Hypertrophic cardiomyopathy outflow obstruction

Editors-In-Chief: C. Michael Gibson, M.S., M.D. [mailto:mgibson@perfuse.org]

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

Classification of the Valve Gradient in Hypertrophic Cardiomyopathy
The valve gradient in HCM can be classified into three categories:


 * 1) A gradient greater than 30 mm Mercury under basal conditions
 * 2) A gradient that is greater than 30 mm Mercury with provocation
 * 3) A gradient that is less than 30 mm Mercury at rest and with provocation

Maneuvers that Increase the Outflow Gradient

 * Amyl nitrite inhalation
 * Valsalva maneuver
 * Premature ventricular contractions (PVCs)
 * Isoproterenol infusion
 * Dobutamine infusion; but this is not recommended as a diagnostic tool
 * Treadmill or exercise stress testing

Causes of Left Ventricular Outflow Obstruction: Systolic Anterior Motion of the Mitral Valve (SAM)
If dynamic outflow obstruction is present in a patient with HCM, it 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 a subaortic bulge of the septum along with narrowing the left ventricular outflow tract, which taken together cause high velocity flow. This in turn is associated with the Venturi effect which is 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. More recently, however, SAM onset has been observed to be instead a low velocity phenomenon. . The role of Venturi forces in the left ventricular outflow tract may be less important than previously thought. While the Venturi effect was thought to cause the abnormality in prior studies, more recent echocardiographic studies indicates that drag, which is more of a pushing force rather than a sucking force like the Venturi effect, may be the dominant hydrodynamic force acting on the mitral leaflets .

The videos below show examples of systolic anterior motion of the mitral valve:





Impact of Systolic Anterior Motion of the Mitral Valve: The Spike and Dome Pattern to the Carotid Pulse
Because the mitral valve leaflet doesn't get pulled into the left ventricular outflow tract (LVOT) until after the aortic valve opens, the initial upstroke of the arterial pulse pressure will be normal. When the mitral valve leaflet gets pushed into the LVOT, the arterial pulse will momentarily collapse and will later be followed by a second rise in the pulse pressure, as the left ventricular pressure overcomes the increased obstruction caused by the SAM of the mitral valve. 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 or a "spike and dome pattern" to the carotid pulse.

Accompanying Mitral Regurgitation
As a result of the drag effect or the Venturi effect, there may be mild to moderate mitral regurgitation in association with hypertrophic cardiomyopathy. Most often the mitral regurgitation jet is directed posteriorly. If the jet is not directed posteriorly then other diagnoses should be considered which include myxomatous degeneration or other anomalies of the mitral valve.

Pathophysiologic Consequences of Outflow Obstruction
Chronic outflow obstruction and result in the following abnormalities :


 * Increased left ventricular wall stress


 * Myocardial ischemia


 * Myocardial necrosis


 * Replacement fibrosis

Prognostic Significance of Outflow Obstruction
The presence of outflow obstruction is associated with a twofold increased risk of death and a 4.4 fold increase in the risk of progression to New York Heart Association class III or IV heart failure. Above a gradient of 30 mm Hg, there was no further increase in the risk of sudden cardiac death or progression of congestive heart failure symptoms.