VO2max is the maximum amount of oxygen a person can take in during exercise. In a VO2max study, a patient walks on a treadmill for about five to 15 minutes and breathes through a valve; the oxygen and carbon dioxide in the expired air are measured. Results are given in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min).
"The best predictor of survival in cardiac patients is their capacity for exercise," according to Thomas Allison, Ph.D., lead author of the study and from the Mayo Clinic in Rochester, Minn.
"When we considered all of the measurable clinical variables – such as whether they had bypass surgery or whether they have diabetes or high blood pressure – the patient's capacity for exercise as measured by VO2max stood clear as the best predictor for 10-year survival."
In this study, the significance of low VO2max levels was examined in patients with coronary artery disease, the top cause of death in the United States and often a precursor to a heart attack and heart failure.
A group of 282 patients, 17 percent of them women, underwent cardiopulmonary treadmill testing at the end of cardiac rehabilitation, and were followed for an average of 9.8 years. The average age was 61 at the time of the test.
In the first two years few deaths occurred, but after that mortality was significantly higher in patients with low VO2max. Fifty-five patients had a low VO2max (less than 18 ml/kg/ min); at 10 years, almost half (42 percent) had died. Of the 227 patients who had a VO2max above or equal to 18 ml/kg/min, only 11.6 percent had died at the 10-year mark.
A Strong Indicator
Even after adjusting for age, sex and ventricular function, VO2max was a strong predictor of mortality.
The results confirm that poor functional capacity – specifically a VO2max of less than 18 ml/kg/min – is a major predictor of long-term mortality, even in a group of cardiac rehabilitation patients with good medical management, close follow-up and excellent short-term prognosis.
"This research suggests that physicians should strongly consider looking at VO2max for heart patients, not just those with heart failure, and help them preserve their exercise capacity," according to Allison. "Patients can do things, too, like exercising regularly and losing weight, to maintain their VO2max.
"It remains to be seen whether improving VO2max through better medical care or surgical intervention versus weight loss and a better exercise program will be the key to improving survival, but we suspect both are important."