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We sought to summarize our recent experience with intraoperative monitoring for management of patients undergoing surgical myectomy for hypertrophic obstructive cardiomyopathy with emphasis on dynamic left ventricular outflow tract obstruction. We also analyzed the impact of these data on surgical decision-making and adequacy of septal myectomy.We retrospectively analyzed the medical records of 198 patients who underwent transaortic septal myectomy and evaluated baseline and provoked left ventricular outflow tract gradients obtained by Doppler echocardiography and by direct measurement of pressures in the left ventricle and aorta.After induction of anesthesia before myectomy, left ventricular outflow tract obstruction, assessed by direct measurement, was less than the gradient documented by preoperative Doppler echocardiography in 119 patients (60%) (41 ± 31 vs 76 ± 40 mm Hg; P < .001). In 75 patients (38%), the obstruction was more severe (64 ± 32 vs 35 ± 31 mm Hg; P < .001); 4 patients (2%) had similar left ventricular outflow tract gradients. After myectomy, left ventricular outflow tract gradient decreased markedly (49 ± 33 vs 4 ± 8 mm Hg [P < .001] by direct measurement; 59 ± 42 vs 4 ± 6 mm Hg [P < .001] by transesophageal echocardiography). Cardiopulmonary bypass was resumed for more extensive myectomy in 8 (4%) patients because of a persistent residual left ventricular outflow tract gradient of 33 ± 14 mm Hg. Of note, for 78 patients (39%) intraoperative Doppler echocardiographic assessment of left ventricular outflow tract gradient was technically inadequate.Direct intraoperative measurement of pressures in the left ventricle and aorta provides important hemodynamic data in addition to intraoperative transesophageal echocardiography findings. This information assists the surgeon in defining the extent of myectomy.