Dynamic Left Ventricular Outflow Obstruction in Hypertrophic Cardiomyopathy Revisited Significance, Pathogenesis and Treatment Mark V. Sherrid, M.D. Hypertrophic Cardiomyopathy Program and Echocardiography Laboratory Division of Cardiology, St. Luke's-Roosevelt Hospital Center Columbia University, College of Physicians and Surgeons, New York, New York Reprinted with permission from Cardiology in Review 1998; 6: 135-145 Lippincott, Williams & Wilkins

Abstract: Systolic anterior motion of the mitral valve and mitral-septal contact is the usual cause of dynamic left ventricular outflow obstruction in hypertrophic cardiomyopathy. That true obstruction actually occurs is now established based on cardiac catherization and echocardiographic evidence. A mid-systolic drop in left ventricular systolic ejection velocity due to obstruction has recently been demonstrated. Echocardiographic data indicates that systolic anterior motion of the mitral valve is initiated by flow drag; the mitral valve is swept toward the septum by the pushing force of flow. After mitral-septal contact, obstruction begets further obstruction as the pressure gradient pushes the mitral valve into the septum. Most symptomatic patients can be treated successfully with negative inotropic drugs. These medications reduce systolic anterior motion and obstruction by decreasing early left ventricular ejection acceleration, decreasing the early systolic pushing force on the protruding mitral leaflet. Patients who do not improve on medication, generally benefit from surgery. Newer interventions to relieve obstruction, dual chamber pacing and percutaneous transluminal septal myocardial ablation (PTSMA) are under active investigation. Hypertrophic cardiomyopathy (HCM) is a condition characterized by thickening of the myocardium without apparent clinical cause. Dynamic obstruction occurs in approximately 25% of HCM patients. Obstruction to left ventricular outflow most often occurs due to systolic anterior motion of the mitral valve and mitral-septal contact. Less commonly an anomalous papillary muscle inserts into the outflow tract causing obstruction (1). Also uncommonly, mid-ventricular obstruction due to systolic apposition of the mid-cavity left ventricular walls can occur (2, 3). This paper will first concentrate on the common form of outflow tract obstruction caused by systolic anterior motion and mitral-septal contact. In the 1970s and 1980s there was considerable debate in the scientific literature as to whether outflow tract obstruction actually occurs in HCM. This debate has now been settled in favor of the presence of obstruction by several observations. First, in patients with obstructive HCM, a significant pressure gradient is found in the catheterization laboratory between catheters situated in the aorta and inflow tract of the left ventricle near the mitral valve (4). This observation resolves the question as to whether the observed pressure difference might be due to catheter entrapment in the left ventricle. Catheter entrapment does not occur in the inflow portion of the left ventricle, near the mitral valve. Second, left ventricular ejection time is prolonged in HCM only when obstruction is present (5). Third, Doppler studies show acceleration of flow velocity at the point of mitral-septal contact in the outflow tract. A gradient can be measured with Doppler across the narrowing (6). Fourth, ejection flow velocity falls in mid-systole both in the aorta and in the left ventricle due to obstruction (7, 8). Figure 1 Upper panel: Pulsed Doppler echocardiogram recorded in the left ventricle 2.5 cm apical of the mitral valve and 1 cm from the septum. Mid-systolic drop in left ventricular systolic ejection velocity is seen, after the arrow. Lower panel: Continuous wave Doppler through the left ventricular outflow tract jet in the same patient. The close temporal relationship between the nadir of the mid-systolic drop in the left ventricle and the peak velocity of the jet is displayed. This close temporal relationship links the cause of the mid-systolic drop to obstruction. Reprinted from Sherrid et al (8) by permission from the Journal of the American Society of Echocardiography 1997;10:707-12 © 1997 Mosby-Year Book Inc. Recently a mid-systolic drop in left ventricular velocity has been observed in the left ventricular cavity itself (8). This unusual flow contour has been termed the lobster claw abnormality, because of its characteristic appearance, shown in figure 1. Such abnormalities are only seen when the measured outflow pressure gradient is greater than about 60 mm Hg. We have not observed this pattern in the left ventricle in non-obstructed HCM. Also, when patients are successfully treated, and outflow obstruction is abolished, the mid-systolic drop is no longer seen. It appears that the mid-systolic drop is due to obstruction; the heart "stutters" in mid-systole due to the sudden imposition of afterload. This abnormality highlights the effect of obstruction on the left ventricle. Patients with obstructive HCM present with symptoms of dyspnea, angina and syncope. The pathophysiology of symptoms in HCM is complex and multifactorial; symptoms develop for a variety of other reasons besides obstruction. These other factors operate in non-obstructed HCM patients and include the hypertrophy itself, diastolic dysfunction, increased diastolic pressure, impaired coronary flow reserve and non-atherosclerotic occlusive disease of the intramural coronary arterioles (9, 10). However, obstruction in addition to these other abnormalities may cause enough symptoms to bring the patient to medical attention. Obstruction, increases systolic left ventricular pressure, systolic wall tension and myocardial work. Coronary perfusion pressure is decreased as aortic diastolic pressure falls and left ventricular diastolic pressure rises. Pacing produced ischemia and anaerobic metabolism is documented (11, 12). Moreover, abnormalities in myocardial metabolism and blood flow are reversed after successful myectomy. (13) In addition, systolic anterior motion causes mitral regurgitation; the functional deformation of the mitral valve causes incomplete coaptation. Syncope and pre-syncope may be caused by arrhythmias but also may be a manifestation of obstruction. Patients will often note pre-syncope after exertion; gradients have been shown to increase after completion of exercise. NEXT PAGE