Facing acid-base disorders in the third millennium - the Stewart approach revisited

Analysis of acid-base disorders in an ICU cohort using a computer script

Background/aims

Acid–base status is important for understanding pathophysiology, making a diagnosis, planning effective treatment and monitoring progress of critically ill patients. Manual calculations are cumbersome, easily result in wrong conclusions. We wanted to develop an automated assessment of acid–base status.

Methods

A simplified adaptive MATLAB script processing all available theory to date was created, evaluated and used on blood gas analyses drawn immediately after admission to ICU. The script was compared to golden standard, calculating manually by two experienced ICU physicians.

Results

Results from the script correlated completely with detailed manual calculations of randomly chosen 100 blood gas results and it was able to deliver complex data on cohort level with advanced graphics. The initial blood gas analyses from 8875 admissions constituted the cohort, of which 4111 (46.3%) were normal. Respiratory acidosis was the primary disturbance in 2753 (31.0%) and metabolic acidosis in 464 (5.2%). Respiratory alkalosis was the primary disturbance in 1501 (17.0%) and metabolic alkalosis in 46 (0.5%). Of the disturbances 74.7% were mixed with two and 2.1% with three simultaneous disturbances. Acidoses were less compensated compared to alkaloses.

Conclusions

Acid–base theories are developed on ideal models and not on critical care patients, they require inputs that might not be available, and therefore, estimations are needed. In our cohort, it was difficult to develop a working script based on Stewart, whereas Boston/Copenhagen worked better. Acidoses were more common and more deviated compared to alkaloses.

Management of Osteoarthritis: Expert Opinion on NSAIDs

Osteoarthritis (OA) is a leading cause of disability among older adults worldwide. Treatment aims are to alleviate inflammatory pain and improve physical function through non-pharmacological and pharmacological interventions. Non-steroidal anti-inflammatory drugs (NSAIDs) are recommended as first-line therapy. However, selection is challenged by patient age, comorbidities and polypharmacy, and by the drug's benefit/risk balance, all of which together influence the risk of cardiovascular (CV), gastrointestinal (GI) and renal adverse events (AEs). While the efficacy profile of the various NSAIDs is delineated, the differences in their safety profile are not straightforward. This narrative review provides practical indications by a multidisciplinary Italian expert panel for general practitioners and specialists managing OA patients with chronic inflammatory pain; the goal is to maximize therapy efficacy while reducing untoward effects caused by inappropriate NSAID use. The discussion on the best approach to NSAIDs spanned the following topics: (1) patient evaluation: investigate pain origin, duration and components together with possible risk factors for CV, GI and renal AEs; (2) non-pharmacological interventions: the physiatrist provides a person-centered, holistic approach accounting for all patient aspects; (3) pharmacological interventions: patient profile and drugs' pharmacological properties affect NSAID selection, which drugs to be used in combination or to be avoided, formulation and therapy duration; (4) the pharmacologist's, general practitioner's and pain therapist's points of view; (5) NSAID safety: the individual baseline risk and the drug's safety profile are major determinants of CV, GI and renal risk; consider possible drug-drug interactions; (6) periodical re-evaluation of treatment response and adherence, using scales to assess pain and function.

Connecting two worlds: positive correlation between physicochemical approach with blood gases and pH in pediatric ICU setting

Objective

Physicochemical approach such as strong ion difference provides a novel concept in understanding and managing acid–base disturbance in patients. However, its application in pediatrics is limited. This study aimed to evaluate a correlation between the physicochemical approach and blood gas pH for acid–base determination in critically ill pediatric patients.

Results

A total of 130 pediatric patients were included, corresponding to 1338 paired measures for analyses. Of these, the metabolic subgroup (743 paired measures) was defined. Among physicochemical parameters, the effective strong ion difference showed the best correlation with the blood gas pH in the whole cohort (R = 0.398; p < 0.001) and the metabolic subgroup (R = 0.685; p < 0.001). Other physicochemical parameters (i.e., the simplified and the apparent strong ion difference, the strong ion gap, and the sodium chloride gap) and the traditional measures (standard base excess, lactate, chloride and bicarbonate) also showed varying degrees of correlation. This study revealed the positive correlation between physicochemical parameters and the blood gas pH, serving as a connecting dot for further investigations using physicochemical approach to evaluate acid–base disturbance in pediatric population.

Citrate: How to Get Started and What, When, and How to Monitor?

In most of the case, regional citrate anticoagulation is using diluted citrate around 1% depending on the types used in clinical practice. Diluted citrate is much more safer when compared to highly concentrated citrate around 4% or even more. In clinical practice, trisodium citrate is used in high concentration (around 30%) as a bactericidal agent with anticoagulant properties for locking deep venous catheters used in hemodialysis (HD; close to 25–30% of citrate). In this review article, buffer and anticoagulant potential of citrate are discussed during renal replacement therapy in critically ill patients with particular focus on the practical approach at the bedside.

Traditional approach versus Stewart approach for acid-base disorders: Inconsistent evidence

Purpose

The traditional approach and the Stewart approach have been developed for evaluating acid-base phenomena. While some experts have suggested that the two approaches are essentially identical, clinical researches have still been conducted on the superiority of one approach over the other one. In this review, we summarize the concepts of each approach and investigate the reasons of the discrepancy, based on current evidence from the literature search.

Methods

In the literature search, we completed a database search and reviewed articles comparing the Stewart approach with the traditional, bicarbonate-centered approach to November 2016.

