Published Online: 1 NOV 2010
Copyright © 2011 American Physiological Society. All rights reserved.
How to Cite
Doolette, D. J. and Mitchell, S. J. 2010. Hyperbaric Conditions. Comprehensive Physiology. 1:163–201.
- Published Online: 1 NOV 2010
Exposure to elevated ambient pressure (hyperbaric conditions) occurs most commonly in underwater diving, during which respired gas density and partial pressures, work of breathing, and physiological dead space are all increased. There is a tendency toward hypercapnia during diving, with several potential causes. Most importantly, there may be reduced responsiveness of the respiratory controller to rising arterial CO2, leading to hypoventilation and CO2 retention. Contributory factors may include elevated arterial PO2, inert gas narcosis and an innate (but variable) tendency of the respiratory controller to sacrifice tight control of arterial CO2 when work of breathing increases. Oxygen is usually breathed at elevated partial pressure under hyperbaric conditions. Oxygen breathing at modest hyperbaric pressure is used therapeutically in hyperbaric chambers to increase arterial carriage of oxygen and diffusion into tissues. However, to avoid cerebral and pulmonary oxygen toxicity during underwater diving, both the magnitude and duration of oxygen exposure must be managed. Therefore, most underwater diving is conducted breathing mixtures of oxygen and inert gases such as nitrogen or helium, often simply air. At hyperbaric pressure, tissues equilibrate over time with high inspired inert gas partial pressure. Subsequent decompression may reduce ambient pressure below the sum of tissue gas partial pressures (supersaturation) which can result in tissue gas bubble formation and potential injury (decompression sickness). Risk of decompression sickness is minimized by scheduling time at depth and decompression rate to limit tissue supersaturation or size and profusion of bubbles in accord with models of tissue gas kinetics and bubble formation and growth. © 2011 American Physiological Society. Compr Physiol 1:163-201, 2011.