THE INFLUENCE OF HEART RATE ON LEFT VENTRICULAR VOLUME IN DOGS*

Abstract

The effects of changing the heart rate on left ventricular stroke volume (SV), end-systolic volume (ESV), end-diastolic volume (EDV), and left ventricular pressure during 160 experimental periods was studied in 11 dogs. Ventricular volumes was estimated from aortic thermodilution curves resulting from left ventricular injection of cooled blood. A stimulating catheter in the right atrium produced tachycardia and efferent right vagal stimulation with and without slow pacing induced bradycardia. After heart rate reached 30 beats/minute above control levels, SV, ESV and EDV fell progressively and proportionally with higher rates. With extreme tachycardia (225 beats/minute), SV fell faster than ESV, resulting in a significantly greater ESV/EDV ratio. Increases of less than 30 beats/minute caused no significant change. In the bradycardia experiments SV, ESV, and EDV rose; the ESV/EDV ratio decreased because SV increased proportionally more than ESV. The decrease in SV resulting from tachycardia is associated with a progressively smaller left ventricle. The increase in SV resulting from bradycardia is associated with a larger ventricle and a lower residual fraction.