PHYSIOLOGY OF AGING
Two important principles must be kept in mind when discussing
the physiology of aging. First, aging is associated with a progressive loss of functional
reserve in all organ systems. Second, the extent and onset of these changes are
highly variable from person to person. In the vast majority of older patients, physiologic
compensation for age-related changes is adequate, but the resultant limitation in
physiologic reserve may become evident only during times of physiologic stress, including
exercise, illness, and the perioperative period.
Central Nervous System
Several important processes occur with aging that are of interest
to the anesthesiologist. For one, memory decline occurs in more than 40% of persons
older than 60 years.[4]
However, studies have suggested
that memory decline in aging is not inevitable. Age-related memory decline is important
because it can dramatically affect the activities of daily living.
Cerebral atrophy occurs with aging, as reflected by decreases
in the volume of both gray and white matter.[5]
The decrease in gray matter volume is thought to be secondary to neuronal shrinkage
as opposed to neuronal loss. Recent studies focusing on the effects of normal aging
on the human cerebral cortex suggest that there is a small overall loss of neurons
from the neocortex.[6]
This decrease in neuron
number is nowhere near as massive as older studies had indicated. In fact, some
neocortical areas do not lose any neurons with aging. On the other hand, there may
be up to 15% loss of white matter with aging.[6]
Such loss results in gyral atrophy and increased ventricular size. It is controversial
whether the aging process alters the number of synapses present in the cortex. However,
data from nonhuman primates suggest significant regional reductions in the neurotransmitters
dopamine, acetylcholine, norepinephrine, and serotonin with aging.[6]
Glutamate levels, the primary neurotransmitter
TABLE 62-1 -- U.S. surgical procedures in the year 2000
|
Procedure |
Number Performed in Patients
Older than 65 Years |
Percentage of Patients Older
than 65 Years Undergoing Specific Procedures |
|
All |
14,737,000 |
37 |
|
Coronary artery bypass surgery |
286,000 |
55 |
|
Cholecystectomy |
149,000 |
36 |
|
Prostatectomy |
134,000 |
73 |
|
Total knee |
211,000 |
71 |
|
Data from Hall MJ, Owings MF: 2000 National Hospital
Discharge Survey. Advance Data, 329 (June 19, 2002), 2002. |
in the cortex, do not appear to be affected. Coupling of cerebral electrical activity,
cerebral metabolic rate, and cerebral blood flow remains intact in the elderly.
Although biochemical and anatomic changes have been described
in the aging brain, the exact mechanisms causing changes in functional reserve are
unclear. Decreases in brain reserve are manifested by decreases in functional activities
of daily living, increased sensitivity to anesthetic medications, increased risk
for perioperative delirium, and an increased risk for postoperative cognitive dysfunction.
