Neurohormonal and autonomic dysregulation
Neurohormonal mechanisms such as the renin-angiotensin-aldosterone
system decline with age. Plasma renin
activity at age of 60 years is 40% to 60% of the levels
found in younger individuals[23]. This has been attributed
to the effect of age-associated nephrosclerosis on the
juxtaglomerular apparatus. Plasma aldosterone levels also
decreases with age. Consequently, elderly patients with
hypertension are more prone to drug-induced hyperkalemia[24].
In contrast, net basal sympathetic nervous system
activity increases with advancing age. Peripheral plasma
norepinephrine concentration in the elderly is double the
level found in younger subjects[25]. The age-associated rise
in plasma norepinephrine is thought to be a compensatory
mechanism for reduction in β-adrenergic responsiveness
with aging[25].
Decreased baroreflex sensitivity with age causes orthostatic
hypotension in the elderly[26,27]. On the contrary, orthostatic
hypertension, where BP increases with postural
change, is also prevalent among the elderly[28]. The orthostatic
hypertension is blocked by α-adrenergic blockade,
indicating that α-adrenergic activity may be a predominant
pathophysiological mechanism