Clinical acid-base pathophysiology: disorders of plasma anion gap

Association between serum anion gap and risk of in-hospital mortality in patients with acute heart failure

A high serum anion gap (AG) at the time of patient admission can lead to the deterioration or even death; data are lacking for patients who suffer acute heart failure (AHF). The present study aimed at exploring the impact of serum AG (SAG) levels on the in-hospital mortality in AHF patients. The study conducted retrospective analysis on the data from the medical information mart for intensive care (MIMIC-IV) database in severe AHF cases. Serum AG, age, sex, concomitant diseases and laboratory tests were collected from patients at admission. Multivariate Cox proportional hazard regression model together with Kaplan Meier (K-M) survival curve served for analyzing the relationship of serum AG with the hospital all-cause mortality (ACM). In addition, subgroup analysis assisted in assessing the concordance. Data from 2774 AHF patients were collected in the study. The hospital ACM rate was 9.2% (254/2774). After correcting potential confounders, multivariate analysis compared the high serum AG level (≥ 16 mmol/L) and the low serum AG level (< 16 mmol/L) (hazard ratio (HR): 1.89 [95% CI 1.42-2.51]). In a similar way, K-M survival curve indicated that hospital survival was lower in patients with high serum, suggesting that high serum AG level could lead to poor AHF prognosis. In patients with AHF, high serum AG level could increase the hospital ACM.

Data on the clinical, analytical, and laboratory factors associated with negative anion gaps at an academic medical center

The anion gap is a calculated parameter derived from the difference between the major plasma cations and anions in serum/plasma or whole blood, with a widely used simple equation utilizing concentrations of sodium, chloride, and bicarbonate. While there is extensive literature on the clinical significance and causes of elevated anion gaps, there is comparatively less data on low anion gaps. Occasionally, anion gap calculations result in a negative number (-1 or less). From the published literature, causes of these 'negative anion gaps' include laboratory error, specimen contamination or interference, hypoalbuminemia, extreme hyperkalemia, bromism, and paraproteins from multiple myeloma or similar pathologic processes. The data in this article present results from retrospective review of clinical chemistry and blood gas analysis testing at an academic medical center. The data include electrolyte concentrations and anion gap values derived from a total of 2,948,574 specimens (2,841,863 serum/plasma specimens analyzed on Roche Diagnostics clinical chemistry analyzers, 93,987 whole blood specimens analyzed on Radiometer blood gas analyzers, and 12,724 whole blood specimens on point-of-care chemistry devices) from 371,925 unique patients, clinical area where testing was ordered (for serum/plasma samples), sex, and age. For serum/plasma specimens with a negative anion gap, the data additionally include information from detailed chart review of possible factors and disease conditions contributing to the negative anion gap, pattern of electrolyte abnormalities, presence or absence of hypoalbuminemia, and corrected anion gap (if hypoalbuminemia is present).

The association between anion gap and in-hospital mortality of post-cardiac arrest patients: a retrospective study

We aimed to determine the association between anion gap and in-hospital mortality in post-cardiac arrest (CA) patients. Extracted the data of patients diagnosed with CA from MIMIC-IV database. Generalized additive model (GAM), Cox regression and Kaplan–Meier survival analysis were used to demonstrate the association between AG levels and in-hospital mortality. ROC curve analysis for assessing the discrimination of AG for predicting in-hospital mortality. Totally, 1724 eligible subjects were included in our study finally. 936 patients (551 males and 385 females) died in hospital, with the prevalence of in-hospital mortality was 54.3%. The result of the Kaplan–Meier analysis showed that the higher value of AG had significant lower survival possibility during the hospitalization compared with the lower-value of AG patients. In the crude Cox regression model, high-level of AG subjects was associated with significant higher HR compared with low-level of AG subjects. After adjusted the vital signs data, laboratory data, and treatment, high-level of AG (group Q3 and group Q4) were also associated with increased risk of in-hospital mortality compared with low-level of AG group, 1.52 (95% Cl 1.17–1.85; P < 0.001), 1.64 (95% Cl 1.21–2.08; P < 0.001), respectively. The ROC curve indicated that AG has acceptable discrimination for predicting in-hospital mortality. The AUC value was found to be 0.671 (95% CI 0.646–0.698). Higher AG levels was associated with poor prognosis in post-CA patients. AG is a predictor for predicting in-hospital mortality of CA, and could help refine risk stratification.

