August 4, 2005
Women Can Gauge what Their Fitness Level Should Be at a Given Age to Reduce Risk of Death
Researchers have developed a nomogram (alignment chart) specifically for women that can be used to predict their expected exercise capacity at any given age, as well as demonstrated that the resulting measure is a predictor of the risk of death. Women whose exercise capacity was less than 85 percent the age-predicted value had twice the risk of death compared to women reaching at least 85 percent. according to research led by Martha Gulati, MD, a cardiologist at the Bluhm Cardiovascular Institute of Northwestern Memorial Hospital, which will be published in the August 4 issue of the New England Journal of Medicine.
"This is the first study of its kind in women," said Dr. Gulati. "Despite extensive research on the role of exercise stress testing and exercise capacity, there has been a lack of data on what is normal or expected for healthy women. Until now, they have been evaluated using the men's nomogram, which does not provide an accurate assessment for women."
Exercise capacity can be estimated by performing a symptom-limited stress test based on the speed and grade of the treadmill. It is defined as the maximal oxygen uptake for a given workload and can be expressed in metabolic equivalents (MET). MET is the amount of oxygen used by an average seated person and increases with the intensity of exercise.
To utilize the nomogram to establish the percentage of predicted exercise capacity for age requires only the woman's age and exercise capacity achieved in MET on the exercise stress test. Drawing a straight line between the age and exercise capacity will allow the determination of the percentage of predicted exercise capacity for age; a value of 100 percent is the mean for any given age. Any result greater than 100 percent indicates better-than-average performance. Any result lower than 100 percent indicates some degree of functional impairment for age.
Use of the women's nomogram provides a more accurate assessment of prognosis among women than does use of the men's nomogram. "Use of the male nomogram to assess women results in a lower sensitivity, translating into more false positives," said Dr. Gulati. "That means a woman might be told she's at a higher risk for death when, in fact, she's not." The sensitivity and specificity of the survival model for predicting death from any cause among the asymptomatic women are 70 percent and 47 percent, respectively, when the women's nomogram is used. In contrast, the sensitivity and specificity are 55 and 64 percent, respectively, when the men's nomogram is used.
"As age increases, so does the difference in predicted exercise capacity between men and women," explains Dr. Gulati. "Use of the men's nomogram in this study group would have resulted in 800 more false positive results," adds Dr. Gulati.
For this study, 5,721 asymptomatic women and 4,471 symptomatic women underwent a symptom-limited treadmill test according to the Bruce protocol, the most commonly performed stress test. The nomogram was then used to determine the percentage of predicted exercise capacity for both cohorts. Survival data were obtained and a survival analysis was used to estimate the rates of death from any cause and cardiac causes in each group.
The relationship between exercise capacity and the risk of death from cardiac cause was remarkably similar for all age groups in the symptomatic cohort, with two exceptions. The youngest (less than 55 years of age) and oldest women (older than 70 years) with a poor exercise capacity had an especially high mortality rate.
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