Comprehensive Performance Limits for Divers' Underwater Breathing Gear: Consequences of Adopting Diver-Focused Limits.

Abstract

An underwater breathing apparatus (UBA) imposes loads on the diver: anything from the UBA's weight to those imposed on the respiratory muscles, some of the weakest muscles in the body. The types of respiratory loads imposed by a UBA are breathing resistance, elastic loads, hydrostatic imbalance (static lung load), inertial loads and CO2. Historically, the limits on resistive efforts have been based on the performance on the best commercially available UBAs around 1980. The limits proposed in this report are based on the diver, not the UBA, and they state how much of each respiratory load is acceptable and how the loads interact; the resistive effort (WOB/V sub T) should not exceed WOB/V sub T=2.49-0.016 * depth (with depth in msw) or WOB/V sub T=2.49-0.00485 * depth (with depth in fsw). The elastance should not exceed 0.7 kPa/L independent of depth and ventilation. The maximum tolerable hydrostatic imbalances, measured relative to the suprasternal notch, should be in the range of +0.4 to +2.9 kPa for a vertical diver and in the range of -0.3 to +1.7 kPa for a horizontal diver. Any CO2 presented to the diver forces an increase respiratory minute ventilation thereby magnifying the effect of the other respiratory loads imposed by the UBA. The dead space in the UBA and the CO2 in the inspired gas can be major influences in determining whether a UBA is acceptable. During test of CO2 scrubber endurance, the empirically determined ratio of CO2 flow to minute ventilation (4%) should be used. Adopting these limits will mean that some rebreathers that had been nominally not acceptable are actually acceptable. The limits make little difference in the acceptability of currently available open circuit UBAs. These physiologically based limits should be adopted for use in the U.S. Navy.