Transcutaneous and End-Tidal CO₂ Measurements in Hypoxia and Hyperoxia.

BACKGROUND: Transcutaneous measurement of carbon dioxide (CO₂) has been proposed for physiological monitoring of tactical jet aircrew because in some clinical settings it mirrors arterial CO₂ partial pressure (Paco₂). End-tidal monitoring in laboratory settings is known to give high-fidelity estimates of Paco₂.METHODS: The correspondence between end-tidal (PETco₂) and transcutaneous Pco₂ (tcPco₂) was examined in healthy volunteers under laboratory conditions of hyperoxia and hypoxia. Rest and exercise, skin heating and cooling, hyperventilation, and induced CO₂ retention were employed.RESULTS: Neither measure followed all known changes in Paco₂ and tcPco₂ changed when the skin temperature near the probe changed. Bland-Altman analysis showed significant nonzero slopes under most conditions. Regression analysis indicated that oxygen partial pressure (Po₂) in tissue measured as transcutaneous Po₂ (tcPo₂) is an important explanatory variable for tcPco₂ in addition to PETco₂, and that local skin temperature also has an effect. Additionally, absorption atelectasis from breathing 100% O₂ may cause PETco₂ to deviate from Paco₂.DISCUSSION: Even as a trend indicator for Paco₂, tcPco₂ is not useful under conditions that resemble those in the highly dynamic tactical jet aircraft environment. PETco₂ is also not a good indicator of CO₂ status in pilots who breathe nearly 100% O₂.Shykoff BE, Lee LR, Gallo M, Griswold CA. Transcutaneous and end-tidal CO₂ measurements in hypoxia and hyperoxia. Aerosp Med Hum Perform. 2021; 92(11):864-872.