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

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.

U-Shaped Association between Serum Chloride Levels and In-Hospital Mortality in Patients with Congestive Heart Failure in Intensive Care Units

Serum chloride level has clinical significance in the prognosis of heart failure. Little is known regarding the association between serum chloride levels and in-hospital mortality in patients with heart failure.This retrospective study used clinical data obtained from the Medical Information Mart for Intensive Care Database. The study cohort comprised patients who were categorized on the basis of their serum chloride levels, and the primary endpoint was in-hospital mortality. To assess the impact of serum chloride levels at the time of intensive care unit admission on in-hospital mortality, we used various statistical approaches, including multivariable logistic regression models, a generalized additive model, and a two-piecewise linear regression model. In addition, subgroup analysis was conducted to examine the robustness of the main findings.This study comprised 15,983 participants. When compared with the reference group (Q5), the groups with the highest (Q7) and lowest (Q1) blood chloride levels exhibited increased in-hospital mortality, with fully adjusted odds ratios (ORs) of 1.36 [95% confidence interval (CI): 1.08-1.71] and 1.25 (95% CI: 1-1.56), respectively. A U-shaped relationship was observed between blood chloride levels and in-hospital mortality, with the lowest risk observed at a threshold of 105.017 mmol/L. The effect sizes and corresponding CIs below and above the threshold were 0.969 (95% CI: 0.957-0.982) and 1.039 (95% CI: 1.002-1.076), respectively. Stratified analyses demonstrated the robustness of this correlation.The relationship between serum chloride levels and in-hospital mortality in patients with heart failure was U-shaped, with an inflection point of 105.017 mmol/L.

Strong ions and charge-balance

It has been shown that the ability to predict the pH in any chemically characterized fluid, together with its buffer-capacity and acid content can be based on the requirement of electroneutrality, conservation of mass, and rules of dissociation as provided by physical chemistry. More is not required, and less is not enough. The charge in most biological fluids is dominated by the constant charge on the completely dissociated strong ions but, nonetheless, a persistent narrative in physiology has problematized the notion that these have any role at all in acid-base homeostasis. While skepticism is always to be welcomed, some common arguments against the importance of strong ions are examined and refuted here. We find that the rejection of the importance of strong ions comes with the prize that even very simple systems such as fluids containing nothing else, or solutions of sodium bicarbonate in equilibrium with known tensions of CO₂ become incomprehensible. Importantly, there is nothing fundamentally wrong with the Henderson-Hasselbalch equation but the idea that it is sufficient to understand even simple systems is unfounded. What it lacks for a complete description is a statement of charge-balance including strong ions, total buffer concentrations, and water dissociation.

Acid content and buffer-capacity: a charge-balance perspective

Rational treatment and thorough diagnostic classification of acid-base disorders requires quantitative understanding of the mechanisms that generate and dissipate loads of acid and base. A natural precondition for this tallying is the ability to quantify the acid content in any specified fluid. Physical chemistry defines the pH-dependent charge on any buffer species, and also on strong ions on which, by definition, the charge is pH-invariant. Based, then, on the requirement of electroneutrality and conservation of mass, it was shown in 1914 that pH can be calculated and understood on the basis of the chemical composition of any fluid. Herein we first show that this specification for [H⁺] of the charge-balance model directly delivers the pH-dependent buffer-capacity as defined in the literature. Next, we show how the notion of acid transport as proposed in experimental physiology can be understood as a change in strong ion difference, ΔSID. Finally, based on Brønsted-Lowry theory we demonstrate that by defining the acid content as titratable acidity, this is equal to SID_ref - SID, where SID_ref is SID at pH 7.4. Thereby, any chemical situation is represented as a curve in a novel diagram with titratable acidity = SID_ref - SID as a function of pH. For any specification of buffer chemistry, therefore, the change in acid content in the fluid is path invariant. Since constituents of SID and titratable acidity are additive, we thereby, based on first principles, have defined a new framework for modeling acid balance across a cell, a whole organ, or the whole-body.

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.

