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Pressure-flow changes during ventricular asystole (but with atrial activity present) were studied in 11 dogs with complete heart block. The influence of respiration on these parameters was evaluated during spontaneous breathing, intermittent positive pressure respiration, and apnea. During spontaneous breathing, the pulmonary artery pressure initially fell below the right atrial pressure following the onset of ventricular asystole. Both pressures then showed a gradual rise which was accompanied by flow of blood into the pulmonary artery (estimated to be 121 to 139 ml./min.). The mean pressure in the left atrium became greater than that in the pulmonary artery, concomitant with a gradual rise in the end-expiratory intrathoracic pressure. Despite this mean pressure change, flow from the pulmonary artery to the left atrium was demonstrated. Analysis of the instantaneous pressures during the atrial cycle showed that flow could occur during early atrial diastole in both phases of respiration and throughout atrial diastole during inspiration. Following each atrial beat, the left atrium received blood from the pulmonary veins and left ventricle (mitral regurgitation). There was a continued fall in the aortic pressure and, when it was exceeded by the left atrial systolic pressure, blood was propelled into the aorta. These pressure-flow changes were affected by positive pressure respiration and apnea. All pressures were significantly lower during these respiratory conditions and flow from the pulmonary artery to left atrium could not be demonstrated. The marked effects of respiration on the circulatory dynamics in animals during ventricular asystole must be considered in the interpretation of the results of studies, particularly in thoracotomized or apneic dogs, whenever there are conditions of low flow or pressure in the central circulation.

Circulatory Dynamics and the Effects of Respiration During Ventricular Asystole in Dogs with Complete Heart Block