EFFECTS OF DUAL-CHAMBER PACING IN HYPERTROPHIC CARDIOMYOPATHY ON LEFT-VENTRICULAR OUTFLOW TRACT OBSTRUCTION AND ON DIASTOLIC FUNCTION

Hypertrophic cardiomyopathy (HC) is characterized by impaired diastoli c function and, in about 1/4 of patients, left ventricular (LV) outflo w tract obstruction. Atrioventricular (AV) pacing diminishes LV outflo w tract gradient in HC, but impairs diastolic function in the experime ntal animal and in different categories of patients. To investigate th e effects of AV pacing on hemodynamics and LV function in obstructive HC, 16 patients with HC were studied by cardiac catheterization and si multaneous radionuclide angiography during atrial and AV pacing. The r esting LV outflow tract gradient decreased witt;AV pacing from 60 +/- 34 to 38 +/- 37 mm Hg (mean +/- SD; p <0.001). Regional ejection fract ion decreased significantly at the septal level, from 0.81 +/- 0.21% t o 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 asynchro ny (i.e., the coefficient of variation of the time to end-systole incr eased from 7 +/- 4% to 14 +/- 10% (p <0.01). The time constant of isov olumetric relaxation (7) 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 obstructio n through a reduction in septal ejection fraction (i.e., an increase i n LV outflow tract width in systole), but impairs active diastolic fun ction and increases filling pressures. These latter effects are potent ially detrimental in patients with HC in whom diastolic dysfunction is present.