Chapter 24: Cold

INTRODUCTION

Sports and physical activity often take place in cold environments. Normal deep body homeostatic temperature in humans is around 37°C (99°F). When ambient air temperature drops below 28°C, the human body must either reduce heat loss (e.g. via insulation) or produce sufficient heat through activity to maintain homeostasis. Hypothermic injury results when these conditions are not met. At the 2018 Winter Olympics, in PyeongChang, temperatures reached −14°C (−7°F), leading to significant challenges for the athletes. As discussed in the previous chapter thermally induced injuries, including drowning, are a common cause of death in people undertaking sport. The primary focus of this chapter is to:

• review thermoregulation of the human body in cold environments

• demonstrate how to measure environmental and body temperatures in the cold

• look at the impact of cold on performance

• discuss the effects of cold air and water: medical risks and mitigation

• briefly review treatment approaches to freezing and non-freezing cold injuries and health screening for sport in cold environments.

THERMOREGULATION IN THE COLD

For the body to be in heat balance, the physical routes of heat exchange (conduction, convection, evaporation and radiation) must match heat production or gain, such that heat storage or debt does not occur. This is important biophysical knowledge, but what is sometimes forgotten is that alongside these physical pathways lies a thermoregulatory system attempting to maintain thermal homeostasis. To achieve homeostasis there are thermal sensors distributed throughout the body, especially the skin and central nervous system. These transient receptor potential cation channels can respond, or be inhibited by, heat and cold. They are also activated by different chemical stimuli (e.g. menthol: TRPM8 ‘cold’; capsaicin: TRPM1 ‘hot’), hence the use of such chemicals in cooling or warming products. It is worth noting these effects are primarily sensory.

The distribution of cold and warm receptors varies throughout the body. Again, this is interesting, but for the clinician it is the relative sensitivity of the different body sites for determining thermoregulatory effector responses that is perhaps more useful. In this regard, the hierarchy for the thermoregulatory effector responses (skin blood flow, sweating, shivering) is:

The predominance of deep body temperature (T_db) in determining thermoregulatory effector responses explains, for example, why hand immersion in cold water can be used to cool hyperthermic individuals (skin blood flow 3 L/min), but hand immersion in warm water will not rewarm hypothermic individuals (skin blood flow 20 mL/min).^{1, 2} It also explains why the best way to avoid cold injuries to the extremities is to maintain, or raise slightly, deep body temperature and thereby maintain blood flow (heat supply) to the hands (Fig. 24.1). ...