Pulmonary function, muscle strength and mortality in old age

https://doi.org/10.1016/j.mad.2008.07.003Get rights and content

Abstract

Numerous reports have linked extremity muscle strength with mortality but the mechanism underlying this association is not known. We used data from 960 older persons without dementia participating in the Rush Memory and Aging Project to test two sequential hypotheses: first, that extremity muscle strength is a surrogate for respiratory muscle strength, and second, that the association of respiratory muscle strength with mortality is mediated by pulmonary function. In a series of proportional hazards models, we first demonstrated that the association of extremity muscle strength with mortality was no longer significant after including a term for respiratory muscle strength, controlling for age, sex, education, and body mass index. Next, the association of respiratory muscle strength with mortality was attenuated by more than 50% and no longer significant after including a term for pulmonary function. The findings were unchanged after controlling for cognitive function, parkinsonian signs, physical frailty, balance, physical activity, possible COPD, use of pulmonary medications, vascular risk factors including smoking, chronic vascular diseases, musculoskeletal joint pain, and history of falls. Overall, these findings suggest that pulmonary function may partially account for the association of muscle strength and mortality.

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.

Section snippets

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

References (42)

  • D.A. Bennett et al.

    Amyloid mediates the association of apolipoprotein E e4 allele to cognitive function in older people

    J. Neurol. Neurosurg. Psychiatry

    (2005)
  • P.A. Boyle et al.

    Parkinsonian signs in subjects with mild cognitive impairment

    Neurology

    (2005)
  • Buchman, A. S., Wilson, R. S., Bienias, J. L., Bennett, D. A., 2009, in press. Change in Frailty and Risk of Death in...
  • A.S. Buchman et al.

    Motor function and mortality in older persons

    J. Am. Geriatr. Soc.

    (2007)
  • A.S. Buchman et al.

    Physical activity and leg strength predict decline in mobility performance in older persons

    J. Am. Geriatr. Soc.

    (2007)
  • N. de Rekeneire et al.

    Is a fall just a fall: correlates of falling in healthy older persons. The Health, Aging and Body Composition Study

    J. Am. Geriatr. Soc.

    (2003)
  • E.M. Drost et al.

    Oxidative stress and airway inflammation in severe exacerbations of COPD

    Thorax

    (2005)
  • R.M. Enoka et al.

    Motor unit physiology: some unresolved issues

    Muscle Nerve

    (2001)
  • P.L. Enright et al.

    Respiratory muscle strength in the elderly. Correlates and reference values. Cardiovascular Health Study Research Group

    Am. J. Respir. Crit. Care Med.

    (1994)
  • L.P. Fried et al.

    Frailty in older adults: evidence for a phenotype

    J. Gerontol. A Biol. Sci. Med. Sci.

    (2001)
  • A. Iqbal et al.

    Worldwide guidelines for chronic obstructive pulmonary disease: a comparison of diagnosis and treatment recommendations

    Respirology

    (2002)
  • Cited by (0)

    View full text