Acid-base physiology: the 'traditional' and the 'modern' approaches

The sodium-chloride difference: A marker of prognosis in patients with acute myocardial infarction

BACKGROUND

The difference between serum sodium and chloride ion concentrations (SCD) may be considered as a surrogate of a strong ion difference and may help to identify patients with a worse prognosis. We aimed to assess SCD as an early prognostic marker among patients with myocardial infarction.

METHODS

Data of 594 consecutive patients with acute myocardial infarction treated with PCI (44.9% STEMI patients; 70.7% males) was analysed for SCD in relation to their 30-day mortality. A restricted cubic spline regression model was used to study the relationship between mortality and SCD. Cox regression models were used to assess the association between SCD and the mortality risk.

RESULTS

Patients with Killip class ≥3 had lower SCD values in comparison to patients with Killip class ≤2: (32.0 [30.0-34.0] vs. 33.0 [31.0-36.0], p = .006). The overall 30-day mortality was 7.7% (n = 46). There was a significant difference in SCD values between survivors and non-survivors groups of patients (median (IQR): (33.0 [31.0-36.0] vs. 31.5 [28.0-34.0] (mmol/L), p = .002). The restricted cubic splines model confirmed a non-linear association between SCD and mortality. Patients with SCD <30 mmol/L (in comparison to SCD ≥30 mmol/L) had an increased mortality risk (unadjusted HR 2.92, 95% CI 1.59-5.36, p = .001; and an adjusted HR 2.30, 95% CI 1.02-5.19, p = .04).

CONCLUSIONS

Low SCD on admission is associated with an increased risk of 30-day mortality in patients with acute myocardial infarction treated with PCI and may serve as a useful prognostic marker for these patients.

Prognostic Potential of Thrombocyte Indices, Acute Phase Proteins, Electrolytes and Acid-Base Markers in Canine Parvovirus Infected Dogs With Systemic Inflammatory Response Syndrome

Dogs with canine parvovirus enteritis (CPVE) that develop systemic inflammatory response syndrome (SIRS) frequently have a poor prognosis. The aim of the study was to assess the prognostic potential of thrombocyte indices, acute phase proteins, electrolytes, and acid-base markers in CPVE puppies with SIRS (CPVE-SIRS+) at admission. A case-controlled, prospective, and observational study was performed on 36 CPVE puppies. Mean concentrations of C-reactive protein (CRP), albumin, thrombocyte count, mean platelet volume (MPV), platelet distribution width (PDW), sodium (Na+), potassium (K+), chloride (Cl-) and ionized calcium (iCa) were measured and strong ion difference 3 (SID3), ATOT-albumin and ATOT-total protein were determined in CPVE-SIRS+ survivors and nonsurvivors. A prognostic cut-off value for predicting the disease outcome was determined by receiver operating characteristic (ROC) curve analysis. The mean values of MPV, PDW and CRP were significantly higher and the mean values of albumin, Cl- and ATOT-albumin were significantly lower in CPVE-SIRS+ nonsurvivor than CPVE-SIRS+ survivor puppies on the day of admission, but the thrombocyte count, Na+, K+, iCa, SID3 and ATOT- total protein values did not differ significantly. The positive predictive values (PPVs) for survival using cut-off value of MPV (≤15.08 fL), PDW (≤14.85%), CRP (≤180.7 mg/L), albumin (≥1.795 g/dL), Cl- (≥96.00 mmol/L), and ATOT-albumin (≥7.539) were determined as 100%, 100%, 100%, 80%, 100%, and 80%, respectively with better area under ROC curve and sensitivity. Based on sensitivity, specificity, and PPVs from ROC analysis, it is concluded that the determination of Cl- concentration and MPV at admission followed by CRP will serve as the most appropriate biomarkers in predicting the disease outcome of CPVE puppies that develop SIRS.

Effect of systemic inflammatory response syndrome on thrombocytogram, acute phase proteins, electrolytes, acid-base indices and cytokine expression in naturally canine parvovirus infected dogs

Systemic inflammatory response syndrome (SIRS) in canine parvoviral enteritis (CPVE) is associated with high mortality in young puppies. Changes in acute phase response, thrombocytogram, inflammatory cytokine profiles, and disturbances in electrolyte and acid-base homeostasis are thought to have a significant impact on the development of SIRS. However, the mechanisms causing these perturbations have not been well described in CPVE puppies, especially with SIRS. The purpose of this study was to assess the changes of electrolytes, acid-base indices using strong ion model, acute phase proteins and thrombocytogram in blood and expressions of inflammatory cytokines in blood mononuclear cells of CPVE puppies with or without SIRS at admission. Additionally, the positive predictive value (PPV) and cut-off value with specificity and sensitivity of the biomarkers were determined by receiver operating characteristic (ROC) curve analysis to predict the development of SIRS in CPVE puppies at admission. A case-controlled, prospective and observational study was conducted on fifteen SIRS-positive CPVE, twenty-one SIRS-negative CPVE and six healthy puppies. Our data showed marked hyponatremia, hypokalemia, hypoalbuminemia and hypoproteinemia, decreased A_Tot-albumin and A_{Tot-total protein} and increased mean platelet volume (MPV), platelet distribution width (PDW) and C-reactive protein (CRP) concentration and up-regulation of TNF-α, IL-8 and IL-10 expressions in SIRS-positive CPVE puppies as compared to SIRS-negative CPVE puppies at admission. Based on sensitivity, specificity and AUC from ROC curve analysis and PPV, the CRP concentration in serum at a cut-off value of 141.9 mg/L and TLC of blood at a cut-off value of 3.355 × 10³/μL were identified as potential prognostic biomarkers followed by A_{Tot-total protein} and total protein at a cut-off value of 11.80 and 4.72 g/dL, respectively to predict the development of SIRS in CPVE puppies at admission. In conclusion, the findings of the current study will help the canine practitioners to institute the time-sensitive and need based interventions to disrupt progression along the continuum of shock and multi-organ dysfunction syndrome in CPVE puppies that develop SIRS at admission.

