Preload-adjusted maximal power: A novel index of left ventricular contractility in atrial fibrillation

Background: Left ventricular contractility in atrial fibrillation is known to change in a beat to beat fashion, but there is no gold standard for contractility indices in atrial fibrillation, especially those measured non-invasively. Objective: To determine whether the non-invasive index of contractility "preload-adjusted PWR_max" (maximal ventricular power divided by the square of end diastolic volume) can accurately measure left ventricular contractility in a beat to beat fashion in atrial fibrillation. Methods: Atrial fibrillation was induced experimentally using 60 Hz stimulation of the atrium and maintained in 12 sheep; four received diltiazem, four digoxin, and four no drugs (control). Aortic flow, left ventricular volume, and left ventricular pressure were monitored simultaneously. Preload-adjusted PWR_max, the slope of the end systolic pressure-volume relation (E_max), and the maximum rate of change of left ventricular pressure (dP/dt_max) were calculated in a beat to beat fashion. Results: Preload-adjusted PWR_max correlated linearly with load independent E_max (p < 0.0001) and curvilinearly with load dependent dP/dt_max (p < 0.0001), which suggested the load independence of preload-adjusted PWR_max. After five minutes of diltiazem administration, preload-adjusted PWR_max, dP/dt_max, and E_max fell significantly (p < 0.0001) to 62%, 64%, and 61% of baseline, respectively. Changes were not significant after five minutes of digoxin (103%, 98%, and 102%) or in controls (97%, 96%, and 95%). Conclusions: Preload-adjusted PWR_max correlates linearly with E_max and is a useful measure of contractility even in atrial fibrillation. Non-invasive application of this method, in combination with echocardiography and tonometry, may yield important information for optimising the treatment of patients with atrial fibrillation.