"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.
About Northwestern Memorial Hospital
Northwestern Memorial Hospital is one of the country's premier academic medical centers and is the primary teaching hospital of Northwestern University's Feinberg School of Medicine. Northwestern Memorial and its Prentice Women's Hospital and Stone Institute of Psychiatry have 744 beds and more than 1,200 affiliated physicians and 5,000 employees. Providing state-of-the-art care, Northwestern Memorial is recognized for its outstanding clinical and surgical advancements in such areas as cardiothoracic and vascular care, gastroenterology, neurology and neurosurgery, oncology, organ and bone marrow transplantation, and women's health.
Northwestern Memorial received the prestigious 2005 National Quality Health Care Award and is listed in eight specialties in this year's US News & World Report's issue of "America's Best Hospitals." The hospital is also cited as one of the "100 Best Companies for Working Mothers" by Working Mother magazine for the past 5 years and has been chosen by Chicagoans for a decade as their "most preferred hospital" in National Research Corporation's annual survey.
About the Bluhm Cardiovascular Institute
The Bluhm Cardiovascular Institute at Northwestern Memorial Hospital is a world-class heart program offering comprehensive services and state-of-the-art surgical treatments in all areas of cardiovascular care. Recently named by Solucient, an industry-leading healthcare information provider, as the only Chicago hospital on its list of the country's 100 Top Cardiovascular Hospitals, Northwestern Memorial Hospital offers a timely response to referrals and a multidisciplinary approach that joins physicians, nurses and a range of other medical specialists and caregivers from Cardiology, Cardiac Surgery, Vascular Surgery, Cardiovascular Anesthesiology and Radiology from evaluation to follow-up. Patients benefit from the latest minimally invasive surgical techniques and are offered the opportunity to participate in a range of clinical research trials.
Advanced Cardiovascular Care
Patients referred to the Bluhm Cardiovascular Institute experience a healthcare environment in which the most advanced diagnostic and treatment options are supported by state-of-the-art technology and a commitment to medical excellence through research. Expertise is available in all areas of cardiovascular care, including: