Low-frequency respiratory system resistance in the normal dog during mechanical ventilation

The low-frequency resistances of the respiratory system, lung, and chest wall were investigated in four anesthetized paralyzed dogs mechanically ventilated at various frequencies between 0.08 and 0.83 Hz. The resistances were calculated by three different methods: 1) as the real part of the complex impedance obtained from regular ventilation data, 2) as the effective resistance of a two-compartment model fitted to the same data, and 3) as the resistance of a single-compartment model fitted to data obtained during sinusoidal ventilation at various frequencies. Alveolar pressures were measured by a closed-chest alveolar capsule technique and afforded a direct measure of airways resistance. All three resistance estimates were very similar and decreased markedly with frequency between 0 and 1 Hz. The real part of lung impedance at the higher frequencies investigated (around 5 Hz) closely approximated airways resistance, as predicted by the eight-parameter viscoelastic model of respiratory mechanics proposed by Bates et al. (J. Appl. Physiol. 67:2276-2285, 1989).