A Proof of Concept Study, Demonstrating Extracorporeal Carbon Dioxide Removal Using Hemodialysis with a Low Bicarbonate Dialysate

Extracorporeal carbon dioxide removal (ECCO2R) devices remove CO2 directly from blood, facilitating ultraprotective ventilation or even providing an alternative to mechanical ventilation. However, ECCO2R is not widely available, whereas dialysis is available in most intensive care units (ICUs). Prior attempts to provide ECCO2R with dialysis, by removing CO2 in the form of bicarbonate, have been plagued by metabolic acidosis. We hypothesized that bicarbonate dialysis is feasible, provided the plasma strong ion difference is maintained. We used a mathematical model to investigate the effects of bicarbonate removal on pH and CO2 in plasma, and performed in-vitro experiments to test CO2 removal using three dialysates with different bicarbonate concentrations (0, 16, and 32 mmol·L). Our modeling predicted a reduction in partial pressures of CO2 (PCO2) and increased pH with progressive lowering of plasma bicarbonate, provided strong ion difference and plasma proteins (Atot) were maintained. In our in-vitro experiments, total CO2 removal, scaled up to an adult size filter, was highest with our dialysate containing no bicarbonate, where we removed the equivalent of 94 ml·min (±3.0) of CO2. Under the same conditions, our dialysate containing a conventional bicarbonate concentration (32 mmol·L) only removed 5 ml·min (±4; p < 0.001). As predicted, pH increased following bicarbonate removal. Our data show that dialysis using low bicarbonate dialysates is feasible and results in a reduction in plasma PCO2. When scaled up, to estimate equivalent CO2 removal with an adult dialysis circuit, the amount removed competes with existing low-flow ECCO2R devices.

Physicochemical Models of Acid-Base

Physicochemical models have played an important role in understanding, diagnosing, and treating acid-base disorders for more than 100 years. This review focuses on recent complex models, solved using computers, and shows how these models provide new understanding and diagnostic approaches in acid-base disorders. These advanced models use the following physicochemical principles: (1) chemical equilibrium, (2) conservation of mass, (3) electroneutrality, and (4) osmotic equilibrium to describe the steady-state distribution of H₂O and ions in the four major body-fluid spaces, cells, interstitium, plasma, and erythrocytes, and show how this distribution is changed by fluid infusions and losses through renal and gastrointestinal physiological processes. Illustrations of model use with a new comprehensive diagnostic approach are the understanding of an important clinical situation, saline acidosis, and the diagnosis of a patient with diabetic ketoacidosis. This new approach predicts a patient's whole-body base excess and partitions this value into 10 individual values, producing the disorder. These data and other data produced by the diagnostic model described in this review provide much more extensive insight than previous approaches.

The effects of severe hemoconcentration on acid-base equilibrium in critically ill patients: the forgotten role of buffers in whole blood

PURPOSE

Idiopathic Systemic Capillary Leak Syndrome (ISCLS) is a paroxysmal permeability disorder characterized by abrupt onset of shock and hemoconcentration due to massive shift of fluids and proteins from the intravascular to the interstitial compartment. We hypothesize that increased hemoglobin concentration has a pivotal role in the acid-base imbalance during life-threatening crises.

MATERIALS AND METHODS

Analysis of the acid-base balance fluctuations during six severe ISCLS flares admitted to ICU of a referral center for ISCLS.

RESULTS

Acid-base equilibrium was assessed for plasma and the whole blood by single and multicompartmental models. The acute phase of ISCLS was characterized by shock, hypoalbuminemia, severe hemoconcentration, and acidosis. The physical-chemical approach for plasma found a remarkable component of unmeasured anions (SIG) during the acute phase. After correction of the physical-chemical model for the whole blood, the SIG variations disappeared because the buffer role of hemoglobin was relevant.

CONCLUSION

Hemoglobin has a remarkable role in buffering metabolic acidosis during the shock phase of ISCLS. In these circumstances, the assessment of acid-base equilibrium in plasma alone may overestimate unmeasured anions. On the contrary, the physical-chemical model corrected for whole blood better explains the metabolic component of acid-base imbalance when marked shift of hemoglobin concentration occurs.

