Variable unstressed volume keeps normal distributions of canine left ventricular contractility and total mechanical energy under atrial fibrillation

We have reported that the contractility index (E_max) and the total mechanical energy (PVA) of arrhythmic beats of the left ventricle (LV) distribute normally in canine hearts under electrically induced atrial fibrillation (AF). Here, E_max is the ventricular elastance as the slope of the end-systolic (ES) pressure-volume (P-V) relation (ESPVR), and PVA is the systolic P-V area as the sum of the external mechanical work within the P-V loop and the elastic potential energy under the ESPVR. To obtain E_max and PVA, we had to assume the systolic unstressed volume (V_o) as the V-axis intercept of the ESPVR to be constant despite the varying E_max, since there was no method to obtain V_o directly in each arrhythmic beat. However, we know that in regular stable beats V_o decreases by ∼7 ml/100 g LV with ∼100 times the increases in E_max from ∼0.2 mmHg/(ml/100 g LV) of almost arresting weak beats to ∼20 mmHg/(ml/100 g LV) of strong beats with a highly enhanced contractility. In the present study, we investigated whether E_max and PVA under AF could still distribute normally, despite such E_max-dependent V_o changes. The present analyses showed that the E_max changes were only ∼3 times at most from the weakest to the strongest arrhythmic beat under AF. These changes were not large enough to affect V_o enough to distort the frequency distributions of E_max and PVA from normality. We conclude that one could practically ignore the slight E_max and PVA changes with the E_max-dependent V_o changes under AF.