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NEUROSURGICAL ANESTHESIA

Capnography Monitoring During Neurosurgery: Reliability in Relation to Various Intraoperative Positions

Bruno Grenier, MD^*, Eric Verchère, MD^*, Abdelghani Mesli, MD^*, Marc Dubreuil, MD, Daniel Siao, MD^*, Monique Vandendriessche, MD^*, Jacques Calès, MD^*, and Pierre Maurette, MD^*

Departments of *Anesthesiology 3 and Anesthesiology 4, University Hospital, Bordeaux, France

Address correspondence and reprint requests to B. Grenier, DAR 3, Hôpital Pellegrin, 33076 Bordeaux Cedex, France. Address e-mail to bruno.grenier{at}chu-aquitaine.fr

	Abstract

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In neurosurgery, estimation of PaCO₂ from PETCO₂ has been questioned. The aim of this study was to reevaluate the accuracy of PETCO₂ in estimating PaCO₂ during neurosurgical procedures lasting >3 h and to measure the effect of surgical positioning on arterial to end-tidal CO₂ gradient (P[a-ET]CO₂) over time. One hundred four neurosurgical patients classified into four groups (supine [SP], lateral [LT], prone [PR], sitting [ST]) were included in a prospective study. PaCO₂, PETCO₂, and P(a-ET)CO₂ were measured after induction of anesthesia (T0), after positioning (T1), each following hour (T2, T3, T4), 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 linear 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, respectively. 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 with the SP, PR, and ST groups) because of a lower PETCO₂ (26 ± 4 mm Hg). Negative P(a-ET)CO₂ (PETCO₂ > PaCO₂) occurred 22 times, only in the SP (n = 9) and ST groups (n = 13). Changes in opposite directions of PETCO₂ and PaCO₂ between two successive measurements were found in 26% of the cases. Correlation 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 Hg. The superior (13–15 mm Hg) and inferior (-5 to 0 mm Hg) limits of agreement were too large to expect PETCO₂ to replace PaCO₂. In conclusion, during neurosurgical procedures of >3 h, capnography should be performed with regular analysis of arterial blood gases for optimal ventilator adjustment.

Implications: This study, which aimed to reevaluate the ability of PETCO₂ to estimate PaCO₂ during neurosurgical procedures according to surgical position, indicates that PETCO₂ cannot replace PaCO₂ for the following reasons: scattering of individual values; occurrence of negative arterial to end-tidal CO₂ gradient (P[a-ET]CO₂; PaCO₂ and PETCO₂ variations in opposite directions; large changes in P(a-ET)CO₂ between two samples; and instability of P(a-ET)CO₂ over time.

	Introduction

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Capnography is essential for patient monitoring during general anesthesia. It allows for the detection of life-threatening situations, such as esophageal intubation, ventilator disconnection, alveolar hypoventilation, or pulmonary embolism. In neurosurgical patients, intraoperative hypercapnia (which increases cerebral blood volume due to vasodilation) and deep hypocapnia (which may cause cerebral ischemia) must be detected reliably. Therefore, capnography may be useful to provide an estimate of PaCO₂. This is true if the arterial to end-tidal CO₂ partial pressure difference (P[a-ET]CO₂) remains constant over time. Nevertheless, this assumption has been questioned, especially during long procedures such as neurosurgery (1,2). To our knowledge, the effect of intraoperative position on P(a-ET)CO₂ has only been evaluated in patients undergoing renal surgery in the "kidney rest" lateral decubitus position (3).

The aim of this study was to reevaluate the accuracy of PETCO₂ in estimating PaCO₂ during neurosurgical procedures of >3 h and to measure the effect of surgical positioning on P(a-ET)CO₂ over time.

	Methods

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The study was approved by our institutional review board, and informed consent was obtained from all subjects. Between January 1995 and February 1996, 104 patients aged 49 ± 17 yr and weighing 64 ± 13 kg (mean ± SD) undergoing intracranial (n = 82) or spinal (n = 22) procedures were enrolled into an open prospective study. Exclusion criteria included <18 yr of age, preoperative endotracheal intubation, coexisting chronic bronchopulmonary disease, intraoperative venous air embolism, or severe hemodynamic instability. According to the surgical position, patients were classified into four groups: supine (SP; n = 26), lateral (LT; n = 32), prone (PR; n = 24), or sitting (ST; n = 22) position.

