The "1st septal unit" in hypertrophic obstructive cardiomyopathy: a newly recognized anatomo-functional entity, identified during recent alcohol septal ablation experience.

In hypertrophic obstructive cardiomyopathy, selective and asymmetric hypertrophy results in a stenotic subaortic channel, which is further narrowed by a Venturi effect (suctioning of the anterior leaflet, manifested by systolic anterior motion of the mitral valve). Better understanding of these essential pathophysiologic mechanisms has led to the definition of a new anatomo-functional entity, the 1st septal unit, which consists of the basal interventricular septal hypertrophy and its related septal arterial branches. As an alternative to surgical myomectomy, alcohol septal ablation is an effective method of reducing subaortic stenosis and improving mitral valve function. After alcohol ablation, global negative remodeling of the hypertrophied left ventricle eventually ensues. This review presents specific anatomic and functional features of a newly identified pathophysiologic entity (the 1st septal unit) in relation to the clinical manifestations and natural history of hypertrophic obstructive cardiomyopathy. This relationship is also relevant during the performance of alcohol septal ablation interventions: related operative suggestions are provided for optimizing subaortic stenosis relief during septal ablation and for preventing complications.