Organ Systems Physiological Changes: Cardiovascular
Heart Structure And Function At Rest
Changes in heart size and shape with advancing age. The left ventricle (LV) is the largest heart chamber in terms of muscle mass and pumps blood, under pressure, to the entire body (see Figure 2, Panels A and B). Studies of volunteer subjects without cardiovascular disease, indicate that, at rest, the LV cavity size at enddiastole (filled) and end-systole (emptied), increases moderately with age in healthy, normotensive, sedentary men, but does not vary with age in women. The LV wall thickness increases progressively with age in both sexes. The age-associated increase in left ventricular wall thickness is caused mostly by an increase in the average size of cardiac myocytes (muscle cells). An increase in the amount of, and a change in the physical properties of, collagen (a protein that holds the myocytes together) also occurs within the myocardium with aging. In summary, the heart of a normal healthy older individual is somewhat larger, and slightly stiffer, than that of a younger person.
There is an increase in elastic and collagenous tissue in all parts of the heart's conduction system with advancing age (see Figure 2C). Fat accumulates around the sinoatrial (SA) node, sometimes producing a partial or complete separation of the node from the atrial musculature. Beginning by age sixty, there is a pronounced decrease in the number of pacemaker cells in the SA node, and by age seventy-five less than 10 percent of the pacemaker-cell number found in the young adult remains. A variable degree of calcification of the left side of the cardiac skeleton, which includes the aortic and mitral annuli, the central fibrous body, and the summit of the interventricular septum, occurs with aging. Because of their proximity to these structures, the atrioventricular (AV) node, AV bundle, bifurcation, and proximal left and right bundle branches may be affected by this process. These structural changes in the cardiac conduction system are associated with a number of functional changes that can be observed in the electrocardiogram of an elderly person.
Age-associated changes in heart function at rest. When a person is in the sitting position, the resting heart rate decreases slightly with age (in both men and women). Tiny beat-to-beat variations in resting heart rate are normal, and become diminished with advancing age. This decreased heart-rate variability is likely related to changes in the parasympathetic and sympathetic nervous systems as an age. The P-R interval is the time it takes for the signal that initiates the heartbeat to travel from the conduction system in the heart (the AV node and the AV bundle). A modest prolongation of the P-R interval (see Figure 2D) of the electrocardiogram occurs with aging in healthy individuals, and is localized to the proximal P-R interval, probably reflecting delay within the atrioventricular junction. An increase in the number of premature beats occurs in healthy older men and women compared to their younger counterparts. These changes in the regulation of the heartbeat by the nervous system and the cardiac conduction system are observed in normal healthy individuals as they age, and although they do not by themselves interfere with heart function, they are associated with an increased risk of adverse heart events. More severe changes in the cardiac conduction system are associated with diseases and are often treated by implanting an artificial pacemaker.
The peak rate at which the LV fills with blood during early diastole is reduced by 50 percent between the ages of twenty and eighty. The time course of early myocardial relaxation becomes prolonged by 40 percent with adult aging in both men and women, probably due to alterations in LV wall structure and collagen properties and changes in the relaxation phase of cardiac muscle contraction. Different parts of the ventricle relax at different rates in hearts of older individuals, and this contributes to the reduction in the filling rate. The age-associated reduction in the early filling rate does not result in a reduced end-diastolic volume in healthy older individuals, because greater filling occurs later in diastole, particularly during the atrial contraction. The enhanced atrial contribution to ventricular filling with advancing age is associated with left atrial enlargement, and a more forceful atrial contraction. So, although ventricular filling in a resting older person is accomplished differently, it is as adequate as ventricular filling in a younger person.
The contractility, or strength, of heart muscle contraction is not reduced at rest with age in either healthy men or women. The LV ejection fraction (EF) is also not altered with aging in healthy men or women at rest. The stroke volume index (SVI; amount of blood pumped per beat/ body surface area) is increased in males, due to a slight increase in the LV end-diastolic volume. Thus, in healthy older men, the cardiac output (amount of blood pumped per minute) is not reduced, because the stroke volume index is increased, due to end-diastolic enlargement Cardiac output at rest is slightly decreased in older women (compared to younger healthy women) as neither the resting end-diastolic volume (EDV) nor SV increases with age to compensate for the modest reduction in heart rate. These gender differences appear to be an artificial effect of differences in body composition—the proportion of body fat increases with age in women to a greater extent than in men. If one compares the younger and older heart at any point during the resting heartbeat, the older heart contains more blood. Therefore, at rest, the heart of an older healthy individual pumps blood as well as the heart of a younger person, but does so in a slightly different manner.
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