4.4: Age Changes in the Heart
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- 83986
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Though aging causes several changes in the heart, these age changes do not result in an alteration in cardiac output when a person is at rest. This is the case because the changes are slight and because adjustments that compensate for detrimental changes occur. These adjustments include changes in the atria and the myocardium that increase heart strength and increases in blood levels of norepinephrine, which stimulates the heart.
Cardiac Adaptability
As aging occurs, changes occur in the way the heart adjusts CO to meet the varying demands of the body. However, as with resting conditions, the changes are not very great, and most detrimental changes in the heart are overshadowed by compensatory changes. For example, the heart compensates for an age-related decrease in maximum heart rate by increasing the amount it pumps per beat. Therefore, the maximum cardiac output which can be achieved when a person is exercising as vigorously as possible (cardiac reserve capacity) remains essentially unchanged. The compensatory change that seems most important involves norepinephrine: As age increases, its blood level rises faster and reaches a higher peak value after vigorous activity begins.
An adverse age change in the heart for which there is no compensatory adjustment is an increase in the amount of blood remaining in the left ventricle after contraction. This residual blood causes a slight inhibition of blood flow from the lungs, resulting in an accumulation of blood in the lungs - pulmonary congestion - which raises the blood pressure in lung capillaries and forces extra fluid out through the capillary walls. This fluid accumulation (pulmonary edema) reduces respiratory functioning and causes people to feel out of breath sooner and more intensely when they exercise strenuously.
Another important age change involves the declining efficiency of the heart. A stiffer, dilated and thickened older heart consumes more oxygen to pump the same amount of blood pumped by a younger heart. This is not important as long as the coronary arteries remain completely normal because these arteries widen and allow blood to flow adequately when the heart needs more oxygen. However, most people do not have completely normal coronary arteries. In these cases, the decreased efficiency of the heart and the resultant increased demand for oxygen can become serious. In fact, individuals who show even slightly low coronary blood flow when exercising are very likely to have a heart attack.
In summary, because there are both positive and negative age changes in the normal heart, its ability to adjust the pumping of blood to supply the varying needs of the body remains essentially unchanged. However, the maximum rate of exercise people can perform normally declines with advancing age. This chapter and Chaps. 5, 6, 8, and 9 will show how this is due to a variety of factors outside the heart, including age changes in other parts of the circulatory system and in the respiratory, nervous, muscle, and skeletal systems. The maximum rate of physical activity decreases even more when diseases of the heart, blood vessels, or other body systems are present.
Exercise and the Aging Heart
Though the effects of initiating exercise programs on the hearts of younger people have been well studied, only a few of the effects of training on the older heart have been elucidated. Results for elders are quite variable among studies and among individuals, and depend heavily on the nature of the exercise program (e.g., type, intensity, frequency of exercise, duration of each exercise session, duration of entire program). For the older heart, these effects include no change in the maximum heart rate attainable. Some exercise programs produce a decrease in the resting heart rate, decreases in the maximum heart rate required for maximum activity, and increases in stroke volume and cardiac efficiency. However, it is expected that other beneficial effects of regular exercise on the older heart will be discovered. We will see later in this chapter that regular exercise has beneficial effects on other parts of the circulatory system.
To affect an older heart, an exercise program must involve fairly vigorous exercise performed for an extended period during each session. The sessions must occur frequently, at least once every few days. The degree of improvement is proportional to the intensity, duration, and frequency of activity. Exercise at a low level, performed for a short time, or conducted infrequently has no effect on the aged heart. Beneficial changes in cardiac output can be observed within days to a few weeks after beginning an exercise program, and regressive detrimental changes in CO occur within the same time frame when a person ends the program.