Effect on breathing of abruptly loading and unloading the canine left heart

Cardiopulmonary bypass and pulmonary vein ligation were used to isolate left hearts of anesthetized open-chest dogs. After external gas exchange, blood was returned at constant flow (approximately 120 ml.min-1.kg-1) directly to the aorta or indirectly through the left heart ("left heart loading"). Loading caused breathing frequency (f) to increase approximately 5 breaths/min (approximately 20%), whereas systemic arterial pressure (Psa) fell approximately 15%. Because Psa was pulsatile during loading, we demonstrated separately the effect of pulsatile pressure and found it to lower mean Psa without changing f. Cooling cervical vagi to 7 degrees C eliminated the f response to loading and slightly decreased the Psa response. Loading was compared with graded distension of the fibrillating ventricle and beating atrium, which also increased f. As measured by an abdominal pneumograph, depth of breathing decreased significantly (approximately 4%) during left heart loading but did not change significantly on distension of the fibrillating heart. I conclude that left heart loading may induce tachypnea and a slightly reduced tidal volume by a vagal reflex most likely originating from the left heart.