Acid-base balance and cerebrovascular regulation

The regulation and defence of intracellular pH is essential for homeostasis. Indeed, alterations in cerebrovascular acid-base balance directly affect cerebral blood flow (CBF) which has implications for human health and disease. For example, changes in CBF regulation during acid-base disturbances are evident in conditions such as chronic obstructive pulmonary disease and diabetic ketoacidosis. The classic experimental studies from the past 75+ years are utilized to describe the integrative relationships between CBF, carbon dioxide tension (PCO₂ ), bicarbonate (HCO₃ ^- ) and pH. These factors interact to influence (1) the time course of acid-base compensatory changes and the respective cerebrovascular responses (due to rapid exchange kinetics between arterial blood, extracellular fluid and intracellular brain tissue). We propose that alterations in arterial [HCO₃ ^- ] during acute respiratory acidosis/alkalosis contribute to cerebrovascular acid-base regulation; and (2) the regulation of CBF by direct changes in arterial vs. extravascular/interstitial PCO₂ and pH - the latter recognized as the proximal compartment which alters vascular smooth muscle cell regulation of CBF. Taken together, these results substantiate two key ideas: first, that the regulation of CBF is affected by the severity of metabolic/respiratory disturbances, including the extent of partial/full acid-base compensation; and second, that the regulation of CBF is independent of arterial pH and that diffusion of CO₂ across the blood-brain barrier is integral to altering perivascular extracellular pH. Overall, by realizing the integrative relationships between CBF, PCO₂ , HCO₃ ^- and pH, experimental studies may provide insights to improve CBF regulation in clinical practice with treatment of systemic acid-base disorders.

The value of oxygen index and base excess in predicting the outcome of neonatal acute respiratory distress syndrome

OBJECTIVE

This study aimed to identify the predictors and threshold of failure in neonatal acute respiratory distress syndrome.

METHODS

Newborns with severe acute respiratory distress syndrome aged 0-28 days and gestational age ≥36 weeks were included in the study if their cases were managed with non-extra corporal membrane oxygenation treatments. Patients were divided into two groups according to whether they died before discharge. Predictors of non-extra corporal membrane oxygenation treatment failure were sought, and the threshold of predictors was calculated.

RESULTS

A total of 103 patients were included in the study. A total of 77 (74.8%) survived hospitalization and were discharged, whereas 26 (25.2%) died. Receiver operating characteristic analysis of oxygen index, pH, base excess, and combinations of these indicators demonstrated the advantage of the combination of oxygen index and base excess over the others variables regarding their predictive ability. The area under the curve for the combination of oxygen index and base excess was 0.865. When the cut-off values of oxygen index and base excess were 30.0 and -7.4, respectively, the sensitivity and specificity for predicting death were 77.0% and 84.0%, respectively. The model with base excess added a net reclassification improvement of 0.090 to the model without base excess.

CONCLUSION

The combination of oxygen index and base excess can be used as a predictor of outcomes in neonates receiving non-extra corporal membrane oxygenation treatment for acute respiratory distress syndrome. In neonates with acute respiratory distress syndrome, if oxygen index >30 and base excess <-7.4, non-extra corporal membrane oxygenation therapy is likely to lead to death.

Comparison of a modified Story approach to traditional evaluation of acid-base disturbances in patients with shock: a cohort study

To compare whether the diagnostic evaluation of metabolic acidosis can be improved by using a modified Story method compared to the traditional evaluation in a population of critically ill patients with shock. This prospective cohort study included shock patients admitted to the ICU of a tertiary hospital in Brazil between May 2018 and November 2019. We collected laboratory data necessary for traditional evaluation and the simplified Stewart's method. During the study period, 149 patients were included in the final analysis. Of the 17 patients with a normal SBE and AG_corrected, 13 (76.5%) presented with metabolic acidosis according to the modified Story assessment. Therefore, of the 149 patients included in the study, the traditional approach failed to identify metabolic acidosis that was identified by the modified Story assessment in 13 (8.7%) patients. In addition, the determination of the severity of metabolic acidosis also differed between the two methods by a mean of - 7.8 mEq/L. We found that a modified Story method can identify and quantify metabolic acidosis in patients with disorders that were not revealed by the traditional approach.

Acid-base effects of continuous infusion furosemide in clinically stable surgical ICU patients: an analysis based on the Stewart model

OBJECTIVES

We sought to test the strength of correlation between predicted and observed systemic acid-base status based on the Stewart model equations during continuous infusion (CI) furosemide therapy.

DESIGN, SETTING AND PARTICIPANTS

This was a prospective, single-center, observational study conducted in the Surgical ICU of a large academic medical center. Ten critically ill patients who received CI furosemide were included.

MAIN OUTCOMES AND MEASURES

The primary purpose was to characterize the relationship between changes in serum electrolyte and acid-base status and the excretion of electrolytes in the urine during infusion of CI furosemide in critically ill patients. As a secondary endpoint, we sought to evaluate the predictive application of the Stewart model. Over 72-h, intake and output volumes, electrolyte content of fluids administered, plasma and urine electrolytes, urine pH, and venous blood gases were collected. Predicted and observed changes in acid-based status were compared for each day of diuretic therapy using Spearman's correlation coefficient.

RESULTS

The mean (SD) strong ion difference (SID) increased from 45.2 (3.2) at baseline to 49.6 (4.0) after 72 h of continuous infusion furosemide. At Day 1, the mean SID (observed) (SD) was 47.5 (3.5) and the predicted SID was 49.5 (5.8). Day 1 observed plasma SID was positively correlated with the predicted SID (r_s = 0.80, p = 0.01). By Days 2 and 3, the correlations of observed and predicted SID were no longer statistically significant.

CONCLUSIONS AND RELEVANCE

Using the Stewart model, increases in SID as an indicator of metabolic alkalosis due to the chloruretic effects of furosemide were observed. Predicted and observed SID correlated well over the first 24 h of treatment.

Effects of mineral waters on acid-base status in healthy adults: results of a randomized trial

Background

The ‘Western diet’ typically consumed in industrialized countries is characterized by high amounts of processed cereal grains and animal products while being low in vegetables, tubers, and fruits. This dietary behavior leads to imbalances of acid–base status in favor of the acids and may cause low-grade metabolic acidosis (LGMA) that is associated with negative effects on health in the long run, including urolithiasis, bone loss, and even cardiometabolic diseases. Therefore, it has become of great interest to find dietary strategies that can be used to neutralize the acid load associated with Western diets.

Objective

The aim of this study was to investigate whether the diet-dependent net acid load can be reduced by the daily consumption of mineral waters with different bicarbonate content and different potential renal acid load (PRAL).

Methods

A single-centered, randomized trial including 129 healthy men and women aged from 18 to 75 years was conducted. Participants consumed 1,500–2,000 mL of one of four mineral waters with different bicarbonate content and different PRAL values daily for 4 weeks: low bicarbonate, high PRAL (LBHP, HCO₃⁻: 403.0 mg/L, PRAL: 10.7); medium-high bicarbonate, medium PRAL (MBMP, HCO₃⁻ : 1816.0 mg/L, PRAL: −10.8); high bicarbonate, low PRAL (HBLP, HCO₃⁻: 2451.0 mg/L, PRAL: −19.3); medium-high bicarbonate, low PRAL (MBLP, HCO₃⁻: 1846.0 mg/L, PRAL: −22.1). Throughout the study, participants were asked to maintain their usual dietary habits. The primary outcome was the net acid excretion (NAE) measured in the 24-h urine output.

