Variation in the arterial to end-tidal Pco2 difference during one-lung thoracic anaesthesia

We have measured the arterial to end-tidal PCO2 difference (PaCO2 − PE′CO2) in 22 patients undergoing pulmonary resection in the lateral thor-acotomy position during two-lung ventilation (TLV) and after transition to one-lung ventilation (OLV). With OLV for each patient, the practice of correcting the estimate by an initial measurement of (PaCO2 − PE′CO2) was evaluated by subtracting the initial (PaCO2 − PE′CO2) from subsequent values measured at 10-min intervals. Net (uncorrected) and corrected differences during OLV were analysed using ANOVA. (PaCO2 − PE′CO2) values during TLV and OLV were similar: mean (SD) 1.3 (0.6) kPa and 1.2 (0.7) kPa, respectively (ns). Mean (PaCO2 − PE′CO2) varied in the range 0.2–2.5 kPa, while maximum (PaCO2 − PE′CO2) was 0.3–2.8kPa. The mean (SD) of 133 pairs of measurements with OLV was 1.1 (0.7) kPa. Even after correction, mean (PaCO2 − PE′CO2) varied in the range -0.7 to 0.8 kPa; individual extreme values were from -1.3 to 1.7 kPa. Variation between patients was found to be greater than variation within patients for both net and corrected differences (F ratio = 37.0 and 10.9, respectively), although calculating a corrected difference did reduce variation between patients from a mean square value of 2.44 to 0.61. The wide variation in (PaCO2 − PE′CO2) suggests that the accuracy of estimation of PaCO2 by monitoring PE′CO2 although improved by the use of a corrected difference, remains questionable during OLV. (Br. J. Anaesth. 1994; 72: 21–24)