Journal of Applied Physiology, Vol 61, Issue 4 1431-1437, Copyright © 1986 by American Physiological Society
Pulmonary mechanics during rapid mechanical ventilation in rabbits with saline-lavaged lungs
J. J. Perez Fontan, B. S. Turner, G. P. Heldt and G. A. Gregory
Infants with respiratory failure are frequently mechanically ventilated at
rates exceeding 60 breaths/min. We analyzed the effect of ventilatory rates
of 30, 60, and 90 breaths/min (inspiratory times of 0.6, 0.3, and 0.2 s,
respectively) on the pressure-flow relationships of the lungs of
anesthetized paralyzed rabbits after saline lavage. Tidal volume and
functional residual capacity were maintained constant. We computed
effective inspiratory and expiratory resistance and compliance of the lungs
by dividing changes in transpulmonary pressure into resistive and elastic
components with a multiple linear regression. We found that mean pulmonary
resistance was lower at higher ventilatory rates, while pulmonary
compliance was independent of ventilatory rate. The transpulmonary pressure
developed by the ventilator during inspiration approximated a linear ramp.
Gas flow became constant and the pressure-volume relationship linear during
the last portion of inspiration. Even at a ventilatory rate of 90
breaths/min, 28-56% of the tidal volume was delivered with a constant
inspiratory flow. Our findings are consistent with the model of Bates et
al. (J. Appl. Physiol. 58: 1840-1848, 1985), wherein the distribution of
gas flow within the lungs depends predominantly on resistive factors while
inspiratory flow is increasing, and on elastic factors while inspiratory
flow is constant. This dynamic behavior of the surfactant-depleted lungs
suggests that, even with very short inspiratory times, distribution of gas
flow within the lungs is in large part determined by elastic factors.
Unless the inspiratory time is further shortened, gas flow may be directed
to areas of increased resistance, resulting in hyperinflation and
barotrauma.
