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

Background. 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 Pa-CO2 in severely obese patients during anaesthesia. Methods. We studied patients with severe obesity (BMI greater than or equal to40 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 Pa-CO2 was averaged for each patient to provide one value, and data compared with a non-paired, two-way t-test, Fisher's exact test. Results. 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 Pa-CO2 was 0.2 (0.2) (mean, sd) kPa while the absolute difference between the FE'(CO2) and Pa-CO2 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 Pa-CO2 than FE'(CO2) in patients with severe obesity.