The electrical-respiratory analogy when gas density is high.

The respiratory system can be considered analogous to a remarkably simple alternating-current electrical system with a resistor, an inductor, and a capacitor in series. This communication extrapolates the analogy from normal to hyperbaric environments with reference to information from experiments involving high-frequency, low-amplitude breaths (forced oscillations) in humans and various animals. Although the natural frequency, where inertial forces balance elastic recoil forces, is decreased by high density, the natural frequency does not become so low that it will be below the frequencies expected in exercising men; it falls from the normal 5 to around 1.5 breaths/s when density is increased 10-fold, but changes little more when density is increased further. When density is normal, total pressure for transporting gas through airways during forced oscillations is little more than pressure for overcoming airflow resistance over a range of frequencies from 1 to more than 15 breaths/s; this range of resistance domination shrinks when gas inertance increases, but the effect of high inertance tends to be countered by the increase of flow resistance that accompanies high gas density.