Changes in the SIDActual and SID Effective Values in the Course of Respiratory Acidosis in Horses With Symptomatic Severe Equine Asthma-An Experimental Study

Equine asthma syndrome is an allergic, inflammatory airway disease that usually affects older horses. Respiratory acidosis is an acid-base imbalance caused by alveolar hypoventilation. The acid-base balance may be assessed using the Henderson-Hasselbalch equation as well as the Stewart model. The authors hypothesized that systemic respiratory acidosis changes the ionic concentrations affecting water dissociation. The study group included 16 Warmblood, mixed breed horses of both sexes with a history of severe equine asthma, and 10 healthy horses were used as controls. Arterial and venous blood were collected from all the horses. The pH, pO₂, and pCO₂ and HCO_3- were assessed in the arterial blood. Na, K, Cl, albumin, and P_inorganic (P_i) were assessed in the venous blood. The obtained results were used to calculate the anion gap (AG), modified AG, actual strong ion difference (SID_a), weak non-volatile acids, and effective strong ion difference (SID_e) values for all the horses. A systemic, compensatory respiratory acidosis was diagnosed in the study group. The concentration of Na in the blood serum in the study group was significantly higher, whereas the concentration of Cl was significantly lower than the values in the control group. The SID_a and SID_e values calculated in the horses from the study group were significantly higher than those in the control group. Significantly higher SID_a and SID_e values confirm the presence of ionic changes that affect water dissociation in the course of respiratory acidosis in horses. The SID_a and SID_e values may be useful in the diagnosis and treatment of respiratory acidosis in horses, which warrant further investigation.

Assessment of renal hemodynamic toxicity of fluid challenge with 0.9% NaCl compared to balanced crystalloid (PlasmaLyte®) in a rat model with severe sepsis

BACKGROUND

According to international guidelines, volume expansion with crystalloids is the first-line treatment for hemodynamic management in patients with severe sepsis or septic shock. Compared to balanced crystalloids, 0.9% sodium chloride (0.9% NaCl) induces hyperchloremia and metabolic acidosis and may alter renal hemodynamics and function. We compared the effects of 0.9% NaCl to a less chloride-concentrated fluid, PlasmaLyte^® (PL) in targeted fluid resuscitation in a randomized, double-blind controlled study in an experimental model of severe sepsis in rats.

RESULTS

A sepsis with hypotension was induced by cecal ligature and puncture (CLP) in 40 male Wistar rats (20 for each crystalloid). Rats received fluid resuscitation over a period of 200 min for a targeted mean arterial pressure of 90 mm Hg. Animals received similar volumes of 0.9% NaCl or PL. Unlike PL-resuscitated rats, 0.9% NaCl-resuscitated rats experienced hyperchloremia and metabolic acidosis, whereas systemic hemodynamics, renal hemodynamics and renal function were not significantly different between both groups.

CONCLUSION

In our model of rats with severe sepsis resuscitated with large amounts of crystalloids, 0.9% NaCl-induced hyperchloremic acidosis, but balanced crystalloid did not improve systemic and renal hemodynamics or renal function.

Extracorporeal CO2 removal by hemodialysis: in vitro model and feasibility

BACKGROUND

Critically ill patients with acute respiratory distress syndrome and acute exacerbations of chronic obstructive pulmonary disease often develop hypercapnia and require mechanical ventilation. Extracorporeal carbon dioxide removal can manage hypercarbia by removing carbon dioxide directly from the bloodstream. Respiratory hemodialysis uses traditional hemodialysis to remove CO₂ from the blood, mainly as bicarbonate. In this study, Stewart's approach to acid-base chemistry was used to create a dialysate that would maintain blood pH while removing CO₂ as well as determine the blood and dialysate flow rates necessary to remove clinically relevant CO₂ volumes.

METHODS

Bench studies were performed using a scaled down respiratory hemodialyzer in bovine or porcine blood. The scaling factor for the bench top experiments was 22.5. In vitro dialysate flow rates ranged from 2.2 to 24 mL/min (49.5-540 mL/min scaled up) and blood flow rates were set at 11 and 18.7 mL/min (248-421 mL/min scaled up). Blood inlet CO₂ concentrations were set at 50 and 100 mmHg.

RESULTS

Results are reported as scaled up values. The CO₂ removal rate was highest at intermittent hemodialysis blood and dialysate flow rates. At an inlet pCO₂ of 50 mmHg, the CO₂ removal rate increased from 62.6 ± 4.8 to 77.7 ± 3 mL/min when the blood flow rate increased from 248 to 421 mL/min. At an inlet pCO₂ of 100 mmHg, the device was able to remove up to 117.8 ± 3.8 mL/min of CO₂. None of the test conditions caused the blood pH to decrease, and increases were ≤0.08.

