Relationship Between Arterial and Venous Bicarbonate Values

We determined the clinical efficacy of using the venous CO₂ value, as obtained with routine "electrolytes," in acid-base management. Venipuncture samples for venous CO₂ content and chloride concentrations were obtained in 336 patients with arterial blood pH, Pao₂ Paco₂, and oxygen saturation determinations. The linear correlation between actual calculated arterial HCO₃and the measured venous CO₂ was significant (P <.001). Using venous CO₂, chloride, and arterial pH values, we present various prediction equations for estimating arterial HCO₃-. We determined the effects of time delay, exposure to air, and acute changes in arterial blood pH and Paco₂ on venous CO ₁evels. Venous CO₂ determinations should not be substituted for the arterial HCO₃- value in the Henderson-Hasselbalch equation to calculate arterial blood pH or Paco₂. Clinically, the venous CO₂ value has little direct use, but when venous CO₂ content is abnormal, it should alert the clinician to the need for obtaining arterial blood gas and pH values.

(Arch Intern Med 139:440-442, 1979).