An Unexpected Complication Due to Metformin Use After Femur Fracture Operation: Metabolic Acidosis Without Lactic Acidosis

A 74-year- old male, who was known to have hypertension, chronic obstructive lung disease, and benign prostate hyperplasia, was evaluated preoperatively in our clinic for a femur fracture. In addition, it was found that the patient was using 1000 mg of metformin per oral due to type 2 diabetes. At the preoperative cardiology evaluation, the ejection fraction was 60% with normal systolic ventricular function. Routine laboratory tests were normal. Metformin was held 24 hours before surgery. Spinal anesthesia was applied with 10 mg bupivacaine and 20 mcg fentanyl. Total blood loss at surgery was 150 cc. After an uneventful surgery, the patient was observed at the surgical postanesthesia care unit. Cardiac and respiratory physical examinations seemed normal but the patient had minimal acidosis and hypoxia in the arterial blood gas analysis. Twelve hours after the operation, compensated high anion gap ( 30 mEq/l) metabolic acidosis emerged, but lactate was normal. The patient's urea and creatinine levels were normal in the control blood tests, and the patient's urine output was above 0.5 ml/kg. Within this period, glucose levels were around 80-140 mg/dl. To overcome metabolic acidosis, bolus intravenous 8.4 % bicarbonate solution was administered. Bicarbonate infusion was started on the continuation of metabolic acidosis and base loss despite the bolus bicarbonate treatment. Since there was no other reason for the metabolic acidosis, metformin usage was considered to cause metabolic acidosis. During this treatment period, despite high anion gap acidosis, there was no lactate elevation. The patient had normal laboratory and hemodynamic values and was discharged from the intensive care unit at postoperative Day 3.

The role of the clinical laboratory in diagnosing acid-base disorders

Acid-base homeostasis is fundamental for life. The body is exceptionally sensitive to changes in pH, and as a result, potent mechanisms exist to regulate the body's acid-base balance to maintain it in a very narrow range. Accurate and timely interpretation of an acid-base disorder can be lifesaving but establishing a correct diagnosis may be challenging. The underlying cause of the acid-base disorder is generally responsible for a patient's signs and symptoms, but laboratory results and their integration into the clinical picture is crucial. Important acid-base parameters are often available within minutes in the acute hospital care setting, and with basic knowledge it should be easy to establish the diagnosis with a stepwise approach. Unfortunately, many caveats exist, beginning in the pre-analytical phase. In the post-analytical phase, studies on the arterial reference pH are scarce and therefore many different reference values are used in the literature without any solid evidence. The prediction models that are currently used to assess the acid-base status are approximations that are mostly based on older studies with several limitations. The two most commonly used methods are the physiological method and the base excess method, both easy to use. The secondary response equations in the base excess method are the most convenient. Evaluation of acid-base disorders should always include the assessment of electrolytes and the anion gap. A major limitation of the current acid-base laboratory tests available is the lack of rapid point-of-care laboratory tests to diagnose intoxications with toxic alcohols. These intoxications can be fatal if not recognized and treated within minutes to hours. The surrogate use of the osmolal gap is often an inadequate substitute in this respect. This article reviews the role of the clinical laboratory to evaluate acid-base disorders.

On the etiology of cardiovascular diseases: A new framework for understanding literature results

The interpretative framework presented here provides a rationale for many well-known features of cardiovascular diseases. Prolonged acidemia with high blood levels of free fatty acids is proposed to shape the basic context for formation of fatty acid micelles and vesicles with an acidic core that fuse with the endothelia, disrupt vital cell processes, and initiate atherosclerotic plaque formation. It offers an explanation for the distributed localization of atherosclerotic lesions, and how mild cases of occurrence of fatty acids vesicles formed within the heart and the arteries close to the heart may cause such lesions. It provides a rationale for how acute events, namely heart attacks and strokes, may arise from stormy development of fatty acid vesicles within the heart. Additionally, a process is proposed for clot development from the existing fatty acid vesicles.

Drug-Induced Metabolic Acidosis

Metabolic acidosis could emerge from diseases disrupting acid-base equilibrium or from drugs that induce similar derangements. Occurrences are usually accompanied by comorbid conditions of drug-induced metabolic acidosis, and clinical outcomes may range from mild to fatal. It is imperative that clinicians not only are fully aware of the list of drugs that may lead to metabolic acidosis but also understand the underlying pathogenic mechanisms. In this review, we categorized drug-induced metabolic acidosis in terms of pathophysiological mechanisms, as well as individual drugs’ characteristics.

