Cardiac performance: independent effects of inotropy and preload at high heart rate.

Linear relationships between stroke volume (SV) and heart rate (HR) were observed during right atrial pacing in open-chest dogs at control inotropy, during intravenous isoproterenol infusion and during blood volume expansion by saline infusion at HR exceeding 150 beats/min. The slope of these relationships remained constant during variations in inotropy, but rose during blood volume expansion. Myocardial chord lengths in the anterior left ventricular wall were continuously recored by ultrasonic technique to estimate left ventricular volume. When heart rate was increased, end-diastolic volume decreased more rapidly after than before blood volume expansion, explaining the increased slope of the SV/HR relationship. The end-diastolic volume and the SV/HR relationship were not influenced by changes in inotropy. After blood volume expansion by 57 +/- 13%, control end-diastolic volume was reestablished by increasing heart rate 84 +/- 20 beats/min. At identical end-diastolic volume, SV was equal at different HR. Thus, the effects on SV of changes in preload and inotropy are separable during right atrial pacing, and SV is independent of HR at constant preload and adrenergic stimulation.