Hymel, Brad J. MD; Townsley, Matthew M. MD
A 33-year-old woman with hypertrophic cardiomyopathy (HCM) presents for a septal myectomy. The preoperative transthoracic echocardiogram shows normal left ventricular (LV) function, severe hypertrophy of the interventricular septum, and systolic anterior motion (SAM) of the anterior mitral valve leaflet (AMVL) with moderate mitral regurgitation (MR). The resting peak gradient through the LV outflow tract (LVOT) is elevated at 67 mm·Hg.
SAM is defined as the anterior translation of 1 or both mitral valve (MV) leaflets into the LVOT during systole.1 The extent of this translation is variable and leads to differing degrees of SAM, ranging from no hindrance to blood flow to profound LVOT obstruction (LVOTO) and cardiovascular collapse. This variability results from the dynamic nature of SAM, because its presence and severity depend on the loading conditions and contractile state of the heart.2 Decreased LV end-diastolic volume and systemic vascular resistance, as well as increased LV contractility and chronotropy can all precipitate or exacerbate SAM. The clinical significance of SAM depends on the degree of LVOTO, which corresponds to the onset, extent, and duration of mitral-septal contact.2 The treatment of hemodynamically significant SAM focuses on alleviating LVOTO. While SAM is a common cause of LVOTO, there are also other potential etiologies (Table 1).
SAM in HCM-Pathophysiology
HCM is a genetic disorder of LV hypertrophy with no identifiable cause (i.e., aortic stenosis, chronic hypertension). The basal septal wall is often severely hypertrophied, leading to narrowing of the LVOT, whose borders include the septum and the AMVL. Systolic septal thickening occurring during LV contraction further diminishes the outflow tract dimension, resulting in elevated LVOT blood flow velocities.
Hypertrophied, anteriorly displaced papillary muscles and elongated MV leaflets shift the mitral apparatus toward the enlarged septum. With this shift, the posterior MV leaflet (PMVL) now coapts closer to the base of the AMVL, resulting in excess slack AMVL tissue protruding beyond the coaptation point. High-velocity flow can then lift the anteriorly displaced, slack MV leaflet into the LVOT in a phenomenon known as the Venturi effect, the initial proposed SAM mechanism.3
More recent evidence, however, proposes a drag force as the more predominant SAM mechanism. This suggests that the slack portion of the MV leaflet is swept into the LVOT and toward the septum by high-velocity LVOT flow. The narrowed mitral-aortic angle, created by septal hypertrophy and structural abnormalities of the mitral apparatus, aligns the MV leaflet in the direction of this flow, subjecting it to the drag force.1,3
During SAM, as the distal AMVL remains in the LVOT during systole, adequate mitral coaptation is inhibited, resulting in a posteriorly directed jet of MR. A channel created by the distal portions of both MV leaflets directs the regurgitant flow posteriorly through this opening.4 The MR occurs in mid-to-late systole after the onset of SAM and LVOTO. Hemodynamic management focuses on alleviating the LVOTO, because this will lead to resolution or improvement of the MR.
SAM in HCM-Echocardiographic Assessment
To assess for the presence of SAM in patients with HCM, the comprehensive transesophageal echocardiographic examination (TEE) begins with focused 2-dimensional (2D) imaging in either a midesophageal long-axis (ME LAX) or ME 5-chamber view. The nonstandard ME 5-chamber view, a variation of the more posteriorly directed ME 4-chamber view, is obtained by slight withdrawal or anteflexion of the probe from a ME 4-chamber view until the LVOT (“fifth chamber”) and aortic valve (AV) are seen in the center of the screen. The anterolateral portion of the MV is seen in this view.
In either view, basal septal hypertrophy, often >2.0 cm (normal 0.6–0.9 cm, severe hypertrophy ≥1.7 cm), is assessed and measured (Fig. 1).5 Measurement is performed at end-diastole to exclude the effects of systolic thickening. When planning for septal myectomy, the distance from the aortic annulus to the point of maximum septal thickness is measured to guide the surgical approach.1,2 The LVOT diameter and distance from the mitral coaptation point to the septum (C-sept distance) are also measured, both at the onset of systole. A narrow LVOT (≤2.0 cm) and short C-sept distance (<2.5 cm) both increase the likelihood of SAM.6,7 The angle formed by the intersection of the MV annulus and AV annulus (mitral-aortic angle) is measured, and if <120° is also associated with an increased risk for SAM.8
SAM is observed as the distal AMVL is displaced into the LVOT during systole (Video 1, see Supplemental Digital Content 1, http://links.lww.com/AA/A879). Applying color flow Doppler reveals aliasing in the LVOT, indicative of high-velocity, turbulent flow, followed by a posteriorly directed MR jet (Video 2, see Supplemental Digital Content 2, http://links.lww.com/AA/A880). If the jet is not posteriorly directed, then examination for other contributing causes of MR is necessary.
In the absence of structural MV disease, the MR associated with SAM is dynamic. The severity of MR varies with the degree of SAM and the magnitude of the pressure gradient in the LVOT. The eccentric nature of the MR, particularly for wall-hugging jets, may lead to underestimation of jet severity based on the size of the color flow Doppler jet area and unreliable estimates of severity based on proximal isovelocity surface area methods. Vena contracta can be used to estimate MR severity in this setting. Systolic flow reversal, seen on Doppler analysis of pulmonary vein flow, is specific, but not sensitive, for severe MR (with the left-sided pulmonary veins most likely affected by the posteriorly directed MR jet). Reversed systolic flow may also occur with the loss of sinus rhythm and normal atrial contraction (i.e., atrial fibrillation).
