Initial Transcutaneous Pco₂ Overshoot with Ear Probe at 42°C

Abstract

Objective. To investigate an unexpectedly high initial skin CO₂ pressure with a new small earlobe probe heated to 42°C containing both transcutaneous (tcPCO₂) and pulse oximeter saturation (SpO₂) sensors. Methods. The probe was mounted on the ear lobe of six patients during abdominal or thoracic surgery and on several awake volunteers. The probe was mounted on a cheek or forearm in two other volunteers. Patients were artificially ventilated under general anesthesia at constant end-expiratory PCO₂. Results. In patients, at 8±3 min after mounting, tcPCO₂ peaked 5 mmHg higher than its final value (p = 0.0067, n = 6, paired t-test). After 25 min, tcPCO₂ was not different from PaCO₂ (arterial). Similar overshoots were recorded with this device when mounted on the arm or cheek and with a standard transcutaneous PCO₂ probe set to 42°C, mounted on the ear lobe, arm or chest of awake volunteers. In two volunteers, we found that heating the sensor to 45°C for the first 15 min on the ear, and then decreasing it to 42°C prevented overshoot, and provided valid tcPCO₂ data 3–5 min after application of the sensor. Conclusions. A temperature of 42°C may increase local skin temperature and metabolism before vasodilating more remote arteriolar control of sub-sensor capillary flow. We suggest that transcutaneous PCO₂ probes be initially set to 44–45°C for 5–15 min to induce prompt vasodilation to prevent this overshoot and then reduced to 42°C to avoid skin thermal injury in case of long-term application.