Thermal balance of the underwater diver
Thermal balance of the underwater diver
Thermoregulation is the ability of an organism to keep its body temperature within specific bounds, even when the surrounding temperature is very different. The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment. If the body is unable to maintain a normal human body temperature and it increases significantly above normal, a condition known as hyperthermia occurs. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. It occurs when the body loses heat faster than producing it.
Body heat is lost by respiratory heat loss, by heating and humidifying (latent heat) inspired gas, and by body surface heat loss, by radiation, conduction, and convection, to the atmosphere, water, and other substances in the immediate surroundings. Surface heat loss may be reduced by insulation of the body surface. Heat is produced internally by metabolic processes and may be supplied from external sources by active heating of the body surface or the breathing gas.[1]
Heat transfer to and via gases at higher pressure than atmospheric is increased due to the higher density of the gas at higher pressure which increases its heat capacity. This effect is also modified by changes in breathing gas composition necessary for reducing narcosis and work of breathing, to limit oxygen toxicity and to accelerate decompression. Heat loss through conduction is faster for higher fractions of helium. Divers in a helium based saturation habitat will lose or gain heat fast if the gas temperature is too low or too high, both via the skin and breathing, and therefore the tolerable temperature range is smaller than for the same gas at normal atmospheric pressure.[1]
The heat loss situation is very different in the saturation living areas, which are temperature and humidity controlled, in the dry bell, and in the water.[2]
The alveoli of the lungs are very effective at heat and humidity transfer. Inspired gas that reaches them is heated to core body temperature and humidified to saturation in the time needed for gas exchange, regardless of the initial temperature and humidity. This heat and humidity are lost to the environment in open circuit breathing systems. Breathing gas that only gets as far as the physiological dead space is not heated so effectively. When heat loss exceeds heat generation, body temperature will fall.[1]
Exertion increases heat production by metabolic processes, but when breathing gas is cold and dense, heat loss due to the increased volume of gas breathed to support these metabolic processes can result in a net loss of heat, even if the heat loss through the skin is minimised.
Hypothermia is reduced body temperature that happens when a body dissipates more heat than it absorbs and produces.[3] Clinical hypothermia occurs when the core temperature drops below 35 °C (95 °F).[4] Heat loss is a major limitation to swimming or diving in cold water.[5] The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries.[5][6] Reduced capacity for rational decision making increases risk due to other hazards, and loss of strength in chilled muscles also affects the capacity to manage both routine and emergency situations. Low tissue temperatures and reduced peripheral perfusion affect inert gas solubility and the rate of ingassing and outgassing, thereby affecting decompression stress and risk.[4] Body heat is lost much more quickly in water than in air, so water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia in inadequately protected divers, although it is not often the direct clinical cause of death.[5]
The thermal status of the diver has a significant influence on decompression stress and risk, and from a safety point of view this is more important than thermal comfort. Ingassing while warm is faster than when cold, as is outgassing, due to differences in perfusion in response to temperature perception, which is mostly sensed in superficial tissues. Maintaining warmth for comfort during the ingassing phase of a dive can cause relatively high tissue gas loading, and getting cold during decompression can slow the elimination of gas due to reduced perfusion of the chilled tissues, and possibly also due to the higher solubility of the gas in chilled tissues.[4]
Thermal management
Factors affecting the thermal status of the diver
- Ambient temperature of the water
- Wind chill
- Air tepmperature
- Insolation
- Active heating
Heat loss
Insulation
Effects on competence
- Dexterity
- Strength
- situation awareness
Influence on risk
The temperature of the diver's body or parts thereof affects the short term risk of:
Long term effects of cold water exposure
Persistent exposure of the external auditory canal to cold water can induce the growth of exostoses.[4]
References
- ^ a b c Neves, João; Thomas, Christian (25 April 2018). "Fighting Exposure – Is Helium a "cold" gas?". www.tdisdi.com. Archived from the original on 8 December 2021. Retrieved 8 February 2024.
- ^ Crawford, J. (2016). "8.5.1 Helium recovery systems". Offshore Installation Practice (revised ed.). Butterworth-Heinemann. pp. 150–155. ISBN 9781483163192.
- ^ a b Brown, D.J.; Brugger, H.; Boyd, J.; Paal, P. (Nov 15, 2012). "Accidental hypothermia". The New England Journal of Medicine. 367 (20): 1930–8. doi:10.1056/NEJMra1114208. PMID 23150960. S2CID 205116341.
- ^ a b c d e Pollock, Neal (20–22 April 2023). Thermal Management. Rebreather Forum 4. gue.tv. Valetta, Malta. Retrieved 30 April 2024.
- ^ a b c Sterba, J.A. (1990). "Field Management of Accidental Hypothermia during Diving". US Navy Experimental Diving Unit Technical Report. NEDU-1-90.
- ^ Cheung, S.S.; Montie, D.L.; White, M.D.; Behm, D. (September 2003). "Changes in manual dexterity following short-term hand and forearm immersion in 10 degrees C water". Aviat Space Environ Med. 74 (9): 990–3. PMID 14503680. Retrieved 11 June 2008.
- ^ Cite error: The named reference
Laden et al 2007was invoked but never defined (see the help page).
Category:Underwater diving physiology