Pulmonary gas exchange in diving

Abstract

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 PO 2. 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 PO 2 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. respiratory dead space; ventilation-perfusion ratio; respiratory mechanics DESPITE HAVING EVOLVED in and adapted to an atmosphere with gas density close to 1 g/l, the performance of the human lung in the diving environment is remarkable. Adequate ventilation and gas exchange have been achieved at an ambient pressure up to 71 atmospheres absolute (ATA) [701 m of sea water (msw); 2,310 ft of sea water (fsw)] with an ambient PO 2 of 0.39 ATA (49), and with a PO 2 of 0.2 ATA up to a gas density of 25 g/l (50). Adequate exchange of oxygen and carbon dioxide while diving requires the ability to maintain ventilation in the face of significantly increased resistive and elastic loads. Resistance is increased primarily by the increase in breathing gas density. Elastic load is enhanced primarily by changes in transrespiratory pressure (P TR ). Inertial mechanical load is also increased, although this has a minimal effect on the diver. Added challenges include blunted respiratory drive due to elevated partial pressures of inert gas and oxygen, and possibly impaired diffusion within the alveolus (7). While in most dives the breathing gas is hyperoxic, thus precluding hypoxemia, hypercapnia is common. Hyperoxia, particularly in the venous blood, can induce a small reduction in CO 2 solubility, and hence an increase in venous PCO 2 via the Haldane effect (27,122). Arterial PCO 2 (Pa CO 2 ) is not affected by the Haldane effect because of regulation of breathing via the chemoreceptors, although hypercapnia does occur for other reasons as discussed below.Studies of pulmonary gas exchange under hyperbaric conditions designed to simulate diving have been performed...

Rest and exercise work of breathing calculated from alveolar pressure-volume loops, submersed and at depth

“…This may be exacerbated by the increased inertia of gas flowing in the airways and breathing apparatus (inertance). Although inertance increases in direct proportion to gas density [29], due to the low V̇E, and specifically slow b f of divers reported here and elsewhere its role is not likely to be the main cause of the increased POB, which is supported by a previous study [30]. Both submersion and depth impose obvious limitations on physical exertion during open water dives due to these increases in POB.…”

Human Physiology in an Aquatic Environment

“…Respiratory muscle fatigue and CO 2 retention are both attenuated (321). The principal causes of hypocapnia are the effects of diving on mechanical load, gas mixing and diffusion, V A /Q matching and respiratory drive (271).…”

Oxygen, the lung and the diver: friends and foes?

“…However, when the diver is in an upright position, he or she will not benefit from the position of the counter lung and has to overcome the normal opening pressure which is somewhat higher than in the prone position. All together there will be more imbalance in the upright dive situation compared with the prone situation [31].…”

“…in toto under water) or immersed ( partially under water, mostly up to the thorax or neck) hydrostatic pressure will have some effect on the body. In the presence of hydrostatic pressure, there is a redistribution of 500-800 mL of blood from the legs to the large veins and pulmonary vessels [31]. Due to intra-thoracic blood pooling, the ventricular wall stretches and the length of the cardiac muscle increases.…”