Schrauben SJ, Negoianu D, Costa C, Cohen RM, Goldfarb S, Fuchs BD, Berns JS: Accuracy of Acid-Base Diagnoses Using the Central Venous Blood Gas in the Medical Intensive Care Unit. Nephron. 2018 doi: 10.1159/000488501
Disturbances of acid-base homeostasis very commonly complicate critical illnesses. Arterial blood gases (ABG) combined with assessment of serum electrolytes (especially the anion gap and delta bicarbonate) are traditionally used to determine the nature of these disturbances.
Schrauben and colleagues studied an alternative approach, using venous blood gas (VBG) analyses employing blood obtained from a central venous catheter in 23 critically ill patients in a medical intensive care unit, 55% of whom were vaso-pressor dependent. The average anion gap was 9mmol/L and 70% has acute kidney injury of whom 25% required dialysis.
Venous blood gas showed an excellent sensitivity compared to with ABG for metabolic acidosis and alkalosis and respiratory acidosis but lesser sensitivity for respiratory alkalosis. The specificity for any acidosis, respiratory or metabolic, was 58%. This agreement between VBG and ABG was very good for acidosis, but less satisfactory for alkalosis, particularly respiratory alkalosis (see Table 1). While these data suggest cautious use of VBG as a substitute for ABG in diagnosis of acid-base disorders in critically ill patients, the performance of such VBG-based diagnostic approaches have inherent weaknesses which must be understood. Arterial blood gas determinations remain as the “gold-standard” for diagnosis of acid-base disorders, but VBG can be utilized as a non-invasive substitute for ABG, as they have high sensitivity for detecting and classifying acidosis, but are less valuable for detecting respiratory alkalosis.
Table 1: Percent agreement of central VBG-based diagnosis compared to ABG-based diagnosis