CONCLUSIONS

When the bench top data is scaled up, the system removes a therapeutic amount of CO₂ standard intermittent hemodialysis flow rates. The zero bicarbonate dialysate did not cause acidosis in the post-dialyzer blood. These results demonstrate that, with further development, respiratory hemodialysis can be a minimally invasive extracorporeal carbon dioxide removal treatment option.

Transient dilutional acidosis but no lactic acidosis upon cardiopulmonary bypass in patients undergoing coronary artery bypass grafting

INTRODUCTION

Dilutional acidosis may result from the introduction of a large fluid volume into the patients' systemic circulation, resulting in a considerable dilution of endogenous bicarbonate in the presence of a constant carbon dioxide partial pressure. Its significance or even existence, however, has been strongly questioned. Blood gas samples of patients operated on with standard cardiopulmonary bypass (CPB) were analyzed in order to provide further evidence for the existence of dilutional acidosis.

MATERIAL AND METHODS

Between 07/2014 and 10/2014, a total of 25 consecutive patients scheduled for elective isolated coronary artery bypass grafting with CPB were enrolled in this prospective observational study. Blood gas samples taken regularly after CPB initiation were analyzed for dilutional effects and acid-base changes.

RESULTS

After CPB initiation, hemoglobin concentration dropped from an average initial value of 12.8 g/dl to 8.8 g/dl. Before the beginning of CPB, the mean value of the patients' pH and base excess (BE) value averaged 7.41 and 0.5 mEq/l, respectively. After the onset of CPB, pH and BE values significantly dropped to a mean value of 7.33 (p < 0.0001) and -3.3 mEq/l (p < 0.0001), respectively, within the first 20 min. In the following period during CPB they recovered to 7.38 and -0.5 mEq/l, respectively, on average. Patients did not show overt lactic acidosis.

CONCLUSIONS

The present data underline the general existence of dilutional acidosis, albeit very limited in its duration. In patients undergoing coronary artery bypass grafting it seems to be the only obvious disturbance in acid-base homeostasis during CPB.

Intravenous fluids: balancing solutions

The topic of intravenous (IV) fluids may be regarded as “reverse nephrology”, because nephrologists usually treat to remove fluids rather than to infuse them. However, because nephrology is deeply rooted in fluid, electrolyte, and acid-base balance, IV fluids belong in the realm of our specialty. The field of IV fluid therapy is in motion due to the increasing use of balanced crystalloids, partly fueled by the advent of new solutions. This review aims to capture these recent developments by critically evaluating the current evidence base. It will review both indications and complications of IV fluid therapy, including the characteristics of the currently available solutions. It will also cover the use of IV fluids in specific settings such as kidney transplantation and pediatrics. Finally, this review will address the pathogenesis of saline-induced hyperchloremic acidosis, its potential effect on outcomes, and the question if this should lead to a definitive switch to balanced solutions.

Old and new approaches to the interpretation of acid-base metabolism, starting from historical data applied to diabetic acidosis

The approach to acid-base chemistry in medicine includes several methods. Currently, the two most popular procedures are derived from Stewart's studies and from the bicarbonate/BE-based classical formulation. Another method, unfortunately little known, follows the Kildeberg theory applied to acid-base titration. By using the data produced by Dana Atchley in 1933, regarding electrolytes and blood gas analysis applied to diabetes, we compared the three aforementioned methods, in order to highlight their strengths and their weaknesses. The results obtained, by reprocessing the data of Atchley, have shown that Kildeberg's approach, unlike the other two methods, is consistent, rational and complete for describing the organ-physiological behavior of the hydrogen ion turnover in human organism. In contrast, the data obtained using the Stewart approach and the bicarbonate-based classical formulation are misleading and fail to specify which organs or systems are involved in causing or maintaining the diabetic acidosis. Stewart's approach, despite being considered 'quantitative', does not propose in any way the concept of 'an amount of acid' and becomes even more confusing, because it is not clear how to distinguish between 'strong' and 'weak' ions. As for Stewart's approach, the classical method makes no distinction between hydrogen ions managed by the intermediate metabolism and hydroxyl ions handled by the kidney, but, at least, it is based on the concept of titration (base-excess) and indirectly defines the concept of 'an amount of acid'. In conclusion, only Kildeberg's approach offers a complete understanding of the causes and remedies against any type of acid-base disturbance.

