Michael S. Kristensen

Background The duration of apnoeic oxygenation with high‐flow nasal oxygen is limited by hypercapnia and acidosis and monitoring of arterial carbon dioxide level is therefore essential. We have performed a study in patients undergoing prolonged apnoeic oxygenation where we monitored the progressive hypercapnia with transcutaneous carbon dioxide. In this paper, we compared the transcutaneous carbon dioxide level with arterial carbon dioxide tension. Methods This is a secondary publication based on data from a study exploring the limits of apnoeic oxygenation. We compared transcutaneous carbon dioxide monitoring with arterial carbon dioxide tension using Bland–Altman analyses in anaesthetised and paralysed patients undergoing prolonged apnoeic oxygenation until a predefined limit of pH 7.15 or PCO 2 of 12 kPa was reached. Results We included 35 patients with a median apnoea duration of 25 min. Mean pH was 7.14 and mean arterial carbon dioxide tension was 11.2 kPa at the termination of apnoeic oxygenation. Transcutaneous carbon dioxide monitoring initially slightly underestimated the arterial tension but at carbon dioxide levels above 10 kPa it overestimated the value. Bias ranged from −0.55 to 0.81 kPa with limits of agreement between −1.25 and 2.11 kPa. Conclusion Transcutaneous carbon dioxide monitoring provided a clinically acceptable substitute for arterial blood gases but as hypercapnia developed to considerable levels, we observed overestimation at high carbon dioxide tensions in patients undergoing apnoeic oxygenation with high‐flow nasal oxygen.