Effect of heart rate on left ventricular geometry and function in conscious dogs

The effect of heart rate on geometry and function of the left ventricle is inadequately defined. Heart rate (HR) was altered by atrial and/or ventricular pacing in conscious dogs chronically instrumented for measurement of left ventricular (LV) axes and volumes from biplane cine exposures of implanted beads and LV, aortic, and left atrial pressure. In 7 studies abruptly increasing HR from a mean of 106 to 202 beats/min resulted in a significant decrease in end diastolic volume (EDV) from a mean of 70 to 59 ml while no change in mean end systolic volume (ESV) was observed. A symmetrical decrease in all 3 measured LV axes (base apex, anterior posterior, septal lateral) accounted for the fall in EDV. To study the importance of HR in determining ESV, aortic pressure was controlled with phenylephrine infusion. In 6 studies at similar peak and mean systolic pressure EDV was consistently smaller at high (188 beats/min) than at low (121 beats/min) HR while ESV was unchanged. Thus in the conscious dog increasing HR results in a predictable fall in EDV but ESV remains unchanged when systolic pressure load is kept constant.