Loss of laryngopharyngeal muscle mass and coordination predispose elderly patients to perioperative airway collapse and pulmonary aspiration, respectively. Decline in lung function is primarily contributed by loss of lung elasticity, increasing chest wall stiffness, and reduced inspiratory muscle strength (Table 43.1). Small airway collapsibility causes air trapping, which leads to “senile emphysema.” However, this must be distinguished from actual emphysema in the preoperative evaluation, as the total lung capacity is decreased significantly in the latter.

Basal hyperinflation leads to flattening of the diaphragm, which accompanied by weaker chest muscles has major consequences. The vital capacity is decreased; hence ‘4-vital capacity breaths’ for preoxygenation in an elderly patient may not be effective. The already compromised thoracic muscles are highly susceptible to postoperative residual weakness with the use of nondepolarizing neuromuscular blockers. A flattened diaphragm not only makes respiration abdominothoracic, but significantly increases the work of breathing, leading to difficult weaning and extubation of these patients. Other notable changes include fall of FEV₁ (6–8 % per decade), increased closing capacity, and residual volume. The closing capacity (volume of air in the lungs at which small airways begin to close) exceeds the functional residual capacity (volume of air remaining in the lungs at the end of a normal expiration) at age 45 years in the supine position, and at age 65 years in the sitting position.

Uneven distribution of ventilation, increased closing capacity, and increased thickness of the interstitial barrier account for age-related fall in diffusion capacity. With age, arterial oxygen tension falls by 0.3–0.4 mmHg yearly; however, the partial pressure of CO₂ remains almost constant due to decreased production and much higher diffusion capacity of CO₂. This predisposes the elderly patient to desaturation under anesthesia. Positive pressure ventilation extends West’s Zone I (see Chap. 28), which increases dead space ventilation.

As a result of decreased peripheral and central chemoreceptor sensitivity with age, the ventilatory drive in response to hypoxia and hypercarbia falls by about 50 % and 40 %, respectively. This becomes even more significant in the perioperative period, where drugs like opioids and benzodiazepines further suppress these ventilatory responses, preventing compensatory responses to desaturation, with resultant CO₂ retention.

Renal function is commonly determined by the glomerular filtration rate (GFR) and serum creatinine. The average age-related loss in GFR is 6–8 % per decade. By the age of 80 years, the number of glomeruli is reduced to half than that at age 30 years, causing increased rate of sclerosis of the residual hyperfunctioning nephrons. As the body muscle mass also decreases, the deteriorating renal function may not be reflected by measurements of serum creatinine (measuring creatinine clearance reflects renal function better). However, the decrease in GFR tends to prolong the half-life of the anesthetic drugs which are primarily dependent upon glomerular clearance. Newer tests with molecules like cystatin C, neutrophil gelatinase-associated lipocalin (NGAL), and kidney injury molecule 1 (KIM-1), which remain unaffected by age, may serve better in measuring perioperative renal function.

The ability to conserve sodium and excrete H⁺ decreases with aging, diminishing the ability to regulate fluids and acid-base balance. Decreased sensitivity to renin-angiotensin and ADH impairs renal ability to compensate for perioperative hypervolemia and extra renal fluid losses. The susceptibly to fluid overload also increases, due to the inability to excrete excess fluids as a result of decreased GFR.

Urinary retention and catheter sensation are typically common problems in the elderly, which are aggravated by the use of opioids. Prostatic hyperplasia in elderly males and decreased estrogen levels in females alter urinary sphincter tone leading to urinary retention, thus promoting urinary tract infections and predisposing the patients to perioperative bacteremia.

Liver blood flow and size both decrease by around 35 % in old age. Any hypotension causing a further decrease in hepatic blood flow can significantly prolong drug action. Phase I metabolic reactions (cytochrome p450 enzyme-based reactions) are prolonged, whereas phase II reactions (conjugation reactions) are well preserved, which may guide the anesthesiologists’ choice to use drugs metabolized via phase II reactions in the elderly. Furthermore, there is a decreased production of albumin and plasma cholinesterase enzyme.

Gastric emptying is prolonged and the gastric pH tends to rise. Age-related changes do not significantly alter anesthetic plan in these patients. Perioperative opioids, may however, aggravate constipation, which already has a higher incidence in the elderly.

Biometric changes in body composition modify drug distributions in the elderly and cause changes in effect site concentrations. Lean muscle mass decreases by 40 %, with a decrease in the total body water. Subsequently, the volume of distribution of water-soluble drugs decreases, which increases the achieved effect site concentration. Conversely the proportion of body fat increases and, therefore, fat-soluble drugs undergo extensive redistribution, thereby, slowing their rate of elimination.

Centrally acting drugs may have a slower onset of action as a result of prolonged brain-arm circulation time. As hepatic and renal function decrease with age, drugs which are dependent on hepatic/renal clearance will have a prolonged duration of action. Lower serum albumin causes an increase in free plasma drug concentration, thus increasing the active form of the drug for the same dose. Initial boluses of drugs may need no dose modification (considering no change in drug sensitivity); however, subsequent doses need reduction or interval lengthening in view of slower elimination (Table 43.2).