Oscillatory pressure-flow relationships of canine airways: a steady-state model for different gases.

We measured the airway pressure drop (delta Paw) between the trachea and the alveolus in three normal anesthetized paralyzed dogs, together with flow (V) at the trachea. Alveolar pressure was measured using a closed-chest modification of the standard alveolar capsule technique. Measurements were made during a period of sinusoidal ventilation at 0.83 Hz with a tidal volume of 35 ml/kg, after a 30-s period of apnea. We found that the amplitude of the swings in delta Paw decreased transiently after onset of sinusoidal ventilation. We established that this decrease was due to a reflex bronchodilatation by making additional measurements on vagotomized dogs. We developed a mathematical model to account for the steady-state data that included both laminar and turbulent flow, airway wall elastance, airway gas inertance, and the Bernoulli effect at the site of tracheal pressure measurement. The model accurately described the data obtained from each dog with four different gas mixtures containing air, helium, neon, and sulfur hexafluoride. We also constructed Moody plots from the measured delta Paw-V relationships, after removing the estimated contributions of gas inertance and the Bernoulli effect from delta Paw, and found evidence of the effects of changing airway dimensions throughout a breath, V unsteadiness, and differences between inspiratory and expiratory resistances.