The Accuracy of the Central Venous Blood Gas for Acid-Base Monitoring

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Background:Routine use of central venous blood gases (VBGs) may reduce complications from prolonged arterial cannulation. We investigated the reliability of the VBG as a substitute for arterial blood gas (ABG) in multiple care settings.Methods:We developed a VBG adjustment rule of ABG pH = VBG pH + 0.05, ABG CO2 = VBG Pco2 −5 mm Hg from prior studies and validated this relationship with simultaneous venous and arterial blood obtained from 187 medical/surgical intensive care, cardiac catheterization laboratory, and coronary care unit patients with central venous access.Results:The overall accuracy of a normal adjusted VBG (aVBG) to predict a normal ABG was 90%. After adjustment, the mean systematic difference (bias) between ABG and VBG pH decreased from 0.035 ± 0.02 to −0.015 ± 0.02 and Pco2 bias decreased from −4.5 ± 3.5 to 0.5 ± 3.5. Intraclass correlation coefficients for agreement improved after applying the adjustment rule to venous pH (from 0.84 to 0.93, P < .001) and Pco2 (from 0.66 to 0.84, P < .001). Overall diagnostic accuracy of VBG improved from 45% to 74% after adjustment. Multiple logistic regression demonstrated that the factor independently associated with discrepancy between VBG and ABG diagnoses was an abnormal aVBG (OR 6.8, 95% CI 2.8–16.5). Conclusions: Because of the high agreement between a normal aVBG with a normal ABG and the small bias between these tests, we recommend use of the adjusted central VBG.

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