CO2 and mechanical factors reduce blood flow in a collapsed lung lobe

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Abstract
Left lower lobe-to-total blood flow ratio (.ovrhdot.Ql/.ovrhdot.QT) was measured with electromagnetic flow probes in anesthetized open-chest dogs. There was a 66% reduction in .ovrhdot.Ql/.ovrhdot.QT during lobar collapse, a 53% reduction during lobar ventilation hypoxia with pulmonary venous PO2 [O2 tension] and PCO2 equal to mixed venous tensions, and a 45% reduction during a similar degree of ventilation hypoxia but with normal end-tidal PCO2. Apparently, the reduction in blood flow during lobar collapse is due predominantly to hypoxic vasoconstriction, but that this mechanism is augmented by the raised PCO2 and mechanical factors present during collapse.