Cardiac dimensions in intact unanesthetized man. VI. Effects of changes in heart rate.

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Abstract
This study was undertaken to define the effects of alterations in heart rate per se on cardiac dimensions in intact, conscious man. At corrective cardiac operations, roentgenopaque markers were sutured to the surfaces of one or both ventricles in 9 patients. Postoperatively, cineradiograms were obtained, and the distances between the markers were measured on each frame. Heart rate was altered in 4 patients with a left ventricular radiofrequency pacemaker, and in 5 patients with a right atrial electrode catheter. As heart rate was increased by an average of 56 beats/min, right ventricular end-diastolic and end-systolic dimensions decreased by averages of 7.2 and 4.9%, respectively. Left ventricular end-diastolic length diminished by an average of 6.0%. The average decrease in external linear dimensions produced by these elevations of heart rate was equivalent to approximately 35 ml, or about 1/2 of a normal stroke volume.