Comparison of ear and chest probes in transcutaneous carbon dioxide pressure measurements during general anesthesia in adults.

OBJECTIVE

For transcutaneous carbon dioxide pressure (tcPCO(2)) measurement, the probe on the trunk or extremities has been used for many years. Our previous study showed that chest was better than arm for tcPCO(2) monitoring. Recently, the ear probe has been developed. The accuracy of tcPCO(2) as a surrogate measurement of arterial carbon dioxide pressure (PaCO(2)) has not been compared between the measurement with probe on the chest and measurement with probe on the earlobe. This study compared the accuracy of tcPCO(2) measured on the chest and tcPCO(2) measured on earlobe during general anesthesia in adults using linear regression analysis and Bland-Altman plot.

METHODS

Ten patients aged 30-70 years scheduled for abdominal surgery under general anesthesia were enrolled. TcPCO(2) by TCM4™ (Radiometer, Copenhagen, Denmark, TtcPCO(2)) with its probe on the chest, tcPCO(2) by Sentec™ (Sentec AG, Therwil, Switzerland, StcPCO(2)) with ear probe, end-tidal carbon dioxide pressure (EtCO(2)), and PaCO(2) were simultaneously measured at four different sets of EtCO(2) levels in each patient. In total, 40 measurements were performed. The Scatter plot and Bland-Altman plot were obtained. Correlation coefficient (R(2)) ≥0.70 and limits of agreement ≤4 mmHg were judged as significant.

RESULTS

TtcPCO(2) showed significant positive correlation with PaCO(2) (R(2) = 0.80) but StcPCO(2) did not (R(2) = 0.55). TtcPCO(2) and PaCO(2), and StcPCO(2) and PaCO(2) had large limits of agreement (-6.56 mmHg, 4.21 mmHg and -11.05 mmHg, 7.64 mmHg, respectively). TtcPCO(2) and StcPCO(2) had no significant correlation (R(2) = 0.63) and large limits of agreement (-8.98 mmHg to 7.91 mmHg).

CONCLUSION

During general anesthesia in adults, both TtcPCO(2) and StcPCO(2) were not interchangeable with PaCO(2), but only TtcPCO(2) had good positive correlation with PaCO(2).