PURPOSE: Although the end-tidal PCO2 (PETCO2) tracks PaCO2, it hardly predicts accurately the PaCO2 in a given individual especially if they have underlying chronic obstructive pulmonary disease (COPD). In our clinical practice, we have observed three different patterns of relationships between PaCO2 and PETCO2 during a maximal exercise test in COPD patients: the parallel, the reverse, and the cross-over patterns (figure). The purpose of the study is to generate a precise prediction equation for PaCO2 using noninvasive variables incorporating these different relationships between PaCO2 and PETCO2.
METHODS: The exercise data of 41 patients with moderate to severe COPD were reviewed. The prediction equations for PaCO2 were generated using multiple regression techniques. The precision of the prediction equations with and without considering PaCO2-PETCO2 relationship was then compared.
RESULTS: The best prediction equation for PaCO2 without considering the pattern is PaCO2pred (mmHg) = 13.9 + 1.2×PETCO2(mmHg) –4.27×FECO2 –0.26×DLCO(ml/mmHg/min) (n = 41, R2 = 0.84, P<0.0001). Eight patients had the |PaCO2-PaCO2pred| of ≥3 mmHg. By constraining the prediction in the patients with parallel relationship between PaCO2 and PETCO2, the best prediction equation is PaCO2pred = 11.3 + 0.88×PETCO2 –2.1×IC(L) –0.64×COHb(%) (n = 34, R2 = 0.92, P<0.0001). Only one patient had the |PaCO2-PaCO2 pred| of ≥3 mmHg. The differences in anthropometrics and lung function between patients with different PaCO2-PETCO2 patterns were observed.
CONCLUSION: This approach provides insight into the mechanisms that determine the difference between PaCO2 and PETCO2. Using the conditioned prediction equation in patients with COPD similar to the “parallel pattern” group, PaCO2 can be more precisely predicted. For those patients with different anthropometrics and lung function from the patients with the parallel pattern, the general prediction formula is an option.
CLINICAL IMPLICATIONS: This study may much improve the interpretation of exercise-induced hypercapnia in patients with COPD undergoing a maximal exercise testing.
DISCLOSURE: Ming-Lung Chuang, No Financial Disclosure Information; No Product/Research Disclosure Information