In vivo response times for a heated skin surface CO2 electrode during rest and exercise

Abstract

The continuous, noninvasive estimation of arterial carbon dioxide tension (P_aCO₂) by heated skin surface electrodes has recently become available for pediatric patients. Skin surface (P_sCO₂) electrodes can enhance the safety of procedures such as intubation, bronchoscopy, ventilator changes, sleep studies, or measurement of the ventilatory response to CO₂ or hypoxia. However, clinical situations that include rapid changes in P_aCO₂ demand knowledge of the in vivo response time. We compared the response of a heated P_sCO₂ electrode to end tidal CO₂ (PetCO₂) during abrupt changes in inspired CO₂ from room air to 7% CO₂ and back to room air. We obtained 54 curves on nine healthy subjects. There was an initial lag time with less than a 10% change in P_sCO₂. Then P_sCO₂ approached PetCO₂ exponentially. For subjects at rest changing from breathing room air to 7% CO₂, the initial lag time was 46 ± 2 seconds and the 50% response time of the exponential portion was 46 ± 3 seconds. Thus, it took 86 seconds for the electrode to record a 60% response to an abrupt increase in inspired CO₂. The initial lag and 50% response time were considerably shorter during exercise (30 ± 2 and 33 ± 2 seconds) and even shorter when switched from breathing 7% CO₂ to room air (23 ± 2 and 21 ± 2 seconds). Exercise did not further reduce the response time when CO₂ was initially elevated, suggesting the faster response time was due to vasodilation of the skin due to elevated CO₂. Users of such electrodes should be aware that changes in ventilation will not be reflected by the CO₂ electrode for at least one minute and reaching equilibrium takes several minutes.