Introduction
The pulmonary system has two central functions that contribute to maintaining energy and metabolism: taking up oxygen, critical for energy generation, and getting rid of the waste products of respiration that can impair metabolism if allowed to accumulate. A decline in pulmonary function could therefore contribute to exhaustion and slowing mobility, key components of physical frailty (Singer et al., 2016). Aging results in a steady decline of lung function after the age of 35 (Zeleznik, 2003), through loss of lung elasticity and decreased surface area for alveolar gas exchange, along with weakened muscles of respiration. Peak lung function, exposures to toxins such as cigarette smoke, and aging related changes in the immune system all play a role in the development of pulmonary impairment in later life.
The relationship between frailty and lung function appears to go in both directions. In the Cardiovascular Heath Study, pre-frail and frail participants had 42% increased likelihood of developing respiratory impairment compared to non-frail participants over 12 years of follow up, while those with lung disease but not frailty at baseline had a 58% increase in the likelihood of becoming pre-frail or frail during follow up (Vaz Fragoso et al., 2012).
Impact of Pulmonary Impairment on Frailty
Chronic obstructive lung disease (COPD) has been associated with a 10.2% prevalence of physical frailty compared to 3.4% over all in one community-based study of 2,146 older adults, with greater risk among those with more frequent exacerbations (Lahousse et al., 2016). Furthermore, while COPD alone did not increase mortality risk, frail patients with COPD were at almost 3-fold increased risk of death during follow up.
Impact of Frailty on Pulmonary Function
Lungs have a tremendous surface area–the greatest surface area of any organ–all of which is at constant risk of exposure to microbes inhaled from ambient air or aspirated (drawn into the lungs from the mouth) from the gastrointestinal system (Green & Pinkerton, 2004). With this constant connectivity of the lungs to the environment, robust immunological responses are a key defense. At the same time, chronic inflammation has been associated with frailty and declining lung function. For example, in a study of lung transplant candidates, frailty was associated with higher levels of IL-6 and TNFR1 as well as clinical deterioration and mortality (Singer et al., 2015). Thus age-associated attenuation of immune defenses in the lung could contribute to the acceleration of frailty by rendering the lungs of older adults more susceptible to infections.
Gas exchange also depends on an adequate volume of breath. The strength of the inter-costal muscles that allow one to fully inflate the lungs could also be impacted by the loss of muscle mass associated with frailty. In those awaiting lung transplant, cachexia and lower IGF-1 levels were both associated with frailty (Singer et al., 2015). Thus, declining physical strength is another possible mechanism by which frailty and pulmonary function interact to accelerate functional declines.
Applying Knowledge of Frailty to Pulmonary Status
Aging, even in the absence of lung-specific disease, results in declines in respiratory reserve, which is manifest as decreases in stamina and may contribute to other components of frailty such as slower walking speed. These changes appear to exacerbate the declines in function among those with lung disease such as COPD, while underlying lung disease may lower the threshold for becoming physically frail. Therefore, clinical strategies to help maintain lung-function during aging would benefit all older adults at risk of frailty as well as those with lung disease. Additional research is needed to provide insight into the factors and mechanisms responsible for reduced reserve such as the immune system and immune response of the lungs. Such research should aim to understand whether any of these factors might be not only preventable, such as limiting smoke exposure, but actually reversible.