Effect of lung-inflating pressure on pulmonary blood pressure and flow

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Abstract
In ten dogs the widely exposed left lower lobe of the lung was inflated to different static pressures over the range 5–20 cm H2O while being perfused with blood from a constant-flow source over a range of flows up to 300 ml/min. Left atrial pressure was kept below 3–4 cm H2O. The relationship between perfusion pressure and inflating pressure was linear and the slope independent of perfusion flow rate except at very low rates of flow. The Δ perfusion pressure-to-Δ inflating pressure ratio, with one exception, varied from 0.6 to 1.0, indicating that the major part of alveolar pressure was transmitted to the capillaries. At normal rates of flow the relationship between perfusion pressure and flow was linear. The findings, together with results obtained on a model, suggest that: 1) Calculation of pulmonary vascular resistance from the pulmonary artery-left atrial pressure gradient is misleading where inflating pressure exceeds left atrial pressure. In such circumstances the precapillary resistance can be measured. 2) At normal rates of flow the dominant resistance in the pulmonary vascular bed is precapillary. Despite the distensibility of pulmonary vessels this resistance does not vary with perfusion pressure or flow.