Transcutaneous carbon dioxide measurements in anesthetized apneic patients with BMI > 35 kg/m 2

Transcutaneous carbon dioxide measurement (TcCO 2 ) offers the ability to continuously and non-invasively monitor carbon dioxide (CO 2 ) tensions when end-tidal monitoring is not possible. The accuracy of TcCO 2 has not been established in anesthetized apneic patients with obesity. In this secondary publication, we present a methods comparison analysis of TcCO 2 with the gold standard arterial PCO 2 , in adult patients with body mass index (BMI) > 35kg/m 2 who were randomized to receive high flow or low flow nasal oxygenation during post-induction apnea. Agreement between PaCO 2 and TcCO 2 at baseline, the start of apnea and the end of apnea were assessed using a non-parametric difference plot. Forty-two participants had a median (IQR) BMI of 52 (40–58.5) kg/m 2 . The mean (SD) PaCO 2 was 33.9 (4.0) mmHg at baseline and 51.4 (7.5) mmHg at the end of apnea. The bias was the greatest at the end of apnea median (95% CI, 95% limits of agreement) 1.90 mmHg (−2.64 to 6.44, −7.10 to 22.90). Findings did not suggest significant systematic differences between the PaCO 2 and TcCO 2 measures. For a short period of apnea, TcCO 2 showed inadequate agreement with PaCO 2 in patients with BMI > 35 kg/m 2 . These techniques require comparison in a larger population, with more frequent sampling and over a longer timeframe, before TcCO 2 can be confidently recommended in this setting.