IMPACT OF NORMAL AGING ON COMPLIANCE OF THE LUNG PARENCHYMA AND CHEST WALL, AND PERFORMANCE OF RESPIRATORY MUSCLES
Normal aging is associated with changes in the respiratory system that have important consequences on the ability of older subjects to cope with the decrease in lung compliance (infiltration by inflammatory cells related to infection) and increase in airway resistance (bronchial edema, secretions) caused by lower respiratory tract infection (LRTI).
When compared to younger individuals, elastic recoil of the lung, compliance of the chest wall, and strength of respiratory muscles are all decreased in older subjects.
Age-related alterations in lung parenchyma (enlargement of alveoli, also referred to as ductectasia or senile emphysema) explain the decline in elastic recoil of the lung and the resulting increase in functional residual capacity (FRC). Indeed, FRC (lung volume at the end of a quiet expiration) is determined by the equilibrium between two opposite forces: the elastic recoil of the lung parenchyma and that of the chest wall, the latter tending to expand the chest. Thus a decrease in elastic recoil of the lung increases the FRC: older patients breathe at higher lung volumes, increasing the workload imposed on respiratory muscles.
Decreased compliance of the chest wall (i.e., "stiffening" of the chest wall) is explained to some extent by calcifications and other structural changes within the rib cage and its articulations. Changes in the shape of the thorax as a result of osteoporosis and vertebral fractures, resulting in dorsal kyphosis and increased antero-posterior (AP) diameter (barrel chest), also affect chest wall compliance. Indeed, prevalence of vertebral fractures in the aged is high and increases with age: in Europe, for female subjects aged over 60, the prevalence of vertebral fractures is 17% in the 60 to 64 age group, increasing to 35% in the 75 to 79 age group (Cummings et al., 2002). Males also have an increase in vertebral fractures with age, but rates are approximately half those of the female population. The increase in AP diameter of the chest decreases the curvature of the diaphragm and has a negative effect on its force-generating capabilities. Respiratory muscle performance in older subjects is thus limited by the increase in FRC, the decrease in chest-wall compliance, and geometric changes in the rib cage (Janssens et al., 1999).
Respiratory muscle strength is also affected by nutritional status, often deficient in the elderly, and by the age-related decrease in muscle mass (sarcopenia) (Enright et al., 1994; Tolep et al., 1995; Polkey et al., 1997). Indeed, normal values for maximal inspiratory pressure in subjects aged over 80 are below the threshold defined in an adult population for clinically relevant respiratory dysfunction (Enright et al., 1994). Situations in which an additional load is placed on the respiratory muscles, such as decreased parenchymal compliance (pneumonia, congestive heart failure) or increased airway resistance (presence of tracheal or bronchial secretions and inflammation, asthma), may lead to hypoventilation and hypercapnic respiratory failure. Patients with preexisting structural changes in lung mechanics (such as COPD, interstitial lung disease, or Kyphoscoliosis) are, of course, at increased risk of hypercapnic respiratory failure.
Respiratory muscle function also depends on energy availability (i.e., blood flow, oxygen content); indeed, decreased respiratory muscle strength has been described in patients with chronic heart failure (CHF), a frequent occurrence in older subjects (Nishimura et al., 1994; Evans et al., 1995). Presence of CHF with a New York
Heart Association (NYHA) functional class 3 or 4 was shown to be associated with a 30% decrease in maximal mouth inspiratory pressure (PIMAX) compared to control subjects. Furthermore, a significant relationship was demonstrated between cardiac index (CI) and PIMAX, the latter decreasing when CI decreased (Nishimura et al., 1994). Other frequent clinical situations decreasing respiratory muscle function in the elderly include Parkinson's disease and sequelae of cerebral vascular disease. Indeed, in hemiplegic patients, limitation of thoracic excursions caused by weakness and uncoordina-tion of the chest muscles may lead to decreased chest wall compliance and compromise respiratory muscle function (Annoni et al., 1990).
AGING, FORCED EXPIRATORY VOLUMES, AND PEAK FLOW: IMPLICATIONS FOR COUGHING AND CLEARANCE OF AIRWAY SECRETIONS
Forced expiratory volumes and peak expiratory flow show an age-related linear decrease, probable reflecting structural changes, and chronic low-grade inflammation in peripheral airways (Enright et al., 1993). Indeed, for a male subject with a height of 180 cm, between the ages of 25 and 75, forced expiratory volume in 1 sec (FEV1) drops by 32%, forced vital capacity (FVC), by 24%, and peak expiratory flow (PEF), by 22% (Quanjer et al., 1993). In the very old, both the decrease in forced expiratory flow rates and in lung elastic recoil may compromise the efficacy of clearing airway secretions by coughing. Critical values for PEF have been reported, under which the risk of pneumonia markedly increases (Tzeng et al., 2000). In patients with neuromuscular disorders, a PEF below 270 L/min is associated with an increase in risk of pulmonary infection, and at PEF values < 160 L/min, cough is ineffective for clearing secretions from the airways ( predicted PEF for an 80-year-old woman, measuring 160 cm, is 300 L/min). Coughing requires a precise coordination of laryngeal and respiratory muscle function: after a rapid inspiration, airways are submitted to a compression phase in which active glottic closure and abdominal contraction are critical, before the "explosive" expiratory phase. A decrease in expiratory muscle strength may thus compromise the efficacy of the cough reflex. The efficacy of glottic closure depends on the integrity of laryngeal muscle function and complex integrated reflexes that may be altered in the elderly by transient or permanent neurological disorders (cerebro-vascular disorders, extra-pyramidal, or cerebellar disorders). Indeed, ischemic stroke increases markedly the risk of pneumonia: in a large series of 13440 patients, pneumonia was the most frequent and serious complication, causing 31% of all deaths (Heuschmann et al., 2004). Glottal gap related to unilateral vocal cord paralysis is a potentially reversible sequel of acute cerebro-vascular events that may also increase the risk and frequency of aspiration (Fang et al., 2004).
Mucociliary clearance (progression of mucus layer lining the tracheal and bronchial epithelium) is also affected by the aging process. Even in the healthy nonsmoking aged population, mucociliary clearance rates are slowed in comparison with the young. Nasal mucociliary clearance and frequency of mucosal ciliary beat are decreased in older subjects; aging also is associated with ultrastructural changes in microtubules of ciliae of the respiratory epithelium. Indeed, both smoking and nonsmoking elderly have reduced tracheal mucus velocity compared with younger individuals (Ho et al., 2001). Dehydration, frequent in older debilitated subjects, may further compromise mucociliary clearance.
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