Poor exercise capacity for the patients age. what does this mean? i received this comment on stress test?

The metric used during the treadmill stress test measured your oxygen uptake during exercise and yours was felt to be too low( probably less than 8). If you achieved 85% of your maximal heart rate, had no EKG changes seen with coronary blockages and had no significant symptoms of breathing difficulty or chest distress, I would question the calibration and accuracy of the gas exchange component