Results

Consumption of the three mineral waters: MBMP, HBLP, and MBLP led to a significant decrease in NAE values. Within the MBMP group, the NAE could be reduced by 48% (P = 0.001), while consumption of HBLP led to a reduction of 68% (P < 0.001) and MBLP to a reduction of 53% (P = 0.001). Moreover, a slight increase in serum bicarbonate could also be observed in the groups that drank HBLP (P = 0.057) and MBLP (P = 0.001).

Conclusion

Daily consumption of at least 1,500–2,000 mL of mineral water rich in bicarbonate (>1800.0 mg/L) with medium or low PRAL (<−11 mEq/L) can effectively reduce the NAE level by reducing the dietary acid load under free-living conditions in healthy adults.

Monitoring of Acid-base and Regulating Variables during Abdominal Lavage

This prospective clinical study was performed to determine acid-base regulating variables during abdominal lavage treatment for patients with severe peritonitis or after abdominal surgery. Arterial blood was sampled from twelve patients with secondary peritonitis and nine patients after abdominal surgery without peritonitis at three time points: immediately before, immediately after and 15 minutes after abdominal lavage with normal saline solution. The total++ – amount of irrigant fluid, the strong ion difference [(Na+ +K+)-(Cl- +lactate-)], and total protein concentrations were determined and standard bicarbonate, standard base excess were calculated from pH and PaCO2. Peritonitis patients developed a moderate alkalaemia (pH 7.440-7.485). The alkalaemia was unmasked after optimization of mild hypoventilation, but was supported by a decrease in protein concentration of about 3.4 mEql/l in the first 15 minutes after the lavage. There was no marked increase in chloride concentration in either the peritonitis or the control group. The data indirectly exclude major fluid absorption during abdominal lavage with 3000 to 6000 ml normal saline, given that we found no clinically relevant electrolyte and acid-base changes that might be expected after rapid fluid absorption. The factors of major influence in acid-base regulation were ventilation and protein loss in the course of abdominal lavage. Monitoring of the Stewart variables is an easily applicable method of monitoring acid-base regulating variables in the perioperative course of patients undergoing abdominal lavage therapy.

Acid-Base Balance during Continuous Veno-Venous Hemofiltration: The Impact of Severe Hepatic Failure

Background

Continuous renal replacement therapy (CRRT) affects acid-base balance but the influence of severe hepatic failure (SHF) on this effect is unknown.

Aim

To assess the effect of SHF on acid-base balance in patients receiving CVVH.

Design

Retrospective laboratory investigation.

Subjects

Forty patients with SHF and acute renal failure (ARF) treated with CVVH and 42 critically ill patients with severe ARF but no liver disease also treated with CVVH (controls).

Intervention

Retrieval of clinical and laboratory data from prospective unit and laboratory databases.

Methods

Quantitative acid-base status assessment using the Stewart-Figge methodology. Comparison of findings between the two groups.

Results

Although CVVH had a major effect on acid base balance in both groups, patients with SHF had a higher mean lactate concentrations (4.8 vs. 3.1 mmol/L; p<0.0005), a greater base deficit compared to controls (-1 vs. 4.1 mEq/L; p<0.0001) and a lower PaCO2 tension (36.8 vs. 42.5 mmHg; p<0.0001), despite the use of bicarbonate replacement fluid. The acidifying effect of hyperlactatemia was slightly worsened by an increased strong ion gap (9.3 vs. 4.9 mEq/L; p<0.0001). It was, however, attenuated by an increased strong ion difference apparent (SIDa) (43.6 vs. 41.9 mEq/L; p<0.05) secondary to hypochloremia (96 vs. 100 mmol/L; p<0.0001) and by hypoalbuminemia, although hypoalbuminemia in SHF patients (26 vs. 23; p<0.005) was less pronounced than in controls.

Conclusion

The use of CVVH does not fully correct the independent acidifying effect of liver failure on acid-base status. Increased lactate and strong ion gap values maintain a persistent base deficit despite the alkalinizing effects of hypoalbuminemia and hypochloremia. The correction of acidosis in SHF patients may require more intensive CVVH.

Acid-base Balance in Combined Severe Hepatic and Renal Failure: A Quantitative Analysis

Background

Severe hepatic failure (SHF) commonly leads to major changes in acid-base balance status. However, the direct effects of liver failure per se on acid base balance are poorly understood because this condition is usually associated with acute renal failure (ARF).

Aim

To assess the effect of SHF on acid-base balance.

Design

Retrospective laboratory investigation.

Subjects

Thirty-seven critically ill patients with SHF complicated by ARF, and 42 patients with severe ARF without liver failure prior to renal replacement therapy.

Intervention

Retrieval of clinical and laboratory data from prospective unit and laboratory databases.

Methods

Quantitative acid-base assessment using Stewart-Figge methodology. Comparison of findings between the two groups. Comparison of demographic and clinical features.

Results

Patients with combined SHF and ARF were younger and had significantly higher mean bilirubin, ALT and INR levels (p<0.0001). Their mean lactate concentration was higher (6.4 vs. 2.1 mmol/L; p<0.0001) leading to a greater anion gap (25.8 vs. 16.1 mmol/L; p<0.0001). The ionized calcium concentration (1.00 vs. 1.15 mmol/L; p<0.0001) was lower but the strong ion difference apparent (SIDa) was greater (42.0 vs. 38.0 mEq/L; p<0.005) due to hypochloremia. The albumin concentration was low but higher than in control patients (28 vs. 24 g/L; p<0.01) and the calculated strong ion gap (SIG) was greater (12.6 vs. 9.3 mEq/L; p<0.01). The base excess was similar to controls and the pH was preserved in the near normal range by marked hypocapnea.

Conclusions

Combined SHF and ARF is a syndrome with unique acid-base changes due mostly to lactic metabolic acidosis and, in smaller part, to the accumulation of unmeasured anions. This acidosis, like that of ARF, is attenuated by hypoalbuminemia, by a unique preservation of the SIDa due to hypochloremia, and by marked hypocapnea.

Metabolic acid-base adaptation triggered by acute persistent hypercapnia in mechanically ventilated patients with acute respiratory distress syndrome

Objective

Hypercapnia resulting from protective ventilation in acute respiratory distress syndrome triggers metabolic pH compensation, which is not entirely characterized. We aimed to describe this metabolic compensation.

Methods

The data were retrieved from a prospective collected database. Variables from patients' admission and from hypercapnia installation until the third day after installation were gathered. Forty-one patients with acute respiratory distress syndrome were analyzed, including twenty-six with persistent hypercapnia (PaCO₂ > 50mmHg > 24 hours) and 15 non-hypercapnic (control group). An acid-base quantitative physicochemical approach was used for the analysis.