Association of Hyperchloremia With Hospital Mortality in Critically Ill Septic Patients

OBJECTIVES

Hyperchloremia is frequently observed in critically ill patients in the ICU. Our study aimed to examine the association of serum chloride (Cl) levels with hospital mortality in septic ICU patients.

DESIGN

Retrospective cohort study.

SETTING

Urban academic medical center ICU.

PATIENTS

ICU adult patients with severe sepsis or septic shock who had Cl measured on ICU admission were included. Those with baseline estimated glomerular filtration rate less than 15 mL/min/1.73 m or chronic dialysis were excluded.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Of 1,940 patients included in the study, 615 patients (31.7%) had hyperchloremia (Cl ≥ 110 mEq/L) on ICU admission. All-cause hospital mortality was the dependent variable. Cl on ICU admission (Cl0), Cl at 72 hours (Cl72), and delta Cl (ΔCl = Cl72 - Cl0) were the independent variables. Those with Cl0 greater than or equal to 110 mEq/L were older and had higher cumulative fluid balance, base deficit, and Sequential Organ Failure Assessment scores. Multivariate analysis showed that higher Cl72 but not Cl0 was independently associated with hospital mortality in the subgroup of patients with hyperchloremia on ICU admission (adjusted odds ratio for Cl72 per 5 mEq/L increase = 1.27; 95% CI, 1.02-1.59; p = 0.03). For those who were hyperchloremic on ICU admission, every within-subject 5 mEq/L increment in Cl72 was independently associated with hospital mortality (adjusted odds ratio for ΔCl 5 mEq/L = 1.37; 95% CI, 1.11-1.69; p = 0.003).

CONCLUSIONS

In critically ill septic patients manifesting hyperchloremia (Cl ≥ 110 mEq/L) on ICU admission, higher Cl levels and within-subject worsening hyperchloremia at 72 hours of ICU stay were associated with all-cause hospital mortality. These associations were independent of base deficit, cumulative fluid balance, acute kidney injury, and other critical illness parameters.

Acid-base disorders associated with serum electrolyte patterns in patients on hemodiafiltration

BACKGROUND

Metabolic acidosis (MAC) is a common aspect of dialysis-dependent patients. It is definitely caused by acid retention; however, the influence of other plasma ions is unclear. Understanding the mechanism of MAC and its correction is important when choosing the dialysis solution. Therefore, we assessed the relationship between intradialytic change of acid-base status and serum electrolytes.

METHODS

We studied 68 patients on post-dilution hemodiafiltration, using dialysate bicarbonate concentration 32mmol/L. The acid-base disorders were evaluated by the traditional Siggaard-Anderson and modern Stewart approaches.

RESULTS

The mean pre-dialysis pH was 7.38, standard base excess (SBE) -1.5, undetermined anions (UA(-)) 7.5, sodium-chloride difference (Diff(NaCl)) 36.2mmol/L. MAC was present in 34% of patients, of which 83% had an increased UA(-) as a major cause of MAC. The mean nPCR was 0.99g/kg/day and correlated negatively with SBE. After dialysis, metabolic alkalosis predominated in 81%. The mean post-dialysis pH was 7.45, SBE 4, UA(-) 2.6, Diff(NaCl) 36.9mmol/L. ΔSBE significantly correlated with ΔUA(-), but not with ΔDiff(NaCl) or ΔCl(-).

CONCLUSIONS

MAC in patients on hemodiafiltration is mainly caused by acid retention and is associated with higher protein intake. We did not prove the effect of sodium or chloride on acid-base balance. Even though we used a relatively low concentration of dialysate bicarbonate, we recorded a high proportion of post-dialysis alkalosis caused by the excessive decrease of undetermined anions, which had been completely replaced by bicarbonate and indicated the elimination of undesirable anions, as well as of normal endogenous anions.

