Predicting walking-induced oxygen desaturations in COPD patients: a statistical model.

Background- Oxygen desaturation during walking can have important consequence on prognosis of COPD patients. However, a standard 6-minute walking test (6MWT) useful to detect walking desaturators (WD+), can be difficult to execute in some settings of COPD management, as in the community health care service. Aim of our study was to validate and evaluate the accuracy of a newly composed score of risk of oxygen desaturation during walking in COPD patients: the Walking Desaturation Score-WDS. Methods- Data on symptomatic COPD inpatients admitted for rehabilitation (derivation cohort) and outpatients referred to the local community health service (validation cohort) were recorded. By pulse-oximetry oxygen saturation (SpO2) was monitored during 6MWT to obtain minimal values (SpO2 nadir); patients were thus divided into WD+ or non-desaturators (WD-). By a regression analysis model we have assigned a weighted score proportional to the measured percentage of explained variance for each variable. Risk estimate was computed by odds ratio (OR). A Receiver Operating Curve (ROC) analysis and a Hosmer-Lemeshow (HL) goodness of fit test were then performed to measure discrimination and calibration of WDS. Results- Baseline characteristics in derivation (n=435, WD+ 74%) and validation (n=238, WD+ 37%) cohorts were different. Resting arterial oxygen saturation-SO2, arterial partial pressure of oxygen-PaO2 and forced expiratory volume in the 1st second-FEV1 % pred. were the variables predicting walking desaturation. The proportion of WD+ patients (and OR estimate) gradually increased according to WDS (range 0 to 6) and associated categories of desaturation risk (low 0-1 in total score of WDS, high 2-3, and very high 4-6) (X2<0.001). A considerable predictive discrimination (area under curve-AUC 0.90, 95% CI 0.86 to 0.93, P< 0.001) and calibration (HL X2 1.31, P=0.859) values have been shown. Conclusions- WDS accurately predicts and classifies the risk of walking desaturation in COPD patients.