Results

The mean ages in the hypercapnic and control groups were 48 ± 18 years and 44 ± 14 years, respectively. After the induction of hypercapnia, pH markedly decreased and gradually improved in the ensuing 72 hours, consistent with increases in the standard base excess. The metabolic acid-base adaptation occurred because of decreases in the serum lactate and strong ion gap and increases in the inorganic apparent strong ion difference. Furthermore, the elevation in the inorganic apparent strong ion difference occurred due to slight increases in serum sodium, magnesium, potassium and calcium. Serum chloride did not decrease for up to 72 hours after the initiation of hypercapnia.

Conclusion

In this explanatory study, the results indicate that metabolic acid-base adaptation, which is triggered by acute persistent hypercapnia in patients with acute respiratory distress syndrome, is complex. Furthermore, further rapid increases in the standard base excess of hypercapnic patients involve decreases in serum lactate and unmeasured anions and increases in the inorganic apparent strong ion difference by means of slight increases in serum sodium, magnesium, calcium, and potassium. Serum chloride is not reduced.

Unmeasured anions and mortality in critically ill patients in 2016

The presence of acid-base disturbances, especially metabolic acidosis may negatively affect the outcome of critically ill patients. Lactic acidosis is the most frequent etiology and has largest impact on the prognosis. Since lactate measurement might not have always been available at bedside, it had been regarded as one of the unmeasured anions. Therefore, anion gap and strong ion gap has been used to as a surrogate of lactate concentration. From this perspective, the relationship between either anion gap or strong ion gap and mortality has been explored. Then, lactate became routinely measurable at bedside and the direct comparison between directly measured lactate and these surrogate parameters can be possible. Currently available evidence suggests that directly measured lactate has larger prognostic ability for mortality than albumin-corrected anion gap and strong ion gap without lactate. In this commentary, the rationale and possible clinical implications of these findings are discussed.

Intravenous maintenance fluid therapy in children

Intravenous fluids are frequently used in paediatrics but have been associated with significant adverse outcomes. Understanding the composition of fluid prescribed and administering an appropriate rate is essential for safe fluid administration, along with regular monitoring. Recent evidence has shown that using an isotonic fluid with a sodium concentration similar to plasma can decrease the risk of hyponatraemia without an increase in adverse effects. This should lead to a change in guidelines: isotonic fluid should now be used as the primary maintenance intravenous fluid given to the majority of children.

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

Acid-base disorders are common in the critically ill. Most of these disorders do not cause harm and are self-limiting after appropriate resuscitation and management. Unfortunately, clinicians tend to think about an acid-base disturbance as a "disease" and spend long hours effectively treating numbers rather than the patient. Moreover, a sizable number of intensive-care physicians experience difficulties in interpreting the significance of or understanding the etiology of certain forms of acid-base disequilibria. Traditional tools for interpreting acid-base disorders may not be adequate for analyzing the complex nature of these metabolic abnormalities. Inappropriate interpretation may also lead to wrong clinical conclusions and incorrectly influence clinical management (eg, bicarbonate therapy for metabolic acidosis in different clinical situations). The Stewart approach, based on physicochemical principles, is a robust physiological concept that can facilitate the interpretation and analysis of simple, mixed, and complex acid-base disorders, thereby allowing better diagnosis of the cause of the disturbance and more timely treatment. However, as the concept does not attach importance to plasma bicarbonate, clinicians may find it complicated to use in their daily clinical practice. This article reviews various approaches to interpreting acid-base disorders and suggests the integration of base-excess and Stewart approach for a better interpretation of these metabolic disorders.

Physicochemical evaluation of acid-base disorders after liver transplantation and the contribution from administered fluids

OBJECTIVES

To analyze the mechanism of acid-base disorders in liver transplant recipients and to examine the relationship between these disorders and the fluids administered during surgery.

METHODS

This prospective study in a university-affiliated hospital intensive care unit (ICU) included 52 patients admitted to the ICU from December 2009 to January 2011. We examined the contributions of inorganic ion differences, lactate, unmeasured anions, phosphate, and albumin to metabolic acidosis. In addition to laboratory variables, we collected demographic and clinical data.

RESULTS

Metabolic acidosis (standard base excess ≤ -2.0 mmol/L) was identified in 37 (71.2%) patients during the immediate postoperative period. The inorganic ion difference was the main determinant of acidosis, accounting for -6.17 mEq/L of acidifying effect. The acidemia was attenuated mainly by the alkalinizing effect of albumin reduction, which contributed +6.03 mEq/L. There was an inverse proportional relationship between the quantity of saline solution used during surgery and the inorganic ion difference during the immediate postoperative period.

CONCLUSIONS

Hyperchloremia is the primary contributor to metabolic acidosis in liver transplant recipients. Possibly the use of chloride-rich solutions increases the incidence of this disorder.

The central role of chloride in the metabolic acid-base changes in canine parvoviral enteritis

The acid–base disturbances in canine parvoviral (CPV) enteritis are not well described. In addition, the mechanisms causing these perturbations have not been fully elucidated. The purpose of the present study was to assess acid–base changes in puppies suffering from CPV enteritis, using a modified strong ion model (SIM). The hypothesis of the study was that severe acid–base disturbances would be present and that the SIM would provide insights into pathological mechanisms, which have not been fully appreciated by the Henderson–Hasselbalch model.

The study analysed retrospective data, obtained from 42 puppies with confirmed CPV enteritis and 10 healthy control dogs. The CPV-enteritis group had been allocated a clinical score, to allow classification of the data according to clinical severity. The effects of changes in free water, chloride, l-lactate, albumin and phosphate were calculated, using a modification of the base excess algorithm. When the data were summated for each patient, and correlated to each individual component, the most important contributor to the metabolic acid–base changes, according to the SIM, was chloride (P < 0.001). Severely-affected animals tended to demonstrate hypochloraemic alkalosis, whereas mildly-affected puppies had a hyperchloraemic acidosis (P = 0.007). In conclusion, the acid–base disturbances in CPV enteritis are multifactorial and complex, with the SIM providing information in terms of the origin of these changes.

Use of sodium-chloride difference and corrected anion gap as surrogates of Stewart variables in critically ill patients

INTRODUCTION

To investigate whether the difference between sodium and chloride ([Na(+)] - [Cl(-)]) and anion gap corrected for albumin and lactate (AG(corr)) could be used as apparent strong ion difference (SID(app)) and strong ion gap (SIG) surrogates (respectively) in critically ill patients.

METHODS

A total of 341 patients were prospectively observed; 161 were allocated to the modeling group, and 180 to the validation group. Simple regression analysis was used to construct a mathematical model between SID(app) and [Na(+)] - [Cl(-)] and between SIG and AG(corr) in the modeling group. Area under the receiver operating characteristic (ROC) curve was also measured. The mathematical models were tested in the validation group.

