The existence of dynamic left ventricular outflow tract obstruction

There had been considerable debate, prominently featured at the annual sessions of the American Heart Association several decades ago, on whether there is true obstruction as opposed to systolic cavity obliteration.3 4 5 6 Today, we recognise that both situations can exist, but there are clearly many patients who have true obstruction in the left ventricular outflow tract with an open, non-emptied residual ventricular cavity. This obstruction is highly dynamic in that the severity of the obstruction is dramatically impacted by simple haemodynamic manoeuvres such as changing posture or even eating meals. Likewise, simple bedside manoeuvres can significantly change the auscultatory and Doppler echocardiographic findings. Ultimately, the impact of this obstruction for individual patients can be highly variable with some requiring no symptom-directed treatment, while others come to need surgical intervention in order to maintain an acceptable quality of life.7

Proposed mechanisms of outflow tract obstruction

Our understanding of the functional and anatomical determinants of dynamic left ventricular outflow tract obstruction has certainly evolved over time. The first patients sent for cardiac surgery on the basis of symptoms, physical examination findings and haemodynamic studies were felt preoperatively to represent patients with some sort of subvalvular ridge or membrane. Much to the early investigators’ consternation, the surgeon did not find such a subvalvular abnormality.2 The initial surgical series described the surgeons’ viewpoint that upon inserting their finger into the outflow tract obstruction it felt as if there were a muscular sphincter that constricted the outflow tract in systole.8 9 10 It was on this basis that a myotomy (based on pyloric sphincterotomy) was proposed as a potential mechanism for relieving the outflow tract obstruction. The notion of the muscular sphincter was eventually disproved.

M-mode echocardiography and ventriculography provided insight into the fact that the mitral valve had an abnormal motion in systole such that it moved anteriorly to contact the basal septum and significantly narrow the left ventricular outflow tract.6 11 This systolic anterior motion of the mitral valve has persisted as one of the hallmark features of obstructive hypertrophic cardiomyopathy. However, the mechanisms of systolic anterior motion defied explanation for some time.

It had been proposed that the rapidly ejecting blood (from the hyperdynamic systolic function felt to be present in hypertrophic cardiomyopathy) resulted in suction of the mitral valve toward the outflow tract.12 These Venturi forces were the leading haemodynamic hypothesis for the mechanism of systolic anterior motion for years.

Careful analysis of two-dimensional echocardiographic images showed, however, that there were more likely pushing and drag forces on the mitral valve similar to that observed with airflow over the wings of an airplane.13 14 Specifically, because of the septal hypertrophy, the direction of flow from the ventricular cavity to the outflow tract was forced to change from a course that is parallel to the mitral valve to one which flowed across the mitral valve, pushing it into the outflow tract and thereby resulting in a phenomenon that is characterised by the sequence of eject–obstruct–leak. In this description it is the anatomical feature of septal hypertrophy that results in an alteration in the flow pattern through the ventricular cavity that then secondarily impacts on function of the mitral valve.

Further assessment of the mitral valve has also disclosed several important features which appear to be important in the generation of outflow tract obstruction. First, it is recognised that in many patients with hypertrophic cardiomyopathy the mitral valve and, in particular, the anterior mitral leaflet is enlarged and elongated giving it a larger surface area on which the abnormal flow vectors can impact. Additionally, there is often anterior displacement of the papillary muscles and other mitral valve support apparatus that positions the mitral valve and its coaptation point more anteriorly in the ventricle, moving it closer to the septum than is seen in hearts that are not affected by hypertrophic cardiomyopathy.15 16 Here the combination of an anteriorly displaced mitral valve with the abnormal flow vectors results in a favourable environment for dynamic outflow tract obstruction.

Cardiac surgeons and cardiac imaging specialists have come to appreciate that there are other less common abnormalities in the mitral valve which impact some patients. These abnormalities include unusual chordal attachments from the mitral valve directly to the anterior septum, hypertrophied papillary muscles and the direct insertion of papillary muscles onto the anterior mitral leaflet (absence of chordae tendineae), all of which can crowd the outflow tract.17

In this issue of Heart there is a description of another potential mechanism contributing to dynamic outflow tract obstruction, specifically, more severe angulation of the long axis of the left ventricle to the main axis of the aorta (see article page 1784).18 The authors propose that such acute angulation results in increased local stress at the basal septum which may represent stimulus for basal septal hypertrophy in a pattern that has previously been described as sigmoid septal hypertrophy or hypertrophic cardiomyopathy of the elderly. Such basal septal hypertrophy might theoretically result in abnormal flow vectors from the mid-ventricle toward the base directing flow more obliquely across the mitral valve leaflets, which also foster the systolic anterior motion of the mitral valve required to produce dynamic outflow tract obstruction. What is not clear is the actual aetiology of this angulation. Is it related to genetic abnormalities or genetic polymorphism that results in an abnormal position of the heart? Is it due to simple stature or thoracic cavity anatomy of individual patients?

How should clinicians approach suspected dynamic outflow tract obstruction?

From a practical standpoint, clinicians must be certain that true obstruction is present before they can direct appropriate pharmacological treatment at the functional components of outflow tract obstruction. The constellation of a dynamic murmur, systolic anterior motion of the mitral valve with an open ventricular chamber and a late-peaking Doppler echocardiographic signal needs to be demonstrated either under resting conditions or with physiological provocation. If these features can be proved, then treatments that optimise loading conditions in order to minimise the tendency to outflow tract obstruction can be successful in most patients. Specifically, decreasing left ventricular contractility, maximising diastolic filling and avoiding dramatic afterload reduction can significantly decrease a patient’s tendency to outflow tract obstruction. However, the anatomy may be sufficiently abnormal in some patients to render such haemodynamic manipulations unsuccessful unless the anatomy can be revised. Here, there is a role for procedures such as surgical septal myectomy, which can debulk the septal hypertrophy, normalise blood flow vectors and deal with some of the abnormalities of the mitral valve.

The 50 years of investigation into the dynamic obstruction of hypertrophic cardiomyopathy have provided us with many wonderful insights into intracardiac haemodynamics, led us to academic debate and have improved our delivery of care for patients with hypertrophic cardiomyopathy.7 Septal hypertrophy, abnormal mitral valve anatomy, angulations and abnormal flow vectors, in various relative combinations, resulting in outflow obstruction, can impact the quality of life. Understanding these predisposing features in order to direct treatments accordingly will ultimately serve the needs of our patients.