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 Peripheral Circulation
Our bodies try to stay in 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 thermoregulation is one aspect of homeostasis.
Our body’s reaction to changes in external temperature is to make changes to keep a thermal equilibrium. When you feel cold, the blood vessels in your limbs shrink (vasoconstriction), you get goosebumps (piloerection) and stop sweating. These simple measures try to prevent temperature loss by minimizing heat loss through radiation, convection and conduction.
If you feel hot, the same blood vessels will dilate (vasodilation) to favor temperature loss through radiation and convection. You also sweat, which further enhances temperature loss through conduction, convection and evaporation.
Temperature and Bubble Formation
A number of factors affect the solubility of a gas in a liquid, such as inert gas in your blood after diving. Temperature is one of those factors. As the temperature increases, gases usually become less soluble in water solutions (such as blood). Thermal stress can contribute to bubble formation, which makes it one of the contributing factors in decompression sickness risk.
If you are cold after diving, you will have diminished circulation in your limbs due to blood vessel constriction. Hot tubs (or hot showers) will warm your extremities and restore circulation faster. If your inert gas load is small, the warming will help eliminate gas more quickly because of the improved blood flow. Larger inert gas loads can cause more problematic responses. Since the solubility of gas is inversely related to temperature, tissues will hold less in solution as they warm. Warming tissues with significant gas loads can promote bubble formation. Because superficial tissues warm before the increased blood flow happens, bubbles formed then can be problematic. These develop before regular circulation can remove them harmlessly.
There is no simple formula to compute what constitutes a minor, significant or substantial peripheral inert gas load. The conditions vary based on the individual as well as their thermal protection, physical activity and dive profile.
- Delay the hot tub or hot shower by five to 30 minutes instead of jumping in immediately.
- If you are unwilling to wait, dive more conservatively.
- Use a lower hot tub or shower temperature if you don’t wait.
Neal W. Pollock, Ph.D.