[CO2 "wash-out" : a factor contributing to increase myocardial performance in the face of hypoxia (author's transl)]

The cardiovascular system of the anaesthetized dog has been used as an experimental model for studying the mechanisms of the haemodynamic responses to hypoxia. Together with aortic and left circumflex coronary artery blood flow (electromagnetic flow transducers), aortic and left ventricular pressures have been recorded and blood has been sampled from the aorta and the coronary sinus (PO2, PCO2, pH). During short periods of hypoxia an improvement of myocardial performance has been observed both before and after administration of a beta-adrenergic receptor blocker and a marked reduction of coronary sinus PCO2 has been noted. When hypoxia was caused by a mixture of nitrogen (95%) and CO2 (5%) an improvement of performance was observed only before administration of the beta-blocker. The slope of the relationship between PCSCO2 and cardiac performance was found to be similar before and after administration of the beta-blocker and also similar to that observed in studies of the response of the isolated heart muscle to acute hypocapnia. Besides beta-adrenergic receptor stimulation, a reduction of coronary sinus PCO2 (CO2 wash-out due to an increase of coronary blood flow) could be a factor contributing to the maintenance of myocardial performance in the face of hypoxia.