Anesth Analg 2003;97:776-779
© 2003 International Anesthesia Research Society
TECHNOLOGY, COMPUTING, AND SIMULATION
A Comparative Evaluation of Transcutaneous and End-Tidal Measurements of CO2 in Thoracic Anesthesia
Motoko Oshibuchi, MD,
Sungsam Cho, MD,
Tetsuya Hara, MD,
Shiro Tomiyasu, MD,
Tetsuji Makita, MD, and
Koji Sumikawa, MD
Department of Anesthesiology, Nagasaki University School of Medicine, Nagasaki, Japan
Address correspondence and reprint requests to Sungsam Cho, MD, Department of Anesthesiology, Nagasaki University School of Medicine, 1-7-1 Sakamoto, Nagasaki 852-8501, Japan. Address e-mail to chos{at}net.nagasaki-u.ac.jp
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Abstract
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We performed this study to assess the accuracy of transcutaneous CO2 (PTCCO2) monitoring compared with end-tidal CO2 (PETCO2) in thoracic anesthesia. Twenty-six patients undergoing pneumonectomy with thoracotomy for which a long period of one-lung ventilation (OLV) was required were studied. The lungs were mechanically ventilated in the lateral decubitus position. PTCCO2, PETCO2, and arterial CO2 (PaCO2) were simultaneously measured during two-lung ventilation (TLV) and during OLV at intervals of 15 min. All patients completed the study protocol. Bland-Altman analysis revealed a bias of -0.4 mm Hg with a precision of ±2.5 mm Hg during OLV and 1.4 mm Hg with ±4.3 mm Hg during TLV when PTCCO2 and PaCO2 were compared and revealed a bias of -5.8 mm Hg with a precision of ±4.1 mm Hg during OLV and -7.1 mm Hg with ±4.6 mm Hg during TLV when PETCO2 and PaCO2 were compared. We conclude that PTCCO2 monitoring is accurate for evaluating CO2 levels during thoracic anesthesia.
IMPLICATIONS: Our study indicates that transcutaneous CO2 is more accurate than end-tidal CO2 during either two- or one-lung ventilation in thoracic anesthesia.
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Introduction
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Mechanical ventilation is adjusted to maintain an acceptable arterial CO2 partial pressure. End-tidal CO2 (PETCO2) monitoring reflects arterial CO2 (PaCO2) and has become standard monitoring during general anesthesia (1,2). However, PETCO2 may inaccurately estimate PaCO2 when there is significant ventilation/perfusion mismatching (3). Areas with large ventilation/perfusion ratios (alveolar dead space) have a low alveolar CO2 tension, which decreases overall ETCO2 concentration.
During thoracic anesthesia, various factors may influence the PETCO2/PaCO2 difference. Most thoracic surgical patients have some degree of preoperative lung dysfunction and a history of smoking. One-lung ventilation (OLV) is often used to improve surgical exposure during thoracic procedures, and this impairs ventilation/perfusion matching (4). In addition, the lateral decubitus position impairs ventilation/perfusion matching in anesthetized patients (5,6).
Transcutaneous CO2 (PTCCO2) monitoring offers the noninvasive and continuous estimation of CO2 by sampling from "arterialized" capillary blood and is not influenced by abnormalities in pulmonary gas exchange (7,8). We therefore considered that PTCCO2 monitoring might provide a closer approximation of PaCO2 during thoracic surgery. The aim of our study was to assess the clinical usefulness and accuracy of a PTCCO2 monitor in thoracic anesthesia as compared with PETCO2.
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Methods
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This study was approved by the Ethics Committee for Human Study of Nagasaki University Hospital, and informed consent was obtained from each patient. Twenty-six ASA physical status I or II patients undergoing a pneumonectomy procedure with thoracotomy for which a long period of OLV was required were studied. Before surgery, simple spirometry (Autospirometry System 9; Minato Medical Science, Japan) and arterial blood gas analysis in room air were performed in all patients. Patients with significant cardiovascular diseases were excluded.
Midazolam 2–3 mg or hydroxyzine 25 mg was given IM 30 min before arrival at the operating room. Before the induction of general anesthesia, an epidural catheter was inserted in all patients between the fourth and the seventh thoracic interspace, depending on the site of surgery, but no epidural medication was administered until all study measurements had been taken.
