The effects of reversing distending pressure sequences in the neonatal piglet.

We studied different sequences of lung inflation in ventilated newborn piglets with normal lungs in order to determine the effects of sequence, magnitude and duration of distending pressure on pulmonary function, and/or hemodynamics. End-expiratory pressure was varied using a continuous negative extrathoracic pressure (CNEP) device. Three groups of ventilated piglets with normal lungs were exposed to 2 cmH2O increments of CNEP from -2 to -12 cmH2O, and to decrements from -12 to -2 cmH2O, or to only -6 cmH2O. Lung inflation sequence, magnitude of inflation pressure, and duration of inflation had significant effects on end-expiratory lung volume and lung compliance at numerically equivalent pressure levels. End-expiratory lung volume and lung compliance varied (at four and five of six inflation pressures studied) by as much as 68% and 104%, respectively. Hemodynamic effects of the lung inflation sequence were more variable; those found to be different at numerically equivalent pressure levels were associated with changes in lung compliance and ventilation. Differences in pulmonary mechanics can best be explained by the effects of lung inflation on alveolar recruitment versus overinflation.