Lactic Acidosis in a Patient with Type 2 Diabetes Mellitus

Lactic acidosis occurs when lactate production exceeds its metabolism. There are many possible causes of lactic acidosis, and in any given patient, several causes may coexist. This Attending Rounds presents a case in point. Metformin's role in the pathogenesis of lactic acidosis in patients with diabetes mellitus is complex, as the present case illustrates. The treatment of lactic acidosis is controversial, except for the imperative to remedy its underlying cause. The use of sodium bicarbonate to treat the often alarming metabolic derangements may be quite efficacious in that regard but is of questionable benefit to patients. Renal replacement therapies (RRTs) have particular appeal in this setting for a variety of reasons, but their effect on clinical outcomes is untested.

Pseudohyponatremia: does it matter in current clinical practice?

Serum consists of water (93% of serum volume) and nonaqueous components, mainly lipids and proteins (7% of serum volume). Sodium is restricted to serum water. In states of hyperproteinemia or hyperlipidemia, there is an increased mass of the nonaqueous components of serum and a concomitant decrease in the proportion of serum composed of water. Thus, pseudohyponatremia results because the flame photometry method measures sodium concentration in whole plasma. A sodium-selective electrode gives the true, physiologically pertinent sodium concentration because it measures sodium activity in serum water. Whereas the serum sample is diluted in indirect potentiometry, the sample is not diluted in direct potentiometry. Because only direct reading gives an accurate concentration, we suspect that indirect potentiometry which many hospital laboratories are now using may mislead us to confusion in interpreting the serum sodium data. However, it seems that indirect potentiometry very rarely gives us discernibly low serum sodium levels in cases with hyperproteinemia and hyperlipidemia. As long as small margins of errors are kept in mind of clinicians when serum sodium is measured from the patients with hyperproteinemia or hyperlipidemia, the present methods for measuring sodium concentration in serum by indirect sodium-selective electrode potentiometry could be maintained in the clinical practice.

Acid-base indicators in the venous and arterial blood of horses affected by recurrent airway obstruction (RAO)

The acid-base equilibrium is closely linked to gas exchange in the lungs, and respiratory exchange ratios are used to evaluate respiratory effectiveness and tissue oxygen levels. Acid-base indicators are determined in both arterial and venous blood samples. This study compares the usefulness of acid-base indicators of venous and arterial blood in monitoring the condition of horses with recurrent airway obstruction. Prior to treatment involving bronchodilating glucocorticoids, expectorant and mucolytic drugs, more pronounced changes were observed in venous blood (pH 7.283, pCO2 61.92 mmHg, pO2 35.541 mmHg, HCO3- 31.933 mmHg, BE 2.933 mmol/l, O2SAT 58.366%, ctCO2 38.333 mmol/l) than in arterial blood (pH 7.309, pCO2 53.478 mmHg, pO2 90.856 mmHg, HCO3- 28.50 mmHg, BE 3.133 mmol/l, O2SAT 93.375%, ctCO2 31.652 mmol/l), indicating compensated respiratory acidosis. The improvement of respiratory efficiency minimized acidosis symptoms in both venous blood (pH 7.365, pCO2 43.55 mmHg, pO2 47.80 mmHg, HCO3 30.325 mmHg, BE 3.050 mmol/l, O2SAT 80.10%, ctCO2 29.80 mmol/l) and arterial blood (pH 7.375, pCO2 39.268 mmHg, pO2 98.476 mmHg, HCO3- 26.651 mmHg, BE 4.956 mmol/l, O2SAT 98.475%, ctCO2 28.131 mmol/l). Venous blood parameters were marked by greater deviations from mean values, both before and after treatment. Acid-base indicators determined in venous blood are indicative of respiratory disturbances, but they do not support a comprehensive evaluation of gas exchange in the lungs.

A practical approach to understanding acid-base abnormalities in critical illness

Acid-base disorders are common in the critically ill. Arterial blood gas (ABG) analysis is frequently used to identify and manage acid-base disturbances. Using a systematic problem-solving approach to acid-base disturbances will facilitate the identification and assess the progression and severity of the metabolic and respiratory abnormality. The intent of this review is to examine acid-base physiology and regulation, provide a method to evaluate a patient's acid-base disorder, and provide therapeutic interventions.

