Environ Med. 2001; 72(5): 477-83. Leffler CT, White JC. Recompression treatments during the recovery of TWA Flight 800. Undersea Hyperb Med. 1997; 24(4):.
Vann RD, Denoble PJ, Pollock NW, eds. Rebreather Forum 3. AAUS/DAN/PADI: Durham, NC; 2014.
THERMAL PHYSIOLOGY AND DIVER PROTECTION Neal W. Pollock Divers Alert Network and Center for Hyperbaric Medicine and Environmental Physiology Duke University Medical Center Durham, NC, USA ABSTRACT Thermal issues can substantially alter decompression stress. The impact will depend on the timing, direction and magnitude of the thermal stress. While divers may be cognitively and physically impaired by cold stress, reaching a true state of hypothermia would be highly unusual. Thermal protection can be provided by a variety of passive systems and an increasing number of active systems. Active systems must be used with particular care since they can markedly alter inert gas exchange and decompression risk. Increased decompression stress will be experienced by divers warm during descent and bottom phases and cool or cold during ascent and stop phases. Decreased decompression stress will be experienced by divers cool or cold during descent and bottom phases and warm during ascent and stop phases. Practically, it is important for divers to remember that while many dive computers measure water temperature, none assess the thermal stress actually experienced by the diver. While real-time monitoring might one day allow for dynamic decompression algorithm adjustment based on thermal status, the current diver must consciously manage thermal status and risk. Keywords: cold stress, cold water, decompression, diving, hypothermia, immersion, insulation
or seven metabolic equivalents [MET]) in a wet chamber compressed to a pressure equivalent to a depth of 37 msw (120 fsw). The bottom phase was followed by a long decompression (87 minutes) that would accommodate increases in bottom time in the event that the rate of decompression sickness (DCS) stayed low during the study. The water temperature was clamped for two phases: descent/bottom and ascent/stop. Clamp temperatures were 36°C (97°F — “Warm”) and 27°C (80°F — “Cold”). The study yielded 22 cases of DCS. The relative risk from high to low can be ranked as “Warm–Cold” (warm in the descent and bottom phases and cold in the ascent and stop phases), then “Cold–Cold,” “Warm–Warm,” and finally “Cold–Warm.” The results of the Gerth et al. (2007) study make sense intuitively since being warm during the descent and stop phase would augment inert gas uptake and being cold during the ascent and stop phase would impair inert gas elimination. Similarly, being cold during the descent and bottom phase would reduce inert gas uptake, and being warm during the ascent and stop phase would increase inert gas elimination. The surprising part was the magnitude of the effect. The “Warm–Cold” combination had a 30-minute bottom time and yielded 22 percent DCS, while the final “Cold–Warm” combination had a bottom time of 70 minutes and yielded only 0.1 percent DCS. While the decompression and stop phase of the dive was disproportionately long
INTRODUCTION Diving is conducted across a broad range of conditions. Water temperatures can exceed 38°C (100°F) and be as low as -1.9°C (29°F). The duration of exposures can also be extreme, with examples of exploration dives lasting tens of hours (Kernagis et al., 2008). While thermal status is probably most obviously associated with individual comfort and then concentration and performance issues, it can also play a critical role in affecting decompression risk. Thermal factors can have complex effects, either increasing or decreasing the net decompression stress, depending on the timing, direction and magnitude of the effect. The best demonstration of the fundamental relationships was provided by Gerth et al. (2007). This study was conducted at the U.S. Navy Experimental Diving Unit (NEDU) Ocean Simulation Facility. The study captured 73 male subjects (37±6 years of age; 27.6±3.1 kg·m-2 body mass index) completing 484 man-dives in eight series. Dives included full immersion and substantial exercise (at a rate of approximately seven times resting effort,
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Figure 1. A diver about to descend in slush-filled hole in Antarctica. Photo courtesy Neal Pollock.
