Six patients with ASH have been studied with incremental atrial and ventricular pacing during diagnostic cardiac catheterization. The data suggest that (1) the detrimental clinical effect of atrial or ventricular tachyarrhythmias is primarily mediated by an elevation in filling pressure and a decrease in cardiac output rather than augmentation of outflow obstruction and (2) at slow rates less than or equal to 10 bpm) atrial pacing is hemodynamically advantageous.