Has Stewart approach improved our ability to diagnose acid-base disorders in critically ill patients?

Physico-chemical characterization of acid base disorders in patients with COVID-19: A cohort study

BACKGROUND

Acid-base imbalance has been poorly described in patients with coronavirus disease 2019 (COVID-19). Study by the quantitative acid-base approach may be able to account for minor changes in ion distribution that may have been overlooked using traditional acid-base analysis techniques. In a cohort of critically ill COVID-19 patients, we looked for an association between metabolic acidosis surrogates and worse clinical outcomes, such as mortality, renal dialysis, and length of hospital stay.

AIM

To describe the acid-base disorders of critically ill COVID-19 patients using Stewart's approach, associating its variables with poor outcomes.

METHODS

This study pertained to a retrospective cohort comprised of adult patients who experienced an intensive care unit stay exceeding 4 days and who were diagnosed with severe acute respiratory syndrome coronavirus 2 infection through a positive polymerase chain reaction analysis of a nasal swab and typical pulmonary involvement observed in chest computed tomography scan. Laboratory and clinical data were obtained from electronic records. Categorical variables were compared using Fisher's exact test. Continuous data were presented as median and interquartile range. The Mann-Whitney U test was used for comparisons.

RESULTS

In total, 211 patients were analyzed. The mortality rate was 13.7%. Overall, 149 patients (70.6%) presented with alkalosis, 28 patients (13.3%) had acidosis, and the remaining 34 patients (16.2%) had a normal arterial pondus hydrogenii. Of those presenting with acidosis, most had a low apparent strong ion difference (SID) (20 patients, 9.5%). Within the group with alkalosis, 128 patients (61.0%) had respiratory origin. The non-survivors were older, had more comorbidities, and had higher Charlson's and simplified acute physiology score 3. We did not find severe acid-base imbalance in this population. The analyzed Stewart's variables (effective SID, apparent SID, and strong ion gap and the effect of albumin, lactate, phosphorus, and chloride) were not different between the groups.

CONCLUSION

Alkalemia is prevalent in COVID-19 patients. Although we did not find an association between acid-base variables and mortality, the use of Stewart's methodology may provide insights into this severe disease.

A Systematic Approach to Understanding Acid-Base Disorders in the Critically Ill

OBJECTIVE

The objective of this review is to discuss acid-base physiology, describe the essential steps for interpreting an arterial blood gas and relevant laboratory tests, and review the 4 distinct types of acid-base disorders.

DATA SOURCES

A comprehensive literature search and resultant bibliography review of PubMed from inception through March 7, 2023.

STUDY SELECTION AND DATA EXTRACTION

Relevant English-language articles were extracted and evaluated.

DATA SYNTHESIS

Critically ill patients are prone to significant acid-base disorders that can adversely affect clinical outcomes. Assessing these acid-base abnormalities can be complex because of dynamic aberrations in plasma proteins, electrolytes, extracellular volume, concomitant therapies, and use of mechanical ventilation. This article provides a systematic approach to acid-base abnormalities which is necessary to facilitate prompt identification of acid-base disturbances and prevent untoward morbidity and mortality.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

Many acid-base disorders result from medication therapy or are treated with medications. Pharmacists are uniquely poised as the medication experts on the multidisciplinary team to assist with acid-base assessments in the context of pharmacotherapy.

CONCLUSION

The use of a systematic approach to address acid-base disorders can be performed by all pharmacists to improve pharmacotherapy and optimize patient outcomes.

