In thoracic anesthesia with one lung ventilation (OLV), end-tidal CO₂ (PeTCO₂) monitoring may not be accurate to estimate PaCO₂ mainly due to ventilation/perfusion mismatching. This study aimed to asses the clinical usefulness and accuracy of a transcutaneous CO₂ (PtCCO₂) monitoring in thoracic anesthesia, compared with PeTCO₂.

18 patients of ASA physical status 1 or 2, required a long period of OLV were researched. The lungs were mechanically ventilated in the lateral decubitus position. We measured PaCO₂, PeTCO₂, PtCCO₂ during two lung ventilation (TLV) and 15 min, 30 min, 60 min after OLV. For comparing the differences of PtCCO₂/PaCO₂ and PtCCO₂/PaCO₂, we figured out bias (mean difference between values) and precision (standard deviation of bias) by using Bland-Altman plot. We compared the differences of each value, PeTCO₂/PaCO₂, PtCCO₂/PaCO₂, using the student's t-test.

The bias ± precision of PeTCO₂/PaCO₂ and PtCCO₂/PaCO₂ was each 7.82 mmHg ± 3.98 mmHg, 1.95 mmHg ± 2.66 mmHg during TLV and that of mean value of PeTCO₂/PaCO₂ and PtCCO₂/PaCO₂ was each 6.93 mmHg ± 2.6 mmHg, 2.35 mmHg ± 1.66 mmHg during OLV. PtCCO₂/PaCO₂ differences were significantly less than PeTCO₂/PaCO₂ differences (P < 0.05).

During one lung ventilation, PtCCO₂ monitoring is more useful and accurate than PeTCO₂ monitoring for assessing PaCO₂ levels.