REFERENCE EQUATIONS FOR THE SIX-MINUTE WALK IN HEALTHY ADULTS

	To the Editor :

Enright and Sherrill (1) reported reference equations for prediction of the total distance walked during six minutes (6MWD) for healthy men (n = 117) and women (n = 173) aged 40 to 80 years. Gender-specific regression equations based on height, weight, body-mass index, and age explained 40% of the variance in the distance walked. The authors suggested that future population-based studies that include additional data, such as knowledge of exercise habits, cardiopulmonary conditioning, and musculoskeletal problems might improve their prediction models and account for the 60% of unexplained variance.

An additional factor that should be considered in future investigations is the presence of carboxyhemoglobin from passive smoke and/or exposure to nontobacco sources of carbon monoxide (CO) in study subjects. In has been known for more than three decades that exposure to low-level environmental CO impairs exercise performance in normal persons as well as those with underlying coronary artery disease (2). The availability of portable, accurate, and relatively inexpensive CO meters provides an inexpensive means for measuring expired CO in healthy adult participants in six-minute-walk studies.

Population studies indicate that more than 75% of the United States population is regularly exposed to environmental tobacco smoke (5). A smaller but still significant fraction of the population, particularly urban dwellers, are exposed to nontobacco sources of CO. It is thus likely that many subjects who participate in six-minute-walk tests will have an increased blood carboxyhemoglobin concentration. The measurement of CO in future study subjects may contribute to our understanding of the sources of variation and reproducibility of the 6 MWD in cross-sectional and longitudinal studies of normal subjects and specific patient populations such as those with chronic heart failure (6).

Departments of Medicine and Public Health, Indiana University School of Medicine, Indianapolis, Indiana

1. Enright, P. L., and D. L. Sherrill. 1998. Reference equations for the six-minute walk in healthy adults. Am. J. Respir. Crit. Care Med. 158: 1384-1387 [Abstract/Free Full Text].

2. Chevalier, R. B., R. A. Krumholtz, and J. C. Ross. 1966. Reaction of nonsmokers to carbon monoxide inhalation. J.A.M.A. 198: 135-138 [Medline].

3. Glantz, S. A., and W. W. Parmley. 1995. Passive smoking and heart disease: mechanisms and risk. J.A.M.A. 273: 1047-1053 [Abstract].

4. Sheps, D. S., M. C. Herbst, A. L. Hinderliter, K. F. Adams, L. G. Ekelund, J. J. O'Neil, G. M. Goldstein, P. A. Bromberg, J. L. Dalton, M. N. Ballenger, S. M. Davis, Gary, and G. Koch. 1990. Production of arrhythmias by elevated carboxyhemoglobin in patients with coronary artery disease. Ann. Intern. Med. 113: 343-351 .

5. Pirkle, J. L., K. M. Flegal, J. T. Bernert, D. J. Brody, R. A. Etzel, Kurt, and R. Maurer. 1996. Exposure of the U.S. population to environmental tobacco smoke. J.A.M.A. 275: 1233-1239 [Abstract].

6. Jay, S. J.. 1997. Passive smoking and the six-minute walk test in heart failure. Chest 112: 289-290 [Free Full Text].

	From the Authors:

We appreciate the suggestion by Dr. Jay that an increased level of carbon monoxide (CO) from smoking (or other sources) is probably an independent factor that reduces exercise performance. We did not have the foresight to measure either exhaled breath CO nor carboxyhemoglobin levels in our study participants. However, we did find that current smoking, but not former smoking (after correcting for cardiopulmonary disease), was significantly associated with a lower six-minute-walk distance, both in the Tucson Airways SCOR cohort and the elderly Cardiovascular Health Study cohort (manuscripts in preparation). We are unaware of the component(s) of cigarette smoke responsible for this effect or the mechanism of action.

We also encourage other investigators to include a measure of CO in new studies of exercise performance (in all age groups), so that this association, its strength and duration of action may be confirmed. Meanwhile, student athletes should consider that cigarette smoking may substantially reduce athletic performance.

PAUL ENRIGHT

Tucson, Arizona