Qu'apporte le modèle de Stewart à l'interprétation des troubles de l'équilibre acide–base?

OBJECTIVES

To explain the different approaches for interpreting acid-base disorders; to develop the Stewart model which offers some advantages for the pathophysiological understanding and the clinical interpretation of acid-base imbalances.

DATA SOURCE

Record of french and english references from Medline data base. The keywords were: acid-base balance, hyperchloremic acidosis, metabolic acidosis, strong ion difference, strong ion gap.

DATA EXTRACTION

Data were selected including prospective and retrospective studies, reviews, and case reports.

DATA SYNTHESIS

Acid-base disorders are commonly analysed by using the traditional Henderson-Hasselbalch approach which attributes the variations in plasma pH to the modifications in plasma bicarbonates or PaCO2. However, this approach seems to be inadequate because bicarbonates and PaCO2 are completely dependent. Moreover, it does not consider the role of weak acids such as albuminate, in the determination of plasma pH value. According to the Stewart concept, plasma pH results from the degree of plasma water dissociation which is determined by 3 independent variables: 1) strong ion difference (SID) which is the difference between all the strong plasma cations and anions; 2) quantity of plasma weak acids; 3) PaCO2. Thus, metabolic acid-base disorders are always induced by a variation in SID (decreased in acidosis) or in weak acids (increased in acidosis), whereas respiratory disorders remains the consequence of a change in PaCO2. These pathophysiological considerations are important to analyse complex acid-base imbalances in critically ill patients. For example, due to a decrease in weak acids, hypoalbuminemia increases SID which may counter-balance a decrease in pH and an elevated anion gap. Thus if using only traditional tools, hypoalbuminemia may mask a metabolic acidosis, because of a normal pH and a normal anion gap. In this case, the association of metabolic acidosis and alkalosis is only expressed by respectively a decreased SID and a decreased weak acids concentration. This concept allows to establish the relationship between hyperchloremic acidosis and infusion of solutes which contain large concentration of chloride such as NaCl 0.9%. Finally, the Stewart concept permits to understand that sodium bicarbonate as well as sodium lactate induces plasma alkalinization. In fact, sodium remains in plasma, whereas anion (lactate or bicarbonate) are metabolized leading to an increase in plasma SID.

CONCLUSION

Due to its simplicity, the traditional Henderson-Hasselbalch approach of acid-base disorders, remains commonly used. However, it gives an inadequate pathophysiological analysis which may conduct to a false diagnosis, especially with complex acid-base imbalances. Despite its apparent complexity, the Stewart concept permits to understand precisely the mechanisms of acid-base disorders. It has to become the most appropriate approach to analyse complex acid-base abnormalities.

CRITICAL CARE ISSUES FOR THE NEPHROLOGIST: Acid-base Disturbances in the Intensive Care Unit: Metabolic Acidosis

This article will discuss metabolic acidosis and, to a lesser extent, metabolic alkalosis in the ICU setting. A classification and clinical approach will be the focus.

Severe lactic acidosis complicating metformin overdose successfully treated with high-volume venovenous hemofiltration and aggressive alkalinization

OBJECTIVE

In this report of a near-fatal metformin ingestion successfully treated with alkalinization and high-volume hemofiltration, we discuss the management of severe lactic acidosis and demonstrate that early aggressive intervention resulted in a positive outcome.

DESIGN

Case report.

SETTING

A tertiary pediatric intensive care unit.

PATIENT

The patient was a healthy 14-yr-old female found by a sibling following a seizure of unknown duration, thought to be secondary to hypoglycemia as a consequence of a self-ingestion of metformin, atenolol, and diclofenac. She responded well to advanced resuscitation but progressively developed severe lactic acidosis, bradycardia, and hypotension in addition to persistent hypoglycemia. The peak lactate level was 37.5 mmol/L with an albumin corrected anion gap of 65 mmol/L.

INTERVENTIONS

She was treated with high-volume venovenous hemofiltration and aggressive alkalinization therapy. The latter facilitated control of severe acidosis, whereas the hemofiltration removed the ingested drugs in addition to endogenously produced lactate precipitated by metformin.

CONCLUSIONS

In this case, early and aggressive treatment of the acidosis and cardiovascular compromise with inotropes, venovenous hemofiltration, and large doses of sodium bicarbonate in metformin overdose resulted in a successful outcome even in the presence of severe acidosis and very high lactate levels.