Letters to the Editor: Letter to the Editor
I thank Professor Emmett for his response1 to my Open Mind piece.2 He uses the bicarbonate approach to correctly conclude that my example patient has “…hyperchloremic (probably NaCl expansion) and anion gap (lactic) metabolic acidosis and respiratory acidosis.” I entirely agree with this. However, the simplified Stewart approach then provides an extended quantitative assessment of this problem: severe metabolic acidosis (base excess −11.5 mEq/L) predominantly due to a relative hyperchloremic effect (−12 mEq/L) that is deceptively severe as the result of both increased chloride (110 mmol/L) and decreased sodium (133 mmol/L) concentrations. The acid-base effect of lactate is relatively minor (−4 mEq/L). One point of convergence is that his calculation to correct the anion gap for decreased albumin in the bicarbonate approach is similar to that for the albumin effect on base excess in the simplified Stewart approach (answer about +6 mEq/L for the example patient).
As a critical care anesthetist, I find this quantitative simplified Stewart approach useful in managing patients at the bedside, particularly in working out the effects of fluid therapy. From this approach, the question: “Does my patient have hyperchloremic acidosis?” is answered with one easy estimate: Is Na-Cl-35 <0? Furthermore, the question, “What will a liter of 4% albumin do?” can be quantitatively estimated.
Our perceptions about acid-base are derived from training, experience, and clinical practice needs. Professor Emmett concludes that the bicarbonate-based approach is “…simpler, more straightforward and easier to understand….” As someone who has used Stewart for more than 15 years, I find the reverse to be true. Stewart becomes intuitive if one stops thinking about bicarbonate and instead focuses on the other routinely measured components of plasma chemistry. If I give a patient 30 mL/kg saline,3 their chloride goes up and they get an acidosis due to the chloride. I do not need to think much about bicarbonate. Again, I encourage clinicians to compare what I have proposed with what they do now, using blood gasses in the operating room, intensive care unit, emergency department, or the internal medicine clinic. If what they do (including the bicarbonate approach) is adequate for their needs, that is fine.
David A. Story, MBBS, MD, BMedSci, FANZCA
Department of Anaesthesia, Perioperative and
Pain Medicine Unit
The University of Melbourne
1. Emmett M. Stewart versus traditional approach to acid-base disorders. Anesth Analg. 2016;123:1063–1064.
2. Story DA. Stewart acid-base: a simplified bedside approach. Anesth Analg. 2016;123:511–515.
3. Story DA, Lees L, Weinberg L, et al. Cognitive changes after saline or plasmalyte infusion in healthy volunteers: a multiple blinded, randomized, cross-over trial. Anesthesiology. 2013;119:569–575.