The Physiology and Pathophysiology of the Hyperbaric and Diving Environments Pulmonary gas exchange in diving

Moon RE, Cherry AD, Stolp BW, Camporesi EM. Pulmonary gas exchange in diving. J Appl Physiol 106: 668–677, 2009. First published November 13, 2008; doi:10.1152/japplphysiol.91104.2008.—Diving-related pulmonary effects are due mostly to increased gas density, immersion-related increase in pulmonary blood volume, and (usually) a higher inspired PO2. Higher gas density produces an increase in airways resistance and work of breathing, and a reduced maximum breathing capacity. An additional mechanical load is due to immersion, which can impose a static transrespiratory pressure load as well as a decrease in pulmonary compliance. The combination of resistive and elastic loads is largely responsible for the reduction in ventilation during underwater exercise. Additionally, there is a density-related increase in dead space/tidal volume ratio (VD/VT), possibly due to impairment of intrapulmonary gas phase diffusion and distribution of ventilation. The net result of relative hypoventilation and increased VD/VT is hypercapnia. The effect of high inspired PO2 and inert gas narcosis on respiratory drive appear to be minimal. Exchange of oxygen by the lung is not impaired, at least up to a gas density of 25 g/l. There are few effects of pressure per se, other than a reduction in the P50 of hemoglobin, probably due to either a conformational change or an effect of inert gas binding.