The choice of drugs used (total IV anesthesia) was made by the anesthetist. After orotracheal intubation, the lungs were ventilated with a ventilator with a tidal volume of 7.5 mL/kg, a respiratory rate of 16 breaths/min, an inspiratory to expiratory ratio of 0.33, and 50% air in oxygen. In all cases, these ventilatory variables led to an initial PETCO₂ <35 mm Hg and a normal shape on the capnograph tracing. After hemodynamic stability was achieved, arterial blood was sampled from a radial catheter. At the same time, PETCO₂ was measured (T0, patient SP) at the proximal end of the tracheal tube using a CO₂ mainstream capnometer. For each patient, the capnometer was calibrated before use according to manufacturer's specifications. PaCO₂ values were analyzed at 37°C and corrected for rectal temperature. Thus, P(a-ET)CO₂ was calculated at T0 as the difference between temperature-corrected PaCO₂ and PETCO₂. Similarly, the following measurements of PaCO₂ and PETCO₂ were performed after positioning (T1), each subsequent hour (T2, T3, T4), and at the end of the procedure after the patient was returned to the SP position (T5). Patients in the ST position were managed with inflatable leg splints and a positive end-expiratory pressure (PEEP) of 10 cm H₂O. In this group, arterial blood gases at T5 were sampled after splint and PEEP withdrawal.

Results are presented as mean ± SD. The normal distribution of values was checked by using a Kolmogorov-Smirnov test. A P value <0.05 was considered significant. The relationship between PaCO₂ and PETCO₂ was determined using a linear regression and the Bland-Altman method (4), which is becoming the standard when two measurement techniques must be compared. It allows a direct and visual analysis of the results, providing a bias (mean of differences) and limits of agreement (bias ± 2 SD). The precision of the bias was defined as 95% confidence intervals. One-way analysis of variance (ANOVA) was used to compare the SP, LT, PR, and ST groups for the following variables: PaCO₂, PETCO₂, and P(a-ET)CO₂. P(a-ET)CO₂ variations over time were analyzed by using ANOVA for repeated measurements. When indicated (P < 0.05), comparison between groups was performed using a projected least significant difference Fisher's test.

	Results

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The mean durations of positioning and surgery were 4.1 ± 1 and 3.7 ± 1.3 h, respectively. Six hundred twenty-four simultaneous measurements of PaCO₂ and PETCO₂ were performed (SP group: n = 156; LT group: n = 192; PR group: n = 145; ST group: n = 131). These data were normally distributed. Mean PaCO₂, PETCO₂, and P(a-ET)CO₂ according to intraoperative position are presented in Table 1. There was no difference in PaCO₂, PETCO₂, and P(a-ET)CO₂ among the SP, PR, and ST groups. In the LT group, PETCO₂ was lower (26 ± 4 mm Hg; P < 0.05), but PaCO₂ was not different. As a result, P(a-ET)CO₂ was higher in this group (7 ± 3 mm Hg; P < 0.05). Negative P(a-ET)CO₂ values (PETCO₂ > PaCO₂) were found 22 times (4% of measurements), but only in the PR (n = 9) and ST (n = 13) groups.

View larger version (30K): [in this window] [in a new window]   Figure 1. The Bland-Altman method. PaCO2 versus PETCO2 with bias (mean) and limits of agreement (bias ± 2 SD) in the supine, lateral, prone, and sitting groups and for all patients.

View larger version (22K): [in this window] [in a new window]   Figure 2. Comparative changes in PaCO2 (PaCO2) and PETCO2 (PETCO2) in the supine, lateral, prone, and sitting groups and for all patients.

	Discussion

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taken into account. As reported by these authors, large individual variations were observed (-14 to 19 mm Hg). However, most studies dealing with P(a-ET)CO₂ have been performed in SP patients (2,7–11). To our knowledge, Pansard et al. (3) are the only investigators to have studied P(-ET)CO₂ in patients in the LT position and to take account of it.