Acid-Base Disorders in the Critically Ill Patient

Acid-base disorders are common in the intensive care unit. By utilizing a systematic approach to their diagnosis, it is easy to identify both simple and mixed disturbances. These disorders are divided into four major categories: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. Metabolic acidosis is subdivided into anion gap and non-gap acidosis. Distinguishing between these is helpful in establishing the cause of the acidosis. Anion gap acidosis, caused by the accumulation of organic anions from sepsis, diabetes, alcohol use, and numerous drugs and toxins, is usually present on admission to the intensive care unit. Lactic acidosis from decreased delivery or utilization of oxygen is associated with increased mortality. This is likely secondary to the disease process, as opposed to the degree of acidemia. Treatment of an anion gap acidosis is aimed at the underlying disease or removal of the toxin. The use of therapy to normalize the pH is controversial. Non-gap acidoses result from disorders of renal tubular H + transport, decreased renal ammonia secretion, gastrointestinal and kidney losses of bicarbonate, dilution of serum bicarbonate from excessive intravenous fluid administration, or addition of hydrochloric acid. Metabolic alkalosis is the most common acid-base disorder found in patients who are critically ill, and most often occurs after admission to the intensive care unit. Its etiology is most often secondary to the aggressive therapeutic interventions used to treat shock, acidemia, volume overload, severe coagulopathy, respiratory failure, and AKI. Treatment consists of volume resuscitation and repletion of potassium deficits. Aggressive lowering of the pH is usually not necessary. Respiratory disorders are caused by either decreased or increased minute ventilation. The use of permissive hypercapnia to prevent barotrauma has become the standard of care. The use of bicarbonate to correct the acidemia is not recommended. In patients at the extreme, the use of extracorporeal therapies to remove CO 2 can be considered.

Clinical Approach to Assessing Acid-Base Status: Physiological vs Stewart

Evaluation of acid-base status depends on accurate measurement of acid-base variables and their appropriate assessment. Currently, 3 approaches are utilized for assessing acid-base variables. The physiological or traditional approach, pioneered by Henderson and Van Slyke in the early 1900s, considers acids as H⁺ donors and bases as H⁺ acceptors. The acid-base status is conceived as resulting from the interaction of net H⁺ balance with body buffers and relies on the H₂CO₃/HCO₃^- buffer pair for its assessment. A second approach, developed by Astrup and Siggaard-Andersen in the late 1950s, is known as the base excess approach. Base excess was introduced as a measure of the metabolic component replacing plasma [HCO₃^-]. In the late 1970s, Stewart proposed a third approach that bears his name and is also referred to as the physicochemical approach. It postulates that the [H⁺] of body fluids reflects changes in the dissociation of water induced by the interplay of 3 independent variables-strong ion difference, total concentration of weak acids, and PCO₂. Here we focus on the physiological approach and Stewart's approach examining their conceptual framework, practical application, as well as attributes and drawbacks. We conclude with our view about the optimal approach to assessing acid-base status.

Revisiting Stewart's Approach toward Assessment of Unidentified or Complex Acid-Base Disorders

How to cite this article: Gopaldas JA. Revisiting Stewart's Approach toward Assessment of Unidentified or Complex Acid-Base Disorders. Indian J Crit Care Med 2022;26(1):5-6.

Effects of alkaline agents on respiratory characteristics in rabbit models of respiratory failure

This study aimed to investigate the effects of alkaline agents on reducing strong inspiratory effort. Rabbits with hypercapnia or lung injury, induced via repeated lung lavage following injurious ventilation, were treated with Saline, NaHCO₃, or Trometamol. In the hypercapnia, minute ventilation and tidal volume were unchanged during NaHCO₃ administration; however, one hour after the end of NaHCO₃ these parameters decreased (82.1+/-7.8 %, 90.8+/-6.0 % of the baseline, respectively, p < 0.05). Trometamol reduced minute ventilation, tidal volume, and respiratory rate after infusion (59.8+/-19.0 %, 87.0+/-9.2 %, 68.2+/-18.4 % of the baseline, respectively, p < 0.05). Alkaline agents did not cause a large change in the cerebrospinal fluid acid-base balance. In the lung injury model, NaHCO₃ and Trometamol had little effect on ventilation. However, Trometamol reduced transpulmonary pressure. Trometamol exerted more inhibitory effects on ventilation than NaHCO₃ in the hypercapnia model, and Trometamol reduced the transpulmonary pressure in the lung injury model.

