As seen by echocardiography or cardiac magnetic resonance imaging (MRI), the most common pattern of hypertrophy is diffuse involvement of the ventricular septum. LV wall thickness typically ranges from 20 to 22 mm; however, 5% to 10% of patients have a wall thickness ranging from 30 to 50 mm, and some patients have SAM with a septal thickness of less than 29 mm.

LVOT obstruction is characterized on Doppler echocardiography by a high-velocity, late-peaking signal, sometimes described as dagger-shaped. Patients with no resting obstruction may reveal latent obstruction with provocative maneuvers, as described in the following.

The surgeon should identify the level of LVOT obstruction relative to SAM because this will ultimately guide the surgical approach. At times, the white endocardial fibrous scar on the septum demarcating the area of contact between the septum and the anterior mitral leaflet can be identified on an echocardiogram.

Transthoracic echocardiography is also used to identify any primary mitral valve pathologies that need to be addressed intraoperatively, including abnormal papillary muscles and their insertion.

Septal myectomy is recommended for patients who have symptoms and diminished functional capacity refractory to initial medical therapy (e.g., beta blockers, calcium channel blockers, and/or disopyramide).

Patients with a resting LVOT gradient of 30 mm Hg or more have reduced late survival rates compared to those with HCM without obstruction.

For those patients with latent obstruction, septal myectomy is indicated because relief of latent obstruction results in similar symptom relief, as does myectomy in patients with resting obstructive HCM.

Midventricular obstruction can cause symptoms similar to those occurring with subaortic obstruction. With isolated midventricular obstruction, there is no SAM of the mitral leaflet; rather, the site of obstruction is the contact point between the anterolateral papillary muscle and the midventricular septum.

Patients with apical HCM and diastolic heart failure may benefit from apical myectomy to enlarge the LV cavity.

Documentation of the severity of the LVOT gradient is made by transthoracic Doppler echocardiography. Many symptomatic patients will have minimal resting outflow tract gradients, and it is important in the preoperative evaluation to perform provocative maneuvers to elicit a gradient. These maneuvers include simple Valsalva maneuver, inhalation of amyl nitrite, exercise echocardiography, and infusion of isoproterenol during a hemodynamic study.

Cardiac MRI may be useful in identifying details of the ventricular anatomy, but it is not necessary for planning the operation in patients with isolated subaortic obstruction. This study is commonly performed, however, to determine LV wall thickness more precisely and the presence or absence of delayed enhancement, which may influence decision making for implantation of a defibrillator to prevent sudden cardiac death.

The level of ventricular outflow obstruction should be determined based on the transthoracic echocardiogram, which will reveal SAM of the mitral valve and the bright endocardial scar of the septal contact area.

It is important to identify midventricular or multilevel obstruction in patients with LVOT obstruction because a simple subaortic septal myectomy may not be adequate to relieve obstruction and the resulting symptoms.

If chest pain is a presenting symptom, coronary angiography or computed tomography (CT) angiography of the coronaries is warranted to assess for the need for concomitant coronary artery bypass surgery if the patient has fixed coronary obstruction or unroofing of intramyocardial vessels, where bridging is thought to produce obstruction.

A median sternotomy is performed with the institution of normothermic cardiopulmonary bypass in the usual fashion, with a single two-staged venous cannula through the right atrium. Before placing the patient on bypass, intracardiac pressures should be measured to ascertain the LV to aortic gradient to help confirm a successful myectomy at the end of the case. This is done by using a 2.5-inch, 22-G spinal needle that is placed into the aorta near the inflow cannula, as well as a 3.5-inch, 22-G spinal needle in the left ventricle through the right ventricular free wall and septum. By measuring pressures from both needles at the same time, the gradient can be calculated. To assess dynamic LVOT gradients, a premature ventricular contraction (PVC) can be induced by tapping the heart, and the gradient in the next beat is recorded ( Fig. 29.2 ).

Direct intraoperative measurement of the left ventricle outflow tract (LVOT) pressure gradient. As shown on the top, a 2.5-inch, 22-G spinal needle is inserted into the aorta near the inflow cannula; a 3.5-inch, 22-G spinal needle is inserted into the left ventricle through the right ventricular free wall and septum. The pressure gradient can be calculated in real time by measuring pressures from both needles simultaneously. On the bottom, dynamic LVOT gradients can be elicited by tapping the heart to induce a premature ventricular contraction (PVC) and recording the gradient in the next beat.

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