VO2Max on a ramped cardiopulmonary test indicates the maximum amount of oxygen that is delivered to the muscles. In most people, this is limited by the strength of the heart, the blood volume and the architecture of the microvascular system in the vicinity of the muscles. In highly trained athletes, or people with substantial lung disease (25%+ reduction in lung function), this can also be limited by pulmonary (lung) factors.
Compared to age/sex norms, VO2Max is primarily an indicator of cardiovascular fitness, specifically the body adapting to intensity of exercise (it is about quality, not quantity - doing 10,000 steps per day at moderate intensity will not lead to high physical fitness).
A sedentary person can easily increase their VO2Max by 6% in a month with as little as 20 minutes of exercise (5 minutes of intense exercise reaching over 80% of true maximal heart rate, once a week for 4 weeks).
Therefore unfortunately, a single CPET test is not useful as a measure of impairment for diseases other than heart/lung diseases.
I have also done a CPET and my VO2Peak and peak heart rate was 10% higher than age/sex predicted norms. Yet despite above average cardiopulmonary fitness (at the time of the test), I still suffered substantial impairment due to symptoms.
