Gulati M, Black HR, Shaw LJ, Arnsdorf MF, Merz CNB, Lauer MS, Marwick TH, Pandey DK, Wicklund RH, Thisted RA
N Engl J Med. 2005;353(5):468-475.
Background:
Exercise capacity is an independent predictor of mortality in men and women. There is a negative linear relationship between exercise capacity and age. A nomogram has been developed to estimate the percentage of predicted exercise capacity for a given age in men. To date, normative values of exercise capacity for age have not been established for women, nor have previous findings in men been validated in a female population.
Objectives:
To construct a nomogram that allows determination of age-predicted exercise capacity in women, and to assess the predictive value of this nomogram with regards to survival.
Methods:
A total of 5721 asymptomatic women from the St. James Women Take Heart Project underwent a symptom-limited, maximal stress test according to the Bruce protocol. Target heart rates were not used as a predetermined end point. Exercise capacity was estimated and measured in metabolic equivalents (MET). Linear regression was then used to estimate the mean MET achieved for age. A nomogram was developed that allowed the percentage of predicted exercise capacity to be estimated from age and the exercise capacity achieved. This nomogram was used to determine the percentage of age-predicted exercise capacity for 2 female cohorts-the original asymptomatic cohort, and a referral population of 4471 symptomatic women (from the Economics of Noninvasive Diagnostic Study) who also underwent a similar symptom-limited stress test. Survival data were obtained for both cohorts over an 8.4-year follow-up period. Cox survival analysis was used to estimate the rates of all cause and cardiac mortality in each group.
Results:
The asymptomatic cohort was younger and had fewer histories of hypertension or diabetes than the symptomatic cohort. The asymptomatic women had higher mean exercise capacities and a lower overall mortality rate during the follow-up period than the symptomatic women. The linear regression equation for predicted exercise capacity (in MET) on the basis of age derived from the original asymptomatic cohort was: predicted MET = 14.7 - (0.13 x age). The risk of all cause and cardiac mortality among asymptomatic women whose exercise capacity was <85% of the age-predicted value was twice that of women whose exercise capacity was at least 85% of the age-predicted value (HR = 2.03 and 2.44, respectively; P <.001). Results were similar for the symptomatic female cohort.
Discussion:
This study describes a nomogram for age-predicted exercise capacity in women. Deviation from the established normal values was a significant predictor of all cause and cardiac mortality among symptomatic and asymptomatic women. Furthermore, there was a greater risk of death with a greater deviation below these predictive normative values. The use of this nomogram seems to provide a more accurate assessment of prognosis among women than using the established "men's" nomogram. These findings can be incorporated into the interpretation of exercise stress tests, providing additional prognostic information for risk stratification in women.
Comment:
This study addresses the paucity of exercise data regarding exercise capacity in women; it proposes a predictive nomogram based upon age and intensity of exercise attained. One limitation of this nomogram is that it was derived from a predominantly white population. Although a decline in exercise capacity with age is well known, an apparent gender difference in this age-related decline is suggested by the study, with greater deterioration in exercise capacity with age in women compared to men. Further studies will be needed to further elucidate these possible differences between the sexes. What is consistent though is that exercise capacity is a strong predictor of mortality and cardiac events. This study highlights the important prognostic information that can be gleaned from a simple exercise stress test beyond electrocardiographic findings. Exercise capacity assessment from the MET achieved from an exercise stress test is a simple and relatively inexpensive method to help stratify patients for mortality risk. This nomogram could indeed be incorporated into clinical practice and needs to be validated in more diverse female populations.
ST