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

The Stewart approach-the application of basic physical-chemical principles of aqueous solutions to blood-is an appealing method for analyzing acid-base disorders. These principles mainly dictate that pH is determined by three independent variables, which change primarily and independently of one other. In blood plasma in vivo these variables are: (1) the PCO₂; (2) the strong ion difference (SID)-the difference between the sums of all the strong (i.e., fully dissociated, chemically nonreacting) cations and all the strong anions; and (3) the nonvolatile weak acids (A_tot). Accordingly, the pH and the bicarbonate levels (dependent variables) are only altered when one or more of the independent variables change. Moreover, the source of H⁺ is the dissociation of water to maintain electroneutrality when the independent variables are modified. The basic principles of the Stewart approach in blood, however, have been challenged in different ways. First, the presumed independent variables are actually interdependent as occurs in situations such as: (1) the Hamburger effect (a chloride shift when CO₂ is added to venous blood from the tissues); (2) the loss of Donnan equilibrium (a chloride shift from the interstitium to the intravascular compartment to balance the decrease of A_tot secondary to capillary leak; and (3) the compensatory response to a primary disturbance in either independent variable. Second, the concept of water dissociation in response to changes in SID is controversial and lacks experimental evidence. In addition, the Stewart approach is not better than the conventional method for understanding acid-base disorders such as hyperchloremic metabolic acidosis secondary to a chloride-rich-fluid load. Finally, several attempts were performed to demonstrate the clinical superiority of the Stewart approach. These studies, however, have severe methodological drawbacks. In contrast, the largest study on this issue indicated the interchangeability of the Stewart and conventional methods. Although the introduction of the Stewart approach was a new insight into acid-base physiology, the method has not significantly improved our ability to understand, diagnose, and treat acid-base alterations in critically ill patients.

Evaporation of free water causes concentrational alkalosis in vitro

BACKGROUND

The development of metabolic alkalosis was described recently in patients with hypernatremia. However, the causes for this remain unknown. The current study serves to clarify whether metabolic alkalosis develops in vitro after removal of free water from plasma and whether this can be predicted by a mathematical model.

MATERIALS AND METHODS

Ten serum samples of healthy humans were dehydrated by 29 % by vacuum centrifugation corresponding to an increase of the contained concentrations by 41 %. Constant partial pressure of carbon dioxide at 40 mmHg was simulated by mathematical correction of pH [pH(40)]. Metabolic acid-base state was assessed by Gilfix' base excess subsets. Changes of acid-base state were predicted by the physical-chemical model according to Watson.

RESULTS

Evaporation increased serum sodium from 141 (140-142) to 200 (197-203) mmol/L, i.e., severe hypernatremia developed. Acid-base analyses before and after serum concentration showed metabolic alkalosis with alkalemia: pH(40): 7.43 (7.41 to 7.45) vs 7.53 (7.51 to 7.55), p = 0.0051; base excess: 1.9 (0.7 to 3.6) vs 10.0 (8.2 to 11.8), p = 0.0051; base excess of free water: 0.0 (- 0.2 to 0.3) vs 17.7 (16.8 to 18.6), p = 0.0051. The acidifying effects of evaporation, including hyperalbuminemic acidosis, were beneath the alkalinizing ones. Measured and predicted acid-base changes due to serum evaporation agreed well.

CONCLUSIONS

Evaporation of water from serum causes concentrational alkalosis in vitro, with good agreement between measured and predicted acid-base values. At least part of the metabolic alkalosis accompanying hypernatremia is independent of renal function.

Effects of gas exchange on acid-base balance

This paper describes the interactions between ventilation and acid-base balance under a variety of conditions including rest, exercise, altitude, pregnancy, and various muscle, respiratory, cardiac, and renal pathologies. We introduce the physicochemical approach to assessing acid-base status and demonstrate how this approach can be used to quantify the origins of acid-base disorders using examples from the literature. The relationships between chemoreceptor and metaboreceptor control of ventilation and acid-base balance summarized here for adults, youth, and in various pathological conditions. There is a dynamic interplay between disturbances in acid-base balance, that is, exercise, that affect ventilation as well as imposed or pathological disturbances of ventilation that affect acid-base balance. Interactions between ventilation and acid-base balance are highlighted for moderate- to high-intensity exercise, altitude, induced acidosis and alkalosis, pregnancy, obesity, and some pathological conditions. In many situations, complete acid-base data are lacking, indicating a need for further research aimed at elucidating mechanistic bases for relationships between alterations in acid-base state and the ventilatory responses.

