Transesophageal echocardiography is essential in guiding the surgical approach for patients with obstructive hypertrophic cardiomyopathy. Septal hypertrophy, elongated mitral valve leaflets, and abnormalities of the subvalvular apparatus are prominent features, all of which may contribute to left ventricular outflow tract obstruction. Surgery aims to alleviate the obstruction via an extended myectomy, often with an intervention on the mitral valve and subvalvular apparatus. The goal of intraoperative echocardiography is to assess the anatomic pathology and pathophysiology in order to achieve a safe intraoperative course and a successful repair. This guide summarizes the systematic evaluation of these patients to determine the best surgical plan.
Highlights
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Intraoperative two- and three-dimensional TEE is crucial for HCM patients.
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Measure septal thickness, mitral leaflet length, and assess subvalvular apparatus.
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The septum is best measured from the three-chamber long-axis and transgastric views.
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Use 3D TEE from the LV side, “looking up”, to visualize mitral valve.
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Interactive communication with the surgeon before and after bypass is key.
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Patients affected by HCM may present in a variety of ways. Signs of disease progression include heart failure with exertional dyspnea, angina, syncope, and atrial fibrillation; infrequently, the course of HCM is unexpectedly ended by sudden cardiac death. An important therapeutic goal in these patients is to decrease or eliminate obstruction. Initially, first-line pharmacologic treatment includes β-blockers, disopyramide, verapamil, or a combination of these medications. In patients with persistent symptoms refractory to medical management or who experience significant side effects, surgical intervention may be required. Guidelines indicate that alcohol septal ablation should be reserved for patients who are too old, too frail for surgery, or inalterably opposed to sternotomy.
Transthoracic echocardiography (TTE) is critical to the initial preoperative workup. A preliminary plan for surgical intervention can be formed on the basis of this examination, and the patient can be recommended for surgery on the basis of the findings. Exercise stress echocardiography may also be used to assess inducible LV outflow tract (LVOT) gradients, mitral regurgitation (MR), and provokable arrhythmia. Increasingly, magnetic resonance imaging is used in the initial evaluation of patients with HCM, including preoperative evaluation of the septum and mitral valve apparatus, though it is our impression that it is not as useful as echocardiography for planning surgery, unless echocardiographic imaging is suboptimal. In such patients, elective outpatient preoperative transesophageal echocardiography (TEE) is indicated to clarify any ambiguity before surgery.
Surgical septal myectomy is a technically demanding procedure. Inadequate resection results in persistent obstruction and symptoms, while excessive or poorly placed myectomy can produce iatrogenic heart block and ventricular septal defects and consequent high morbidity and mortality, reported as high as 5.9%. In sharp contrast, data collected from dedicated HCM centers have shown remarkably decreased operative mortality (0.4%), explained primarily by surgical experience, advances in surgical technique, improved myocardial preservation, and decreased cardiopulmonary bypass times.
Patients with HCM not only have myocardial hypertrophy but also malformations of the mitral valve and subvalvular apparatus. Current guidelines recommend surgical intervention over alcohol septal ablation in patients with concomitant abnormalities of the mitral valve and the submitral apparatus, as a class I recommendation. The decision of whether to intervene on the mitral valve and the subvalvular apparatus in addition to myectomy is controversial. Dedicated HCM centers have found success with both approaches: some perform extended myectomy with mitral valve or subvalvular interventions in selected patients ; at other centers, extended myectomy is performed without operating on the mitral valve. Clinical judgment and experience determines the best operative choices for these patients.
Preoperative Examination
The comprehensive assessment of these abnormalities in a systematic fashion directs the surgical plan for the operation.
According to the 2011 American College of Cardiology Foundation/American Heart Association guidelines, TEE is recommended for the intraoperative guidance of surgical myectomy as a class IB recommendation. Because the patient already has confirmation of HCM on preoperative TTE, TEE is generally performed after the induction of general anesthesia in the operating room; a final surgical plan may then be formed among the cardiologist, cardiac anesthesiologist, and surgeon at that time. Because HCM pathophysiology is complex and may involve, to varying degrees, septal thickening of a heterogeneous nature, mitral leaflet elongation, and subvalvular abnormalities, at our hospital we find it useful for the referring HCM cardiologist to participate in operative planning; pursuant to this end, we have established a secure Internet-based system so that live images in the operating room can be viewed remotely from the office and discussed.
Interventricular Septum
LV hypertrophy, particularly of the septum, is a cardinal feature of HCM. The nomenclature for HCM has been refined in recent years. Idiopathic hypertrophic subaortic stenosis has been largely discarded after the realization that patients with nonobstructive HCM can have limiting symptoms and be prone to sudden cardiac death; current nomenclature acknowledges this by the dichotomy between obstructive and nonobstructive HCM.
