Cardiac and sinoaortic reflexes during aortic constriction in awake calves.

The hemodynamic responses to increased afterload of the left ventricle were studied in conscious calves before and during cooling of the cervical vagus nerves bilaterally. The calves were chronically instrumented to measure (or derive) heart rate, stroke volume, cardiac output, iliac, superior mesenteric and renal flows and resistances, mean aortic, right atrial, pulmonary artery, and left atrial pressures, the systemic and pulmonary pressure gradients, and total systemic and total pulmonary resistances. The calves were also instrumented to produce reversible partial constriction of the ascending aorta and for cooling of the cervical vagus nerves. The hemodynamic responses to increased afterload were characterized before and during bilateral cervical vagus nerve cooling to 6-7 degrees C. Aortic constriction causes stroke volume, cardiac output and aortic pulse pressure to decrease. Left atrial pressure and total peripheral resistance increase. Mean aortic pressure is constant during aortic constriction alone, despite a continued decrease in pulse pressure, due to a balance between sinoaortic reflexes which attempt to increase arterial pressure in response to the decreased pulse pressure and cardiopulmonary reflexes which attempt to decrease arterial pressure in response to the increased left atrial and cardiopulmonary pressures. Vagal cooling removes cardiopulmonary reflex modulation of the sinoaortic reflexes. During aortic constriction and vagal cooling, the carotid sinus reflex, acting alone, causes large increases in renal and total peripheral resistance and mean aortic pressure.