Study was determined to be focused on Quality Assurance (QA) and Quality Improvement (QI), rather than research, based on its primary purpose and activities. Study was closed prior to enrollment.

This is a follow up investigation to our previous study entitled "On-pump intraoperative echocardiography (OPIE)" (clinicaltrials.gov NCT03094325) whereby we determined that left ventricular septal thickness as measured by the OPIE technique correlates highly with traditional methods of transthoracic and transesophageal echocardiography during septal myectomy for hypertrophic cardiomyopathy. OPIE may be especially useful in patients with a thin ventricular septal thickness as adequate treatment may rely on mere millimeters of myocardial resection. We therefore propose a study in which OPIE is compared to transthoracic and transesophageal echocardiography in patients with a thin interventricular septum. Subjects will receive the same perioperative care regardless of their involvement in the study. Patients who enroll in the study will undergo an additional intraoperative echocardiographic measurement that adds less than five minutes to total operative time.

Inclusion Criteria:

• All hypertrophic cardiomyopathy patients, inclusive of male and female and all racial/ethnic origins, age 18 or older who are scheduled to receive a septal myectomy with a preoperative transthoracic echocardiograph suggesting an interventricular septal width of less than 2.0 cm.

Exclusion Criteria:

• Those hypertrophic cardiomyopathy patients who are scheduled to receive a septal myectomy who are less than 18 years old or whose preoperative transthoracic echocardiograph suggests an interventricular septal width of greater than 2.0 cm.

The de-identified participant data from the final research dataset used in the published manuscript will be shared upon reasonable request beginning 9 months and ending 36 months following article publication or as required by a condition of awards and agreements supporting the research provided the investigator who proposes to use the data executes a data use agreement with New York University Langone Health. Requests may be directed to: daniel.swistel@nyulangone.org. The protocol and statistical analysis plan will be made available on Clinicaltrials.gov only as required by federal regulation or as a condition of awards and agreements supporting the research.

Beginning 9 months and ending 36 months following article publication or as required by a condition of awards and agreements supporting the research.