Regional function of the heart

The assessment of regional function of the heart is a powerful tool for the detection of deterioration of a certain parts of the heart wall early on and before it diffuses further. One of the most accurate measures of changes in regional function is the use of strain as a practical, sensitive, and accurate measure of regional function of cardiac muscle.

One of the most important indicators of heart problems is described as weakening of the cardiac function. This means that the heart’s ability to pump blood to the body is diminished. Before this becomes a problem for other organs of the body, it is beneficial to detect early progress of the disease in the heart wall muscle itself. This can be detected, when the proper device is used, as weakening of the contractility of some part of the heart muscle. Hence, it is described as change in regional function. It is important to note that the weakening of regional function does not always lead to weakening in overall cardiac function. This will depend on a number of factors, including the degree of regional weakening and its extent inside the wall.

The myocytes of the heart (also called the myocardial fibers) are arranged in a general circumferential direction in the ventricles. In the LV, the fibers will change gradually in direction from a certain longitudinal-cirumferential direction in the outer layer of the heart (epicardium) to another angulated direction almost orthogonal in the inner wall (endocardium), becoming overwhelmingly circumferential somewhere halfway in the middle of the wall. It is, therefore, perceived that measuring the strain in the circumferential direction is a good indicator of the contractility of the fibers.

There are different ways to measure the regional function of the wall. It has been proposed to measure the speed of the LV wall motion, the thickening of the wall, or other changes in the shape of small regions of the wall as it contracts and relaxes. The latter is best measured using the mechanical quantity called “strain." Strain can be described as the percentile change in spacing between two points on a deforming object. Here are some important points about strain:

Because strain is measured between two points, that strain, therefore, describes the change in distance along the direction connecting the two points. If we think of a rubber band that is stretched, the strain along the band will have a positive value for stretching; i.e. when selecting two points placed along the band length. At the same time, the band width will decrease, resulting in a negative strain orthogonal to the band's length. In the case of the heart, it has become conventional to use certain directions for measuring strain:

This is the strain measured along the thickness of the wall. In fact, it measures the wall thickening. In a normal heart, the radial strain is positive as measured from end-diastole.

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