RESULTS

in the modeling group, SID(app) and SIG were well predicted by [Na(+)] - [Cl(-)] and AG(corr) (R(2) = 0.973 and 0.96, respectively). Accuracy values of [Na(+)] - [Cl(-)] for the identification of SID(app) acidosis (<42.7 mEq/L) and alkalosis (>47.5 mEq/L) were 0.992 (95% confidence interval [CI], 0.963-1) and 0.998 (95%CI, 0.972-1), respectively. The accuracy of AG(corr) in revealing SIG acidosis (>8 mEq/L) was 0.974 (95%CI: 0.936-0.993). These results were validated by showing excellent correlations and good agreements between predicted and measured SID(app) and between predicted and measured SIG in the validation group (R(2) = 0.977; bias = 0±1.5 mEq/L and R(2) = 0.96; bias = -0.2±1.8 mEq/L, respectively).

CONCLUSIONS

SID(app) and SIG can be substituted by [Na(+)] - [Cl(-)] and by AG(corr) respectively in the diagnosis and management of acid-base disorders in critically ill patients.

Desenvolvimento de software para interpretação de dados gasométricos aplicável em unidades de terapia intensiva

O objetivo deste estudo foi desenvolver um software para interpretação de dados gasométricos aplicável em UTIs. Trata-se de estudo de caráter experimental, sendo selecionada uma base teórica em Java com a IDE NetBeans 6.8 por meio de parceria com profissionais capacitados em Sistemas de Informação. O desenvolvimento do programa foi baseado na criação de um algoritmo, uma sequência de instruções bem definidas e não ambíguas a serem executadas mecanicamente com a finalidade de fornecer um diagnóstico desejado. Foi criado um software aplicável em UTIs denominado InterGas, que é um programa de fácil instalação, possui interface de fácil compreensão e utilização, além de processar os dados rapidamente e de forma precisa, oferecendo como resultado final o diagnóstico para o distúrbio do equilíbrio ácido-básico. O desconhecimento de outra ferramenta que reúna todos os componentes do InterGas o torna um software pioneiro que facilita a tomada de decisão à medida que caracteriza a ocorrência de distúrbios mistos utilizando fórmulas de compensação. Com isso, futuros estudos deverão ser feitos com o objetivo de avaliar aspectos relacionados à implementação e eficácia do software desenvolvido.

Early chloride intake does not parallel that of sodium in extremely-low-birth-weight infants and may impair neonatal outcomes

BACKGROUND AND OBJECTIVE

Accurate data on the optimal chloride (Cl) intake in premature infants are scarce. The aim of the present study was to describe Cl intakes in the first 10 days of life and to assess the relations between high Cl intakes and corrected serum Cl level or markers of severe acidosis in infants <28 weeks' gestation.

METHODS

Retrospective cohort study including all of the infants <28 weeks admitted to the neonatal intensive care unit during a 3-year period and cared for from birth until day 10 or more.

RESULTS

Fifty-six infants were included. Cumulative total Cl intakes reached 9.6 ± 3.7 mmol/kg at day 3 and 49.2 ± 13.5 mmol/kg at day 10. Inadvertent intakes (from intravenous fluids other than parenteral nutrition) represented on average 70% of total Cl intakes in the first 3 days. Difference between Cl and sodium intakes reached 7.8 ± 4.8 mmol/kg at day 10 and mainly originated from parenteral nutrition. By multivariate analysis, cumulative Cl intake >10 mmol/kg during the first 3 days was an independent risk factor of base excess <-10 mmol/L. Cumulative Cl intake >45 mmol/kg during the first 10 days was an independent risk factor of corrected chloremia >115 mmol/L and of base excess <-10 mmol/L.

CONCLUSIONS

Cumulative Cl intake >10 mmol/kg during the first 3 days (ie, 3.3 mmol · kg (-1) · day(-1) on average) and >45 mmol/kg during the first 10 days (ie, 4.5 mmol · kg (-1) · day(-1) on average) may have unwanted metabolic consequences and should be avoided. Imbalance between electrolytes provided by the parenteral nutrition solution need to be detected and corrected.

Acid-base disorders in patients with chronic obstructive pulmonary disease: a pathophysiological review

The authors describe the pathophysiological mechanisms leading to development of acidosis in patients with chronic obstructive pulmonary disease and its deleterious effects on outcome and mortality rate. Renal compensatory adjustments consequent to acidosis are also described in detail with emphasis on differences between acute and chronic respiratory acidosis. Mixed acid-base disturbances due to comorbidity and side effects of some drugs in these patients are also examined, and practical considerations for a correct diagnosis are provided.

Alterations of acid-base balance, electrolyte concentrations, and osmolality caused by nonionic hyperosmolar contrast medium during pediatric cardiac catheterization

OBJECTIVE

This prospective clinical observational study was conducted to investigate the effects of contrast medium on acid-base balance, electrolyte concentrations, and osmolality in children.

BACKGROUND

For pediatric cardiac catheterization, high doses of nonionic hyperosmolar contrast medium are widely used.

METHODS

Forty pediatric patients (age 0-16 years) undergoing cardiac angiography with more than 3 ml·kg(-1) of nonionic hyperosmolar contrast medium (Iomeprol) were enrolled, and the total amount of the contrast agent given was documented. Before and after contrast medium administration, a blood sample was collected to analyze electrolytes, acid-base parameters, osmolality, hemoglobin, and hematocrit.

RESULTS

After cardiac catheterization, pH, hemoglobin, hematocrit, bicarbonate, base excess, sodium, chloride, calcium, anion gap and strong ion difference decreased, whereas osmolality increased significantly (base excess -1.8 ± 1.8 vs -3.4 ± 2.3, sodium 138 ± 2.9 vs 132 ± 4.1 mm, osmolality 284 ± 5.7 vs 294 ± 7.6 mosmol·kg(-1), P < 0.01). Seventy-eight percent of the children developed hyponatremia (sodium <135 mm). No changes were seen in pCO(2) , lactate, and potassium levels.

CONCLUSIONS

Regarding the differential diagnosis of metabolic disturbances after pediatric cardiac catheterization, low-anion gap metabolic acidosis and hyponatremia should be considered as a possible side effect of the administered contrast medium.

Acid-base disorders in critically ill neonates

Objective:

To study acid–base imbalance in common pediatric diseases (such as sepsis, bronchopneumonia, diarrhea, birth-asphyxia etc.) in neonates.

Design and Setting:

An observational study was conducted in an emergency room of a tertiary teaching care hospital in Haryana, India.

