The cardiopulmonary exercise test grey zone; optimising fitness stratification by application of critical difference: Cardiorespiratory fitness and surgical risk stratification

Background. Cardiorespiratory fitness (CRF) can inform patient care, though to what extent natural variation in CRF influences clinical practice remains to be established. We calculated natural variation for cardiopulmonary exercise test (CPET) metrics, which may have implications for fitness stratification.

Methods. In a two-armed experiment, critical difference (CD) comprising analytical imprecision and biological variation was calculated for CRF, and thus defined the magnitude of change required to claim a clinically meaningful change. This metric was retrospectively applied to 213 patients scheduled for colorectal surgery. These patients underwent CPET and the potential for misclassification of fitness was calculated. We created a model with boundaries inclusive of natural variation (CD applied to oxygen uptake at anaerobic threshold (V̇O2-AT): 11mL O2 kg-1 min-1, peak oxygen uptake (V̇ O2 peak): 16mL O2 kg-1 min-1, and ventilatory equivalent for carbon dioxide at AT (V̇ E/V̇ CO2-AT): 36).

Results. The CD for V̇ O2-AT, V̇ O2 peak, and V̇ E/V̇ CO2-AT was 19%, 13%, and 10%, resulting in false negative and false positive rates of up to 28 and 32% for unfit patients. Our model identified boundaries for unfit and fit patients: AT < 9.2 and ≥ 13.6mL O2 kg-1 min-1, V̇O2 peak < 14.2 and ≥ 18.3mL kg-1 min-1, V̇ E/V̇ CO2-AT ≥ 40.1 and < 32.7, between which an area of indeterminate-fitness was established. With natural variation considered, up to 60% ofpatients presented with indeterminate-fitness.

Conclusions. These findings support a reappraisal of current clinical interpretation of CRF highlighting the potential for incorrect fitness stratification when natural variation is not accounted for.