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COLD STRESS

Fig. 1. Physiological effects of WBC exposure

At 20°C, the body temperature of a resting individual wearing light clothes requires no regulation. Cold stress occurs in any environment causing heat loss constituting a threat to homeostasis. A drop in skin or blood temperature leads to a hypothalamic response, activating the mechanisms to counteract cold, and thus increasing the body's heat production. The first reactions are shivering, non-shivering thermogenesis and peripheral vasoconstriction. Shivering is a reflex response to cold, which translates as a succession of involuntary muscle contractions. It increases resting metabolic heat production four- to five-fold. Non-shivering thermogenesis is the result of stimulation of the metabolism by the sympathetic nervous system. Resting metabolic rate is controlled by the thyroid hormones, and an increase in metabolic rate leads to increased endogenous heat production. Sympathetic stimulation also leads to peripheral vasoconstriction by acting on smooth muscles located in the walls of the arterioles near the surface of the skin. Their contraction reduces the size of the vessels and thus the blood flow to the skin, thereby preventing excessive heat loss. As the skin's temperature drops, the metabolic rate of skin cells is also reduced, thus limiting the need for peripheral oxygen.

 

In contrast to the response to high external temperatures, the body's thermoregulatory process to counteract cold is quite limited. Once this limit has been reached, a significant amount of heat is lost if appropriate protective clothing is not worn. In a cold environment, heat exchange by conduction, convection and radiation can lead to a loss of calories greater than what is produced by endogenous systems. It is difficult to define the precise conditions leading to this excessive heat loss and hypothermia. Indeed, numerous factors, both intrinsic (adipose tissue, circulatory system, hormonal system, level of training, state of fatigue) and extrinsic (type of clothing, external temperature, wind, humidity, altitude) are involved in the thermal equilibrium and affect the gradient between loss and gain of heat. The general principle is that the greater the temperature differences between the skin and the environment, the greater the loss of heat. But, as we have just explained, the rate of heat loss also depends on anatomic and environmental factors. Wind, for example, is a cooling factor, increasing heat loss by conduction and convection. Similarly, thermal stress is increased by high humidity and more intense cold. The same actual temperature can feel much more uncomfortable when the air is very humid or in windy conditions.

 

 Local cold therapy or cryotherapy is commonly used to relieve pain, particularly pain caused by inflammatory diseases, injuries and overuse symptoms. A specific form of cold therapy or stimulation was proposed 30 years ago to treat rheumatic diseases. This therapy involves brief exposure of the whole body in special temperature-controlled cryochambers, where the air is maintained below -100 °C. The treatment was named whole-body cryotherapy (WBC) or whole-body cryostimulation and its effects are presented in Figure 1. 

 

WBC is generally applied for 2 to 3 minutes, but in some protocols it can last for up to 4 minutes. Exposure can involve a single subject, or a small group of subjects - up to four people can be present in the chamber at the same time. Before entering the cryochamber, each subject spends 30 seconds in a vestibule at a temperature of -60 °C to allow temperature adaptation. During exposure, subjects wear minimal clothing; to avoid frostbite they wear shorts (bathing suit), socks, clogs or shoes, surgical mask, gloves, and a hat (or headband) covering the ears. Subjects are dried of any sweat before entering the cryo facility, where the air is clear and dry. This treatment can help to relieve pain and inflammatory symptoms caused by numerous disorders, particularly those associated with rheumatic conditions, and it is recommended for the treatment of arthritis, fibromyalgia and ankylosing spondylitis. Very recently, whole-body exposure to cold was shown to aid post-exercise recovery by altering blood flow (Pournot et al. 2011) and improving perceived recovery. Exposure of the body to cold may also exert significant effects on post-exercise recovery at the cardiovascular level. As exercise causes intensity-dependent parasympathetic withdrawal and a sympathetic increase, prompt recovery of parasympathetic activity is desirable after exercise. The effects of dry air whole-body cryostimulation (under -100 °C) on post-exercise autonomic recovery are not well documented, even though this recovery method is increasingly used in the sporting realm and positive results are frequently reported.

 

TISSUE TEMPERATURES

WBC is mainly offered as a tool to rapidly cool various tissues by applying a strong temperature gradient between the inside and the outside of the body. Examination of recent studies makes it possible to objectively review whether this approach is effective. Indeed, despite extreme temperatures of down to -180 °C measured at the outlet in nitrogen-based devices, the decreases in temperature recorded on the skin (-4 °C to -14 °C), in the muscles (≈ -1.1 °C) or for core temperature (≈ -0.3 °C) remain modest. In particular, these differences are lower in magnitude than those measured when ice packs are applied or with protocols involving immersion in cold water (Beakley et al. 2014). The reason for these differences is relatively simple to understand given the laws of thermodynamics, in particular for the coefficient of thermal conductivity (k = W/m2 - K) which reflects a material's capacity to transfer heat. As air has a lower heat transfer coefficient (0.024 k) than ice (2.18 k) or water (0.58 k), its capacity to extract cold from the body is limited compared to the two other cryotherapy methods. Nevertheless, in the case of WBC, this reduction in heat-extraction is partly compensated for by the fact that a much larger body surface area is exposed than with the other techniques. The extent of this surface appears to play an important role in skin cooling, as recent studies have shown significant differences depending on whether the system is open (e.g. open gas cooled cylinders) or closed (e.g. icelab -110 °C). Thus, it appears that in open systems the cooling effect on the body is less significant since the gas rises, whereas in closed systems, cooling is more homogeneous, inducing a greater reduction in mean skin temperature over the whole body. (Another technical explanation is, a different temperature measurement, e.g. in most open gas cooled cylinders the manufacturers are talking about the temperature at the evaporation nozzle, compared to icelab -110 °C where the temperature is measured inside the room meters away from the cold intake.)   All of these studies indirectly raise the question of the optimal duration of exposure; only one study has attempted to provide an answer so far (Selfe et al. 2014). According to the authors of this study, exposure to under -100 °C for 2 min after 30 s at -60 °C would be adequate for a population of professional rugby players. However, they also indicate that, given the specificity of the population and the markers examined, it is difficult to derive a general protocol from these results. Despite this drawback, this study clearly posed the question and developed a method to attempt to glean a response. Other methods could also be envisaged, for example mathematical modelling of the individual thermal characteristics based on simple data (height, % body fat, gender, etc.) as is already the case for other cold-based therapies. Because of the lack of data, it appears difficult today to set out precise recommendations on the optimal durations, frequencies and/or exposure surfaces without having previously defined thresholds for the desired target temperature. These thresholds depend on the tissues, what took place before the therapy, and the objective of the WBC (i.e., therapeutic, recovery, etc.). In this context, current recommendations mainly advise caution to avoid potential side effects, but further investigations should lead to tailored protocols with improved efficacy.