The venous-arterial difference in CO 2 should be interpreted with caution in case of respiratory alkalosis in healthy volunteers.

The venous-arterial difference in CO2 (ΔCO2 ) has been proposed as an index of the adequacy of tissue perfusion in shock states. We hypothesized that the variation in PaCO2 (hyper- or hypocapnia) could impact ΔCO2 , partly through microcirculation adaptations. Fifteen healthy males volunteered to participate. For hypocapnia condition (hCO2 ), the subjects were asked to hyperventilate, while they were asked to breathe a gas mixture containing 8 % CO2 for hypercapnia condition (HCO2 ). The 2 conditions were randomly assigned. Blood gases were measured at baseline before each condition, and after 5-7 min of either hCO2 or HCO2 condition. Microcirculation was assessed by the muscle reoxygenation slope measured with near infrared spectroscopy following a vascular occlusion test and by skin circulation with in vivo reflectance confocal microscopy. ΔCO2 was significantly increased with hCO2 while it tended to decrease with HCO2 (non-significant). HCO2 induced a moderate increase of the resaturation slope of NIRS oxygenation. Skin microcirculatory blood flow significantly dropped with hCO2 , while it remained unchanged with hypercapnia. Our results warrant cautious interpretation of ΔCO2 as an indicator of tissue perfusion during respiratory alkalosis.