground is cold enough. 😉

  5. Steve Whitehead says:

    @Reader My 1-2 degree limb temp also came from the EMT text. It didn’t specify where in the limb and I don’t think the difference would add any value to the teaching point.

    The key here was recognizing that the temperature of the caregivers hand should be colder than the core of the patient and if the patients core feels cold to the touch, they are most likely significantly hypothermic.

  6. Steve, the effects of hypothermia at different core temperatures is merely interesting to field providers Core temperature assessment with an oral or rectal thermometer lags actual core temperature in both the cooling and warming processes.

    Yes, we can estimate core temp with assessment findings. At Wilderness Medical Associates (I am a lead instructor) we teach students that the key assessment findings in determining hypothermia severity are 1) presence or absence of shivering and 2) mental status changes for an awake patient or level of consciousness for a patient that is not awake. With those two assessments you can determine mild or severe.

    Dr. Gordon Giesbrecht aka Dr. Ice Cube has done extensive hypothermia research including human subject cooling in controlled laboratory conditions. http://www.umanitoba.ca/faculties/kinrec/about/giesbrecht_faqs.html

    Finally, the hand to the abdomen assessment assumes that you, the rescuer, are normothermic and have not been exposed to the same relative cold environment as the patient. Stepping out of the ambulance and touching the patient’s belly is much different than riding a rescue snowmobile for 45 minutes to reach an injured patient in the back country.

  7. Steve,
    Sorry for going all sourcey on you! I must have been a student too long if I get suspicious whenever there aren’t primary source citations.

    Indeed, the differences wouldn’t generally be clinically significant (e.g. classifying “severe” hypothermia at 82 vs. 80°F — either way, your patient is still probably comatose or just about, and you still have to gentle). I’m not sure that “stupor” is the word you mean for the beginning of “moderate,” and “inability to move voluntarily” didn’t quite convey “appears dead” to me (as one of my instructors put it) or “coma” (as the AAFP labeled that temperature range) for severe. Also, a mention of bradycardia (and/or initial/unpredictable tachy) might be helpful? But probably most readers have already been taught that. But anyway, the specific breakdown to temperatures and the 1-2 degree comment led me to start looking for sources and otherwise splitting hairs. And perhaps the most incredible thing about having found another two “coldest” resusses is that apparently at least a few people have been revived from “about 60.”

    I might still argue that it’s useful to recognize that the hands of you-the-caregiver are possibly >10 degrees cooler than core temp, but could vary wildly, and thus may not be particularly reliable indicators. Anyone colder than my hands (which ordinarily are sufficient to *induce* hypothermia) is in deep trouble. But if I find a (hypothetical) patient who is warmer than my hands, (s)he could still be hypothermic (but will probably be warm enough to mumble to you, unless there was another reason for diminished consciousness). Or if for some reason a patient is trapped and unconscious somewhere and it’s only possible to access the arm, which comes in at, say, 85 or lower, the patient may still be not bad core-temp-wise. Common sense will probably still prevail to tell you that someone found under various conditions may need warming up or at least a few blankets, anyway, even if the patient is not shivering but warmer than the hand (which most people should still recognize as much colder than normal).

    Ouch, I hadn’t heard about the connection to the Nazi experiments. That would make sense.

    And thanks for including source links on Part 2 (as you often have in the past, but I appreciate it in particular this time 🙂 ).

  8. Took it upon myself to convert to Celcius for any that are interested…
    39.4-35.7 103.0-96.4 The normal core temperature range. Remember that body heat fluctuates quite a bit during a typical day. Exercise might raise core temperature from a standard 98.6 to temperatures as high as 103.0. Body temperature may also fall as low as 96.4 at rest. Temperatures in our limbs tend to stay 1-2 degrees lower than our core.
    35 95.0 Mild hypothermia. Shivering begins.
    34 93.2 Amnesia and minor errors in judgment begin. An individual may become overly emotional or easily irritated.
    33 91.4 Major motor coordination begins to fade.
    32.2 90.0 Moderate hypothermia. More significant changes in mentation including confusion and stupor.
    31 87.8 Shivering stops. One this vital heat generation mechanism disappears core temperature will drop more rapidly.
    30 86.0 Irregular heart rhythms emerge. Atrial fibrillation is the most common hypothermia arrhythmia.
    29.55 85.2 Loss of consciousness more likely. Pupils Dilate.
    28 82.4 Spontaneous ventricular fibrillation possible.
    27 80. 6 Severe Hypothermia. Inability to move voluntarily.
    26 78.8 Unresponsiveness to pain.
    24 75.2 Significant hypotension sets in. Pulses become difficult to discern.
    22 71.6 Ventricular fibrillation likely.
    13.72 56.7 Coldest successful adult resuscitation.
    14.2 57.6 Coldest pediatric resuscitation.

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