Influence of the Cardiac Sympathetics on Left Ventricular Dynamics during Tachycardia

Increasing heart rate by atrial pacing in conscious, resting dogs resulted in declines in stroke volume (-0.40 ml/beat-kg/100 beats/min; p < 0.001), and left ventricular transverse internal diameter at end-diastole (-3.40 mm/100 beats/min; p < 0.001) and end-systole (-2.14 mm/100 beats/min; p < 0.001). When pacing was repeated following β-adrenergic blockade, stroke volume declined slightly more than had occurred during the control response. The decrease in end-diastolic diameter was -2.13 mm/100 beats/min while the end-systolic diameter declined at a rate of -0.93 mm/100 beats/min. It is concluded that loss of β-adrenergic tone limits stroke volume at high heart rates due to a diminished ability of the myocardium to shorten, as indicated by elevation in the left ventricular end-systolic diameter.