Errors in monitoring transcutaneous Pco2 on the ear*

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).

Assessing PaCO2 in acute respiratory disease: accuracy of a transcutaneous carbon dioxide device

BACKGROUND

Pulse oximetry non-invasively assesses the arterial oxygen saturation of patients with acute respiratory disease; however, measurement of the arterial partial pressure of carbon dioxide (PaCO(2)) requires an arterial blood gas. The transcutaneous partial pressure of carbon dioxide (PtCO(2) ) has been used in other settings with variable accuracy. We investigated the accuracy of a PtCO(2) device in the assessment of PaCO(2) in patients with asthma and suspected pneumonia attending the emergency department.

METHODS

Patients with severe asthma (FEV(1) < 50% predicted) or suspected pneumonia (fever, cough and respiratory rate >18/min) were enrolled. Subjects were excluded if they had a history of chronic obstructive pulmonary disease or other conditions associated with respiratory failure. Arterial blood gases were taken at the discretion of the investigator according to clinical need, and paired with a simultaneous reading from the PtCO(2) probe.

RESULTS

Twenty-five patients were studied with one set of data excluded because of poor PtCO(2) signal quality. The remaining 24 paired samples comprised 12 asthma and 12 pneumonia patients. The range of PaCO(2) was 19-64 mmHg with a median of 36.5 mmHg. Bland-Altman analysis showed a mean (SD) PaCO(2) - PtCO(2) difference of -0.13 (1.9) mmHg with limits of agreement of plus or minus 3.8 mmHg (-3.9 to +3.7).

CONCLUSION

A PtCO(2) device was accurate in the assessment of PaCO(2) in patients with acute severe asthma and suspected pneumonia when compared with an arterial blood gas. These bedside monitors have the potential to improve patient care by non-invasively monitoring patients with acute respiratory disease at risk of hypercapnia.

Optimal clinical time for reliable measurement of transcutaneous CO2 with ear probes: counterbalancing overshoot and the vasodilatation effect

OBJECTIVES:

To determine the optimal clinical reading time for the transcutaneous measurement of oxygen saturation (SpO₂) and transcutaneous CO₂ (TcPCO₂) in awake spontaneously breathing individuals, considering the overshoot phenomenon (transient overestimation of arterial PaCO₂).

EXPERIMENTAL SECTION:

Observational study of 91 (75 men) individuals undergoing forced spirometry, measurement of SpO₂ and TcPCO₂ with the SenTec monitor every two minutes until minute 20 and arterial blood gas (ABG) analysis. Overshoot severity: (a) mild (0.1–1.9 mm Hg); (b) moderate (2–4.9 mm Hg); (c) severe: (>5 mm Hg). The mean difference was calculated for SpO₂ and TcPCO₂ and arterial values of PaCO₂ and SpO₂. The intraclass correlation coefficient (ICC) between monitor readings and blood values was calculated as a measure of agreement.

RESULTS:

The mean age was 63.1 ± 11.8 years. Spirometric values: FVC: 75.4 ± 6.2%; FEV₁: 72.9 ± 23.9%; FEV₁/FVC: 70 ± 15.5%. ABG: PaO₂: 82.6 ± 13.2; PaCO₂: 39.9.1 ± 4.8 mmHg; SaO₂: 95.3 ± 4.4%. Overshoot analysis: overshoot was mild in 33 (36.3%) patients, moderate in 20 (22%) and severe in nine (10%); no overshoot was observed in 29 (31%) patients. The lowest mean differences between arterial blood gas and TcPCO₂ was −0.57 mmHg at minute 10, although the highest ICC was obtained at minutes 12 and 14 (>0.8). The overshoot lost its influence after minute 12. For SpO₂, measurements were reliable at minute 2.

CONCLUSIONS:

The optimal clinical reading measurement recommended for the ear lobe TcPCO₂ measurement ranges between minute 12 and 14. The SpO₂ measurement can be performed at minute 2.

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.