Comparison of transcutaneous with arterial and end tidal carbon dioxide during thoracic surgery - A prospective observational study

Introduction Transcutaneous carbon dioxide (PtCO2) monitoring is a noninvasive alternative to measuring carbon dioxide in the blood (PaCO2) and End-Tidal Carbon Dioxide (EtCO2) during thoracic surgery [1,2]. Continuous non-invasive monitoring of CO2 is done using capnometry. PaCO2 derived from arterial blood gas sampling is the gold standard for accurate monitoring. Recently V-Sign Sensor 2, SenTec Digital Monitoring System: SenTec AG, Therwil, Switzerland has been introduced for assessment of PtCO2 and SpO2. This study was conducted to validate the V-Sign digital sensor 2 in patients undergoing thoracic surgery by comparing PtCO2 with EtCO2 and PaCO2. Methods In 34 adult patients aged 18-81 years, simultaneous measurements of PaCO2, EtCO2 and PtCO2 gold standard at the following time points: Immediately after placement of the arterial line, just prior to initiation of one lung ventilation, 30 minutes after the initiation of one lung ventilation and at 30 minutes intervals thereafter till the end of the procedure and finally 10 minutes after resuming two lung ventilation. Statistical analyses were performed using SAS 9.4 or R. For method comparison, End-Tidalagreement between PaCO2 and PtCO2 or EtCO2 was assessed using the Bland-Altman approach. Limits of agreement were defined as mean bias ± 1.96 standard deviation (SD). Passing and Bablok plot was used as a complementary plot to estimate agreement between PaCO2 and PtCO2 or EtCO2. A mean difference of more than arterial blood mmHg was taken as clinically unacceptable. Results PtCO2 data from 5 patients were excluded from the study because of calibration/ sensor failure. General information of the patients is summarized in table 1. A Bland-Altman comparison of PtCO2 and PaCO2 revealed that the limits of agreement ranged from -14.40 to 13.36 (mean difference -0.52). The limits of agreement between EtCO2 and PaCO2 were between -32.52 to 1.19 (mean difference -15.66). (Figure 1) Discussion PtCO2, as measured by the SenTec Digital Monitor, is equally accurate and as reliable as PaCO2, whereas EtCO2 is not.