Redistribution of pulmonary blood flow in the dog with PEEP ventilation

The effects of positive end-expiratory pressure (PEEP) at 20 cmH2O on the distribution of pulmonary blood flow was studied in intact dogs and isolated lung preparations. Measurements were made of a) the continuous distribution of ventilation-perfusion ratios (VA/Q), b) the vertical distribution of pulmonary blood flow, and c) the dimensions of the microvasculature. Without PEEP the distributions of ventilation and perfusion were unimodal and centered on a VA/Q close to one. Dependent regions received 5-10 times more of cardiac output than uppermost regions. With PEEP the distribution showed a bimodal character, one mode of normal VA/Q and the other comprising one-third of ventilation, lying between VA/Q of 10 and 100. Cardiac output was reduced two- to threefold and blood flow in the uppermost regions was grossly reduced but not eliminated. Bimodal distributions were also found in isolated lungs with PEEP, and histological examination of rapidly frozen lung tissue showed that alveolar capillaries were closed in the uppermost, poorly perfused regions, whereas alveolar corner vessels remained open. We suggest that the blood flow through these corner vessels is responsible for the additional, high VA/Q mode during PEEP.