Results

Our literature review included 17 relevant articles, 5 of which compared their diagnostic abilities, 9 articles compared their prognostic performances, and 3 articles compared both diagnostic abilities and prognostic performances. These articles show a discrepancy over the abilities to detect acid-base disturbances and to predict patients' outcomes. There are many limitations that could yield this discrepancy, including differences in calculation of the variables, technological differences or errors in measuring variables, incongruences of reference value, normal range of the variables, differences in studied populations, and confounders of prognostic strength such as lactate.

Conclusion

In conclusion, despite the proposed equivalence between the traditional approach and the Stewart approach, our literature search shows inconsistent results on the comparison between the two approaches for diagnostic and prognostic performance. We found crucial limitations in those studies, which could lead to the reasons of the discrepancy.

Modern and traditional approaches combined into an effective gray-box mathematical model of full-blood acid-base

Background

The acidity of human body fluids, expressed by the pH, is physiologically regulated in a narrow range, which is required for the proper function of cellular metabolism. Acid-base disorders are common especially in intensive care, and the acid-base status is one of the vital clinical signs for the patient management. Because acid-base balance is connected to many bodily processes and regulations, complex mathematical models are needed to get insight into the mixed disorders and to act accordingly. The goal of this study is to develop a full-blood acid-base model, designed to be further integrated into more complex human physiology models.

Results

We have developed computationally simple and robust full-blood model, yet thorough enough to cover most of the common pathologies. Thanks to its simplicity and usage of Modelica language, it is suitable to be embedded within more elaborate systems. We achieved the simplification by a combination of behavioral Siggaard-Andersen’s traditional approach for erythrocyte modeling and the mechanistic Stewart’s physicochemical approach for plasma modeling. The resulting model is capable of providing variations in arterial pCO2, base excess, strong ion difference, hematocrit, plasma protein, phosphates and hemodilution/hemoconcentration, but insensitive to DPG and CO concentrations.

Conclusions

This study presents a straightforward unification of Siggaard-Andersen’s and Stewart’s acid-base models. The resulting full-blood acid-base model is designed to be a core part of a complex dynamic whole-body acid-base and gas transfer model.

Electronic supplementary material

The online version of this article (10.1186/s12976-018-0086-9) contains supplementary material, which is available to authorized users.

Acid-base disturbances in nephrotic syndrome: analysis using the CO2/HCO3 method (traditional Boston model) and the physicochemical method (Stewart model)

Background

The Stewart model for analyzing acid–base disturbances emphasizes serum albumin levels, which are ignored in the traditional Boston model. We compared data derived using the Stewart model to those using the Boston model in patients with nephrotic syndrome.

Methods

Twenty-nine patients with nephrotic syndrome and six patients without urinary protein or acid–base disturbances provided blood and urine samples for analysis that included routine biochemical and arterial blood gas tests, plasma renin activity, and aldosterone. The total concentration of non-volatile weak acids (A_TOT), apparent strong ion difference (SIDa), effective strong ion difference (SIDe), and strong ion gap (SIG) were calculated according to the formulas of Agrafiotis in the Stewart model.

Results

According to the Boston model, 25 of 29 patients (90%) had alkalemia. Eighteen patients had respiratory alkalosis, 11 had metabolic alkalosis, and 4 had both conditions. Only three patients had hyperreninemic hyperaldosteronism. The Stewart model demonstrated respiratory alkalosis based on decreased PaCO₂, metabolic alkalosis based on decreased A_TOT, and metabolic acidosis based on decreased SIDa. We could diagnose metabolic alkalosis or acidosis with a normal anion gap after comparing delta A_TOT [(14.09 − measured A_TOT) or (11.77 − 2.64 × Alb (g/dL))] and delta SIDa [(42.7 − measured SIDa) or (42.7 − (Na + K − Cl)]). We could also identify metabolic acidosis with an increased anion gap using SIG > 7.0 (SIG = 0.9463 × corrected anion gap—8.1956).

Conclusions

Patients with nephrotic syndrome had primary respiratory alkalosis, decreased A_TOT due to hypoalbuminemia (power to metabolic alkalosis), and decreased levels of SIDa (power to metabolic acidosis). We could detect metabolic acidosis with an increased anion gap by calculating SIG. The Stewart model in combination with the Boston model facilitates the analysis of complex acid–base disturbances in nephrotic syndrome.

Chloride content of solutions used for regional citrate anticoagulation might be responsible for blunting correction of metabolic acidosis during continuous veno-venous hemofiltration

Background

Citrate, the currently preferred anticoagulant for continuous veno-venous hemofiltration (CVVH), may influence acid-base equilibrium.

Methods

The effect of 2 different citrate solutions on acid-base status was assessed according to the Stewart-Figge approach in two consecutive cohorts of critically ill adult patients. The first group received Prismocitrate 10/2 (PC10/2; 10 mmol citrate/L). The next group was treated with Prismocitrate 18/0 (PC18; 18 mmol citrate/L). Both groups received bicarbonate-buffered fluids in post-dilution.

Results

At similar citrate flow, the metabolic acidosis present at baseline in both groups was significantly attenuated in PC18 patients but persisted in PC10/2 patients after 24 h of treatment (median pH 7,42 vs 7,28; p = 0.0001). Acidosis in the PC10/2 group was associated with a decreased strong ion difference and an increased strong ion gap (respectively 43 vs. 51 mmol/L and 17 vs. 12 mmol/L, PC10/2 vs. PC18; both p = 0.001). Chloride flow was higher in PC10/2 than in PC18 subjects (25.9 vs 14.3 mmol/L blood; p < 0.05).

Conclusion

Correction of acidosis was blunted in patients who received 10 mmol citrate/L as regional anticoagulation during CVVH. This could be explained by differences in chloride flow between the applied citrate solutions inducing hyperchloremic acidosis.