Determining the extent and location of interventricular septal wall thickening is crucial and directs the resection. The reader should be aware that the thickness of the septum may vary from initial TTE to the ensuing preoperative TEE and to the subsequent arrested heart on cardiopulmonary bypass. Most anesthetics are vasodilators, and positive pressure ventilation after induction of general anesthesia decreases venous return. Perhaps because of both of these effects, the heart may be unloaded. We have noticed that wall thickness is greater on postinduction TEE than on preoperative ambulatory TTE. Decreased chamber size could possibly explain the increase in wall thickness; alternatively, this might be explained by differences in planes between imaging modalities.
The best views to evaluate the interventricular septum are the midesophageal long-axis view (135°), transgastric midpapillary short-axis view (0°), and midesophageal four-chamber view (0°).
The midesophageal long-axis view is useful for measuring anterior septal wall thickness, which is the principal target for extended myectomy. This view is correlated with wall thickness on the transgastric short-axis views. The myectomy is directed primarily toward the anterior septum. The goal is to thin the septum adequately to divert outflow anteriorly, away from the mitral valve, and allow it to leave the heart without catching the mitral valve leaflets. To accomplish this goal the myectomy often must be extended down to the level of the papillary muscles, resecting the mid-anterior septum. Measurements of the interventricular septum are made near the end of diastole at or before the R wave, when the ventricle is at its largest diastolic dimension.
The portion of the septum just below and adjacent to the aortic annulus generally is not involved in the pathophysiology of obstruction in HCM. Thinning this portion of the septum should be avoided during myectomy to preserve the atrioventricular node and to avoid the thin membranous portion of the septum.
Three-dimensional (3D) volume acquisition can provide precise confirmatory midesophageal long-axis views by acquiring a 3D data set and then aligning a plane to bisect the mitral valve at A2:P2. The septum in this view correlates with the anterior septum ( Figure 1 ). Although one can align a plane to create a midesophageal five-chamber view (0°) from the 3D data set to assess the anterior septum, we prefer the midesophageal long-axis plane, as this view and measurement correspond anatomically to the surgical approach through the aortotomy. Tomographic measurements of the anterior septal thickness can then be checked at both the subaortic and midventricular levels ( Figure 2 ).
The transgastric midpapillary short-axis view is useful for measuring the anterior septum, the posterior septum, and the anterior LV free wall in the same plane. As we advance the probe, we progressively visualize the basal, mid, and apical portions. Alternatively, anteflexion of the probe shows more of the basal layer, while retroflexion reveals more of the apical segments. Biplane imaging can be used to ensure the image is a transverse cut of the left ventricle. These measurements supplement findings from the midesophageal long-axis view ( Figure 3 ).
The midesophageal four-chamber view will often show portions of the posterior septum. Generally, less myectomy is performed in the subaortic posterior septum to avoid complete heart block, though the excision may be extended into the mid-LV posterior septum, so its thickness at this level should be defined.
Mitral Valve
Septal hypertrophy and anterior displacement of the mitral valve by anteriorly positioned papillary muscles decreases the distance from the septum to the plane of mitral valve coaptation. There is a crucial overlap between the inflow and outflow portions of the left ventricle. As flow sweeps around the septal bulge, flow drag, the pushing force of flow, catches the mitral valve from behind and sweeps the anterior mitral valve leaflet (AML) into the septum. Systolic anterior motion (SAM) can disrupt mitral valve coaptation and result in MR. MR secondary to SAM often is directed posteriorly and laterally ( Figure 4 ). A regurgitant jet aimed anteriorly suggests a structural defect in the mitral valve itself, such as calcification, thickening, or a cleft. During the postinduction examination, patients often have decreased preload and decreased afterload, both of which can predispose to SAM.
Compounding the problem of anterior mitral displacement, the majority of patients with HCM have elongated mitral valve leaflets. The anterior leaflet may protrude into the LVOT and contribute to obstruction. We have called this the “nightcap mitral valve” because of its characteristic appearance on the apical three-chamber view on TTE, midesophageal long-axis view on TEE, or transgastric long-axis view on TEE ( Figure 5 ).
Leaflet lengths are measured in the midesophageal long-axis view (135°). On two-dimensional imaging, we have measured this length from the tip of the AML in A2 to the aortic annulus, at the insertion of the noncoronary leaflet. This measure includes the intervalvular fibrosa but is more reproducible than the distance from the tip to the hinge point of the leaflet. Although one may measure only the length of the leaflet up to the point of coaptation, we believe it is important to include the entire leaflet, particularly the portion past the coaptation point, because that is the part that is caught by flow and pushed into the outflow tract. In a previous study, the AML was 34, 30, and 24 mm in patients with obstructed HCM, those with nonobstructed HCM, and those without HCM, respectively. Three-dimensional TEE allows precise measurements, allowing one to identify AML lengths at A2, as well as anywhere along A1:A3 ( Figure 6 ). As defined in this previous study, leaflets >16 mm/m 2 are elongated in HCM and associated with obstruction.
