The formula used to calculate peak exercise heart rate in tests to diagnose heart conditions may be flawed because it does not account for differences between men and women, new research suggests.
The formula of "220 minus age" has been widely used to calculate the maximum number of heart beats per minute a person can achieve. Many people also use it to derive their target heart rate during a workout. Doctors use it to determine how hard a patient should exercise during a common diagnostic test known as the exercise stress test.
Stress tests, which are commonly used to help diagnose conditions such as coronary heart disease, and heart failure, require patients to exercise while their cardiac performance is monitored. Researchers from the Mayo Clinic analysed more than 25,000 stress tests undertaken by patients between 1993 and 2006. The sample included men and women 40 to 89 years of age who had no history of cardiovascular disease. They found significant differences between men and women.
The study revealed that although everybody's peak heart rate declines with age, the decline is more gradual in women. As a result, the previous formula overestimates the peak heart rate younger women can achieve and underestimates the peak heart rate of older women. An updated formula was devised to reflect those nuances.
"The standard that's currently in use is somewhat outdated," said Dr Thomas Allison, who is a cardiologist and senior author of the new study. "We want to make sure that when people do the stress test, they have an accurate expectation of what a normal peak heart rate is. Every so often, you need to recalibrate what's considered normal."
The new formula can help people better optimise their workouts and also improve the accuracy of test results.
Women in the age range of 40 to 89 years should expect their maximum heart rate to be 200 minus 67 percent of their age. In men, the formula is 216 minus 93 percent of their age. For women younger than 40, the relationship of heart rate to age may be different, as an insufficient number of tests on women younger than 40 were available to provide reliable results.
The study also showed that younger men have a lower resting heart rate and higher peak heart rate than women and that in men the heart rates rises more dramatically during exercise and returns to normal more quickly after stopping. The study did not investigate the physiological reasons behind the differences, although the researchers suggest hormones, especially testosterone, may play a role.
The previous formula was based on a study researchers now recognize as having serious limitations. For example, it included few women, a weakness common among older studies.
"It's logical that an equation developed 40 years ago based on a group that was predominantly men might not be accurate when applied to women today," Dr Allison said. "But sometimes things just get stuck."
Dr Allison pointed out that changes since the 1970s in terms of average body weight, fitness levels and attitudes toward exercise, particularly among women, justify a re-evaluation of peak heart rate norms. Other recent studies have offered updates to the formula, but this study used a larger sample size and is the first to include data from both men and women.
The research was presented at the American College of Cardiology's 63rd Annual Scientific Session.
