Hypertrophic cardiomyopathy (HC) is characterized by impaired diastolic function and, in about 1/4 of patients, left ventricular (LV) outflow tract obstruction. Atrioventricular (AV) pacing diminishes LV outflow tract gradient in HC, but impairs diastolic function in the experimental animal and in different categories of patients. To investigate the effects of AV pacing on hemodynamics and LV function in obstructive HC, 16 patients with HC were studied by cardiac catheterization and simultaneous radionuclide angiography during atrial and AV pacing. The resting LV outflow tract gradient decreased with AV pacing from 60 +/- 34 to 38 +/- 37 mm Hg (mean +/- SD; p <0.001). Regional ejection fraction decreased significantly at the septal level from 0.81 +/- 0.21% to 0.69 +/- 0.27% (p <0.01). Pulmonary artery wedge pressure increased from 10 +/- 5 to 15 +/- 6 mm Hg (p <0.001). AV pacing induced asynchrony (i.e., the coefficient of variation of the time to end-systole increased from 7 +/- 4% to 14 +/- 10% (p <0.01). The time constant of isovolumetric relaxation (t) increased from 58 +/- 24 to 74 +/- 33 ms (p <0.02), and peak filling rate decreased from 491 +/- 221 to 416 +/- 184 ml/s (p <0.05). Thus, AV pacing greatly diminishes resting obstruction through a reduction in septal ejection fraction (i.e., an increase in LV outflow tract width in systole), but impairs active diastolic function and increases filling pressures. These latter effects are potentially detrimental in patients with HC in whom diastolic dysfunction is present.