Summary
The World Health Organization predicts that the number of people aged 60 years and over will grow to an estimated 1.2 billion by 2025 and to 2 billion by 2050. In parallel, the number of both elective and emergent surgeries on elderly people also will increase. It is of key importance that anesthesiologists understand the fundamental characteristics of physiology, anatomy, and pharmacology associated with aging, as well as have the ability to optimize care to improve outcomes in this population.
Introduction
The World Health Organization predicts that the number of people aged 60 years and over will grow to an estimated 1.2 billion by 2025 and to 2 billion by 2050. In parallel, the number of both elective and emergent surgeries on elderly people also will increase. It is of key importance that anesthesiologists understand the fundamental characteristics of physiology, anatomy, and pharmacology associated with aging, as well as have the ability to optimize care to improve outcomes in this population.
Age-Related Physiological Changes
Aging is associated with a decline in all organ functions, with an associated reduction in reserve capacity. This overall functional reduction puts the geriatric population at increased risk during anesthesia and surgery.
Nervous System
Normal aging is accompanied by decreased brain mass, reduced neuronal density, decreased cerebral blood flow proportional to neuronal loss, increased ventricular volume, and widening of sulci in the central nervous system. In the peripheral nervous system, there is a reduction in myelinated fibers, with potential alterations in pain perception. In addition, there is a decrease in the number of neuroreceptors and a reduction in neurotransmitter synthesis. The glial cells increase in number to compensate for the loss in neuronal mass. These functional changes correlate with an increased incidence of postoperative delirium and cognitive dysfunction and longer recovery from general anesthesia, as well as increased sensitivity to inhaled anesthetic agents and centrally acting anticholinergic drugs (see Table 26.1).
Table 26.1 Physiologic changes in the nervous system and anesthetic implications
| System | Structural changes | Functional changes | Anesthetic implications |
|---|---|---|---|
| Nervous |
|
|
|
BBB, blood–brain barrier; POCD, postoperative cognitive dysfunction; MAC, minimum alveolar concentration; GA, general anesthesia.
Cardiovascular System
Significant structural changes in the cardiovascular system among aging adults include left ventricular hypertrophy with decreased wall compliance, increased vascular rigidity, decreased compliance of vessels from gradual loss of elastin, deposition of collagen, and calcification of valves and vasculature. Consequences of these changes include increased systolic pressure, increased mean arterial pressure, and decreased diastolic function.
Fatty infiltration of pacemaker cells, decreased conduction fiber density, and sinoatrial node cell number may increase the susceptibility to arrhythmias, such as sick sinus syndrome, atrial fibrillation, premature atrial contractions, and exaggerated bradycardia, after opioid administration in the elderly population.
Autonomic changes include increased sympathetic activity and decreased parasympathetic tone. Combined with increased vascular stiffness, this leads to greater lability in blood pressure. Decreased sensitivity of beta receptors results in limited ability to increase the heart rate pharmacologically, leading to dependence on preload and vascular tone. Decreased cardiac reserve may manifest as exaggerated hypotension upon induction of general anesthesia (see Table 26.2).
Table 26.2 Physiologic changes in the cardiovascular system and anesthetic implications
| System | Structural changes | Functional changes | Anesthetic implications |
|---|---|---|---|
| Cardiovascular |
|
|
|
LV, left ventriclular; SA, sinoatrial; MAP, mean arterial pressure; SV, stroke volume; CO, cardiac output; SVR, systemic vascular resistance; SBP, systolic blood pressure; BP, blood pressure; CHF, congestive heart failure.
Pulmonary System
Overall, pulmonary reserve decreases with aging. Increased chest wall stiffness, decreased elasticity of the lung parenchyma, flattening of the diaphragm, and reduction of respiratory muscle strength are predictable changes during aging which may lead to diaphragmatic fatigue, a predisposition to respiratory failure during the postoperative period, and difficulty in weaning from mechanical ventilation.
Alveolar gas exchange is negatively impacted by decreased functional alveolar surface area, decreased diffusing capacity of carbon monoxide, ventilation–perfusion mismatch, and an increase in dead space. The elderly population has decreased expiratory reserve volume, vital capacity, functional vital capacity, and arterial oxygen tension, as well as increased residual volume (RV), RV/TLC (total lung capacity) ratio, closing volume, and closing capacity (CC). By the age of 65 years, CC exceeds functional residual capacity, which, in turn, leads to closure of small airways, thus resulting in hypoxemia and an increased incidence of respiratory adverse events (see Table 26.3).
Table 26.3 Physiologic changes in the pulmonary system and anesthetic implications
| System | Structural changes | Functional changes | Anesthetic implications |
|---|---|---|---|
| Pulmonary |
|
|
|
VC, vital capacity; FVC, forced vital capacity; FEV1, forced expiratory volume in 1 second; ERV, expiratory reserve volume; RV, residual volume; CC, closing capacity; DLCO, diffusing capacity of carbon monoxide; PAP, pulmonary arterial pressure; V/Q, ventilation–perfusion.
Renal System
Aging causes a variable decline in renal blood flow, glomerular filtration rate (GFR), creatinine clearance, renal mass, tubular function, concentrating ability, and diluting capacity. Other changes include impaired fluid handling, decreased drug excretion, decreased responsiveness of the renin–angiotensin–aldosterone system, and impaired potassium excretion. Plasma concentrations of renally excreted drugs may be elevated and/or prolonged (see Table 26.4).
Table 26.4 Physiologic changes in the renal system and anesthetic implications
| System | Structural changes | Functional changes | Anesthetic implications |
|---|---|---|---|
| Renal |
|
|
|
GFR, glomerular filtration rate; RAAS, renin–angiotensin–aldosterone system; NSAID, nonsteroidal antiinflammatory drug.
Gastrointestinal and Hepatic System
Esophageal motility decreases and gastric emptying time becomes prolonged, putting older adults at increased risk of perioperative aspiration.
Aging causes a decline in liver mass and hepatic blood flow, hence diminishing hepatic function, resulting in slower metabolism of medications. There is decreased protein synthesis, resulting in increased free drug fractions of highly protein-bound drugs, including benzodiazepines and propofol. Impaired hepatic function influences the synthesis of coagulation factors, thus putting older patients at increased risk of perioperative bleeding (see Table 26.5).
Table 26.5 Physiologic changes in the gastrointestinal and hepatic system and anesthetic implications
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