Cardiac performance: optimal heart rate for maximal cardiac output

Abstract

To determine optimal heart rate for the maximal cardiac output at various levels of inotropy and blood volume, the relationship between heart rate (HR) and stroke volume (SV) was examined in anaesthetized dogs during right atrial pacing. Myocardial inotropy was raised by intravenous infusion of isoproterenol, a stimulator of adrenergic β-receptors, and reduced by propranolol, an inhibitor of adrenergic β-receptors. Circulating blood volume was increased by saline infusion. Within the range of optimal heart rate, SV and HR were inversely related: SV = k (HR₀–HR), where k indicates the relationship between changes in SV and HR. The intercept with the HR axis is HR₀. At constant HR a rise in inotropy increased SV and a fall in inotropy reduced SV. These changes in SV were equal at every HR, and k was therefore constant. In contrast, blood volume expansion increased SV more at low than at high HR (k increased), but HR₀ was not significantly changed. Calculated maximal cardiac output: k ± HR₀²/4, and optimal heart rate: HR₀/2, agreed with observations when maximal cardiac output was raised from 1900 to 4500 ml/min by increasing blood volume and inotropy. Optimal HR was not influenced by changes in blood volume, but was increased from 160 to 200 beats/min by increasing inotropy. We conclude that the optimal heart rate and the maximal cardiac output can be predicted from the linear relationship between SV and HR during right atrial pacing.

Key Words:

• blood volume
• myocardial inotropy
• isoproterenol
• propranolol
• right atrial pacing
• stroke volume