Patients and Methods:

Fifty neonates (from first hour to one month) attending pediatric emergency services with various ailments. Blood gas analysis, electrolytes, plasma lactate, and plasma albumin were estimated in neonates.

Results:

Metabolic acidosis was the most common acid–base disorder. Hyperlactatemia was observed in more than half of such cases. Birth asphyxia was another common disorder with the highest mortality in neonates followed by bronchopneumonia and sepsis. Significant correlation between mortality and critical values of lactate was observed.

Conclusion:

Birth asphyxia with high-lactate levels in neonates constituted major alterations in acid–base disorders seen in an emergency room of a tertiary teaching care hospital. Plasma lactate concentration measurement provides an invaluable tool to assess type of metabolic acidosis in addition to predicting mortality in these neonates.

A critique of Stewart's approach: the chemical mechanism of dilutional acidosis

OBJECTIVE

While Stewart's acid-base approach is increasingly used in clinical practice, it has also led to new controversies. Acid-base disorders can be seen from different viewpoints: on the diagnostic/clinical, quantitative/mathematical, or the mechanistic level. In recent years, confusion in the interpretation and terminology of Stewart's approach has arisen from mixing these different levels. This will be demonstrated on the basis of a detailed analysis of the mechanism of "dilutional acidosis." In the classical dilution concept, metabolic acidosis after resuscitation with large volumes is attributed to the dilution of serum bicarbonate. However, Stewart's approach rejects this explanation and offers an alternative one that is based on a decrease in a "strong ion difference." This mechanistic explanation is questionable for principal chemical reasons. The objective of this study is to clarify the chemical mechanism of dilutional acidosis.

METHODS

Experimental data and simulations of various dilution experiments, as well as theoretical and chemical considerations were used.

RESULTS

1. The key to understanding the mechanism of dilutional acidosis lies in the open CO2/HCO3 (-)-buffer system where the buffer base (HCO3(-)) is diluted whereas the buffer acid is not diluted (constant pCO2). 2. The categorization in independent and dependent variables depends on the system regarded. 3. Neither the principle of electroneutrality, nor a change in [SID], nor increased H2O dissociation plays a mechanistic role.

CONCLUSION

Stewart's approach is valid at the mathematical level but does not provide any mechanistic insights. However, the quantification and categorization of acid-base disorders, using Stewart approach, may be helpful in clinical practice.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s00134-009-1528-y) contains supplementary material, which is available to authorized users.

Reference values for strong ion difference--a novel tool for fetal metabolic assessment

OBJECTIVES

The aim of this pilot study was to establish reference values for strong ion difference (SID) in umbilical cord blood and investigate the feasibility of evaluating fetal metabolism according to the comprehensive approach to acid-base abnormalities, based on Stewart's physiochemical theory.

STUDY DESIGN

A prospective observational study. Women who underwent an elective cesarean section at term (n=40) were compared to women who completed a normal spontaneous delivery at term (n=40). The primary outcome was the establishment of normal values for SID in the umbilical cord vein. We also compared acid-base variables in the umbilical vein between the groups.

RESULTS

The apparent SID in the umbilical vein was 34.61+/-3.92 mequiv./L after normal delivery and 35.98+/-2.56 mequiv./L after elective cesarean section (the effective SID is 37.43+/-1.93 and 38.29+/-2.38 mequiv./L, respectively). The pH values were similar in both groups, but the pCO(2) was significantly higher and the plasma principal weak acids (albumin and phosphate) were significantly lower after cesarean sections.

CONCLUSIONS

SID enables a comprehensive approach to acid-base abnormalities in the neonate, making it a potential additional tool for evaluating fetal acid-base status.

Acid-base disorders evaluation in critically ill patients: we can improve our diagnostic ability

PURPOSE

To determine whether Stewart's approach can improve our ability to diagnose acid-base disorders compared to the traditional model.

METHODS

This prospective cohort study took place in a university-affiliated hospital during the period of February-May 2007. We recorded clinical data and acid-base variables from one hundred seventy-five patients at intensive care unit admission.

RESULTS

Of the 68 patients with normal standard base excess (SBE) (SBE between -4.9 and +4.9), most (n = 59; 86.8%) had a lower effective strong ion difference (SIDe), and of these, 15 (25.4%) had SIDe < 30 mEq/L. Thus, the evaluation according to Stewart's method would allow an additional diagnosis of metabolic disorder in 33.7% patients.

CONCLUSIONS

The Stewart approach, compared to the traditional evaluation, results in identification of more patients with major acid-base disturbances.

Blood gas analysis and the fundamentals of acid-base balance

UNDERSTANDING BLOOD GAS values and acid-base balance are fundamental skills of neonatal nursing. This is because, in the NICU, blood gases are probably ordered more than any other laboratory test. The bedside nurse not only obtains the specimen, but is also crucially involved in interpreting the results because blood gases cannot stand alone; they need to be evaluated in the context of the entire clinical picture. This article provides basic information on the components of a blood gas, acid-base balance, as well as a systematic approach to blood gas analysis.

Acid-base analysis: a critique of the Stewart and bicarbonate-centered approaches

When approaching the analysis of disorders of acid-base balance, physical chemists, physiologists, and clinicians, tend to focus on different aspects of the relevant phenomenology. The physical chemist focuses on a quantitative understanding of proton hydration and aqueous proton transfer reactions that alter the acidity of a given solution. The physiologist focuses on molecular, cellular, and whole organ transport processes that modulate the acidity of a given body fluid compartment. The clinician emphasizes the diagnosis, clinical causes, and most appropriate treatment of acid-base disturbances. Historically, two different conceptual frameworks have evolved among clinicians and physiologists for interpreting acid-base phenomena. The traditional or bicarbonate-centered framework relies quantitatively on the Henderson-Hasselbalch equation, whereas the Stewart or strong ion approach utilizes either the original Stewart equation or its simplified version derived by Constable. In this review, the concepts underlying the bicarbonate-centered and Stewart formulations are analyzed in detail, emphasizing the differences in how each approach characterizes acid-base phenomenology at the molecular level, tissue level, and in the clinical realm. A quantitative comparison of the equations that are currently used in the literature to calculate H+concentration ([H+]) is included to clear up some of the misconceptions that currently exist in this area. Our analysis demonstrates that while the principle of electroneutrality plays a central role in the strong ion formulation, electroneutrality mechanistically does not dictate a specific [H+], and the strong ion and bicarbonate-centered approaches are quantitatively identical even in the presence of nonbicarbonate buffers. Finally, our analysis indicates that the bicarbonate-centered approach utilizing the Henderson-Hasselbalch equation is a mechanistic formulation that reflects the underlying acid-base phenomenology.

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.

