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4.0.0 RESPIRATION In the foregoing sections of this report, physical and physiological effects of individual gases and gas mixtures have been considered primarily with respect to their behavior when contained within the blood stream and tissues. If all of the respiratory problems related to gas toxicities could be eliminated, the suc- cess of a diver's attempts to perform effectively would still depend upon a satis- factory delivery of a respirable medium to the lungs themselves. To be com- pletely satisfactory, this delivery must be such that tolerable limits of breathing resistance are not exceeded and adequate pulmonary ventilation is assured at all times. Most of the problems in the field of the mechanics of breathing under- water are related to one or the other of these conditions. 4. 1. 0 Breathing Resistance With the establishment of acceptable limits for both positive and negative breathing resistance, providing a breathing device to function within these limits would be reduced to a matter of engineering. While the problems of breathing re- sistance at normal pressure in air are similar to those existing under water, there is no assurance that the same values for sea-level conditions would apply to the rather dissimilar underwater environment. Noncontroversial limits have not been determined for either positive or negative breathing resistance for ex- tended periods of time even at one atmosphere. Responses to positive pressure breathing include changes in alveolar and pCOz, peripheral vasoconstriction and general blood pressure changes, and alteration of the ventilation/perfusion ratio. Most of the work relating these re- sponses has been conducted with the subjects in air, at sea-level pressure or at altitude. In an underwater environment, some of these responses undoubtedly would be modified by hydrostatic influences upon the circulation, a normal slow- ing of the heart rate on immersion in water below body temperature and by the inertial energy involved in displacing water with the chest wall during the respir- atory cycle. 4. 2. 0 Dead Space Almost any practical breathing device for underwater use must contain some mechanical dead space. While tolerable limits for added dead space has been defined for resting conditions such as air or oxygen breathing at sea-level pressure and at altitude, these limits are not necessarily acceptable in under- water breathing apparatus either at rest or at work. Both added external dead space and an increase in the physiological dead space tend to decrease the effec- tive pulmonary ventilation and may readily result in values which are prohibitively low. In air, this effect may be overcome to a certain extent by increasing the tidal volume but, as briefly mentioned earlier, the inertial effects of displacing larger volumes of water, particularly at high exercise levels where the total ven- tilation is already appreciably increased, may be sufficiently great to make this compensatory response impractical while submerged. ZO
4. 3. 0 Rapid Changes in Depth During changes in depth, the alveolar ventilation may be effected if either ascent or descent is accomplished at a fairly high rate. On descent the gas in the respiratory spaces will be compressed and, while the respiratory movements may not vary from a normal pattern, the total amount of gas inspired will be greatly in excess of that expired and the effective ventilation for carbon dioxide elimination is thereby reduced. During ascent the process is reversed and the amount of gas exhaled is in excess of that inhaled and effective ventilation for car- bon dioxide elimination is increased. If these changes in depth are extremely rapid and the respiratory movements normal, the net result in descent may ap- proach that accomplished in breathholding; in ascent may resemble hyperventila- tion. If true breathholding is practiced, as in skin diving, the mass of air in the lungs remains constant, except for pulmonary exchange, and the lung volume varies inversely with depth; in this situation, effective pulmonary ventilation is essentially absent and man's performance is limited by individual tolerance to changes in lung volume and to partial pressures of oxygen and carbon dioxide in the lungs. Little study has as yet been made of the dynamics of gas exchange under any of these conditions. 4.4. 0 Gas Mixtures The effects of various gas mixtures on alveolar exchange have already been taken up in other sections of this report. These effects may be due in part to changes in the density of gases with increased pressure. In addition to the dif- fusion problems existing at the alveolar wall, an understanding of the flow char- acteristics of various gas mixtures at increased pressure in a system where the gas velocities are constantly changing is necessary for the design of adequate breathing apparatus. Information is also needed on underwater breathing patterns and instantaneous breath velocities. Since it appears from the limited amount of data available to date that individual breathing patterns exert a strong influence on the performance of CO2 absorption canisters, these studies become increas- ingly important in semi-closed and closed-circuit equipment where the efficient removal of CO2 is necessary. Again, the work in this area has thus far been limited almost entirely to an air environment at sea level or at high altitude. 4. 5. 0 Carbon Dioxide Removal A considerable amount of information is available on the requirements for CO£ absorption systems in anesthesia equipment where the relationships between canister size and breathing characteristics for resting conditions are rather well established. Design of underwater breathing equipment cannot be based upon this information because of the wider ranges for tidal volume, breath velocity, and rate of carbon dioxide production encountered in diving. The present require- ments for closed-circuit fire and mine rescue equipment are such that the breath- ing resistance and levels of CO2 considered permissible in this work are intoler- able at the high ambient pressures encountered in diving. While discussion of the development of specific underwater breathing equipment is outside the scope of this report, it must be emphasized that the na- ture of the equipment employed necessarily influences the physiological responses 21
to increased ambient pressure. Therefore, a sound approach to the development of such equipment cannot be made until the physical and physiological aspects of diving are more firmly established. 22
