In vivo response times for a heated skin surface CO 2 electrode during rest and exercise

The continuous, noninvasive estimation of arterial carbon dioxide tension (P a CO 2 ) by heated skin surface electrodes has recently become available for pediatric patients. Skin surface (P s CO 2 ) electrodes can enhance the safety of procedures such as intubation, bronchoscopy, ventilator changes, sleep studies, or measurement of the ventilatory response to CO 2 or hypoxia. However, clinical situations that include rapid changes in P a CO 2 demand knowledge of the in vivo response time. We compared the response of a heated P s CO 2 electrode to end tidal CO 2 (PetCO 2 ) during abrupt changes in inspired CO 2 from room air to 7% CO 2 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 s CO 2 . Then P s CO 2 approached PetCO 2 exponentially. For subjects at rest changing from breathing room air to 7% CO 2 , 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 CO 2 . 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% CO 2 to room air (23 ± 2 and 21 ± 2 seconds). Exercise did not further reduce the response time when CO 2 was initially elevated, suggesting the faster response time was due to vasodilation of the skin due to elevated CO 2 . Users of such electrodes should be aware that changes in ventilation will not be reflected by the CO 2 electrode for at least one minute and reaching equilibrium takes several minutes.