Equivalent of Carbon Dioxide at the Anaerobic Threshold Predicts Acute Exacerbations in Patients with Chronic Obstructive Pulmonary Disease: A Retrospective Cohort Study.

PURPOSE: To investigate the predictive value of the equivalent of carbon dioxide at the anaerobic threshold (EqCO2@AT) for acute exacerbations (AE) in patients with chronic obstructive pulmonary disease (COPD).

PATIENTS AND

METHODS

This retrospective cohort study included 79 patients with COPD who underwent baseline pulmonary function testing and cardiopulmonary exercise testing (CPET). EqCO2@AT = 30 was used only as an empirical threshold for descriptive stratification, rather than as an outcome-derived cutoff. Patients were also classified into non-AE and AE groups according to annualized exacerbation frequency. Baseline characteristics, pulmonary function, and CPET parameters were compared between groups. Logistic regression was used to evaluate the association between EqCO2@AT and AE risk. Predictive performance was assessed using receiver operating characteristic curves, calibration analysis, decision curve analysis, Kaplan-Meier analysis, and a simplified nomogram.

RESULTS

Of the 79 patients, 16 were classified into the low EqCO2@AT group and 63 into the high EqCO2@AT group. Patients with higher EqCO2@AT were older, had lower BMI, greater smoking exposure, lower FEV1% predicted and DLCO% predicted, reduced exercise capacity, and higher ventilatory inefficiency parameters. Compared with the non-AE group, the AE group had worse lung function, lower exercise capacity, and higher VE/VCO2 slope and EqCO2@AT. EqCO2@AT was associated with AE risk in univariable analysis (OR 1.122, 95% CI 1.027-1.226, P = 0.011) and remained significant after adjustment for age, BMI, and FEV1% predicted (OR 1.269, 95% CI 1.110-1.450, P < 0.001). The AUC of EqCO2@AT alone was 0.739, whereas the combined model achieved an AUC of 0.850. Kaplan-Meier analysis showed significantly lower AE-free survival in the high EqCO2@AT group (log-rank P = 0.002). The simplified nomogram provided an individualized visual tool for AE risk estimation.

CONCLUSION

Elevated EqCO2@AT is associated with impaired ventilatory efficiency, reduced exercise capacity, and increased AE risk in COPD. A model incorporating EqCO2@AT, age, BMI, and FEV1% predicted showed good predictive performance and may support individualized AE risk stratification. These findings suggest that CPET-derived ventilatory efficiency parameters may provide clinically useful information beyond conventional pulmonary function assessment, although validation in larger multicenter prospective cohorts is still required.