Correspondence

Nomogram for Exercise Capacity in Women

N Engl J Med 2005; 353:2301-2303November 24, 2005

Article

To the Editor:

The report by Gulati et al. (Aug. 4 issue)1and the accompanying editorial by Kraus and Douglas2 claim that the nomogram for predicting exercise capacity in women according to age and level of metabolic equivalents (MET) is unique. However, an article by Bruce et al.3 (cited in the report by Gulati et al.) involved the use of age and exercise duration to provide nomographic assessments of functional aerobic impairment for men and women separately. The percentage of age-predicted normal exercise capacity equals 100 percent minus functional aerobic impairment. Bruce and colleagues provided nomograms for use in calculating functional aerobic impairment. A report4 comparing age-related changes in maximal cardiac function among sedentary, healthy men and women addressed a question that was also raised in the editorial. The regression line relating age and maximal oxygen consumption had a significantly slower age-related reduction in women than in men.

Bruce et al.5 reported the prognostic value of functional aerobic impairment in men and women separately and confirmed the prognostic value of this variable for cardiac events. The prognostic value of variables other than electrocardiographic changes obtained from symptom-limited exercise tests in asymptomatic persons, which was championed by Bruce and colleagues more than three decades ago, has been confirmed by the report by Gulati et al. in this large cohort of women.

Kenneth F. Hossack, M.B., B.S.
St. Andrew's Hospital, Brisbane 4000, Australia
khossack@bigpond.net.au

5 References

1. 1

   Gulati M, Black HR, Shaw LJ, et al. The prognostic value of a nomogram for exercise capacity in women. N Engl J Med 2005;353:468-475
   Full Text | Web of Science | Medline

2. 2

   Kraus WE, Douglas PS. Where does fitness fit in? N Engl J Med 2005;353:517-519
   Full Text | Web of Science | Medline

3. 3

   Bruce RA, Kusumi F, Hosmer D. Maximal oxygen intake and nomographic assessment of functional aerobic impairment in cardiovascular disease. Am Heart J 1973;85:546-562
   CrossRef | Web of Science | Medline

4. 4

   Hossack KF, Bruce RA. Maximal cardiac function in sedentary normal men and women: comparison of age-related changes. J Appl Physiol 1982;53:799-804
   Web of Science | Medline

5. 5

   Bruce RA, Hossack KF, DeRouen TA, Hofer V. Enhanced risk assessment for primary coronary heart disease events by maximal exercise testing: 10 years' experience of Seattle Heart Watch. J Am Coll Cardiol 1983;2:565-573
   CrossRef | Web of Science | Medline

To the Editor:

We question the attempt by Gulati et al. to address the historical lack of fitness data for women by converting estimated peak oxygen consumption from the equation of Foster et al.1 to MET. That equation, based on a study involving 200 men, was never validated in the population studied by Gulati et al. The Bland–Altman plot (Figure 1Figure 1Bland–Altman Plot Evaluating Measured and Predicted Levels of Oxygen Consumption.)2 comparing Foster's equation with measured oxygen consumption identifies a large overestimation bias (mean [±SD] bias, 13.9±13.0 ml of oxygen per kilogram per minute). Although Gulati and colleagues present exercise capacity in multiples of MET, the dogmatic statement that 1 MET=3.5 ml of oxygen per kilogram per minute has never been substantiated and may never have been obtained through acceptable laboratory methods. Byrne et al.3 determined 1 MET to be significantly lower — 2.6±0.4 ml of oxygen per kilogram per minute — suggesting that the current equation underestimates exercise capacity by approximately 26 percent.

Despite statistical significance, the equations of Gulati et al. are poorly predictive. Only 25 percent of the variance is accounted for by age. Since oxygen consumption is known to be associated with body composition, the inclusion of body-mass index,3,4 for example, may improve these equations.

We suggest that future studies such as this validate equations and constants in the relevant population.

François Haas, Ph.D.
New York University School of Medicine, New York, NY 10016
francois.haas@med.nyu.edu

Nuala M. Byrne, Ph.D.
Queensland University of Technology, Brisbane Q4059, Australia

Mariano Rey, M.D.
New York University School of Medicine, New York, NY 10016

4 References

1. 1

   Foster C, Jackson AS, Pollock ML, et al. Generalized equations for predicting functional capacity from treadmill performance. Am Heart J 1984;107:1229-1234
   CrossRef | Web of Science | Medline

2. 2

   Bland JM, Altman GD. Measuring agreement in method comparison studies. Stat Methods Med Res 1999;8:135-160
   CrossRef | Web of Science | Medline

