Transcutaneous PCO2 monitors are more accurate than end-tidal PCO2 monitors.

PURPOSE: The accuracy of monitors for measuring transcutaneous PCO2 (TcPCO2), end-tidal PCO2 (EtPCO2), and nasal EtPCO2 was evaluated. METHODS: The measuring devices included a TcPCO2 monitor (TCM3; Radiometer Trading), an EtPCO2 monitor (Ultima; Datex-Ohmeda), and a nasal EtPCO2 monitor (TG-920P; Nihon Kohden). The sensor electrode of the TCM3 TcPCO2 monitor was applied to the skin of the subject's upper arm. A sampling tube attached to the proximal end of the tracheal tube was connected to the Ultima EtPCO2 monitor. The miniature sensor of the TG-920P nasal EtPCO2 monitor was attached to the nostril. The values obtained were compared with direct measurements of arterial PCO2 (PaCO2) obtained by means of an ABL700 blood gas analyzer (Radiometer Trading) in surgically treated patients. The means +/- 2 SD of the differences between variables were calculated. RESULTS: The TcPCO2 monitor (0.19 +/- 4.8 mmHg, mean +/- 2-SD) was more accurate than the EtPCO2 monitor (-4.4 +/- 6.5 mmHg, mean +/- 2-SD) in patients receiving artificial ventilation via an endotracheal tube and the TcPCO2 monitor was also more accurate than the nasal EtPCO2 monitor (-6.3 +/- 9.8 mmHg, bias +/- 2-SD) in patients breathing spontaneously. CONCLUSION: We found that the TcPCO2 monitor was more accurate than the EtPCO2 or nasal EtPCO2 monitor in surgically treated patients.

[Method of evading the overshoot phenomenon of transcutaneous P(CO2) measurement at the ear lobe].

BACKGROUND: Kagawa et al. confirmed the overshoot of transcutaneous P(CO2) (Ptc(CO2)) during the early stage of measurement with Ptc(CO2) monitor (TOSCA, Linde Medical Sensors AG, Basel, Switzerland). We examined the method to evade this phenomenon. METHODS: Eight adult patients under general anesthesia were examined. Two probes were mounted each on the left and right ear lobes after constant end expiratory P(CO2) had been obtained for ten to fifteen minutes. One P(CO2) probe was set at 42 degrees C. Another one was set at 45 degrees C for the first 15 minutes and then decreased to 42 degrees C. RESULTS: With the probe heated at 42 degrees C, overshoot was observed in 5 out of 8 cases, the peak of which is 44 +/- 5.5 mmHg at 8.2 +/- 1.1 minutes and then these values went down to 39 +/- 4.0 mmHg at 19 +/- 1.3 minutes and stayed stable. Probes heated at 45 degrees C and placed on the opposite side did not show this phenomenon and were stabilized at 5 +/- 0.9 minutes. CONCLUSIONS: With this apparatus, we found that it is effective to avoid overshoot to heat the probe at 45 degrees C and to change at 42 degrees C after 15 minutes.