When SAM results in LVOTO, stroke volume is reduced, leading to abrupt midsystolic closure of the AV leaflets. This is seen as fluttering of the AV leaflets in a ME LAX view. M-mode directed perpendicularly to the AV also demonstrates early closure of the AV leaflets (Video 3, see Supplemental Digital Content 3, http://links.lww.com/AA/A881). To further evaluate LVOTO, either a deep transgastric LAX or transgastric LAX view is obtained. In either view, blood flow is moving away from the probe through the LVOT. Continuous-wave Doppler is aligned parallel to flow to measure the peak systolic velocity and pressure gradient through the LVOT (Fig. 2). With dynamic LVOTO, the spectral display consists of a high-velocity envelope in the shape of a dagger. The delayed peak of this “dagger-shaped” profile occurs in mid-to-late systole, as opposed to the more homogenous, parabolic profile seen with fixed obstruction (i.e., aortic stenosis). A resting peak gradient ≥30 mm·Hg and/or a provoked gradient ≥50 mm·Hg is considered significant.1
Three-dimensional imaging demonstrates the spatial relationship of the structures involved in SAM and is useful for visualizing the location and extent of LVOT occlusion by the AMVL (Video 4, see Supplemental Digital Content 4, http://links.lww.com/AA/A882). Since SAM is such a rapidly occurring event, however, there are limitations imposed by the relatively slow frame rate associated with 3D imaging.
SAM Following Mitral Valve Repair-Pathophysiology
SAM after MV repair occurs in up to 8.4% of cases.9 As in HCM, the risk for post MV repair SAM escalates with an anteriorly displaced mitral coaptation point and/or an increased amount of slack MV leaflet tissue able to migrate into the LVOT. Alterations in the mitral apparatus from the repair itself have been shown to shift the coaptation point toward the septum.10 This is particularly true with an undersized, complete annuloplasty ring, as it may lead to anterior motion of the posterior/inferior LV wall during systole that narrows the mitral-aortic angle.11
A long AMVL or PMVL may contribute to the development of SAM. A long PMVL tends to coapt near the base of the AMVL, which displaces the coaptation point toward the septum and leaves excess slack AMVL tissue distal to the coaptation point.7 A long AMVL may also result in an increase in residual slack leaflet tissue vulnerable to entering the LVOT after MV repair.7
SAM Following Mitral Valve Repair-Echocardiographic Assessment
TEE evaluation before MV repair is important since the identification of echocardiographic predictors for SAM following repair may alter the surgical approach. MV leaflet lengths are measured, from the annulus to the leaflet tip, in either a ME LAX or ME 5-chamber view at the onset of ventricular ejection.7,8 A prerepair PMVL length >1.5 cm is associated with an increased risk for SAM.8 The ratio of AMVL to PMVL length is calculated to assess the contribution of each leaflet to mitral coaptation. A relatively greater contribution of the PMVL, expressed by a AMVL:PMVL ratio of <1.3, increases the likelihood of post repair SAM. Other predictors of after MV repair SAM include a C-sept distance <2.5 cm, reduced mitral-aortic angle (<120°), long AMVL (>2.0 cm), and enlarged interventricular septum (≥1.5 cm).7,8
The echocardiographic measurements discussed are summarized in Table 2 and illustrated in Figure 3.
Management of Intraoperative SAM
Although SAM following MV repair is a potentially devastating complication, it may be medically managed in most cases, underscoring the importance of intraoperative transesophageal echocardiography in both diagnosing SAM, as well as monitoring the efficacy of immediate treatment. Evidence supports a conservative, nonsurgical approach (i.e., manipulation of loading conditions, avoidance of inotropic drugs, and β blockade) to managing post MV repair SAM, as opposed to immediate surgical intervention.9,12 The success of this treatment approach is evaluated based on reduction of the LVOTO (i.e., peak pressure gradient <30 mm·Hg) and diminished severity of the associated MR jet (i.e., less than mild MR).9,12
Key Teaching Points
* While systolic anterior motion (SAM) of the mitral valve (MV) is often associated with left ventricular outflow tract obstruction (LVOTO) and hemodynamic instability, it may also exist without LVOTO. Because SAM is a dynamic process, its severity varies based on changes in cardiac loading conditions, contractility, and chronotropy.
* Anterior displacement of the MV coaptation point toward the septum is a common theme in the development of SAM. The distance from the coaptation point to the septum (C-sept distance) can be measured and if <2.5 cm is associated with an increased risk for SAM.
* Other echocardiographic predictors of SAM include LVOT diameter ≤2.0 cm, basal septal hypertrophy >1.5 cm, and a mitral-aortic angle <120°. Before MV repair, a posterior MV leaflet length >1.5 cm and a ratio of anterior:posterior MV leaflet lengths <1.3 predict an increased risk for SAM after repair.
* In most cases, SAM that occurs after MV repair does not require surgical intervention and resolves with a combination of volume loading, discontinuation of inotropic drugs, increasing systemic vascular resistance, and β blockade.
* Treatment focuses on measures to reduce LVOTO and the peak pressure gradient through the LVOT, ideally to <30 mmHg. As LVOTO is alleviated, so too should the severity of the associated mitral regurgitation to less than mild.
Name: Brad J. Hymel, MD.
Contribution: This author helped write the manuscript.
Attestation: Brad J. Hymel approved the final manuscript.
Name: Matthew M. Townsley, MD.
Contribution: This author helped write the manuscript.
Attestation: Matthew M. Townsley approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
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