Left ventricular basal muscle bundle in hypertrophic cardiomyopathy: insights into the mechanism of left ventricular outflow tract obstruction

Aims Many factors cause left ventricular outflow tract obstruction (LVOTO) in hypertrophic cardiomyopathy (HCM). Previous studies reported that left ventricular basal muscle bundle (BMB) may be associated with LVOTO. We aimed to evaluate the role of BMB in LVOTO by echocardiography. Methods and results Two hundred fifty-six patients diagnosed with HCM were recruited. The morphologic characteristics of left ventricular outflow tract (LVOT) were analysed. BMB was detected in 178 (69.5%) patients by echocardiography. Patients were separated by a resting or provocative LVOT gradient >= 30 mmHg or not. Compared to patients without LVOTO, patients with LVOTO had a significantly thicker basal septum, elongated anterior mitral leaflet (AML), shorter distance between the AML-free margin and the septum or BMB (M-sept/bundle), larger angle between the plane of the mitral valvular orifice and the ascending aorta (MV-AO angle), and higher prevalence of BMB (P < 0.05). According to multivariate analysis, the independent predictors of LVOTO were the presence of BMB, a large basal septum thickness, a short M-sept/bundle, a large MV-AO angle, and a large AML [odds ratio (95% confidence interval): 5.207 (1.381-19.633), 1.386(1.141-1.683), 0.615(0.499-0.756), 1.113(1.054-1.176), and 1.343(1.076-1.677), respectively, P < 0.05]. Of the 256 included patients, 139 underwent surgical myectomy. The transthoracic echocardiography, compared with surgical specimen, showed: sensitivity 98.3%, specificity 82.3%, positive predictive value 97.6%, negative predictive value 87.5%, and accuracy 96.4% to detect BMB. Conclusions BMB is common in HCM. BMB is a risk factor for LVOTO.