Hypertrophic cardiomyopathy outflow obstruction
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Editors-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Depending on the degree of obstruction of the outflow of blood from the left ventricle of the heart, HCM can be defined as obstructive or non-obstructive. About 25% of individuals with HCM demonstrate an obstruction to the outflow of blood from the left ventricle during rest. In other individuals obstruction only occurs under certain conditions. This is known as dynamic outflow obstruction, because the degree of obstruction is variable and is dependent on the amount of blood in the ventricle immediately before ventricle systole (contraction).
Location Of The Left Ventricular Outflow Obstruction
The left ventricular obstruction can be either
- Mid-cavitary: the middle of the ventricle or
- Sub-aortic: just below the aortic valve
Systolic Anterior Motion of the Mitral Valve (SAM)
Dynamic outflow obstruction (when present in HCM) is usually due to systolic anterior motion (SAM) of the anterior leaflet of the mitral valve. Systolic anterior motion of the mitral valve (SAM) may be due to the septal subaortic bulge, narrowing the outflow tract, which in turn causes a high velocity flow and a Venturi effect — a local low pressure zone in the left ventricular outflow tract. This low pressure zone was thought to suck the mitral valve anteriorly into the septum. But SAM onset is observed to be a low velocity phenomenon: SAM begins at velocities no different from those measured in normals [1] [2]. Hence, the magnitude and importance of Venturi forces in the outflow tract are may be less than previously thought, and Venturi forces may not be the main forces that initiates SAM.
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Recent echocardiographic evidence indicates that drag, the pushing force of flow is the dominant hydrodynamic force on the mitral leaflets [1] [2] [3] [4] [5] [6]. In obstructive HCM the mitral leaflets are often large [7] and are anteriorly positioned in the LV cavity [1] [8] due to anteriorly positioned papillary muscles[1] that at surgery are often "agglutinated" onto the LV anterior wall by abnormal attachments [5] [6].
The mid-septal bulge aggravates the malposition of the valve and redirects outflow so that it comes from a lateral and posterior direction[3]. The abnormally directed outflow may be visualized behind and lateral to the enlarged mitral valve, where it catches it, and pushes it into the septum [1] [2] [3] [4]. There is a crucial overlap between the inflow and outflow portions of the left ventricle [9]. As SAM progresses in early systole the angle between outflow and the protruding mitral leaflet increases. A greater surface area of the leaflets is now exposed to drag which amplifies the force on the leaflets – drag increases with increasing angle relative to flow[3]. An analogy is an open door in a drafty corridor: the door starts by moving slowly and then accelerates as it presents a greater surface area to the wind and finally it slams shut. The necessary conditions that predispose to SAM are: anterior position of the mitral valve in the LV, altered LV geometry that allows flow to strike the mitral valve from behind, and chordal slack [1] [2] [3] [4]. SAM may considered anteriorly directed mitral prolapse [2] [3] [4]. In both conditions the mitral valve is enlarged and is displaced in systole by the pushing force of flow resulting in mitral regurgitation.
Because the mitral valve leaflet doesn't get pulled into the LVOT until after the aortic valve opens, the initial upstroke of the arterial pulse will be normal. When the mitral valve leaflet gets pushed into the LVOT, the arterial pulse will momentarily collapse and be followed by a second rise, as the left ventricular pressure overcomes the increased obstruction that SAM of the mitral valve causes. This can be seen on the physical examination as a double tap upon palpation of the apical impulse and as a double pulsation upon palpation of the carotid pulse, known as pulsus bisferiens.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Jiang L, Levine RA, King ME, Weyman AE. An integrated mechanism for systolic anterior motion of the mitral valve in hypertrophic cardiomyopathy based on echocardiographic observations. Am Heart J 1987; 113:633–44
- ↑ 2.0 2.1 2.2 2.3 2.4 Sherrid MV, Gunsburg DZ, Moldenhauer S, Pearle G. Systolic anterior motion begins at low left ventricular outflow tract velocity in obstructive hypertrophic cardiomyopathy. J Am Coll Cardiol 2000; 36:1344–54
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Sherrid MV, Chu Ck, DeLia E, Mogtader A, Dwyer Jr. EM, An echocardiographic study of the fluid mechanics of obstruction in hypertrophic cardiomyopathy. J Am Coll Cardiol 1993; 22:816–25
- ↑ 4.0 4.1 4.2 4.3 Levine RA, Vlahakes GJ, Lefebvre X, et al. Papillary muscle displacement causes systolic anterior motion of the mitral valve. Circulation 1995; 91:1189–95
- ↑ 5.0 5.1 Messmer BJ. Extended myectomy for hypertrophic obstructive cardiomyopathy. Ann Thorac Surg 1994; 58:575–7
- ↑ 6.0 6.1 Schoendube FA, Klues HG, Reith S, Flachskampf FA, Hanrath P, Messmer BJ. Long-term clinical and echocardiographic follow-up after surgical correction of hypertrophic obstructive cardiomyopathy with extended myectomy and reconstruction of the subvalvular mitral apparatus. Circulation 1995; 92:II-122–7
- ↑ Klues HG, Maron BJ, Dollar AL, Roberts WC. Diverstiy of structural mitral valve alterations in hypertrophic cardiomyopathy. Circulation 1992; 85:1651–60
- ↑ Henry WL, Clark CE, Griffith JM, Epstein SE. Mechanism of left ventricular outflow obstruction in patients with obstructive asymmetric septal hypertrophy (idiopathic hypertrophic subaortic stenosis). Am J Cardiol 1975; 35:337–45
- ↑ Schwammenthal E, Levine RA. Dynamic subaortic obstruction: a disease of the mitral valve suitable for surgical repair? J Am Coll Cardiol 1996; 28:203–6