INFLUENCE OF BETA-ADRENERGIC-BLOCKADE UPON HEMODYNAMIC-RESPONSE TO EXERCISE ASSESSED BY DOPPLER ECHOCARDIOGRAPHY

Peak aortic blood flow acceleration and velocity measured by Doppler echocardiography have been documented to be accurate descriptors of left ventricular systolic function. Both acceleration and velocity are reduced in the presence of β-blockade at rest and during exercise. Whether and to what extent the simultaneous alterations in heart rate (HR) due to β-blockade affect these parameters has received little study. In order to determine the influence of alterations in HR on Doppler measurements of velocity and acceleration, 10 healthy men were studied during upright exercise under control conditions, following propranolol administration, and following propranolol plus transesophageal atrial pacing. In addition, we assessed the response of stroke volume (measured as flow velocity integral) during β-blocked and control exercise. Propranolol significantly reduced acceleration and velocity during all stages of exercise compared with control values (p < 0.05). Increasing the HR during exercise via pacing had no effect on acceleration or velocity compared with propranolol administration alone, thus demonstrating that during upright exercise, changes in acceleration and velocity are independent of alterations in HR. At low levels of exercise, propranolol significantly reduced flow velocity integral (FVI) compared with control (-1.14 cm, p < 0.05.). At high levels of exertion, however, FVI exceeded values obtained during control conditions (1.2 cm at stage 4). Pacing during β-blockade reduced FVI at high levels of exercise but had no effect at lower levels. Our results suggest that during low levels of exercise stroke volume is increased as a consequence of both increased contractility and augmented left ventricular filling. At high levels of exertion, stroke volume plateaus and then falls due to its primary dependence on the Frank-Starling mechanism, which is limited and then diminished by tachycardia. © 1990.