Regional ventilation during spontaneous breathing and mechanical ventilation in dogs.

We evaluated the effects of the different patterns of chest wall deformation that occur with different body positions and modes of breathing on regional lung deformation and ventilation. Using the parenchymal marker technique, we determined regional lung behavior during mechanical ventilation and spontaneous breathing in five anesthetized recumbent dogs. Regional lung behavior was related to the patterns of diaphragm motion estimated from X-ray projection images obtained at functional residual capacity (FRC) and end inspiration. Our results indicate that 1) in the prone and supine positions, FRC was larger during mechanical ventilation than during spontaneous breathing; 2) there were significant differences in the patterns of diaphragm motion and regional ventilation between mechanical ventilation and spontaneous breathing in both body positions; 3) in the supine position only, there was a vertical gradient in lung volume at FRC; 4) in both positions and for both modes of breathing, regional ventilation was nonlinearly related to changes in lobar and overall lung volumes; and 5) different patterns of diaphragm motion caused different sliding motions and differential rotations of upper and lower lobes. Our results are inconsistent with the classic model of regional ventilation, and we conclude that the distribution of ventilation is determined by a complex interaction of lung and chest wall shapes and by the motion of the lobes relative to each other, all of which help to minimize distortion of the lung parenchyma.