Evaluation of lung mechanics in rabbits using short duration flow pulses

An alternative method of evaluating pulmonary mechanical function that uses short duration flow pulses was applied in five adult rabbits with high frequency flow interruption (HFFI) at 4 cycles per second. Variations in the total pressure across the respiratory system were partitioned into separate lungs and chest wall components, and the mechanical impedance of each component was evaluated in the range of 4 to 40 Hz using cross and power spectral analysis and spectral averaging. The resistances, R, of the respiratory system, lungs, and chest wall were 23.5 (2.3 SD), 18.6 (2.8), and 4.9 (1.5) cm H2O.L-1.s, respectively. These values were similar to R obtained by the conventional method at ordinary rates of ventilation, with the exception of chest wall resistance, which was approximately 50% smaller than R obtained by the conventional method. The compliances of the respiratory system, lungs and chest wall were 1.40 (0.19), 2.88 (0.94), and 2.85 (0.86) ml.cm H2O-1, respectively. In all cases, compliance values obtained by pulse analysis were roughly 70% of the values obtained by the conventional method. The smaller compliance values obtained with the pulse method may reflect the multicompartment behavior of the lungs and chest wall at the high frequencies found in the pulse spectra. We conclude that the response of the respiratory system to short duration flow pulses may be used to evaluate pulmonary mechanical function. The brief duration of the flow pulse suggests that this method may be well suited to evaluate lung function in uncooperative subjects, particularly newborns and premature infants.