Monitoring equipment included a pulse oximeter, an electrocardiogram, a rectal thermistor probe, urine output, peak airway pressure, and a radial artery catheter for direct arterial blood pressure measurement and arterial blood gas sampling. General anesthesia was induced with propofol 1–2 mg/kg and fentanyl 2 µg/kg IV, and vecuronium 0.2 mg/kg was used to facilitate endotracheal intubation with a double-lumen tube by direct laryngoscopy. Patients were mechanically ventilated with an Ohmeda anesthesia ventilator in a volume-controlled manner in the lateral decubitus position. The fresh gas flow, tidal volume, respiratory rate, and inspiratory/expiratory ratio were set at 6 L/min, 10 mL/kg, 10 breaths/min, and 1:2, respectively, during either OLV or two-lung ventilation (TLV). Patients received 1.5%–2.0% sevoflurane in oxygen from the time of intubation until 60 min after starting OLV.
The double-lumen tube was advanced into the left main stem bronchus under direct vision with a fiberoptic bronchoscope. The patients were then turned to the lateral decubitus position. The tube position was checked just before OLV was started, and the effectiveness of lung collapse was monitored during OLV by direct observation in the operative hemithorax. IV fluid and phenylephrine or fentanyl was administered so that the mean arterial blood pressure (MAP) deviated by <15% from the preinduction value. Additional doses of vecuronium were administered to achieve approximately 95% blockade of the twitch response as indicated by a blockade monitor.
PTCCO2 was measured with a TCM3 transcutaneous CO2/oxygen device (Radiometer, Copenhagen, Denmark). The monitoring technique was standardized by applying the probe to the upper part of the patient’s dependent arm in the lateral decubitus position. Monitor calibration, placement, and maintenance were performed by the staff. Before each study, the device was calibrated by using a two-point self-calibration (5% and 10% CO2). In accordance with the manufacturer’s recommendations, the working temperature of the electrode was maintained at 42°C to "arterialize" the skin capillary blood flow, and the monitor used an internal adjustment to compensate for the effects of the heated probe on CO2 tension. It took approximately 20 min for initial stabilization after probe attachment. The end-tidal concentrations of the anesthetics and CO2 were measured by a Capnomac Ultima multigas analyzer (Datex-Ohmeda, Helsinki, Finland) that was calibrated in 5% CO2 and 20.9% oxygen gas before the study. Continuous sampling was obtained from a connector attached to the proximal end of the endotracheal tube. Arterial blood samples were obtained during TLV, just before the initiation of OLV, and every 15 min during OLV for 60 min. The blood samples were analyzed with an ABL-250 blood gas analyzer (Radiometer). PTCCO2, PETCO2, and PaCO2 were measured simultaneously. The heart rate, MAP, oxygen saturation, and temperature were also recorded.
The data were analyzed by using the Bland-Altman technique (9). Bias, the mean difference between the values, and precision, the SD of the bias, were calculated for PETCO2/PaCO2 and PTCCO2/PaCO2 differences. Student’s unpaired t-tests were also used to compare PETCO2/PaCO2 and PTCCO2/PaCO2 differences. A probability value of <0.05 was regarded as significant.
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Results
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Twenty-six patients completed the study protocol, and the demographic data are shown in Table 1. Surgical procedures included lobectomy for lung cancer and granuloma (n = 18), partial resection for biopsy (n = 4), or open drainage for empyema (n = 4). A total of 130 data sets consisting of the simultaneous measurements of PTCCO2, PETCO2, and PaCO2 were obtained. The heart rate and MAP did not significantly change from the preoperative values during the study period. The body temperature remained constant between 35.5°C and 36.8°C.
Overall, PETCO2/PaCO2 differences were more than PTCCO2/PaCO2 differences. Bland-Altman analysis revealed a bias of -0.4 mm Hg with a precision of ±2.5 mm Hg during OLV and 1.4 mm Hg with ±4.3 mm Hg during TLV when PTCCO2 and PaCO2 were compared and revealed a bias of -5.8 mm Hg with a precision of ±4.1 mm Hg during OLV and -7.1 mm Hg with ±4.6 mm Hg during TLV when PETCO2 and PaCO2 were compared (Figs. 1 and 2 ). Bias and precision values were stable throughout the period of OLV (Table 2).
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Figure 1. Bland-Altman analysis of end-tidal CO2 (PETCO2) versus arterial CO2 (PaCO2) during two-lung ventilation (TLV) and one-lung ventilation (OLV). Bias and precision are labeled.
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Figure 2. Bland-Altman analysis of transcutaneous CO2 (PTCCO2) versus arterial CO2 (PaCO2) during two-lung ventilation (TLV) and one-lung ventilation (OLV). Bias and precision are labeled.
