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

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 (Pco₂). End-tidal monitoring in laboratory settings is known to give high-fidelity estimates of Pco₂. The correspondence between end-tidal (Pco₂) 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. Neither measure followed all known changes in Pco₂ 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 Pco₂, and that local skin temperature also has an effect. Additionally, absorption atelectasis from breathing 100% O₂ may cause Pco₂ to deviate from Pco₂. Even as a trend indicator for Pco₂, tcPco₂ is not useful under conditions that resemble those in the highly dynamic tactical jet aircraft environment. Pco₂ is also not a good indicator of CO₂ status in pilots who breathe nearly 100% O₂..