Increased density also increases the viscosity of the gas, which means that it is more difficult to breathe the same volume of gas at depth. A volume of gas at depth contains more molecules than the same volume of gas at the surface. Breathing-Gas Density Increases with Depthĭivers’ breathing gas is compressed by the increased ambient pressure at depth. The following eight factors are among the primary reasons for why this happens. Divers’ bodies have to work harder to eliminate CO 2 during dives and have a limited capacity to perform this work. A given amount of exercise also causes a corresponding amount of CO 2 to be produced by the body, and the same amount of alveolar ventilation is required to eliminate the CO 2 regardless of whether a person is at the surface or at depth. To do so, a certain volume of air is used to dilute the CO 2 in the alveoli of the lungs by means of inhalation and exhalation. To maintain the pH balance in the blood, the lungs must be able to “blow off” the CO 2 that is produced by the body. 1 The change in the pH of the blood is sensed by the brain and serves as the trigger to stimulate breathing. The CO 2 combines with the water in the blood to form carbonic acid (which dissociates as hydrogen and bicarbonate ions). Of these two processes, the exhalation of CO 2 is actually the main driver of breathing. The lungs’ most important function is to facilitate gas exchange: supplying oxygen to the bloodstream and extracting and exhaling carbon dioxide (CO 2). Thus it is important that divers know what limitations to pulmonary function occur while diving. Lung conditions are among the most common reasons people fail diving medical examinations - and of course we depend on our lungs for survival. The second equation requires an estimated Vd an and is applicable when Pa CO 2 is not measured or does not plateau (as in exercise).Diving imposes significant challenges to the respiratory system. This fraction is ( Pa CO 2/Pa CO 2) 2, where Pa CO 2 and Pa CO 2 are the mean partial pressures of expired alveolar and of arterial CO 2 in the other equation this fraction is 2 where Pe CO 2 is mixed expired Pco 2 and Vd an is anatomical dead space. The fraction of Vd m subtracted from Vd n is the square of the ratio of effective alveolar to total alveolar ventilation and is never > 1. With only a small modification, these equations are suitable for routine clinical use and give Vd p/ Vt within 0.02 of that by the validated equations (32 of 33 comparisons). To make the proper correction for Vd m, two equations have been derived and validated with seven subjects having Vd p/ Vt from 0.29 to 0.87, using Vd m's from 120 to 322 ml. Under these conditions the traditional subtraction of Vd m from Vd n leads to underestimation of Vd p and can give a falsely small ratio of Vd p to tidal volume ( Vt) when, in fact, an abnormally large Vd p/ Vt exists. When physiological dead space ( Vd p) is calculated for a patient who has alveolar dead space, e.g., after pulmonary vascular occlusion, less than the full volume of attached mechanical dead space ( Vd m) appears in the measured dead space ( Vd n).
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