Ventilation-perfusion relationships with high cardiac output in lobar atelectasis

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Abstract
Pulmonary gas exchange was evaluated in 10 anesthetized mechanically ventilated dogs. Cardiac output (.ovrhdot.QT) was increased approximately 50% by opening peripheral arteriovenous fistulas. With both lungs ventilated, increasing .ovrhdot.QT increased mixed venous O2 pressure (PO2) and pulmonary arterial pressure, but neither shunt fraction nor the distribution of ventilation-perfusion was consistently altered. During left lung atelectasis, increasing .ovrhdot.QT again increased mixed venous PO2 and pulmonary arterial pressure, but 2 different responses in shunt-like perfusion were measured. In 4 dogs, left lung atelectasis caused a shunt fraction of 46 .+-. 6% that was not changed by high .ovrhdot.QT (P > 0.05). In 6 dogs, atelectasis caused a shunt fraction of 24 .+-. 3% during normal .ovrhdot.QT that increased to 42 .+-. 2% during high .ovrhdot.QT (P < 0.001). Dogs whose shunt fraction during atelectasis was high and unchanged by .ovrhdot.QT had lower arterial pH (7.24 .+-. 0.03) than dogs whose shunt fraction was initially lower and was increased with .ovrhdot.QT (7.36 .+-. 0.02, P < 0.01). Increased .ovrhdot.QT can worsen shunt flow during lobar atelectasis when hypoxic vasoconstriction has been effective in limiting perfusion to the collapsed region at normal levels of .ovrhdot.QT.