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AD 2016 Q3

Alert Diver is the dive industry’s leading publication. Featuring DAN’s core content of dive safety, research, education and medical information, each issue is a must-read reference, archived and shared by passionate scuba enthusiasts. In addition, Alert Diver showcases fascinating dive destinations and marine environmental topics through images from the world’s greatest underwater photographers and stories from the most experienced and eloquent dive journalists in the business.


[ BY THE PHYSIOLOGY OF COMPRESSED-GAS DIVING SIMON MITCHELL, MB, CHB, PH.D. [ The breathing of compressed gas while submerged and exposed to increased ambient pressure imposes significant homeostatic challenges on the body (i.e., challenges maintaining physiological equilibrium). This article discusses the important mechanisms of these challenges, with particular attention to the respiratory system. I. THE RESPIRATORY SYSTEM COMPRESSED-GAS BREATHING EQUIPMENT Scuba equipment is the most commonly used recreational compressed-gas system, and it provides examples of important features and functions relevant to diving physiology. Basic scuba equipment consists of a cylinder of air at high pressure, a demand-valve regulator and a device for holding this equipment on the diver’s back, typically a buoyancy control device (BCD). Together with a wetsuit (necessary for temperate-water diving) and a weight belt, this apparatus may constitute a significantly restrictive force over the diver’s chest and abdomen. STEPHEN FRINK 86 | SUMMER 2016 STEPHEN FRINK The regulator reduces the high-pressure air in the cylinder to ambient pressure and supplies air on demand. Thus, at a depth of 100 feet, where the absolute pressure is 4 atmospheres, the regulator supplies air at 4 atmospheres, and the air is four times as dense as air at sea level (1 atmosphere). The ambient pressure is measured by the regulator’s second stage (attached to the mouthpiece), which in an upright diver is approximately 8 inches above the center of the chest. The water pressure acting on the chest will therefore be approximately 8

inches of water depth greater than that of the inspired gas, creating negative transmural pressure (pressure difference across the chest wall) that’s greatest at the base of the lungs. The breathing resistance of a regulator is inversely related to the quality of manufacture and standard of maintenance. Furthermore, breathing resistance tends to increase with depth as denser air flows through the regulator mechanism. Finally, it should be noted that the internal volume of a portion of the regulator second stage is effectively an extension of anatomical respiratory dead space. MECHANICS OF BREATHING Changes in compliance: Changes in compliance are seen in the lungs and chest wall. The negative transmural pressure across the chest wall of the upright scuba diver causes some pulmonary capillary engorgement. This effect is enhanced by the relative centralization of blood volume that occurs with immersion, especially in cold water. This engorgement of the pulmonary capillaries causes reduced compliance in the lung tissue. This reduces the vital capacity of the lung by 10-15 percent. Scuba equipment, wetsuits and weight belts exert a restrictive force on the chest wall and abdomen. This effect is potentially significant if equipment is excessively tight fitting. The compliance of the chest wall is reduced, and diaphragmatic breathing is impeded. Changes in airways resistance: Airways resistance is affected by changes in gas density. Resistance is defined as the pressure Laminar Flow decrease across a tube divided by flow. In laminar flow, Turbulent Flow flow is largely independent of the density of the gas. In turbulent flow, however, flow is inversely related to gas density. Therefore, in turbulent flow, for a given pressure decrease, flow will be decreased if gas density is increased, and by definition resistance to flow will be greater. According to Reynolds number predictions (a method of predicting flow), flow within the lungs and airways is largely laminar; this assumption, however, is likely to be invalid because of the vortices that occur in inspired air at each division of the bronchial tree. Indeed, it is likely that turbulent flow occurs widely in the large airways, particularly during rapid breathing when flow rates are much higher. Changes in the work of breathing: Work of breathing in diving is consequently increased. Work is performed by the respiratory muscles in stretching the elastic tissues of the lungs and chest wall, moving inelastic tissues and moving air through the respiratory passages. The preceding discussion demonstrates that in the immersed scuba diver there is an increase in elastic work (due to decreased compliance in the lungs and chest wall), work of moving inelastic tissues (due to constrictive equipment) and work of moving air through airways (due to increased air density). The airways-resistance component of this increase in work of breathing is dependent upon depth. VENTILATION/PERFUSION MATCHING IN DIVING The single most important determinant of efficient gas exchange is the matching of alveolar ventilation to the perfusion of the alveolar capillaries. The optimum ratio of these two factors is unity. Underventilated and overperfused lung units represent a right-to-left shunt. The mixture of hypoxic blood from underventilated and/or overperfused units into systemic arterial blood is an important cause of a significant alveolar-arterial oxygen gradient. The lungs of a scuba diver are subjected to changes in both perfusion and ventilation. There is an increase in perfusion of lung units due to capillary engorgement (particularly at the bases of the lungs) and the relative centralization of blood volume that occurs with immersion. There is a decrease in ventilation due to reduced lung- and chest-wall compliance, abdominal constriction and increased airways resistance. The net effect is toward an increase in underventilated and/or overperfused units and thus the shunting of blood from right to left. CHANGES IN GAS TRANSPORT Oxygen: Oxygen is transported in the blood either bound to hemoglobin (Hb) or dissolved in plasma. The solubility of oxygen in plasma is low, and in normobaric conditions the greatest proportion of oxygen by far is transported bound to Hb. ALERTDIVER.COM | 87

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