The hidden secrets of a neutral pH—blood gas analysis of postoperative patients according to the Stewart approach

Abstract

Background The superiority of either the traditional or Stewart based approach to acid-base balance has focused primarily on analyzing metabolic acidemia, with little attention given to patients with neutral pH. In this study, we evaluate metabolic disturbances in patients in the immediate postoperative period focusing on patients with neutral pH, while comparing the Stewart and traditional approach. Methods We conducted a single center retrospective observational cohort study. Over a 17-month period, data on arterial blood gas analysis, electrolytes, and albumin on the morning after surgery were retrieved from patients admitted to the postsurgical high dependency unit (HDU). Albumin-corrected anion gap (AG), apparent (SIDa) and effective strong ion difference (SIDe), and strong ion gap (SIG) were calculated. Results Out of 1207 HDU admissions, 400 cases had a complete set of laboratory-data including albumin of which 281 presented with neutral pH (7.35 ≤ pH ≤ 7.45), 64 with acidemia (pH < 7.35) and 55 with alkalemia (pH > 7.45). In pH neutral patients, the following acidifying disturbances were found: SIDa was lowered in 101 (36%), and SIG was raised in 60 (21%). Base excess (BE) was decreased in 16 (6%) and corrected AG raised in 107 (38%). The alkalizing effect of hypoalbuminemia was present in 137 (49%). Out of 134 cases with normal BE and corrected AG, SIDa was lowered in 58 (43%). Out of 136 cases with normal SIDa and SIG, none had lowered BE and 28 increased AG (21%). Length of stay was significantly longer in patients with hypoalbuminemia, lowered SIDa, and increased corrected AG, but not decreased BE (hypoalbuminemia: 16 days vs. 10 days, P < 0.001; low SIDa: 15 days vs. 12 days, P = 0.015; increased AG: 16 days vs. 11 days, P < 0.001; low BE: 14 days vs. 13 days, P = 0.736). Conclusions Metabolic disturbances, characterized mainly by the presence of lowered SIDa, increased AG, and hypoalbuminemia, are frequent in our population with apparent neutral acid-base balance based on pH and base excess. These changes on the morning after surgery are associated with increased length of stay.

Acid–Base Analysis in the Operating Room: A Bedside Stewart Approach

“…In the presence of hypoalbuminemia, the correction calculation for the anion gap is the same as estimating for the albumin effect on base excess: 0.25 × (42 – albumin, g/l). 10,27,29 A less apparent point is that due to effects on both base excess and the anion gap, hypoalbuminemia can mask acidosis 13 from causes including relative hyperchloremia, increased lactate, and other ions (example 2 in Box 2). For any patient with hypoalbuminemia, the severity of acidifying strong ion and weak acids changes is masked.…”

“…For any patient with hypoalbuminemia, the severity of acidifying strong ion and weak acids changes is masked. 12,13…”

“…These translate to base excess effects of +1.5 and +2.5 mM, respectively. In the absence of intraoperative albumin therapy, plasma albumin concentrations after major surgery may be lower still, around 30 g/l, 13 a base excess effect of +3 mM. Therefore, in the absence of a known albumin concentration, the other-ion effect on base excess will often be between 1 to 4 mM more negative than calculated if not corrected for changes in albumin (examples 1, 2, and 4 in Boxes 1, 2, and 4).…”

“…In contrast, bicarbonate-based approaches cannot easily explain this inverse relationship between plasma albumin and bicarbonate despite advocates recommending correcting the anion gap for decreased albumin. 10 Further, studies of ICU patients, including postoperative patients, conclude that the Stewart approach can be superior in detecting important acid-base changes despite patient pH being in the reference range (7.35 to 7.45) [11][12][13] and no apparent acid-base disorder using bicarbonate or base excess analysis (example 4 in Box 4). In this review focusing on clinical anesthesia, I am proposing a simplified Stewart approach that incorporates base excess because it is associated with greater insight into the underlying causes including some masked by other processes.…”

Hyperlactatemia and other perioperative metabolic disturbances in neuroanesthesia

“…However, the final effect on pH is always the result of the interplay between all components of the SID and A TOT . As shown in our recent study, normal pH does not mean normal acid–base balance as opposing disturbances in SID and A TOT can balance each other out [ 4 ▪ ]. Furthermore, acid–base balance is a dynamic process.…”