Pulmonary arterial pressure at high pulmonary flow.

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Abstract
The relationship between pulmonary arterial pressure, left atrial pressure, and pulmonary blood flow was studied in 3 groups of dogs. Group 1. In 9 open-chest dogs, constant and equal systemic and pulmonary flow rates were evaluated during cardiac arrest. Pulmonary flow of 100 ml/kg/min. was considered normal. Flows above 200% of normal through one lung were obtained by balloon or snare occlusion of the pulmonary artery to the other lung. Group 2. In 6 animals, similar studies were made with constant left atrial pressure. Group 3. In 4 open-chest animals, pulmonary pressure/flow relationships were studied while the beating heart perfused only the left lower lobe. Adequate blood oxygenation and pH control were obtained by an artificial oxygenator. In Group 1 animals, pulmonary vascular resistance fell as the flow ratewas increased from 50% of normal to 500% of normal. Similar results were obtained in Group 2. In Group 2, at 200% of normal flow, pulmonary arterial pressure ratio to control was 1.28; at 300% of normal flow, 1.53; at 400% of normal flow, 1.78, and at 500% of normal flow, the ratio was 2.04. Pulmonary vascular resistance did not rise, although the rate of decrease in resistance fell with increasing flow rates. These observations do not support the concept that the limit of pulmonary vascular distensibility is reached at pulmonary flow rates of 350 to 400% of normal, nor the observation that pulmonary arterial pressure does not rise until pulmonary blood flow is trebled.