Comparison of end-tidal and transcutaneous measures of carbon dioxide during general anaesthesia in severely obese adults

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Patients with severe obesity (body mass index (BMI) greater than 35 kg m–2) present difficulties for end-tidal carbon dioxide (FE′CO2) monitoring. Previous studies suggest that transcutaneous (TC) carbon dioxide measurements could be valuable, so we compared FE′ and TC measures with PaCO2 in severely obese patients during anaesthesia.


We studied patients with severe obesity (BMI ≥40 kg m–2) 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, Fisher’s exact test.


We studied 30 adults (aged 18–54 yr, mean 41, SD 8.0 yr; weight: 115–267 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.


Transcutaneous carbon dioxide monitoring provides a better estimate of PaCO2 than FE′CO2 in patients with severe obesity.

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