Optimal Clinical Time for Reliable Measurement of Transcutaneous CO₂ with Ear Probes: Counterbalancing Overshoot and the Vasodilatation Effect

Abstract: OBJECTIVES: To determine the optimal clinical reading time for the transcutaneous measurement of oxygen saturation (SpO ) 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.