Neal W. Pollock
in comparison with many operational dive profiles, the study clearly shows that thermal status can have dramatic effects. Far beyond comfort, this work reinforces the importance of understanding thermal issues associated with diving. MAJOR AVENUES OF HEAT EXCHANGE There are four primary avenues of heat exchange important in the diving environment: radiation, conduction, evaporation and convection. Radiation represents the electromagnetic energy radiating from any object to any cooler object separated by space (air or vacuum). Conduction represents the heat flow between objects in physical contact. Insulation represents the inverse of conduction. The standard unit of insulation is the “clo,” with 1.0 clo approximating the insulative protection of a summer-weight British suit from the 1950s (1 clo = 0.18°C·m2·h·kcal-1 = 0.155°C·m2·W-1 = 5.55 kcal·m2·h-1). Evaporation represents the heat energy expended to convert liquid water to gaseous state. Evaporative heat loss results from humidifying inspired gases and the evaporation of sweat on the skin. Convection represents the heat flow through circulating currents in liquid or gas environment. The concern in most diving environments is the minimization of heat loss. Even tropical waters can produce substantial cold stress over long exposures. Radiative heat loss is a relatively minor concern in diving. Vasoconstriction will decrease skin temperature, effectively reducing the radiative gradient. Radiative barriers have been added to the inside of some wetsuits and drysuits, but probably with little actual benefit given the minimal (or non-existent) physical separation. Conduction is the primary avenue for heat loss in water. The heat capacity of water (density x specific heat) is >3500 times greater than air, yielding conductive loss rates 20-27 times greater than air. Protection against conductive losses is gained through improved insulation. The best insulator is a vacuum layer evenly distributed over the body surface. Next would be gas, then non-metals and, finally, the worst insulator would be highly conductive metals. The key to effective insulation is persistent loft, a challenge in drysuits since hydrostatic forces compromise loft by shifting air to the highest point of a suit during immersion. Respiratory evaporative heat losses increase with depth as a function of increasing gas density. There is a high heat loss associated with breathing open-circuit gas that can fall far below ambient temperature upon expanding from the compressed source. Inspired gases must be heated during deep dives (Piantadosi and Thalmann, 1980; Burnet et al., 1990). Table 1 indicates minimum recommended inspired gas temperatures for open-circuit divers to avoid body cooling.
Neal W. Pollock
Table 1. Minimum recommended inspired gas temperatures for open-circuit deep diving.
Minimum Tinsp (°C) (°F) -3.1 26.4 1.2 34.2 7.5 45.5 11.7 53.1
Depth (msw) (fsw) 107 350 122 400 152 500 183 600
Closed-circuit rebreathers reduce respiratory evaporative heat loss by retaining high humidity in the closed loop. The exothermic carbon-dioxide scrubber reaction warms the circulating gas sufficiently to provide additional thermal benefit. Evaporative heat loss from the skin is not a concern in high relative humidity environments. A fully saturated environment exists during unprotected immersion or in a wetsuit. A fully saturated environment develops very quickly in a sealed drysuit. Convective heat loss can vary substantially, depending on the stability of the near-skin microclimate. Drysuits provide a stable environment, wetsuits provide a reasonably stable environment if the design and fit effectively minimize water circulation. Convective losses can be substantial in a poorly fitting wetsuit. UNPROTECTED COLD WATER IMMERSION Even the modest protection of a poorly fitting wetsuit or drysuit likely provides sufficient thermal protection for hypothermia to be extremely unlikely to develop in most divers. It is, however, possible that unprotected immersions or extreme expeditionary dives can produce significant stress. For that reason, extreme impacts should be understood. The response to unprotected cold water immersion can be described as four phases. The first is characterized by the initial immersion response or ‘cold shock’ that develops in the first two minutes. In this phase heart rate, respiratory rate and blood pressure rapidly increase and cerebral blood flow velocity decreases as hyperventilation reduces the carbon dioxide level in the blood (Mantoni et al., 2008). The impact of cold shock increases as water temperature falls below 15°C (59°F). The second phase is characterized as short term immersion or ‘swimming failure.’ A rapid chilling of superficial skeletal muscles creates a crippling weakening much faster than is likely expected. This is an effect of the conductive heat sink provided by water. It is this phase that is most likely to kill unprotected swimmers that do not have sufficient buoyancy for their airway to remain protected.
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The third phase is described as long-term immersion, when hypothermia might develop. The evolution of hypothermia will vary dramatically with thermal protection, total mass, surface-to-volume ratio, the amount of subcutaneous fat to serve as passive insulation, the amount of skeletal muscle able to generate heat through shivering, and water temperature. Average rates of core temperature decline in human immersion studies range from 0°C·h-1 at 25°C (77°F), -0.75°C·h-1 at 18°C (64°F), -2.6°C·h-1 at 10°C (50°F) (Tipton et al, 1999), through -3.9°C·h-1 at 4.6°C (40°F) (Hayward et al., 1975) and -6°C·h-1 at 0°C (32°F) (Hayward and Eckerson, 1984). As mentioned previously, a victim will survive to this stage only if an effective airway is maintained. Core temperature is normally maintained at 37±1°C (98.6±2°F). Mild hypothermia is defined as a core temperature of 35-32°C (95-90°F); moderate hypothermia 32-28°C (90-82°F), and severe hypothermia
