Arterial Oxygenation

To the Editor.—  The report by Sinha et al, warns that reduced "arterial oxygenation," already well-described after nonthoracic injury, may also occur following central nervous system (CNS) trauma. Yet, their conclusions are based upon inadequate documentation resulting from poor study design.Their interest in arterial oxygenation apparently arose retrospectively for their only measurement was oxygen tension (Po₂), which is insufficient for the assessment of oxygenation. The term implies tissue oxygen availability that is quantitated (per minute) as the product of the hemoglobin oxygen saturation (So₂), cardiac output, hemoglobin value, and a constant.¹ Since the oxyhemoglobin dissociation curve is almost "flat" down to 60 torr, there is little physiological difference between most of the measured oxygen pressures (eg, 60 torr = 91% saturation, 95 torr = 97% saturation).Presumably the authors had simultaneous hemoglobin values, and the hemoglobin oxygen saturation values may be approximated from the measured oxygen pressures with