Complex physiologic changes occur in the respiratory system with aging (Morgan et al. 2007, Leung 2007, Sadean and Glass 2003, Stoelting and Miller 2007). Respiratory mechanics is altered due to changes in the lung parenchyma, the thorax, and the medullary respiratory centers. These changes account for the increased degree of ventilation/perfusion (V/Q) mismatching and intrapulmonary shunting that is observed with normal aging. In addition, resting arterial oxygen tension, compensatory responses to hypoxia and hypercarbia, protective upper airway reflexes, and the work of breathing are all affected with age (Morgan et al. 2007, Leung 2007). It is estimated that arterial oxygen tension decreases by an average rate of 0.35 mm Hg per year (Morgan et al. 2007). However, because the arterial partial pressure of carbon dioxide (PaCO2) does not change much with age, there is a compensatory increase in minute ventilation (Sadean and Glass 2003).
In addition to changes in pulmonary architecture, there are corresponding changes in the medullary respiratory centers of the central nervous system. This results in impaired ventilatory responses to hypoxia and hypercarbia with increasing age (Morgan et al. 2007,
Leung 2007). This phenomenon partially accounts for the exaggerated respiratory effects of benzodiazepines and opioids in this patient population. There is a higher incidence of transient apnea and episodic breathing in the elderly after exposure to central nervous system depressants (Stoelting and Miller 2007).
Protective airway reflexes (laryngeal, pharyngeal, and cough) are also impaired with increasing age, which can contribute to a higher incidence of pulmonary aspiration and postoperative respiratory morbidity and mortality in this population (Stoelting and Miller 2007).
Finally, the work of breathing in the elderly population is increased as compared to younger patients. This is attributed to the gradual stiffening and calcification of the costo-chondral joints of the thorax, resulting in a less compliant chest wall. This, in conjunction with the increased incidence of smaller airway closure, makes breathing more laborious in the elderly. Elderly patients are more predisposed to acute postoperative respiratory failure than their younger counterparts.
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