How Does Hypertrophic Cardiomyopathy Alter the Heart?

The Normal Heart

In order to understand the abnormalities in HCM, it is first helpful to be familiar with the structure and function of the normal heart. A normal heart has four chambers (left and right ventricles as the lower chambers; left and right atria as the upper chambers) and four valves (mitral and tricuspid; aortic and pulmonary). The walls of the heart are composed of heart muscle cells, as well as collagen and small veins and arteries (called venules and arterioles, respectively). The left ventricular wall is normally about three times thicker than the wall of the right ventricle. The left ventricular wall is usually of uniform thickness in all areas, and in adults measures 12 millimeters (mm) or less on the echocardiogram (in the relaxation phase, diastole). The normal course of blood flow through the heart is shown below.

It is helpful to be familiar with the structure and function of the normal heart in order to understand the abnormalities in HCM. This drawing shows a normal heart with heart chambers, valves and the direction of blood flow. The walls of the heart are composed of specialized muscle known as the myocardium. This part of the heart is abnormal in HCM. The arrows show the direction of blood flow through the heart. The right atrium receives blood from the body, transfers it to the right ventricle which pumps it into the lungs to receive oxygen. Blood returns to the heart from the lungs into the left atrium and is transferred to the left ventricle which pumps it into the systemic circulation and another cycle.

Every normal heart beat results from an electrical signal starting in the right atrium, and passing down through the conducting system of the heart and into the ventricles. The contraction of the heart follows the same sequence.

The Heart in HCM

In HCM, the left ventricular wall is abnormal by virtue of excessive thickening, while the cavity of the left ventricle is of normal or small size (See figures below). HCM has often been referred to as an "enlarged heart," but is probably more accurately regarded as "thickened" or "muscular." The distribution of this muscle thickening (or hypertrophy) may take many forms and differ greatly from among patients (even among those who are related). The most common structural form of HCM involves the muscle thickening located predominantly in the ventricular septum - the portion of left ventricular wall that divides the right and left chambers of the heart.

Comparison of heart structure in normal and HCM. Note that compared to normal, the HCM heart usually shows thickening of the ventricular septum which is greater than other parts of the left ventricular (LV) wall, whether or not there is obstruction. However, the exact pattern of hypertrophy in HCM can be quite diverse and not limited to that shown here.

In addition, the absolute thickness of the wall may also vary greatly among patients. HCM may reach thicknesses that far exceed any other cardiac disease Ð up to five times normal. The upper limit of normal wall thickness is 10-12 mm; remarkably, some patients may have thicknesses as much as 40 to 60 mm.

A stop-frame photograph from a two-dimensional echocardiogram of a HCM heart with extreme left ventricular wall thickening. Note that the ventricular septum measures 52 mm in thickness which is about 5 times normal.

Usually, the hypertrophy in HCM is described as asymmetric, which means that some parts of the wall are thicker than other parts. It is usually the septum that is thickest, and portions of the left ventricular free wall (those not part of the septum) which is usually thinner. The term "concentric" means that all portions of the wall are of about the same thickness. This form of hypertrophy is uncommon (present in only about two percent of HCM patients), though it is possibly more common in specific patient groups, such as the elderly. Also, in a small proportion of patients (approximately two percent), wall thickening is predominantly at the tip, or apex, of the heart. This form of HCM seems to be more common in Japan, and always occurs without obstruction.

Outflow obstruction, although present in the resting state in only about 20 to 25 percent of HCM patients, may account for symptoms such as shortness of breath, fatigue, chest pain and fainting. The descriptive term, obstruction, often has a strong connotation to patients that often may not be entirely deserved. This word refers to only partial obstruction (or impedance) to the flow of blood from left ventricle to aorta. Obstruction is not necessarily unfavorable to patients and can be tolerated for many years with no or few symptoms or difficulty. On the other hand, in some patients, severe limiting symptoms can be attributed directly to the presence of obstruction.

Adapted from Hypertrophic Cardiomyopathy: For Patients, Their Families, and Interested Physicians, by Barry J. Maron, MD, et al., Copyright 2001, with permission from Futura Media Services, Inc., Armonk, New York