What physiological changes occur in patients over 65 years that affect anesthesia planning, drug dosing, and peri‑operative risk?

Physiological Changes in Elderly Patients Affecting Anesthesia

Elderly patients require 30-50% reduction in anesthetic induction doses due to age-related physiological decline affecting all organ systems at approximately 1% per year after age 40, with reduced organ reserve limiting stress responses and increasing perioperative risk of myocardial and cerebral ischemia. 1, 2

Cardiovascular System Changes

The aging cardiovascular system renders elderly patients effectively "beta-blocked" with profoundly limited ability to respond to hemodynamic stress. 1, 3

Key cardiovascular alterations include:

• Reduced beta-receptor responsiveness limits cardiac output increases and impairs response to fluid losses 1, 3
• Decreased cardiac compliance further restricts cardiac output capacity 1
• Baroreceptor dysfunction and reduced angiotensin II responsiveness severely impair compensation for hypovolemia 1, 3
• Co-morbid atherosclerotic disease and cardiac polypharmacy compound these limitations 1

Clinical implication: Titrate anesthetic agents slowly and await response with patience to prevent circulatory collapse, as hemodynamic instability is more pronounced and recovery is delayed. 4, 5

Respiratory System Changes

Lung function declines due to loss of both lung and chest wall compliance, with chest wall compliance dropping from 350 ml/cmH₂O in young adults to 136 ml/cmH₂O in those aged 60-69 years. 1, 6

Critical respiratory changes:

• Closing volume increases with greater ventilation/perfusion mismatch, particularly when supine 1, 6
• Oxygen diffusion capacity decreases, more severely in smokers 1, 6
• Reduced oxidative capacity combined with cardiopulmonary changes decreases both oxygen uptake and delivery 1

This creates substantially increased risk of perioperative myocardial and cerebral ischemia. 1

Central Nervous System Changes

Age-related cerebral and cerebrovascular decline contributes to high prevalence of postoperative delirium and cognitive dysfunction, delaying discharge and functional recovery. 1

Anesthetic management implications:

• Depth of anesthesia monitoring using BIS or processed EEG is strongly recommended for patients over 60 years at risk of postoperative delirium 2
• Decreased neural plasticity increases vulnerability to anesthetic overdosage 4
• Increased sensitivity to anesthetics means desired effects occur at lower doses with more pronounced hemodynamic depression 7, 5

Renal System Changes

Renal function decline is variable but affects pharmacokinetics and pharmacodynamics of anesthetic drugs, requiring routine assessment. 1

• Nephrotoxic effects from co-morbidities (hypertension, diabetes) and drugs (NSAIDs, ACE inhibitors) accelerate decline 1
• Drugs eliminated renally require dosage adjustments based on residual function 4
• Decreased renal blood flow, mass, and functioning nephrons alter drug clearance 4

Hepatic Changes

Liver mass, hepatic blood flow, and intrinsic metabolic activity decrease with age, causing variable drug half-lives and requiring dose reduction for hepatically metabolized drugs. 4

Hematological and Immunological Changes

Anemia occurs in approximately 10% of elderly patients, often of unexplained etiology related to erythropoietin resistance and stem-cell aging. 1

• "Immunosenescence" describes multifactorial immune system deterioration reducing infection-fighting capacity and wound healing 1
• Inflammation-mediated organ dysfunction may develop more readily 1

Musculoskeletal System Changes

General decline in muscle volume and function, combined with arthritic and osteoporotic changes, increases fragility fracture likelihood and impairs rehabilitation. 1

• Immobility contributes to greater thromboembolism and pressure necrosis prevalence 1

Critical Anesthetic Dosing Principles

All induction doses must be reduced by 30-50% from standard adult dosing in elderly patients. 2

Key dosing considerations:

• Potency of anesthetic drugs is increased in the elderly due to receptor-level changes and altered functional connectivity 7
• Dose reduction extent is underappreciated by practitioners and not uniformly applied 7
• Neuromuscular blocking agent doses should rarely be reduced for intubation, but duration of action is prolonged and unpredictable, requiring mandatory perioperative neuromuscular monitoring 2, 5

Perioperative Risk Stratification

Age-related physiological decline, multi-morbidity, and frailty are independently associated with increased perioperative risk. 2

Essential management strategies:

• Pre-operative assessment of higher-risk elderly patients must involve both senior geriatrician and senior anesthetist with geriatric subspecialty training 2
• Patients with predicted perioperative mortality >10% must be admitted to level 2 or 3 critical care 2
• Complete end-of-surgery checklist before leaving operating theater for all patients >75 years undergoing major/emergency surgery, including core temperature, hemoglobin, age-adjusted analgesia doses, fluid plan, and safe destination confirmation 2

Common Pitfalls to Avoid

• Intraoperative hypotension and excessive CNS depression are associated with poor perioperative outcomes, particularly consequential in frail elderly with reduced reserves 7
• Failure to titrate carefully leads to circulatory collapse given limited compensatory mechanisms 1, 5
• Inadequate neuromuscular monitoring risks prolonged paralysis and respiratory complications 2, 5
• Standard adult dosing results in overdosage and increased complications 2, 7