Effects of the pattern of ventilation and of an increase in cardiac output on the distribution of blood flow to a hypoxic lung lobe.

Electromagnetic flow probes were used to measure the fraction of the pulmonary blood flow perfusing the left lower lobe (Ql/Qt) in 16 anaesthetized, open-chest dogs in order to study the effects of right lung airway pressure on the distribution of blood flow to a hypoxic lung lobe. Ventilation of the lobe with 7% oxygen in nitrogen resulted in a 37% reduction in Ql/Qt at the beginning and end of the main procedure, thus confirming that the hypoxic pulmonary vasoconstrictor response was unchanged throughout the study. The effects of varying mean airway pressure to the right lung by changing inspiratory:expiratory time ratio and by the addition of a positive end-expiratory pressure were studied when the left lower lobe was insufflated with oxygen or 7% oxygen in nitrogen, or was collapsed. Insufflation of the lobe with 7% oxygen in nitrogen and collapse reduced Ql/Qt by 61% and 66%, respectively. However, changing mean airway pressure in the right lung produced no significant changes in Ql/Qt in any of the different lobar conditions or when collapse was produced after fluid loading. Fluid loading during collapse increased cardiac output and pulmonary vascular pressures and increased Ql/Qt to a value which was not significantly different from lobar ventilation with 7% oxygen in nitrogen. It is concluded that moderate increases in mean airway pressure do not increase Ql/Qt when this has been reduced by exposure of the lobe to mixed venous blood-gas tensions.