Background The mechanism for early improvement in cardiac function after cardioversion from atrial fibrillation is unknown.

Methods We measured ventricular volumes and load-independent contractility during atrial fibrillation and within 24 hours after cardioversion to sinus rhythm in 15 adult patients (10 men, 5 women; mean age 63 ± 4 years, range 31 to 81 years). Duration of atrial fibrillation ranged from <1 day to 6 months.

Results After cardioversion, left ventricular ejection fraction increased from 51% ± 4% to 61% ± 4% (P = .001, 95% confidence intervals for the difference, 7% to 15%), stroke volume increased from 57 ± 4 mL to 76 ± 6 mL (P < .001, 95% confidence intervals 8 to 32 mL), and mean cycle length increased from 0.77 ± .04 seconds in atrial fibrillation to 1.02 ± .04 seconds in sinus rhythm (P = .002, 95% confidence intervals, 0.1 to 0.4 seconds). Cardiac contractility, as expressed by the slope and the intercept of the relation between rate-corrected circumferential velocity of fiber shortening and end-systolic wall stress (Vcfc/ESWS) remained unaltered in 13 of 15 patients, suggesting that intrinsic inotropic state was unchanged immediately after return of normal sinus rhythm. Finally, a significant correlation was observed between improvement in stroke volume and peak A-wave velocity (r = 0.79, P = .035).

Conclusion Both left ventricular stroke volume and ejection fraction increase immediately after cardioversion, whereas intrinsic cardiac contractility is largely unchanged. These data suggest that the mechanism of this increase is enhanced left ventricular diastolic filling due mostly to increased cycle length and return of left atrial mechanical function. (Am Heart J 1998;136:435-42.)