[Determination of respiratory system compliance from resonance frequency--animal experiment]

10 anaesthetized and tracheotomized rabbits were used to test the possibility for calculation of total respiratory system compliance from resonance frequency (CR). Reference values were obtained by inflation method compliance (CI). A new servocontrolled infant ventilator was used with an integrated negative ventilator resistance mode for resistive unloading of combined resistances of lung, endotracheal tube and ventilator. This evoked oscillation at resonant frequency. By analogy with electrical circuit theory compliance was calculated as follows: CR = 1/(4 pi 2 x square root I.RF2). Pairs of CI and CR values were obtained from each animal during spontaneous breathing, after pancuronium relaxation and after surfactant depletion by lavage. There was found a significant linear correlation between CR and CI values (CR = 1.1 x CI-0.74; r = 0.97).