Transcutaneous Oxygen and Carbon Dioxide Tensions Compared to Arterial Blood Gases in Normals

BACKGROUND: Most hyperbaric medicine centers do not monitor arterial oxygen (P_aO2) and carbon dioxide (P_aCO2) tensions during hyperbaric oxygen, but many can transcutaneously monitor oxygen (P_tcO2) and carbon dioxide (P_tcCO2).

METHODS: We compared P_tcO2 and P_tcCO2 measurements to simultaneous P_aO2 and P_aCO2 measurements in 10 healthy volunteers to determine if P_tcO2 and P_tcCO2 measurements are surrogates for P_aO2 and P_aCO2 in the hyperbaric environment. We took blood samples via arterial catheter and took P_tcO2 and P_tcCO2 chest measurements while the subjects were compressed in a monoplace hyperbaric chamber at pressures between 0.85 atmospheres absolute (atm abs) (our local atmospheric pressure, at altitude 1,300 m) and 3.0 atm abs, while the subjects breathed air, then oxygen.

RESULTS: The P_tcO2 correlated with P_aO2 (r² = 0.99). Under all the conditions, the P_tcO2 values were lower than P_aO2 values by approximately 10%. The P_tcCO2 was 2-6 mm Hg higher than the P_aCO2, but the correlation was low (r² = 0.21).

CONCLUSIONS: The P_tcO2 in normal humans may be used to estimate the P_aO2. The P_tcCO2 may not be an adequate reflection of the P_aCO2. It is unknown if P_tcO2 and P_tcCO2 measurements in critically ill patients can replace P_aO2 and P_aCO2 measurements.