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Discussion
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This study demonstrates that PTCCO2 monitoring provides a more accurate estimation of PaCO2 than PETCO2 over a wide range during both TLV and OLV in thoracic surgery. This finding is consistent with previous studies documenting the usefulness of PTCCO2 monitoring in older pediatric patients receiving mechanical ventilation for respiratory failure (10) and adult patients during general anesthesia (7,11).
PETCO2 has become standard for monitoring CO2 concentrations during general anesthesia (1,2). However, PETCO2 is sometimes inaccurate because of ventilation/perfusion mismatching (3). In thoracic anesthesia, various factors—such as preoperative pulmonary function, smoking, position, and OLV—influence ventilation/perfusion matching. A high ventilation/perfusion ratio and deadspace tend to cause low PETCO2 relative to PaCO2, whereas a low ventilation/perfusion ratio and shunt have little effect on causing a small PETCO2 relative to PaCO2. In this study, the PETCO2/PaCO2 difference during TLV was 7.3 ± 4.2 mm Hg, which was slightly higher than that reported by Whitesell et al. (3) or Nun and Hill (12). Their variables were generally obtained in spine patients. Pansard et al. (5) and Grenier et al. (6) have shown that the PETCO2/PaCO2 difference was higher in the lateral position than in the supine position (7.9 versus 4.8 mm Hg and 7 versus 6 mm Hg, respectively). The PETCO2/PaCO2 difference during TLV in our study might have been influenced not only by the patients’ position, but also by their preoperative pulmonary function and smoking. In fact, 15 patients (58%) were smokers, and 10 patients had respiratory dysfunction (% vital capacity <80% or forced expiratory volume in 1 second <70%). Fletcher and Jonson (13) have shown that, even in the absence of respiratory symptoms, smoking increases the PETCO2/PaCO2 difference during anesthesia and artificial ventilation.
Effective PTCCO2 monitoring is dependent on both technical and patient factors. Staff must be trained in the proper use and accuracy of the technique to avoid technical problems such as trapped air bubbles, improper placement, damaged membranes, or inappropriate calibration. Only one staff member in this study, who was trained in the proper use of the technique, used the PTCCO2 monitor. Patient factors, such as tissue hypoperfusion, low cardiac output, impaired peripheral perfusion, or the administration of vasoconstricting drugs, may cause artifactual low PTCCO2 reading (14). These factors affect the ability of CO2 to diffuse from the capillary bed to the membrane of the monitor. Thus, we placed the device on the upper part of the dependent arm in the lateral position to ensure proper blood flow. Our patients were considered hemodynamically stable and did not receive large doses of vasoactive drugs during the study. This study did not address the possible influence of decreased tissue perfusion on the accuracy of the PTCCO2 monitor. In a low cardiac output state, these devices may also have limitations, as does capnography, because of their dependence on local tissue perfusion (15). In such situations, it is probably best to rely on arterial blood gas sampling for the accurate measurement of CO2 levels and acid status.
The PTCCO2 sensor is heated to improve the response time of the measurements. The working temperature of the electrode was maintained at 42°C to "arterialize" the skin capillary blood flow. Increased temperature, however, increases local blood and tissue PCO2 tensions. Transcutaneous monitors apply a temperature correction factor (anaerobic heating coefficient of blood) to estimate the PTCCO2 value at 37°C. The corrected PTCCO2 measurements, which we analyzed in this study, correlated well in virtually all clinical conditions with PaCO2 values.
It is unlikely that the PTCCO2 monitor will replace capnography, because capnography allows for the detection of life-threatening situations, such as esophageal intubation, ventilator disconnection, or pulmonary embolism. Limitations of the PTCCO2 monitor include the possibility of electrical drift of the signal, the requirements of regular maintenance and change of the electrode membrane, no display of respiratory wave form, and a risk of thermal burns. The PTCCO2 monitor could be a valuable addition to capnography in patients with an increased PETCO2/PaCO2 difference and in situations in which the continuous, noninvasive, and precise control of the CO2 level is required.
In summary, this study evaluated the accuracy of the PTCCO2 monitor as compared with PETCO2 in hemodynamically stable ASA status I or II patients. PTCCO2 monitoring could provide a more accurate estimation of PaCO2 than PETCO2 in the range of CO2 30–50 mm Hg during both TLV and OLV in thoracic anesthesia. We believe that this technique may be useful in situations such as OLV, in which PaCO2 control is essential to management.
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References
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Accepted for publication April 15, 2003.
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Abstract
Introduction
