Pulmonary function, muscle strength and mortality in old age
Introduction
Numerous studies have reported that extremity muscle strength is associated with an increased risk of death in older persons (Al Snih et al., 2002, Laukkanen et al., 1995, Metter et al., 2002, Newman et al., 2006, Phillips, 1986, Rantanen et al., 2000, Rantanen et al., 2003, Rolland et al., 2006). However, the mechanisms underlying this association are not known. In some cases, loss of lower extremity muscle strength likely leads to mobility disability, falls, and death (de Rekeneire et al., 2003). However, it is possible that loss of extremity muscle strength may serve as an indicator of systemic disease and represents an early sign of physical frailty, which is associated with mortality (Buchman et al., 2009). A third possibility is that extremity muscle strength is a surrogate for weakness in other skeletal muscles such as respiratory muscles which may be more directly linked to mortality.
Respiratory muscle strength plays a key role in the respiratory network, which depends on intact neural circuitry which orchestrates the interplay between respiratory muscles and intrinsic pulmonary function to maintain adequate ventilation (Kim and Sapienza, 2005, Polkey and Moxham, 2001, Rantanen et al., 2003). In the absence of respiratory muscle activation, pressure gradients cannot be developed and air exchange at the alveolar surface cannot occur. Thus, impaired respiratory muscle strength can lead to pulmonary dysfunction, respiratory distress and even death. Therefore, we hypothesized that extremity muscle strength is a surrogate for respiratory muscle strength, and we also hypothesized that pulmonary function would mediate the association of respiratory muscle strength with mortality. While previous studies have examined the association of extremity muscle strength, respiratory muscle strength, and pulmonary function with mortality separately (Mannino et al., 2003, Sin et al., 2005), we are unaware of any prior study that examined the joint effects of these three indices on risk of death.
We used data from more than 900 older persons without dementia participating in the Rush Memory and Aging Project, a longitudinal study of common chronic conditions of aging, to investigate the associations of extremity muscle strength, respiratory muscle strength and pulmonary function with mortality (Bennett et al., 2005a). In a series of proportional hazards models, we first tested the hypothesis that extremity muscle strength is a surrogate for respiratory muscle strength. In subsequent models, we tested a second hypothesis that pulmonary function is a step in the causal chain linking respiratory muscle strength to death.
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Participants
All participants are from the Rush Memory and Aging Project, a longitudinal investigation of common chronic conditions of old age (Bennett et al., 2005a). The study was conducted in accordance with the latest version of the Declaration of Helsinki and was approved by the Institutional Review Board of Rush University Medical Center. Clinical evaluations for the study commenced in 1997 but pulmonary function measures were not introduced into the study until 2001. Eligibility for these analyses
Descriptive properties of extremity muscle strength and respiratory function
Extremity muscle strength ranged from −1.6 to 4.4 (mean = −0.005; S.D. = 0.73) with higher scores indicating greater muscle strength. Respiratory muscle strength ranged from −2.0 to 2.9 (mean = 0.022; S.D. = 0.89), with higher scores indicating greater muscle strength. Pulmonary function ranged from −2.3 to 3.3 (mean = 0.002; S.D. = 0.90) with higher scores indicating better performance.
Extremity muscle strength, respiratory muscle strength and pulmonary function were inversely related to age and
Discussion
In a cohort of 960 older persons without dementia, we examined the associations of extremity muscle strength, respiratory muscle strength and pulmonary function with mortality. Consistent with our first hypothesis, that extremity muscle strength is a surrogate for respiratory muscle strength and that respiratory muscle strength may account for the well-established association between extremity muscle strength and mortality, we found that the association of extremity muscle strength with
Acknowledgments
This work was supported by National Institute on Aging grants R01AG17917, R01AG24480, and K23 AG23040, the Illinois Department of Public Health, and the Robert C. Borwell Endowment Fund. We thank all the participants in the Rush Memory and Aging Project. We also thank Traci Colvin and Tracey Nowakowski for project coordination; Barbara Eubeler, Mary Futrell, Karen Lowe Graham, and Pamela Smith for participant recruitment; John Gibbons and Greg Klein for data management; and the staff of the
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