Diver respiratory responses to a tunable closed-circuit breathing apparatus.

Respiratory impedance of closed-circuit underwater breathing apparatus (UBA) is comprised of resistive, elastic, and inertial elements in series. Impedance is at a minimum when a UBA operates at its resonant frequency (f(n)). This study investigated the respiratory responses of 12 male U.S. Navy divers to changes in the f(n) of a simulated closed-circuit UBA. Respiratory effort, breathing comfort and ventilatory parameters were assessed during open- and closed-circuit breathing at rest and while exercising at 75 W on a bicycle ergometer in the dry at 1 atm abs. During closed-circuit breathing, the f(n) of the system was adjusted to different frequencies between 0.2 Hz and 0.4 Hz (12 and 24 breaths/min) by varying UBA inertance. When the simulated UBA was switched from open to closed-circuit breathing the subjects changed their breathing frequency in a direction toward the f(n) of the system and attempted to maintain minute ventilation constant by adjusting tidal volume. Results suggest that when divers breathe on a closed-circuit system with different f(n)'s they attempt to improve breathing comfort and reduce respiratory effort by adopting a breathing pattern that reduces their peak-to-peak mouth pressures.