In this review, the authors intend to present the physiological alterations caused by aging such as changes in respiratory system dynamics, gas exchange abnormalities, and changes in exercise capacity and respiratory muscle strength.
Structural changes induce an increase in the anterior and posterior diameters of thoracic cage, causing it to assume a round shape. Increased stiffness and reduced compliance of the thoracic wall is induced by aging-related calcification of the joint cartilage of the thorax and spinal scoliosis caused by osteoporotic changes.
A study that measured the compliance of the thoracic wall in 61 healthy adults ranging from 24 to 75 years old, reported that the thoracic compliance was reduced according to the participants' age. Aging-induced changes in the respiratory system, such as reduction of compliance and of respiratory muscle strength, result in changes in the results of pulmonary function tests. Lung function reaches its peak when individuals are in their early 20s, is maintained for some time, and then decreases normally with aging.
Total lung capacity TLC does not change or slightly reduces during aging, but residual volume RV is increased and vital capacity VC is reduced. These reductions indicate small airway disease based on the flow rate-volume curve. The diffusion capacity for carbon monoxide DLCO also decreases with age by 0. The degree of decreases in women is reported to be lesser than that in men, possibly due to the influence of estrogen, which maintains integrity of the blood vessel.
A study by Stam et al. This decrease in diffusing capacity may likely be caused by aging-induced changes in the alveolar-capillary membrane. However, the minute ventilation is not decreased and there is no occurrence of the carbon dioxide exchange impairment by aging, therefore, the occurrence of respiratory acidosis should be considered as pathological in old ages as well. Decreased muscle strength is a general consequence of aging, and affects the respiratory muscles as well.
All these parameters have been found to be decreased in elderly people. The reduced strength of the diaphragm with aging is associated with muscle atrophy and with decrease of fast fibers that create a higher level of tension. Reduction of diaphragmatic muscle strength increases the respiratory rate in the elderly, which results in fatigue to the diaphragm, and may eventually cause ventilatory failure. Thus, the reduction of respiratory muscle strength due to aging may not be a serious problem in healthy individuals, but if the ventilation demands are increased due to pulmonary disease, it can be a major problem resulting in respiratory failure.
The cross-sectional area of the intercostal muscles is slightly reduced, while that of the expiratory muscles is reduced to a much greater extent, at the age of 50 years or higher. However, the reduction of compliance in the chest wall results in a reduction in the curvature causing a reduction in Pdi max. Reduction in respiratory muscle strength in the elderly has shown to be associated with nutritional status, and BMI has been reported to have a significant correlation with the MIP as well as with the maximal expiratory pressure.
Minute ventilation remains the same between the younger and the older age groups. Although the tidal volume is reduced with age, the minute ventilation is maintained by increasing the respiration rate. However, the ventilator response to hypoxemia or to increased carbon dioxide concentration may be inadequate in the elderly. Although the mechanism is unclear, these changes might have been caused by dysfunction of chemosensory receptors and by structural changes in the lung and thoracic cage.
In a longitudinal study on the effects of aging on lung function at rest and during exercise in healthy older people, McClaran et al. The cough reflex weakens with aging. This change may be attributed to the decrease in cough sensation, increased activation threshold of the vagus and occipital nerves, decreased tension of smooth muscles, and reduction of cognitive capacity.
Such weakening of the cough reflex contributes to the increase in the incidence of aspiratory pneumonia in elderly individuals. Figure 3. Table 2 Static lung volumes. Tidal volume TV Volume of air inspired or expired during quiet breathing Inspiratory reserve volume IRV Maximum volume of air inspired above the tidal volume Expiratory reserve volume ERV Maximum volume of air expired below the tidal volume Residual volume RV Amount of air in the lungs after maximum expiration.
Table 4 Lung function studies. Dyspnea and response to hypoxia and hypercapnia Minute ventilation, a product of volume inhaled per breath and respiratory rate over one minute, is identical in younger and older individuals. Bronchial hyper-responsiveness and age-related pulmonary receptor changes Age had a significant effect on airway reactivity and response to bronchodilator therapy.
Figure 4. Mechanism of action of beta agonist. Muscarinic receptor The data on age-related changes in pulmonary muscarinic receptor in humans is limited. Cysteinyl-Leukotriene CysLT 1 receptor The stimulation of cysteinyl-leukotriene CysLT 1 receptor located on the airway smooth muscles induces bronchoconstriction. Exercise capacity The effect of aging on exercise capacity is highly variable and depends upon individual fitness and regular physical activity.
Table 5 Ventilatory response to hypoxia and hypercapnia. Summary There is marked variation in the effect of aging on lung function. Table 6 Anatomical and physiological changes of respiratory system with aging. Figure 2. Relationship between lung volumes and lung capacities. Table 3 Lung capacities are sum of two or more static lung volumes. Catalytic unit of adenylate cyclase: reduced activity in aged-human lymphocytes.
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