In the recent past, when a long leaflet participated in the pathophysiology of SAM, horizontal plication mattress sutures were placed. Patients who were selected to undergo plication had longer AMLs (32 vs 28 mm). A long redundant leaflet can thus be shortened and stiffened, decreasing the predilection for SAM and maintaining intact coaptation with the posterior leaflet.
MR often is present in patients with HCM. Three-dimensional TEE is an excellent modality to evaluate the mitral valve, particularly in identifying segmental mitral valve disease. Careful assessment of the mitral valve and regurgitant jet will determine whether the etiology is secondary to SAM, structural mitral valve abnormalities, nonphysiologic loading conditions, or a combination of the three. MR due to SAM will be abolished or markedly reduced by correction of the outflow tract obstruction and the SAM. Significant intrinsic mitral valve pathology such as prolapse may warrant a concomitant repair. Infrequently, mitral valve replacement is deemed appropriate, usually because of extensive calcification.
Residual Leaflet
Additionally, there may be a residual part of the AML that extends past the point of coaptation, termed a residual leaflet. This portion may become patulous and thinned, does not participate in coaptation, and can protrude into the LVOT ( Figure 7 , Video 1 ; available at www.onlinejase.com ). For the last four years, we have applied a more nuanced approach to redundant leaflets. Mitral-septal contact can occur in two different ways: first, there may be billowing of the central portion of the leaflet because of excess tissue in the horizontal axis of the leaflet, leading to a “cowl” appearence of the anterior leaflet; or, second, and more commonly, there may be increased tissue in the vertical axis and excessive excursion of the leading edge, the residual leaflet . Differentiation of the two during preoperative echocardiography is now considered to be instrumental in planning the surgical approach to the mitral contribution of obstruction. The first type, with horizontal redundancy, lends itself to repair with a horizontal plication, described above. For the second type, patients with very long discrete residual leaflets that extend past the coaptation point of the mitral valve and protrude into the left ventricle, we now perform a direct residual leaflet excision termed “ReLex.” In making this decision, the length of the coaptation zone is carefully measured, along with the length of excessive leading edge. In general, ≥1 cm of coaptation length must remain after excision to maintain competence, so only the length of the protruding leaflet in excess of the 1 cm of coaptation zone can be considered safe for excision. At times, a portion of the 1-cm coaptation area protrudes into the LVOT, but that area is not safely available for excision as it is necessary for the maintenance of competence. Additionally, excessive chordal tissue must not be confused with residual leaflet, as this may suggest that more tissue can be considered for resection than in actuality. These measurements are best accomplished during TEE just before surgery.
At surgical inspection, once the myectomy is accomplished, visualization of the mitral valve and supporting apparatus is greatly enhanced. If there is a significant residual leaflet, it invariably is attached to a number of loose, curlicued chords that are very myxomatous in appearance and can be identified within the collapsed heart ( Figure 8 ). These chords are too long to prevent prolapse. Nerve hooks are used to lift the anterior leaflet into the LVOT, and the leading edge of A2 is visualized. The section attached to the redundant chordal tissue, usually anywhere from 2–5 mm, can be safely excised as determined by the preoperative echocardiographic measurements. This is performed only if the chordal attachments to the leaflet margin on either side of the residual leaflet are deemed competent enough to prevent leaflet flail when the redundant tip is excised. Competence results from a combination of maintenance of appropriate supporting chords and an adequate coaptation zone; both are necessary. This approach is attractive because it addresses directly an aspect of the primary pathology, which is the protruding residual leaflet that is caught by the flow and actually comes into apposition with the septum.
Edge-to-edge repair also has a place in the surgical management of obstruction in HCM, most notably in patients with minimal septal thickness. We usually restrict this technique to older patients presenting for surgical management, as we are uncertain as to the long-term effects of this repair over a period of decades. In situations in which the body of the leaflet does not lend itself to plication or the leading edge to excision, edge-to-edge repair can be useful, either through the aortotomy or a counter incision in the left atrium. Although we have done both, we prefer the left atrial approach, which allows the additional placement of an oversized mitral ring to stabilize the edge-to-edge suture. The need for the supporting ring remains controversial. Quite rarely, when overzealous residual leaflet excision has occurred with subsequent central mitral insufficiency, edge-to-edge repair has been an effective solution.
Subvalvular Apparatus
A thorough analysis of the subvalvular structures—the papillary muscles and chordae tendineae—is vital for determining the surgical plan.
Papillary Muscles
Patients with HCM may have abnormalities of the papillary muscles that significantly contribute to obstruction. The two most common features noted are (1) anterior and basal displacement of the anterolateral papillary muscle and (2) abnormal attachments of the anterolateral papillary muscle to the anterolateral wall near the A1 scallop ( Figure 9 ). Both of these abnormalities predispose to SAM by dint of their anterior displacement of the mitral valve, by increasing the overlap between outflow and the mitral valve. Flow strikes the anteriorly displaced mitral leaflets from behind and sweeps them toward the septum. In addition, anterior displacement of the papillary muscles decreases the posterior restraint on the leaflets predisposing to the slack that allows SAM.
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