Capnography monitoring during neurosurgery: Reliability in relation to various intraoperative positions

In neurosurgery, estimation of PaCO2 from PETCO2 has been questioned. The a im of this study was to reevaluate the accuracy of PETCO2 in estimating PaC O2 during neurosurgical procedures lasting >3 h and to measure the effect o f surgical positioning on arterial to end-tidal CO2 gradient (P[a-ET]CO2) o ver time. One hundred four neurosurgical patients classified into four grou ps (supine [SP], lateral [LT], prone [PR], sitting [STI) were included in a prospective study. PaCO2, PETCO2, and P(a-ET)CO2 were measured after induc tion of anesthesia (T0), after positioning (T1), each following hour (T2, T 3, T3), and at the end of the procedure after return to the SP position (T5 ). Data are expressed as the mean +/- SD, and statistical analysis used lin ear regression, the Bland-Altman method, and analysis of variance. The mean durations of positioning and surgery were 4.1 +/- 1 h and 3.7 +/- 1.3 h, r espectively. We performed 624 simultaneous measurements of PaCO2 (33 +/- 5 mm Hg) and PETCO2 (27 +/- 4 mm Hg), leading to a mean P(a-ET)CO2 of 6 +/- 4 mm Hg. P(a-ET)CO2 of the LT group (7 +/- 3 mm Hg) was larger (compared wit h the SP, PR, and ST groups) because of a lower PETCO2 (26 1 4 mm Hg). Nega tive P(a-ET)CO2 (PETCO2) Pace,) occurred 22 times, only in the SP (n = 9) a nd ST groups (n = 13). Changes in opposite directions of PETCO2 and PaCO2 b etween two successive measurements were found in 26% of the cases. Correlat ion coefficients in the four groups (PaCO2 versus PETCO2) were not in good agreement (0.46 to 0.62; P < 0.001). The mean bias was between 5 and 7 mm H g. The superior (13-15 mm Hg) and inferior (-5 to 0 mm Hg) limits of agreem ent were too large to expect PETCO2 to replace PaCO2. In conclusion, during neurosurgical procedures of >3 h, capnography should be performed with reg ular analysis of arterial blood gases for optimal ventilator adjustment. Im plications: This study, which aimed to reevaluate the ability of PETCO2 to estimate Pace, during neurosurgical procedures according to surgical positi on, indicates that PETCO2 cannot replace Pace, for the following reasons: s cattering of individual values; occurrence of negative arterial to end-tida l CO2 gradient (P[a-ET]CO2; PaCO2 and PETCO2 variations in opposite directi ons; large changes in P(a-ET)CO2 between two samples; and instability of P( a-ET)CO2 over time.