Pulmonary Blood Pressure and Flow during Atelectasis in the Dog

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Abstract
To measure the time course, direction and magnitude of the hypoxic pulmonary vasoconstriction (HPV) response to atelectasis, 6 dogs were anesthetized with pentobarbital, and, with the chest open, each lung was ventilated separately. Pulmonary blood flow was measured with electromagnetic flow probes. Pulmonary arterial, left atrial and systemic arterial pressures were measured via indwelling catheters. The right lung was ventilated continuously with 100% O2, while the left lung was either ventilated with 100% O2 (control phase), unventilated (4 h of atelectasis) or ventilated with a gas mixture containing 4% O2, 3% CO2 and 93% N2 (hypoxia phase). Left lung atelectasis resulted in a reduction of the percent left lung blood flow from 43 .+-. 4% (mean .+-. SE)-25 .+-. 7% at 15 min and to 12 .+-. 1% at 60 min which persisted for the remaining 4 h period. The percent left lung blood flow was significantly lower (8 .+-. 1) and the PaO2 [arterial pressure] significantly higher (356 .+-. 38 mm Hg) during the maximal response to atelectasis as compared to 15 min of hypoxic ventilation (23 .+-. 5%; 211 .+-. 21 mm Hg). With atelectasis or hypoxic ventilation, pulmonary perfusion pressure was increased significantly from the control value of 7.9 .+-. 0.8 mm Hg to approximately 11 mm Hg. In the open chest model without systemic hypoxemia, the response to acute atelectasis is a regional increase in pulmonary vascular resistance which develops quickly (15 min), is maximal by 60 min and is maintained thereafter. As a result, there is a sustained diversion of blood flow away from the atelectatic lung and a generalized increase of pulmonary perfusion pressure.