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.

Metabolic acidosis and the role of unmeasured anions in critical illness and injury

Acid-base disorders are frequently present in critically ill patients. Metabolic acidosis is associated with increased mortality, but it is unclear whether as a marker of the severity of the disease process or as a direct effector. The understanding of the metabolic component of acid-base derangements has evolved over time, and several theories and models for precise quantification and interpretation have been postulated during the last century. Unmeasured anions are the footprints of dissociated fixed acids and may be responsible for a significant component of metabolic acidosis. Their nature, origin, and prognostic value are incompletely understood. This review provides a historical overview of how the understanding of the metabolic component of acid-base disorders has evolved over time and describes the theoretical models and their corresponding tools applicable to clinical practice, with an emphasis on the role of unmeasured anions in general and several specific settings.

Effect of Intravenously Administered Crystalloid Solutions on Acid-Base Balance in Domestic Animals

Intravenous fluid therapy can alter plasma acid-base balance. The Stewart approach to acid-base balance is uniquely suited to identify and quantify the effects of the cationic and anionic constituents of crystalloid solutions on plasma pH. The plasma strong ion difference (SID) and weak acid concentrations are similar to those of the administered fluid, more so at higher administration rates and with larger volumes. A crystalloid's in vivo effects on plasma pH are described by 3 general rules: SID > [HCO3-] increases plasma pH (alkalosis); SID < [HCO3-] decreases plasma pH (alkalosis); and SID = [HCO3-] yields no change in plasma pH. The in vitro pH of commercially prepared crystalloid solutions has little to no effect on plasma pH because of their low titratable acidity. Appreciation of IV fluid composition and an understanding of basic physicochemical principles provide therapeutically valuable insights about how and why fluid therapy can produce and correct alterations of plasma acid-base equilibrium. The ideal balanced crystalloid should (1) contain species-specific concentrations of key electrolytes (Na+ , Cl- , K+ , Ca++ , Mg++ ), particularly Na+ and Cl- ; (2) maintain or normalize acid-base balance (provide an appropriate SID); and (3) be isosmotic and isotonic (not induce inappropriate fluid shifts) with normal plasma.

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.

Impact of chloride and strong ion difference on ICU and hospital mortality in a mixed intensive care population

BACKGROUND

Abnormal chloride levels are commonly observed in critically ill patients, but their clinical relevance remains a matter of debate. We examined the association between abnormal chloremia and ICU and hospital mortality. To further refine findings and integrate them into the ongoing discussion on the detrimental effects of chloride-rich solutions, the impact of strong ion difference (SID) on the same end points was assessed.

METHODS

Retrospective cohort study in an academic tertiary intensive care unit on 8830 adult patients who stayed at least 24 h in the ICU was carried out. Patients admitted after elective cardiac surgery were treated as a separate subgroup (n = 2350). Analyses were performed using multivariable logistic regression. All statistical models were extensively adjusted for confounders, including comorbidity, admission diagnosis, other electrolytes and acid-base parameters.

RESULTS

Severe hyperchloremia (>110 mmol/L), but not low (SID) was significantly associated with increased mortality in the ICU (odds ratio vs. normochloremia 1.81; 95 % CI 1.32-2.50; p < 0.001) and the hospital (odds ratio 1.49; 95 % CI 1.14-1.96; p = 0.003). Hyperchloremia and low (SID) were encountered in the majority of patients admitted after cardiac surgery (in 86.9 and 47.2 %, respectively), but were not negatively associated with mortality.

CONCLUSIONS

In the ICU, hyperchloremia at admission was associated with negative outcome. On the other hand, decreased strong ion difference did not have an impact on mortality, precluding a simple extrapolation of these findings to the ongoing discussion on the detrimental effects of chloride-rich solutions. This notion is fueled by the finding that hyperchloremia after cardiac surgery, frequently encountered and probably fluid-induced, did not seem to be deleterious.