Mean P(a-ET)CO₂ in the SP group was 6 ± 3 mm Hg. This value is similar to those reported by Askrog (7) in healthy subjects under general anesthesia (5 ± 2 mm Hg) and Kerr et al. (8) in severely head-injured patients (6 ± 6 mm Hg). However, our values are slightly lower than those reported by Russell and Graybeal (2,10) in neurosurgical patients in the operating room (7 ± 3 mm Hg), as well as in the intensive care unit (7 ± 4 mm Hg). Large individual variations were present in our study (P[a-ET]CO₂ values of 0–19 mm Hg) and were comparable to those reported by Russell and Graybeal (2) during neurosurgical procedures (P[a-ET]CO₂ values of -1 to 17 mm Hg). In the LT group, the mean P(a-ET)CO₂ (7 ± 3 mm Hg) was significantly higher than that in other groups but was comparable to the P(a-ET)CO₂ measured by Pansard et al. (3) (8 ± 3 mm Hg) in 35 patients undergoing renal surgery. These results could be explained by alterations in ventilation-perfusion ratios leading to an increase in alveolar dead space and increased P(a-ET)CO₂. This hypothesis is enforced by the fact that, in the LT group, increased P(a-ET)CO₂ resulted from a low PETCO₂, whereas PaCO₂ was the same in the four groups.

No data were found in the literature to compare results obtained in the PR and ST groups. Mean P(a-ET)CO₂ in ST patients was only 5 ± 5 mm Hg, similar to that measured in the PR group (5 ± 5 mm Hg), although the physiological mechanisms are probably different. Indeed, P(a-ET)CO₂ in the ST position was expected to be large (i.e., increase in ventilation-perfusion ratio) because of higher hemodynamic instability related to position and PEEP leading to a decrease in the perfusion component (8). Moreover, the ST position contributes to blood flow redistribution to pulmonary bases and create West's zone I (12) (collapsed pulmonary capillaries with high ventilation-perfusion ratios). However, our ST group included the more scattered values (-14 to 19 mm Hg) and 13 of the 22 negative P(a-ET)CO₂ values. This may explain a lower than expected mean P(a-ET)CO₂. In the PR position, functional residual capacity, already reduced by general anesthesia (13), further decreases because of increased abdominal pressure and impaired diaphragmatic excursion. This results in ventilation-perfusion heterogeneity with shunt effect (low (P[a-ET]CO₂). In our study, other mechanisms were probably involved because the mean P(a-ET)CO₂ was larger than expected. Consequently, further study that includes more patients should be conducted in the PR and ST groups. Direct measurements of alveolar dead space, cardiac output, pulmonary flow and ventilation distribution, and CO₂ production (and, thus, depth of anesthesia) would be especially relevant.

The mean P(a-ET)CO₂ in patients placed in the LT and PR positions did not change during the procedure. In SP patients, P(a-ET)CO₂ was lower during surgery than after induction of anesthesia. Intraoperative hypothermia could have played a role by causing a decrease in PaCO₂. However, blood gas measurements were corrected for body temperature, which excludes this hypothesis. A decrease in functional residual capacity (and then in P[a-ET]CO₂) is present during general anesthesia. It occurs from the start of anesthesia, is immediately maximal, and does not depend on the anesthetic used (13). Why this mechanism may not be involved with the LT, PR, and ST groups is unknown. In ST patients, the mean P(a-ET)CO₂ was significantly larger at the end of the operation, when patients were placed back into the SP position. This suggests an increase in alveolar dead space, which could be due to blood volume redistribution to the legs when the inflatable leg splints were removed at the time of the measurement. Despite apparent hemodynamic stability, no definitive conclusion can be made because cardiac output (and, thus, pulmonary blood flow) was not measured.

