Biphasic left ventricular outflow and its mechanism in hypertrophic obstructive cardiomyopathy.

Background: Biphasic systolic velocity in the left ventricular (LV) outflow tract (LVOT) occurs in hypertrophic obstructive cardiomyopathy (HOCM). The cause and importance of this observation remain poorly understood.
Methods: A total of 25 patients with HOCM were matched to 30 control subjects. A function derived from the relation of flow in the proximal descending aorta to that in the LVOT was used to estimate the LVOT systolic flow rate in HOCM. Patients with HOCM were grouped by absence (group I) or presence (group II) of biphasic LVOT velocity.
Results: Biphasic LVOT velocity was associated with biphasic estimated LVOT outflow (P =.002). The LVOT pressure gradient was inversely related to LV outflow rate at the time of the peak gradient (r = -.64, P <.001). Dobutamine increased the gradient and reduced LVOT outflow at the time of the peak gradient. In group II, mitral-septal separation occurred despite a LVOT gradient (36 mm Hg).
Conclusion: Biphasic LVOT flow in HOCM occurs and may be caused by "afterload mismatch." The late systolic increase in flow is related to mitral-septal separation. Resolution of systolic anterior motion occurs despite a persistent LVOT pressure gradient, implying a role for forces other than pressure differences.