Hypopnea consequent to reduced pulmonary blood flow in the dog

The ventilatory responses to diminished pulmonary blood flow (Qc), as a result of partial cardiopulmonary bypass (PCB), were studied in chloralose-urethan-anesthetized dogs. Qc was reduced by diverting vena caval blood through a membrane gas exchanger and returning it to the ascending aorta. PCB flows of 400--1,600 ml/min were utilized for durations of 2--3 min. Decreasing Qc, while maintaining systemic arterial blood gases and perfusion, results in a significant (P less than 0.05) decrease in expiratory ventilation (VE) (15.9%) and alveolar ventilation (VA) (31.0%). The ventilatory decreases demonstrated for this intact group persist after bilateral cervical vagotomy (Vx), carotid body and carotid sinus denervation (Cx), and combined Vx and Cx. The changes in VE and VA were significantly (P less than 0.001) correlated with VCO2 changes, r = 0.80 and r = 0.93, respectively. These ventilatory changes were associated with an overall average decrease in left ventricular PCO2 of 2.1 Torr; this decrease was significant (P less than 0.05) only in the intact and Cx groups. Decreasing pulmonary blood flow results in a decrease in ventilation that may be CO2 related; however, the exact mechanism remains obscure but must have a component that is independent of vagally mediated cardiac and pulmonary afferents and peripheral baroreceptor and chemoreceptor afferents.