Equivalent heart rate during ventricular fibrillation in the dog heart: mechanoenergetic analysis

We propose equivalent heart rate (eHR) as an estimate of the frequency of contractions of individual myocytes in a fibrillating ventricle by analyzing mechanics and energetics of the ventricle. Using the isolated, cross-circulated dog heart preparation, we determined eHR in two different ways. First, we obtained eHR (eHR1) from myocardial O2 consumption (Vo2)-equivalent pressure-volume area (ePVA) data points during ventricular fibrillation (VF) by utilizing the Vo2-pressure-volume area (PVA) relation in the beating state. PVA is the area surrounded by the end-systolic and end-diastolic pressure-volume relations and the systolic pressure-volume trajectory in the pressure-volume diagram. PVA has been shown to represent the total mechanical energy generated by each contraction. We have recently proposed ePVA as a measure of the total mechanical energy generated by single contractions of all individual asynchronously contracting myocytes in a fibrillating ventricle. ePVA is the area surrounded by the horizontal line at the VF pressure and the end-systolic and end-diastolic pressure-volume relations in the beating state. Second, we measured Vo2 in beating state at various heart rates and Vo2 during VF under a mechanically unloaded condition. By comparing these fibrillating and beating Vo2 values, we determined eHR (eHR2) for the fibrillating state. eHR1 was 216 +/- 27 beats/min and eHR2 was 223 +/- 26 beats/min. These two values were not significantly different. We conclude that the average frequency of contractions of individual myocytes in a fibrillating ventricle is equivalent approximately to 220 beats/min in terms of ventricular energetics.