Several studies have focused on the relationship between PaCO₂ and PETCO₂, but the duration of the procedure has not been considered in the context of surgical position. Sharma et al. (6) did not report any change in P(a-ET)CO₂ in 21 patients undergoing aneurysm or brain tumor surgery of >4 h. Fifteen patients were examined SP, three PR, and three LT. Unfortunately, the authors did not specify patient position in their results, preventing any comparison with our study. Pansard et al. (3) included patients in the LT position (n = 35) undergoing renal surgery (mean duration 102 ± 41 min). After positioning and hemodynamic stability, they reported a significant increase in P(a-ET)CO₂ over time, which they explained by ventilation-perfusion heterogeneity with increased alveolar dead space. Large inter- and intraindividual variations were noted, as in our study.

PETCO₂ values greater than PaCO₂ were observed 22 times (4% of the measurements), and only in the PR and ST groups. This can be compared with results provided by Russell and Graybeal (2), who found only one negative P(a-ET)CO₂ value in 35 SP neurosurgical patients. Isert (1) identified negative P(a-ET)CO₂ values in 13% of patients undergoing brain surgery, with no mention of intraoperative position.

Unexplained negative P(a-ET)CO₂ was first reported by Nunn and Hill in 1960 (5). It was later demonstrated that negative P(a-ET)CO₂ is more frequent (12% of cases) in patients ventilated with high tidal volumes and low respiratory rates (14,15). Indeed, with such ventilation settings, there is a slow emptying of alveoli with a long time constant. However, this mechanism cannot be accepted. Ventilatory variables were the same in all of our patients. However, PEEP in the ST group could favor the emptying of alveoli with low ventilation-perfusion ratios (16). This assumption is valid only if the pulmonary blood flow remains constant. Negative P(a-ET)CO₂ has also been reported in children, during cesarean section and laparoscopies, or after cardiac surgery (11,17–20). In pregnant patients, a decrease in functional residual capacity and an increase in cardiac output are responsible for a high frequency of negative P(a-ET)CO₂ values (up to 50%). In addition to slow emptying of alveoli with a long time constant, a decrease in functional residual capacity, as well as an increase in CO₂ production, have been held responsible for negative P(a-ET)CO₂ values in children (18). Finally, during laparoscopic procedures, a further decrease in functional residual capacity is induced by the Trendelenburg position.

Linear regression is the method most frequently used to compare PETCO₂ and PaCO₂. In our study, poor correlation coefficients were obtained for all the patients and for each of the position groups. This correlation seemed too weak to reliably estimate PaCO₂ from PETCO₂. The same results are reported by several authors. For example, Russell and Graybeal (2) noted a correlation coefficient of 0.63 in 35 neurosurgical patients. In an intensive care unit values reported by the same team (9) were only slightly better (r2 = 0.71) and were comparable to those observed by Christensen et al. (21). According to Isert (1), PETCO₂ can provide a correct estimation of hyper-, normo-, or hypocapnia in only 82% of cases. Because of poor statistical validity of linear regression, the Bland-Altman method is more appropriate when two measurement techniques must be compared. However, the superior (13–15 mm Hg) and inferior (-5 to 0 mm Hg) limits of agreement in our study were too large to expect PaCO₂ and PETCO₂ to be interchangeable for clinical purposes.

We also looked for large variations of P(a-ET)CO₂ between two successive samples—11% varied by >5 mm Hg, especially in the PR and ST groups. Similarly, Isert (1) showed that 18% of the variations were above the threshold of 4 mm Hg. Finally, >25% of all changes in PaCO₂ and PETCO₂ between two successive samples varied in opposite directions. Similarly, in neurosurgical patients, Russell and Graybeal (2) reported the same pattern in 18.4% of the cases.

Although the mean P(a-ET)CO₂ value in our study is similar to values reported in the literature, a reliable estimation of PaCO₂ from PETCO₂ cannot be made for the following reasons: scattering of individual values as demonstrated by using the Bland-Altman method; occurrence of negative P(a-ET)CO₂; PaCO₂ and PETCO₂ variations in opposite directions; large changes in P(a-ET)CO₂ between two samples; and instability of P(a-ET)CO₂ over time according to position. As a result, in neurosurgical procedures of >3 h, capnography monitoring should be performed with regular analy- sis of arterial blood gases for ventilator optimal adjustment.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999