Getting into a hot tub immediately after diving alters decompression stress. Hot tubs could cause a positive or negative response depending on the magnitude of the inert gas load and the heat stress.
Temperature and Heat Exchange
Our bodies maintain a state of internal dynamic stability. While many biological processes are occurring, the net result is a relative balance of forces and reactions. This is called homeostasis, and temperature regulation (thermoregulation) is an important aspect.
Physiological responses to changes in external temperature promotes thermal equilibrium. Cold stress causes blood vessels in the periphery (limbs and surface tissues) to shrink (vasoconstriction). This reduces peripheral blood flow and makes the outer shell of the body more of a passive thermal barrier to help protect core temperature. Substantial cold stress can also promote shivering, with the potential to increase the metabolic rate five to seven times that of rest. Shivering is effective in warming the body.
Heat stress causes the same blood vessels to dilate (vasodilation), increasing blood flow through the outer shell. A warmer shell increases radiative heat loss if the surrounding temperature is lower than skin temperature. The combination of a high skin temperature and an increased sweat rate enhances heat loss through the evaporation of moisture on the skin in air. Evaporation will not occur in a wet environment, but substantial heat energy can be transferred to the water through contact (conduction) some through convection as current replace warmed water with cooler air to be heated.
Peripheral Temperature and Inert Gas Exchange
Vasodilation of peripheral tissues increases blood flow and inert gas uptake during the descent and bottom phases of a dive. Peripheral vasoconstriction during these phases reduces inert gas uptake. Peripheral cooling during the ascent and stop phase will impair inert gas elimination. Mild warming during ascent can promote inert gas elimination, but only effectively when modest. Rapid warming will promote bubble formation. Mild warming might occur when a diver crosses a thermocline into warmer surface water. Rapid warming can be induced by electrically heated garments turned up too quickly during the ascent or post-dive by the hot tub or hot shower.
Temperature and Bubble Formation
The solubility of gas in tissues decreases as the temperature rises. Rapidly warming tissues with high inert gas loads can promote bubble formation that can produce symptoms of decompression sickness before an increase in blood flow can remove the gas. Symptoms of skin bends are most likely, but more serious symptoms can also develop.
Dive Computer and Thermal Information
Dive computers typically measure water temperature, but none measure the thermal status of the diver. Thermal status can vary dramatically based on the protective equipment worn and the physical activity of the diver. Even if the thermal status was known, no decompression models currently incorporates this information into decompression computations in a meaningful way.
Post-Dive Practice
Divers are responsible to consider the impact of their dive and post-dive activity. Active warming during dives increases inert gas uptake. Being cold at the end of a dive traps inert gas. Jumping into a hot tub (or hot shower) immediately post-dive increases the decompression stress and, if gas loads are substantial, the risk of decompression sickness. It may not be easy to hear, but delayed gratification is the best approach for safety. You can think about the hot tub or shower while you eat, take care of your equipment, fill out a logbook, etc. Allowing time for offgassing, especially when it is impaired by your cool state, reduces risk. The more severe the decompression stress, the longer you should wait. If you need the post-dive warming immediately post-dive you should moderate your dive profile to reduce the risk.
Recommendations
Delay the hot tub or hot shower. The longer the delay the better, particularly with high gas loads. Think of this as a surface interval, with the hot tub or shower safest as the next dive.
If you are unwilling to wait, dive more conservatively.
Use a lukewarm shower temperature if you can’t wait.
Neal W. Pollock, Ph.D.