Effects of dietary supplementation of DL-methionine or 2-hydroxy-4-(methylthio)-butanoic acid on food intake, nutrient digestibility, nitrogen balance, and urinary and blood metabolites in healthy, growing dogs

The aim of this study was to evaluate effects on nutritional responses of supplemental DL-methionine and 2-hydroxy-4-(methylthio) butanoic acid (HMTBA) in a commercial-type diet in growing dogs. A nitrogen balance study was conducted as a randomized complete block design using 30 Pointer puppies (72-d-old; 5.5 kg). A corn and poultry byproduct meal based diet was supplemented with 0.1 or 0.2% DL-methionine or HMTBA on an equimolar basis. Organic matter and gross energy tended (p < 0.10) to be less digestible by dogs fed the 0.1% HMTBA diet compared with the 0.2% DL-methionine diet, but other nutrients were unaffected. Postprandial urinary calcium tended (p < 0.10) to be lower for the basal and HMTBA treatments. Fecal ammonia tended (p < 0.10) to be lower for the 0.1% HMTBA diet than for the 0.2% DL-methionine diet. At the levels tested, DL-methionine and HMTBA appear to act similarly when included in a corn and poultry by-product meal diet fed to young dogs.

The role of preservation solution on acid-base regulation during machine perfusion of kidneys

To meet the current clinical organ demand, efficient preservation methods and solutions are needed to increase the number of viable kidneys for transplantation. In the present study, the influence of perfusion solution buffering strength on renal pH dynamics and regulation mechanisms during kidney ex vivo preservation was determined. Porcine kidneys were hypothermically machine perfused for 72 h with either Unisol-UHK or Belzer-Machine Perfusion solution, Belzer-MP solution. Renal perfusate samples were periodically collected and biochemically analyzed. The UHK solution, a Hepes-based solution (35 mM), provided a more efficient control of renal pH that, in turn, resulted in minor changes in the perfusate pH relative to baseline, in response to tissue CO2 and HCO3- production. In the perfusate of Belzer-MP kidney group a wider range of pH values were recorded and a pronounced pH reduction was seen in response to significant rises in pCO2 and HCO3- concentrations. The Belzer-MP solution, containing phosphate (25 mM) as its main buffer, and only 10 mM Hepes, had a greater buffering requirement to attenuate larger pH changes.

Hyperchloremic Acidosis in the Critically Ill: One of the Strong-Ion Acidoses?

Decreases in plasma bicarbonate are associated with hyperchloremic acidosis and lactic acidosis. According to the Stewart approach to acid-base physiology, the strong-ion difference regulates plasma bicarbonate, with chloride and lactate being the only strong anions routinely measured in clinical chemistry. We hypothesized that the plasma strong-ion difference, both with and without lactate, would have a stronger association with plasma bicarbonate than plasma chloride alone would have with bicarbonate. We used plasma acid-base data from 300 critically ill patients. The correlation with bicarbonate became progressively weaker (P < 0.001): all measured strong ions, r = 0.60; measured strong ions without lactate, r = 0.42; chloride alone, r = -0.27. In a subgroup of 26 patients with traditional hyperchloremic acidosis (base excess < -2 mmol/L and anion gap <17 mmol/L), the measured strong-ion difference (without lactate) had a stronger correlation (P < 0.001) with bicarbonate than chloride had: r = 0.85 versus r = -0.60. We conclude that hyperchloremic acidosis and lactic acidosis are strong-ion acidoses. Hyperchloremia should be viewed relative to the plasma strong cations. A practical conclusion is that both managing and preventing acid-base disorders with IV fluid therapy involves manipulating each of the plasma strong ions, particularly sodium and chloride.

Clinical review: the meaning of acid-base abnormalities in the intensive care unit part I - epidemiology

Acid-base abnormalities are common in critically ill patients. Our ability to describe acid-base disorders must be precise. Small differences in corrections for anion gap, different types of analytical processes, and the basic approach used to diagnose acid-base aberrations can lead to markedly different interpretations and treatment strategies for the same disorder. By applying a quantitive acid-base approach, clinicians are able to account for small changes in ion distribution that may have gone unrecognized with traditional techniques of acid-base analysis. Outcome prediction based on the quantitative approach remains controversial. This is in part due to use of various technologies to measure acid-base variables, administration of fluid or medication that can alter acid-base results, and lack of standardized nomenclature. Without controlling for these factors it is difficult to appreciate the full effect that acid-base disorders have on patient outcomes, ultimately making results of outcome studies hard to compare.

Identifying the sick: can biochemical measurements be used to aid decision making on presentation to the accident and emergency department

BACKGROUND

Early and accurate identification of patients who may benefit from aggressive optimal medical intervention is essential if improved outcomes in terms of survival are to be achieved. We studied the usefulness of routine clinical measurements and/or markers of metabolic abnormality in the early identification of those patients at greatest risk of deterioration on presentation to the accident and emergency department.

METHODS

We conducted a prospective observational study in the accident and emergency department of a 602-bed district general hospital. Routine clinical measurements (heart rate, systolic blood pressure, temperature, oxygen saturation in room air, level of consciousness and ventilatory frequency) and venous blood analysis for metabolic markers (pH, bicarbonate, standard base excess, lactate, anion gap, strong ion difference, and strong ion gap) and biochemical markers (Na+, K+, Ca2+, Cl-, PO4- albumin, urea and creatinine) were recorded from unselected consecutive hospital admissions over two 3-month periods (September-November 2002 and February-April 2003).

RESULTS

Logistic regression analysis showed that neither conventional clinical measurements upon presentation to the accident and emergency department nor venous biochemical and metabolic indices have good discriminatory ability when used as single predictors of either hospital mortality or length of hospital stay. Selecting variables from all the clinical and venous blood measurements gave a parsimonious model containing only age, heart rate, phosphate and albumin (area under the receiver operating characteristic curve, 0.82 [95% CI 0.76, 0.87]).

CONCLUSIONS

A combination of clinical and venous biochemical measurements in the accident and emergency department proved the best predictors of hospital mortality. Consequently, they may be helpful as a triage tool in the accident and emergency department to help identify patients at risk of deterioration.

Comparison of three strong ion models used for quantifying the acid-base status of human plasma with special emphasis on the plasma weak acids

Currently, three strong ion models exist for the determination of plasma pH. Mathematically, they vary in their treatment of weak acids, and this study was designed to determine whether any significant differences exist in the simulated performance of these models. The models were subjected to a "metabolic" stress either in the form of variable strong ion difference and fixed weak acid effect, or vice versa, and compared over the range 25 < or = Pco(2) < or = 135 Torr. The predictive equations for each model were iteratively solved for pH at each Pco(2) step, and the results were plotted as a series of log(Pco(2))-pH titration curves. The results were analyzed for linearity by using ordinary least squares regression and for collinearity by using correlation. In every case, the results revealed a linear relationship between log(Pco(2)) and pH over the range 6.8 < or = pH < or = 7.8, and no significant difference between the curve predictions under metabolic stress. The curves were statistically collinear. Ultimately, their clinical utility will be determined both by acceptance of the strong ion framework for describing acid-base physiology and by the ease of measurement of the independent model parameters.

