VARIATION IN THE ARTERIAL TO END-TIDAL PCO(2) DIFFERENCE DURING ONE-LUNG THORACIC ANESTHESIA

We have measured the arterial to end-tidal PCo2 difference (Pa-CO2-PE' (CO2)) in 22 patients undergoing pulmonary resection in the lateral th oracotomy position during two-lung ventilation (TLV) and after transit ion to one-lung ventilation (OLV). With OLV for each patient, the prac tice of correcting the estimate by an initial measurement of (Pa-CO2 - PE'(CO2)) was evaluated by subtracting the initial (Pa-CO2 - PE'(CO2) ) from subsequent values measured at 10-min intervals. Net (uncorrecte d) and corrected differences during OLV were analysed using ANOVA. (Pa -CO2-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 (Pa-CO2-PE'(CO2)) v aried in the range 0.2-2.5kPa, while maximum (Pa-CO2-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 (Pa-CO2 - PE'(CO2)) varied in th e range - 0.7 to 0.8 kPa; individual extreme values were from -1.3 to 1.7kPa. Variation between patients was found to be greater than variat ion within patients for both net and corrected differences (F ratio = 37.0 and 10.9, respectively), although calculating a corrected differe nce did reduce variation between patients from a mean square value of 2.44 to 0.61. The wide variation in (Pa-CO2 - PE'(CO2)) suggests that the accuracy of estimation of Pa-CO2 by monitoring PE'(CO2), although improved by the use of a corrected difference, remains questionable du ring OLV.