Abstract
To the Editor:
The recent article by Grissom and colleagues 1 is an interesting report regarding core temperature afterdrop in simulated avalanche survival. This paper has a number of strengths including near-comprehensive physiological monitoring, detailed methodology, and clear practical implications. However, there is much still to be determined regarding the underlying mechanisms of afterdrop under these conditions.
This study was conducted outdoors, which is a major strength considering that most acute cooling studies are conducted in a laboratory setting. 2 –4 The number of dependent variables collected is impressive: esophageal and rectal temperature, heart rate, oxygen saturation, and minute ventilation, as well as inspired and expired gases. Measuring both rectal and esophageal temperature is a tremendous strength in afterdrop research because of the clear differences between sites. 5 However, the title of the paper and the main reason for conducting the study was based on shivering thermogenesis, which was not measured. The lack of oxygen consumption data and/or electromyography (EMG) prevents clinicians and researchers from discerning why the afterdrop occurred. The underlying mechanisms will need to be identified before widespread guidelines for rewarming avalanche victims can be implemented.
The methods section and figures in this paper 1 present very detailed information. The authors make it clear that the extrication and blanket wrapping were done quickly in an effort to minimize exercise-induced increases in afterdrop. Although technically difficult, it would have been ideal to keep posture consistent during the study. Going from seated (snow burial) to supine conditions (rewarming) likely loaded the baroreceptors, which might have caused increased blood flow and heat loss in the peripheral limbs. Furthermore, cold stress itself causes increased stroke volume in the seated position, 3 but not in the supine position 4 in young people. This, along with the lack of a control group, makes it impossible to definitively state that rewarming in an insulated bag is more effective than another intervention.
Burial in snow would predominantly involve conductive cooling. This type of cooling is more rapid than convective cooling, (eg, cold air exposure). Other factors such as clothing insulation, age, gender, ethnicity, fitness, and body fatness can also play a role in thermoregulation. The article by Grissom et al 1 employed a relatively small sample size (3 men and 3 women), and the individuals were smaller on average than other reports.2,4 The cited studies by the authors show that avalanche survival is possible in young athletic people, even those with considerable hypothermia. Outdoor athletes would tend to have larger muscle mass and thus more shivering capacity. It is yet to be determined how fitness and body fatness might impact thermoregulation during and after snow burial.
Overall, the recent work by Grissom et al 1 is a huge step forward in regards to the afterdrop phenomenon. The study has clear practical implications and was relatively well controlled for being conducted in a real-life (ie, nonlaboratory) environment. However, shivering was not quantified in this study, and it remains to be determined how acute (eg, exercise, postural stress) and chronic (eg, age, diseases) processes may impact cooling rates during snow burial. Perhaps future studies could determine how individual factors play a role in the afterdrop phenomenon.