3. 3

   Byrne NM, Hills AP, Hunter GR, Weinsier RL, Schutz Y. Metabolic equivalent: one size does not fit all. J Appl Physiol 2005;99:1112-1119
   CrossRef | Web of Science | Medline

4. 4

   Jette M, Sidney K, Blumchen G. Metabolic equivalents (METs) in exercise testing, exercise prescription, and evaluation of functional capacity. Clin Cardiol 1990;13:555-565
   CrossRef | Web of Science | Medline

To the Editor:

Figure 3 in the article by Gulati et al. demonstrates essentially equivalent absolute death rates for symptomatic women over 70 years of age and those younger than 55 years after the difference between the observed and the expected exercise capacity (O−E) was taken into account. Expected exercise capacities are lower for older women, so this means that a 71-year-old woman with an exercise capacity of 5.5 MET (O−E=0) has a lower risk of dying than a 54-year-old woman with the same exercise capacity of 5.5 MET (O−E=−2). Given that age is a strong predictor of death both generally and in the asymptomatic cohort of the St. James Women Take Heart Project, as Dr. Gulati's group has previously reported,1 we believe this finding to be either an error (perhaps these rates should be relative within age strata and not absolute) or a striking reversal in the usual effect of age that deserves mention and discussion.

Mark J. Pletcher, M.D., M.P.H.
Charles McCulloch, Ph.D.
University of California, San Francisco, San Francisco, CA 94143-0560
mpletcher@epi.ucsf.edu

1 References

1. 1

   Gulati M, Pandey DK, Arnsdorf MF, et al. Exercise capacity and the risk of death in women: the St James Women Take Heart Project. Circulation 2003;108:1554-1559
   CrossRef | Web of Science | Medline

Author/Editor Response

With regard to the comments of Dr. Haas and colleagues about the conversion of estimated peak oxygen consumption to MET: this conversion could not be validated in our population from the St. James Women Take Heart Project, because direct measurements of oxygen consumption were never taken in these women. In clinical practice, we also estimate MET, and in this context, our findings are clinically relevant.

In response to Drs. Pletcher and McCulloch, you can see in our Figure 3 that we display the rate of death from cardiac causes in symptomatic women. We must point out that the trend we found in symptomatic women was not seen in asymptomatic women and that among the symptomatic women, death from cardiac causes occurred in only 1 percent. We found that the rate of cardiac-related death was highest in the youngest women who were farthest from their age-predicted fitness level, as we displayed in Figure 3; however, we cannot comment on the symptomatic women older than 70 years in whom the value of O–E was less than −2 MET, because there were no women in that cohort who were in this category. We think this is an important finding, as we commented in our article. Our findings do, in fact, suggest that younger, symptomatic women who have a low exercise capacity for their age may have the highest rate of death from cardiac causes.

We were aware of the reference values described in 1973 by Bruce et al.,1 whose study included not only men, but also 157 women, as Dr. Hossack mentions. Despite this important work, we chose to compare our finding with those of the previous study by Morris et al.2 that set the guidelines for the “normal standards” of the American College of Cardiology and the American Heart Association.3 In addition, we wanted to compare our findings with those of this smaller study by Morris et al., in which the women were much younger (mean [±SD] age, 41±11 years); other characteristics of the women in the study by Bruce et al. were not well described. Moreover, in the 1973 study, Bruce and colleagues did not look at the prognostic implications of not achieving the age-predicted fitness level as assessed in their nomogram. In addition, it is critically important that the normative values we might apply to the general population be updated for a contemporary population.

Martha Gulati, M.D.
Northwestern University, Chicago, IL 60611
m-gulati@northwestern.edu

Leslee J. Shaw, Ph.D.
Cedars–Sinai Medical Center, Los Angeles, CA 90048

Morton F. Arnsdorf, M.D.
University of Chicago, Chicago, IL 60637

3 References

1. 1

   Bruce RA, Kusumi F, Hosmer D. Maximal oxygen intake and nomographic assessment of functional aerobic impairment in cardiovascular disease. Am Heart J 1973;85:546-562
   CrossRef | Web of Science | Medline

2. 2

   Morris CK, Myers J, Froelicher VF, Kawaguchi T, Ueshima K, Hideg A. Nomogram based on metabolic equivalents and age for assessing aerobic exercise capacity in men. J Am Coll Cardiol 1993;22:175-182
   CrossRef | Web of Science | Medline

3. 3

   Gibbons RJ, Balady GJ, Bricker JT, et al. ACC/AHA 2002 guideline update for exercise testing: summary article: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1997 Exercise Testing Guidelines). Circulation 2002;106:1883-1892
   CrossRef | Web of Science | Medline