Sharma and Goodwin determines the rate and force of expiration and the thoracic compliance determines the elastic load during inspiration. With aging there are structural changes to the thoracic cage causing reduction in chest wall compliance. Age-related osteoporosis results in reduced height of the thoracic vertebrae. Stiffening of the thoracic cage from calcification of the rib cage and age-related kyphosis from osteoporosis reduces the ability of the thoracic cage to expand during inspiration and places the diaphragm at a mechanical disadvantage to generate effective contraction. Mittman et al (1965) studied total respiratory compliance in 42 healthy male subjects, ages 24 to 78 yrs, with 5 subjects being age ≥70 years. Lung compliance was similar, but the chest wall compliance was lower in older subjects. Moreover, subjects with lower chest wall compliance had higher residual volume (RV), suggesting an impediment to complete emptying of the lungs from stiff chest wall (Mittman et al 1965). Respiratory muscle function The diaphragm is the most important respiratory muscle and plays an essential role during inspiration. Exact measurement of diaphragmatic strength can only be done in vivo. There is very little information on the effect of aging on the contractile properties of the diaphragm. Respiratory muscle strength can be measured by transdiaphragmatic pressure (Pdi), maximum voluntary ventilation (MVV), and maximum inspiratory pressure (MIP). MIP is an index of strength of the diaphragm measured using a mechanical pressure gauge with a closed valve at the mouth during an inspiration. MIP is an indicator of inspiratory muscle strength and a determinant of vital capacity. Decline in MIP can lead to inadequate ventilation and impaired clearance of airway secretions, as seen in patients with neuromuscular disease. Studies measuring MIP have been cross-sectional, and their primary objective has been to establish reference values rather than to evaluate the longitudinal impact of age on diaphragmatic function. MIP is 30% higher in men compared with women at all age groups and decreases by 0.8 cm to 2.7 cm of H20/year between the ages of 65 and 85 years, with larger age-related declines seen in men (Enright et al 1994). Tolep et al (1995) showed a 25% lower Pdi measured using Mueller maneuver in healthy older subjects (age 65–75, n=10) compared with young healthy adults (age 19–28, n=9). Similar results were seen in a study by Polkey et al (1997). The degree of reduction in Pdi in healthy older compared with healthy young controls was less (13%) in the latter study, partly explained by different measurement 254 Table 1 Aging and respiratory muscle strength Study Technique Pdi (cm of H20) Reduction p-value Tolep et Mueller al 1995 Polkey et Sniff al 1997 Abbreviations: Pdi, transdiaphragmatic pressure. Young 171±8 Elderly 128±9 25% 136±17 119±22 13% <0.003 0.05 techniques (Table 1). Maximum voluntary ventilation is also reduced with age, and one longitudinal study showed a 12% decline over 6 years in older trained athletes (McClaran et al 1995). Likely explanation for reduced diaphragmatic strength with age is related to muscle atrophy and age-related decrease in fast twitch fibers, responsible for generating higher peak tensions. This age-related decline in diaphragmatic strength may predispose older individuals to diaphragmatic fatigue and ventilatory failure during increased ventilatory load on the respiratory system. Anatomical changes Senile hyperinflation of the lungs is a well known entity in the medical literature. It is not clear whether it reflects aging- associated destruction of lung parenchyma or loss of supporting structures within the lung parenchyma. Gillooly and Lamb (1993) studied 38 (age ≥65 yrs, n=14) autopsy or surgical lung resection specimens in lifelong nonsmokers and showed increase in airspace size with advancing age. There is homogeneous degeneration of the elastic fibers around the alveolar duct starting around 50 years of age resulting in enlargement of airspaces. Reduction in supporting tissue results in premature closure of small airways during normal breathing and can potentially cause air trapping and hyperinflation, hence “senile emphysema”. Immunologic changes Bronchoalveolar lavage (BAL) fluids in healthy older subjects have consistently shown an increased proportion of neutrophils and lower percentage of macrophages compared with younger adults. There is age-associated increase in immunoglobins IgA and IgM in the BAL fluid. Ratio of CD4+/CD8+ lymphocyte increases with age in the BAL fluid, suggesting the presence of primed T-cell from repeated antigenic stimuli of the lower respiratory tract mucosa (Meyer et al 1996). Moreover, there is increased ability of alveolar macrophages to release superoxide anion Clinical Interventions in Aging 2006:1(3)