Ventilation-perfusion relationships with high cardiac output in lobar atelectasis

Pulmonary gas exchange was evaluated in 10 anesthetized mechanically ventilated dogs. Cardiac output (QT) was increased approximately 50% by opening peripheral arteriovenous fistulas. With both lungs ventilated, increasing 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 QT again increased mixed venous PO₂ and pulmonary arterial pressure, but two 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 QT (P > 0.05). In 6 dogs, atelectasis caused a shunt fraction of 24 ± 3% during normal QT that increased to 42 ± 2% during high QT (P < 0.001). Dogs whose shunt fraction during atelectasis was high and unchanged by QT had lower arterial pH (7.24 ± 0.03) than dogs whose shunt fraction was initially lower and was increased with QT (7.36 ± 0.02) (P < 0.01). We conclude that increased 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 QT.