Evidence for true obstruction to left ventricular outflow in obstructive hypertrophic cardiomyopathy (muscular or hypertrophic subaortic stenosis)

Hypertrophic cardiomyopathy (HCM) is characterized by symmetrical or asymmetrical hypertrophy of the left and/or right ventricles (Table 1). It is a primary disorder of heart muscle in which the site and extent of the hypertrophic process are believed to be of importance in determining the disease manifestations (30). In this discussion we will limit our remarks to the commonest form of HCM, that is associated with ventricular (asymmetrical) septal hypertrophy (Table 1). It has been traditional to hemodynamically classify this form of HCM into obstructive and nonobstructive types, depending on whether or not there is a pressure gradient across the left ventricular outflow tract due to mitral leaflet-septal contact (5, 21, 30). In obstructive HCM this pressure gradient may be persistent (gradient at rest), labile (spontaneously variable), or latent (provocable) (5, 30, 31). In discussing whether there is true obstruction to left ventricular outflow in obstructive HCM, we will focus our attention on the situation in which there is a persistent subaortic pressure gradient at rest.