Exposure to extreme cold results in a series of predictable changes in human physiology, which can be broadly described as "everything stops working". From the coagulation cascade to the the nephron to the interactions of haemoglobin with oxygen, all things become slower and more disorganised. Ultimately, the borders of survival can be found somewhere around 13.7°C, which was the temperature of the coldest survivor (unreliable accounts suggest it may be as low as 9°C for children). Beyond this point, the defrosted organism tends to be permanently damaged by their experience.
Question 1 from the second paper of 2000 asked the candidates to list the clinical effects of severe accidental hypothermia. The defining feature of this question was the accidental nature of the injury: the college were not interested in the pathophysiology of carefully maintained therapeutic hypothermia. Then, extreme hypothermia was ignored by the college for almost twenty years, ultimately reappearing in Question 7 from the second paper of 2019, where the examiners were mainly interested in the consequences of hypothermia for the management of cardiac arrest.
Among the primary exam revision sections, there is already a chapter on the physiology of hypothermia which was probably too extensive for a last minute cram. The time-poor CICM Second Part candidate may safely ignore those details. A summary is all that is required for answering Question 1, and it is offered in a table below. If one were to read only one peer-reviewed article, it should be the 2012 NEJM review by Brown et al. The hypothermia enthusiast may also derive some amusement from reading K.C. Wong's 1983 article, or these two thorough reviews by Kees H. Polderman.
Primary hypothermia is the process an otherwise healthy person being heat-depleted by exposure to some extreme cold environment. In contrast, secondary hypothermia can occur accidentally at a relatively normal temperature, and reuires the person to be in some way susceptible. Table 1 from Brown et al (2012) lists about thirty different conditions which may predispose a person to hypothermia. The notables include alcohol intoxication, hypoadrenalism, hypothyroidism, malnutrition and impaired shivering (e.g by quadriplegia or stroke).
A definition of the stages of hypothermia offered in the college model answer is as follows:
This staging system is well known, but requires the measurement of core temperature, which may not be possible in the prehospital environment. There, the Swiss system is probably going to be more useful (Durrer et al, 2003). The Swiss system classifies hypothermia according to clinical features rather than temeperature:
The first three HT grades correlate roughly with the mild-moderate-severe temperature range; HT 4 spans the interval of 24-28 degrees and is defined as "apparent" death because historically people retrieved from frozen lakes with this sort of temperature have been asystolic, and yet went on to make reasonable neurological recoveries. The "real" death in HT 5 is associated with a temperature below 13.7 degrees, which is the lowest temperature ever survived by anybody (perhaps referring to this case report of a seven year old Swedish girl who fell into the winter sea).
Hypothermia results in a series of predictable physiological derangements in the human organism.The nature and severity of these derangements depends on the depth of hypothermia. A number of excellent free resources are available for this topic. K.C. Wong's 1983 article on this issue is one such resource, and I will refer to it constantly. Additionally, Kees H. Polderman has written two brilliant reviews on the topic of low temperature physiology; I am not sure whether he is the all-knowing guru of hypothermia, but he certainly has his articles available as free full-text, which earns massive brownie points with me.
Endocrine and metabolic consequences
Haematological consequences
Respiratory consequences
Acid-base changes: alkalosis and hypocapnea
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Cardiovascular consequences
Renal consequences
Central nervous system effects
Immunological consequences
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These effects can be represented on a large confusing diagram.
In their comments to Question 7 from the second paper of 2019, the cranky CICM examiners complained that many of the candidates "listed things that were not clinical signs e.g. ECG changes and ETCO2". These days of course the intensivist has an absolute orgy of data available at the bedside, including potentially core temperature, mixed venous oxygen saturation, cardiac output and EEG; but the examiners wanted to reach back to a simpler time. For this, an excellent review by Rosin et al (1964) was highly informative. A slightly more modern take by Aslam et al (2006) is also available for free.
The ARC ALS2 manual is perhaps the best reference point for resuscitation of hypothermia. They offer a list of important differences in the initial management of a hypothermic patient in cardiac arrest. In addition to this resource, Chris Nickson's LITFL page on hypothermia is an excellent overview of resuscitation from hypothermia, including all the relevant techniques.
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