Effects of inspiratory and expiratory resistance in divers' breathing apparatus.

This study was performed to determine if inspiratory breathing resistance causes greater or smaller changes than expiratory resistance. Unacceptable inspiratory resistances were also determined. Five subjects exercised at 60% of their VO2max while immersed in a hyperbaric chamber. The chamber was pressurized to either 147 kPa (1.45 atm abs, 4.5 msw, 15 fsw) or 690 kPa (6.8 atm abs, 57 msw, 190 fsw). Breathing resistance was imposed on the inspiratory or expiratory side and was as high as 0.8-1.2 kPa liter(-1) x s(-1) (8-12 cm H2O x liter(-1) x s(-1)) at a flow of 2-3 liter x s(-1) at 1 atm abs., the other side being unloaded. The subjects reacted to the imposed load by prolonging the phase of breathing that was loaded. Inspiratory breathing resistance caused greater changes than expiratory resistance in end-tidal CO2, dyspnea scores, maximum voluntary ventilation, and respiratory duty cycle. Using previously published criteria for acceptable levels of dyspnea scores and the CO2 levels, we found that an inspiratory resistance inducing a volume-averaged pressure of 1.5 kPa is not acceptable. Similarly, an expiratory resistance should not induce a volume-averaged pressure exceeding 2.0 kPa