A low fraction of inspired oxygen (FiO₂) increases venous deoxygenated hemoglobin concentrations, making the color of the blood darker. The present study was aimed to determine the effects of FiO₂ on the ability to discriminate venous from arterial blood.

One-hundred and sixty surgical patients undergoing percutaneous central venous access of the internal jugular vein were randomly assigned to receive an FiO₂ of 0.2, 0.4, 0.6, or 1.0 (n = 40 each) for at least 20 min prior to central line placement under general anesthesia. Vascular access was achieved with a 22-gauge needle; 2 ml of blood was withdrawn and shown to three physicians including the operator. Each of them was asked to identify the blood as 'arterial', 'venous' or 'not sure'. Simultaneous blood gas analysis of the samples was performed on blood taken from the puncture site and the artery after visual comparison to confirm blood's origin and hemodynamic measurements.

Lowering FiO₂ progressively increased venous deoxygenated hemoglobin concentrations (2.24 ± 1.12, 3.30 ± 1.08, 3.66 ± 1.15, and 3.71 ± 1.33 g/dl) in groups having an FiO₂ of 1.0, 0.6, 0.4 and 0.2, respectively (P < 0.001), thereby facilitating the 'venous' blood identification (P < 0.001). Neither heart rate nor mean arterial pressure differed among the groups. None developed hypoxemia (percutaneous hemoglobin oxygen saturation < 90%) in any group during the study period.

A low FiO₂ increases venous deoxygenated hemoglobin levels, thereby facilitating the recognition by clinicians of its venous origin in percutaneous central venous catheterization under general anesthesia.