Beneficial effect of atrial pacing in severe acute aortic regurgitation and role of M-mode echocardiography in determining the optimal pacing interval. The effect of atrial pacing on cardiac performance was assessed in 11 men (aged 20 to 64 years) with recent-onset severe aortic regurgitation (AR), all of whom had diastolic closure of the mitral valve on the echocardiogram. Thermodilution cardiac outputs were determined, and aortic, left ventricular and pulmonary arterial wedge pressures recorded. Once baseline recordings were completed, the pacing rate was increased by increments of 10 beats/min (70, 80, 90...) to a maximal rate of 140 beats/min. The optimal pacing interval, obtained from hemodynamic data, was defined as that at which the lowest filling pressure was associated with the highest cardiac index. This was then compared with a pacing interval derived from the R wave of the electrocardiogram to the diastolic mitral closing point on the M-mode echocardiogram. Such an interval would shorten diastole without affecting forward mitral flow. Atrial pacing improved the overall hemodynamic state in all patients; the most favorable hemodynamics were achieved at heart rates between 110 and 130 beats/min (mean: 120 +/- 8). At the optimal rate, left ventricular end-diastolic pressure decreased from 46 +/- 7 to 23 +/- 12 mm Hg (p less than 0.001), and the pulmonary arterial wedge pressure from 28 +/- 8 to 16 +/- 7 mm Hg (p less than 0.001), while the cardiac index increased from 2.34 +/- 0.46 to 2.63 +/- 0.49 liters/min/m2 (p less than 0.01). The mean difference between the optimal pacing interval determined from the hemodynamic data and the interval derived from the echocardiogram was 18 +/- 21 ms. 