Metabolic acidosis developing during cardiopulmonary bypass is related to a decrease in strong ion difference

Metabolic acidosis is a frequent complication of cardiopulmonary bypass (CPB). Commonly, its cause is ascribed to hypoperfusion; however, iatrogenic causes, related to the composition and volume of intravascular fluids that are administered, are increasingly being recognized. The aim of this study was to determine if metabolic acidosis during CPB was associated with hypoperfusion, change in strong ion difference (SID) or haemodilution. Forty-nine patients undergoing cardiac surgery using CPB in the Royal Infirmary of Edinburgh (RIE) or the HCI, Clydebank were included in the study. Arterial blood samples were aspirated before induction of anaesthesia and the end of CPB. Samples were subjected to blood gas analysis and measurement of electrolytes and lactate. Changes in concentrations were then calculated. Change variables that were found to be significant (p < 0.1) univariate correlates of the change in hydrogen ion concentration were identified and entered into a multivariate regression model with hydrogen ion concentration at the end of CPB as the outcome variable (r2 = 0.65, p < 0.001). Change variance in hydrogen ion concentration was created by first entering the baseline hydrogen ion concentration into the model. Next, any variance resulting from the respiratory component of acidosis was removed by entering the change in arterial carbon dioxide tension (regression coefficient (beta)=0.67, p < 0.01). Change in SID (beta = -0.34, p < 0.01) and surgical institution (beta = 0.40, p < 0.01) were then found to be predictors of the remaining variance whilst change in concentration of lactate (beta in = 0.16, p = 0.07) and volume of intravascular fluid that was administered (beta = -0.07, p = 0.52) were rejected from the model. These findings suggest that the metabolic acidosis developing during CPB is partially the result of iatrogenic decrease in SID rather than hypoperfusion, as estimated by lactate concentration, or haemodilution.

Bench-to-bedside review: a brief history of clinical acid-base

The history of assessing the acid–base equilibrium and associated disorders is intertwined with the evolution of the definition of an acid. In the 1950s clinical chemists combined the Henderson–Hasselbalch equation and the Bronsted–Lowry definition of an acid to produce the current bicarbonate ion-centred approach to metabolic acid–base disorders. Stewart repackaged pre-1950 ideas of acid–base in the late 1970s, including the Van Slyke definition of an acid. Stewart also used laws of physical chemistry to produce a new acid–base approach. This approach, using the strong ion difference (particularly the sodium chloride difference) and the concentration of weak acids (particularly albumin), pushes bicarbonate into a minor role as an acid–base indicator rather than as an important mechanism. The Stewart approach may offer new insights into acid–base disorders and therapies.

Das Stewart-Modell

About twenty years ago, Peter Stewart had already published his modern quantitative approach to acid-base chemistry. According to his interpretations, the traditional concepts of the mechanisms behind the changes in acid-base balance are considerably questionable. The main physicochemical principle which must be accomplished in body fluids, is the rule of electroneutrality. There are 3 components in biological fluids which are subject to this principle: a)Water, which is only in minor parts dissociated into H+ and OH-, b)"strong", i.e. completely dissociated, electrolytes, which thus do not interact with other substances, and body substances, such as lactate, and c)"weak", i.e. incompletely dissociated, substances. Peter Stewart strictly distinguished between dependent and independent variables and thus indeed described a new order of acid-base chemistry. The 3 dependent variables (bicarbonate concentration [Bic(-)], pH, and with this also hydrogen ion concentration [H(+)]) can only change if the 3 independent variables allow this change. These 3 independent variables are: 1. Carbon dioxide partial pressure, 2.the total amount of all weak acids ([A-] (Stewart called these ATOT), and 3.strong ion difference (SID). [A(-)] can be calculated from the albumin (Alb) and the phosphate concentration (Pi): [A(-)]=[Alb x (0.123 x pH - 0.631)] + [Pi x (0.309 x pH - 0.469)]. An apparent SID (or "bedside" SID) can be calculated using measurable ion concentrations: SID=[Na(+)] + [K(+)] - [Cl(-)]-lactate. Regarding the metabolic disturbances of acid-base chemistry, according to Stewart's terminology, changes in pH, [H(+)], and [Bic(-)] are only possible if either SID or [A(-)] itself changes. If, for example, SID decreases (e.g. in case of hyperchloremia), this increase in independent negative charges leads to a decrease in dependent negative charges in terms of [Bic(-)] resulting in acidosis (and vice versa). Therefore, according to Stewart, the decrease in SID during hyperchloremic acidosis results from the increase in serum chloride concentration and is the causal mechanism behind this acidosis. Contrary for example, a decrease in [A(-)] (e. g. during hypoalbuminemia) leads to an increase in [Bic(-)] and therefore to an alcalosis (and vice versa). Thus, by Stewart's approach, completely new acid-base disturbances, like "hyperchloremic acidosis" or "hypoalbuminemic alcalosis" (which, of course, can also exist in combination) can be detected, which had been unrecognised by the classic acid-base concepts. Consequently, Stewart's analysis can lead to a better understanding of the mechanisms behind the changes in acid-base balance.

New aspects of acid-base balance in intensive care

PURPOSE OF REVIEW

For 20 years, an alternative view of the universe has been available for acid-base physiology. The Stewart approach emphasizes mathematically independent and dependent variables. With the Stewart approach bicarbonate and hydrogen ions are dependent variables that represent the effects rather than the causes of acid-base derangements. Neither bicarbonate nor pH can be regulated directly; rather they are controlled by the independent variables. In plasma there are three independent variables: the partial pressure of carbon dioxide, strong ion difference, and weak acids. In plasma, sodium and chloride are the principal strong ions, and albumin is the principal weak acid. Critically ill patients often have changes in these variables.

RECENT FINDINGS

Recent studies have examined various aspects of the Stewart approach, including the effects of buffers and haemofiltration as well as bedside assessment of a patient's acid-base status. While sodium bicarbonate increases the strong ion difference by increasing plasma sodium, tris-hydroxymethyl aminomethane acts by increasing plasma weak base concentration and weak cations. Several studies support correcting the anion gap for changes in albumin (and even phosphate). One study raises a cautionary note on the poor agreement between central laboratory and point-of-care measurements of important biochemical variables, including plasma sodium and chloride.

SUMMARY

The Stewart approach to acid-base physiology continues to develop as a comprehensive method to diagnose and manage acid-base disorders.