Departments of 1 Anesthesiology and 2 Surgery, The University of Missouri, Columbia, Missouri, USA
Corresponding author. E-mail: tobiasj@health.missouri.edu
Accepted for publication: March 30, 2003
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Abstract |
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Methods. We studied patients with severe obesity (BMI 40 kg m2) undergoing gastric bypass surgery. Carbon dioxide was measured with both FE' and TC devices. The difference between each measure (FE'CO2 and TC-CO2) and the PaCO2 was averaged for each patient to provide one value, and data compared with a non-paired, two-way t-test, Fishers exact test.
Results. We studied 30 adults (aged 1854 yr, mean 41, SD 8.0 yr; weight: 115267 kg, mean 162, SD 35 kg). The absolute difference between the TC-CO2 and PaCO2 was 0.2 (0.2) (mean, SD) kPa while the absolute difference between the FE'CO2 and PaCO2 was 0.7 (0.4) kPa (P<0.0001). The bias and precision were +0.1 (0.3) kPa for TC vs arterial carbon dioxide and 0.7 (0.4) kPa for FE' vs arterial carbon dioxide.
Conclusions. Transcutaneous carbon dioxide monitoring provides a better estimate of PaCO2 than FE'CO2 in patients with severe obesity.
Br J Anaesth 2003; 91: 498501
Keywords: anaesthesia, general; arterial pressure, measurement; complications, obesity; equipment, monitors
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Introduction |
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Transcutaneous (TC) measurement can also be used for the continuous non-invasive monitoring of carbon dioxide. We found TC measurements were accurate intraoperatively, in infants and children and adults.47 In the severely obese patient, excessive adipose tissue beneath the skin may possibly affect the accuracy of TC devices, and data regarding the accuracy of FE'CO2 monitoring in obese patients are limited. We compared the accuracy of FE'CO2 and TC-CO2 monitoring with PaCO2 in adult obese patients.
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Methods |
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The anaesthetic technique was determined by the attending anaesthesiologist. After intubation of the trachea, ventilation was controlled using a tidal volume of 8001000 ml, an I:E ratio of 1:2, positive end-expiratory pressure of 35 cmH2O, FIO2 of 0.40.6, and the ventilatory frequency adjusted to maintain normocarbia. FE'CO2 was measured using an infrared analyser with side-stream sampling at a flow rate of 150 ml min1 (Capnomac Ultima, Datex-Ohmeda, Madison, WI, USA). The FE'CO2 analyser was calibrated with a cylinder containing a calibration gas according to the manufacturers recommendations. The digital readout of the FE'CO2 is based on an algorithm that evaluates two successive waveforms and the valley between them. The FE'CO2 reported by the monitor is the maximum value from the first waveform. TC-CO2 was measured using a TCM-3 device (Radiometer, Copenhagen, Denmark). One of the authors calibrated, placed, and maintained the monitor. Before placement, the electrode was cleaned, a new membrane applied, and calibration done according to the manufacturers recommendations using a calibration gas supply. The working temperature of the electrode was set at 45°C and the electrode was placed on the palmar surface of the forearm. The area where the electrode was placed was swabbed with alcohol before placement to facilitate adhesion of the disk to the skin. The electrode was removed, recalibrated, and replaced in a different location on the forearm every 2 h to avoid thermal injury. Samples were taken for arterial blood gas (ABG) analysis at 37°C as clinically indicated throughout the surgical procedure. When a sample was obtained, the FE'CO2 and TC-CO2 were recorded on the data sheet. The FE'CO2 to PaCO2 and TC-CO2 to PaCO2 differences were averaged for each patient and counted as one value. This was done to avoid biasing the data by over-representation of any one patient as the number of blood gases obtained from each patient was not equal.
The absolute difference between the FE'CO2 and the PaCO2 was compared with the absolute difference between the TC-CO2 and the PaCO2 with a non-paired, two-way t-test. A Fishers exact test and a two-way contingency table were used to compare the number of FE'CO2 vs TC-CO2 values whose absolute difference deviated 0.5 kPa or less and 1.0 kPa or less from the actual PaCO2 value. All data are expressed as the mean (SD) with P<0.05 considered significant.
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Results |
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Discussion |
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The BlandAltman plots show that the gradient between the FE'CO2 and the arterial CO2 was greatest with hypercarbia, and we presume that these patients had a greater degree of ventilation-perfusion inequality. We found no pattern between the alveolar-arterial oxygen gradient and the FE'CO2 to arterial carbon dioxide gradient. However, further study of the alveolar-arterial oxygen gradient and its relationship to the FE'CO2 to arterial carbon dioxide gradient may be warranted.
The success of TC-CO2 monitoring depends on monitor and patient factors. Although TC-CO2 monitoring gave a more accurate reflection of PaCO2 in most of the patients in the current study, several technical factors may affect this accuracy such as trapped air bubbles, improper placement, damaged membranes, and inappropriate calibration techniques. Patient-related factors may also affect the accuracy of monitoring, such as variations in skin thickness, the presence of oedema, tissue hypoperfusion, or the use of vasoconstricting drugs.5 810 This study shows that the TC-CO2 monitor can be used in the severely obese, who may have excessive s.c. adipose tissue. We could not measure skin thickness in our patients, but no tissue oedema was present, and no patients were given vasoactive agents or had poor skin perfusion.
To limit problems related to s.c. fat, we placed the probe on the ventral aspect of the forearm where s.c. tissue may be less than in other areas. We do not know of studies comparing the accuracy of TC-CO2 from different sites.
The type and manufacturer of the TC-CO2 monitor and the intrinsic calibration factor used by the monitor can affect the accuracy of the technique. The TC-CO2 monitor of Radiometer, used in the current study, corrects for the alterations in carbon dioxide related to heating the skin to 45°C. With heating of the skin, the uncorrected TC-CO2 could be significantly greater than the PaCO2, which is measured at 37°C, because of the increased carbon dioxide production from the elevated local tissue metabolism and the altered solubility of carbon dioxide at higher temperatures.11 12 More information concerning these issues is available in the Radiometer Handbook (Radiometer, Copenhagen, Denmark).
Previous reports have described TC-CO2 monitoring in adults in the following circumstances: (i) intraoperatively during neurosurgical procedures and laparoscopy; (ii) mechanical ventilation in the ICU; (iii) non-invasive mechanical ventilation; (iv) sleep studies; (v) fibre-optic bronchoscopy; and (vi) in the immediate postoperative period.7 11 1316 Our results support previous studies using intraoperative TC-CO2 monitoring. Reid and colleagues17 compared FE'CO2 and TC-CO2 during general anaesthesia in 22 adults. The minute ventilation was adjusted at three different levels. Sixty-six data sets with the PaCO2 ranging from 28 to 62 mm Hg showed an FE' to arterial difference of 7.0 (3.1) mm Hg, and a TC to arterial difference of 2.3 (2.4) mm Hg (P<0.05). The differences were greatest at the higher PaCO2 values. Phan and colleagues18 also compared FE' and TC-CO2 during general anaesthesia in 24 adults. The bias and precision for the FE' to arterial comparison were 7.8 (6.1) and 1.6 (4.3) mm Hg, respectively, for the TC to arterial comparison.
The carbon dioxide monitors compared in the current study are not mutually exclusive. Although TC-CO2 is more accurate than FE'CO2, it needs more time to set up and has a longer response time (2030 s) compared with the FE'CO2 device. Gas monitoring is widely available and provides additional useful information aside from the carbon dioxide value such as correct position of the tracheal tube, a ventilator disconnection, and analysis of the waveform.
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References |
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