Partial-pressure effects: The partial pressure of a gas is determined by the concentration of the gas and the ambient pressure, eg, the concentration of O2 in air is about 21%, and the partial pressure of O2 in air at surface (1 atm abs) is about 0.21 atm. The concentration of O2 in air remains the same at depth, but the partial pressure reflects the increasing pressure and compression of the gas. At 2 atm abs, the number of O2 molecules per unit volume is twice what it is at the surface, and the partial pressure is double.
Partial pressures of O2 and CO2 in alveolar gas are modified by the pressure of depth in breath-hold diving and in underwater swimming without breathing apparatus. The impulse to return to the surface and resume breathing depends largely upon CO2 buildup in the body. A breath-holding diver may hyperventilate beforehand to extend time underwater; this blows off CO2 but adds little to stores of O2, and may then cause unconsciousness from hypoxia without warning before PCO2 rises enough to become an effective stimulus.
dioxide poisoning: In
individuals on land, hyperpnea or breathlessness usually provides ample
warning of increased CO2 in inspired gas. Such a response may be more
exception than the rule under water, especially where high PO2 and
are also factors. Some individuals develop spontaneous CO2 retention
an inadequate increase in pulmonary ventilation during exertion.
the source, abnormally high PCO2 per se can cause loss or impairment of
consciousness at depth and can also increase the likelihood of O2
and augment the severity of nitrogen narcosis. The tendency to retain
may be suspected in divers who frequently experience post-dive
or pride themselves on low air-use rates.
Ernest Campbell, MD, FACS All Rights Reserved.