Muscular Subaortic Stenosis

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Abstract
Eleven patients with muscular subaortic stenosis were studied by use of combined right and transatrial septal left heart catheterization. A variety of stimulating maneuvers were performed to induce or increase the functional outflow tract obstruction; 150 µg/kg of propranolol was administered intravenously, and the stimulating maneuvers were repeated. Outflow obstruction was latent (present only with stimulation) in four patients, labile (present intermittently at rest) in five patients, and persistent and severe in two patients. Propranolol abolished or significantly decreased the spontaneous variations in outflow obstruction occurring at rest, during the enhanced gradient in the post-exercise state, and during variations induced by isoproterenol infusion. The outflow obstruction induced by inhalation of amyl nitrite, the Valsalva maneuver, and premature ventricular contractions was only inconsistently prevented. Long-term oral propranolol therapy has been of significant symptomatic benefit in all the patients with latent or labile outflow obstruction and is considered the treatment of choice in these groups. In one patient with persistent outflow obstruction, an increase in symptoms occurred when oral propranolol was administered, and the drug was discontinued. Evidence is presented to support the concept of a cycle of obstruction and hypertrophy in the natural history of this disease. This vicious cycle might be prevented by propranolol.