Influence of the Cardiac Sympathetics on Left Ventricular Dynamics during Tachycardia

Abstract

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.

Quantitative assessment of cardiac pump performance.

“…We have also shown that elevation in heart rate at rest, either through electrical pacing (Bishop & Horwitz, 1974), vagal blockade (Bishop & Horwitz, 1971) or infusion of isoprenaline decreases left ventricular end-diastolic diameter. We have shown that left ventricular internal transverse diameter measured with a sonocardiometer is an accurate index of left ventricular volume in normal dogs (Bishop, Horwitz, Stone, Stegall & Engelken, 1969;Bishop & Horwitz, 1970).…”

“…Therefore, afterload changes were not relevant to our analysis under these experimental conditions. However, acute afterload changes in the conscious dog at rest can limit cardiac output (O'Rourke, Pegram & Bishop, 1974).…”