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

Abstract

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.

Figure 1

Relations between preload-adjusted PWR_max and E_max at baseline. Baseline, before drug administration; E_max, the slope of end systolic pressure–volume relation; PWR_max, maximal ventricular power.

Figure 2

Relations between preload-adjusted PWR_max and dP/dt_max at baseline. Two different random effects models (square root fit and asymptotic fit) were used for this analysis. Baseline, before drug administration; dP/dt_max, maximum rate of change of left ventricular pressure; PWR_max, maximal ventricular power.

Figure 3

Changes in contractility after drug administration. All values are means (with SD) and are expressed as percentages of the values before drug administration. *p < 0.0001 compared with values before drug administration. E_max, the slope of the end systolic pressure–volume relation; dP/dt_max, maximum rate of change of left ventricular pressure; PWR_max, maximal ventricular power.

Figure 4

Representative relation between preload-adjusted PWR_max and RR1 and its change after diltiazem administration. There was a positive trend at baseline, which changed significantly (p < 0.006) after diltiazem administration. Baseline, before diltiazem administration; PWR_max, maximal ventricular power; RR1, preceding RR interval; 5 min after diltiazem, five minutes after the completion of the initial diltiazem load (0.30 mg/kg).

Figure 5

(A) Representative relation between preload-adjusted PWR_max and RR2 and the change after diltiazem administration. There was no trend in this relation at baseline, which changed significantly (p < 0.006) to flat after diltiazem administration. (B) The same relation at baseline after excluding beats that had an RR1 of less than 350 ms. There was a negative trend after this arrangement. Baseline, before diltiazem administration; PWR_max, maximal ventricular power; RR1, preceding RR interval; RR2, pre-preceding RR interval; 5 min after diltiazem, five minutes after the completion of the initial diltiazem load (0.30 mg/kg).

Figure 6

Relations between V_ED and dP/dt_max, preload-adjusted PWR_max, and E_max during atrial fibrillation before drug administration in all 12 sheep. E_max, the slope of end systolic pressure–volume relation; dP/dt_max, maximum rate of change of left ventricular pressure; PWR_max, maximal ventricular power; V_ED, end diastolic left ventricular volume.