MODULE SUPPLEMENT: CARDIOVASCULAR SYSTEM
Response to Stress

What happens to the older cardiovascular system in response to stress?

Changes seen with stress, as exemplified by changes that occur during exercise, can help in identifying why older adults are vulnerable to cardiac dysfunction.

Normally with exercise there's an increase in heart rate to meet the increased metabolic demands, and there is a shunting of blood to the active muscles.

The older heart, because it is less responsive to beta-adrenergic stimulation, can't increase its rate as much as a younger heart. However, cardiac output is maintained, at least for a while through the use of adaptive strategies.

First, end diastolic volume increases about 30% through ventricular dilation. Secondary to this increased stretch there's an increase in the Stroke Volume via the Frank Starling mechanism. However, we need to keep in mind that there is a limit of stretch, so if the ventricle is dilated to start with, adaptation may not be possible.

Although these processes are adaptive, there may be some pump dysfunction in many older adults because end systolic volume does not decrease as much as might be expected. That is, older adults don't empty their ventricles more efficiently to meet need; ejection fraction doesn't go up as much as expected.

Interestingly, most of these changes appear to be secondary to the changes in beta-adrenergic responsiveness (Fleg, Schulman, et al., 1994); if younger adults are given a beta blocker, their hearts adapt in the same way when stressed with exercise.

Also related to the diminished beta adrenergic response is a decrease in the peripheral vasodilation response to exercise which may contribute to the decline in exercise capacity with age; e.g. muscular fatigue.