How the heart changes in hypertrophic cardiomyopathy

In the healthy heart shown on the left in this picture, the ventricular septum, separating the left and right ventricles, is relatively thin - approximately 8 mm to 12 mm thick. Blood flows freely into the left ventricle through the mitral valve when the heart relaxes between beats. When the heart muscle contracts, the blood is pumped out through the aortic valve into the aorta, beginning its journey to deliver oxygen and nutrients to the rest of the body.

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In hypertrophic cardiomyopathy, the ventricular septum thickens usually to 20-25 millimeters but sometimes to over 60 millimeters. The thickening primarily occurs inside the heart and can be evenly distributed throughout the heart or limited to a specific region such as in asymmetric septal hypertrophy, shown in the heart on the right in this picture. Other types of hypertrophy include mid-cavity hypertrophy, characterized by thickening in the mid-portion of the heart, or apical hypertrophy, in which thickening is limited to the bottom tip of the heart. Not only does the enlarged muscle occupy space formerly used for blood, but it also does not relax normally, further reducing the space for blood in the left ventricle.

If the myocardium thickens such that the flow of blood through the heart is obstructed, the condition is known as hypertrophic obstructive cardiomyopathy (HOCM). Between 50 and 75 percent of people with HCM may develop an obstruction. In most people obstruction is transient; triggered by any circumstances that lead to decrease in blood pressure or an increase in heart rate such as salt content of food, body position, exertion level, or temperature of the room. In some people, the obstruction is always present, but the degree of obstruction varies over time.

Obstruction is an active process that decreases the space available for blood flow in the left ventricle. This, in turn, decreases the volume of blood flowing in the heart. The heart compensates by pumping harder and faster. In some people, this faster blood flow can force open the mitral valve during muscle contraction, when it should be closed--allowing blood to flow backwards into the left atrium. More importantly, the opened mitral valve can physically block the opening to the aorta, obstructing the flow of oxygenated blood to the rest of the body as shown in Figure 3. The blood flow actually worsens the harder the heart squeezes, because the thickened septum gets even thicker during contraction, protruding even farther into the pathway of the blood. When the flow of blood is obstructed within the heart, the symptoms of heart failure can develop, sometimes suddenly.

In addition to thickening, the muscle fibers of the heart become disorganized in HCM. Normally, the fibers of the heart muscle interlock with one another forming a parallel array. In a hypertrophied heart, the fibers no longer interlock properly, forming a myocardial disarray.