Acid-base changes after fluid bolus: sodium chloride vs. sodium octanoate

Objectives

This study aims to test the hypothesis that fluid loading with sodium chloride (150 mmol Na and 150 mmol Cl) or sodium octanoate (150 mmol Na, 100 mmol Cl, and 50 mmol octanoate) would lead to different acid–base changes.

Design

We performed a double-blind crossover experimental study.

Setting

The study was done at a University Physiology Laboratory.

Subjects

Eight Merino ewes were used as subjects.

Measurements and main results

We randomly assigned animals to a rapid intravenous infusion (1 L over 30 min) of either normal saline (NS) or sodium octanoate solution (OS). We collected blood samples at 0.5, 1, 2, 4, and 6 h after the start of the infusion for blood gas analyses and biochemistry. We calculated strong ion difference apparent (SIDa), effective strong ion difference, and strong ion gap (SIG). Animals in the NS group developed metabolic acidification immediately after fluid administration (pH 7.49 to 7.42, base excess 3.0 to -1.6 mEq/L), while the OS group did not (pH 7.47 to 7.51, base excess 1.1 to 1.4 mEq/L; P < 0.001). Additionally, the OS group had higher SIDa (36.2 vs. 33.2 mEq/L) and SIG (7.4 vs. 6.2 mEq/L) at the end of the infusion.

Conclusions

Our findings provide further evidence that acidification induced by intravenous fluid loading is dependent on fluid composition and challenges the paradigm of the so-called dilutional acidosis.

Electronic supplementary material

The online version of this article (doi:10.1186/2197-425X-1-4) contains supplementary material, which is available to authorized users.

Whole body acid-base and fluid-electrolyte balance: a mathematical model

A cellular compartment was added to our previous mathematical model of steady-state acid-base and fluid-electrolyte chemistry to gain further understanding and aid diagnosis of complex disorders involving cellular involvement in critically ill patients. An important hypothesis to be validated was that the thermodynamic, standard free-energy of cellular H(+) and Na(+) pumps remained constant under all conditions. In addition, a hydrostatic-osmotic pressure balance was assumed to describe fluid exchange between plasma and interstitial fluid, including incorporation of compliance curves of vascular and interstitial spaces. The description of the cellular compartment was validated by close comparison of measured and model-predicted cellular pH and electrolyte changes in vitro and in vivo. The new description of plasma-interstitial fluid exchange was validated using measured changes in fluid volumes after isoosmotic and hyperosmotic fluid infusions of NaCl and NaHCO3. The validated model was used to explain the role of cells in the mechanism of saline or dilutional acidosis and acid-base effects of acidic or basic fluid infusions and the acid-base disorder due to potassium depletion. A module was created that would allow users, who do not possess the software, to determine, for free, the results of fluid infusions and urinary losses of water and solutes to the whole body.

Why is saline so acidic (and does it really matter?)

Commercial 0.9% saline solution for infusion has a pH around 5.5. There are many reasons for this acidity, some of them still obscure. It is also true that infusion of normal saline can lead to metabolic acidaemia, yet the link between the acidity of saline solution and the acidaemia it can engender is not straightforward. This commentary draws together the known and putative sources of acidity in saline solutions: it turns out that the acidity of saline solution is essentially unrelated to the acidaemia complicating saline infusion.

Factors related to post-operative metabolic acidosis following major abdominal surgery

BACKGROUND

Metabolic acidosis is frequently observed in perioperative patients, especially those who undergo major surgery. The aim of this study was to evaluate the factors related to post-operative metabolic acidosis and to attempt to identify the clinical effect of metabolic acidosis following major abdominal surgery.

METHODS

We included 172 patients admitted to a surgical intensive care unit (ICU) following major abdominal surgery. All cases were divided into either the acidosis or the normal group using immediate post-operative standard base excess (SBE). The following clinical data were retrospectively obtained from the chart and ICU database: basic clinical characteristics, operative data, type and volume of fluid infused during the operation, post-operative arterial blood gas analysis, lactate, and central venous oxygen saturation.

RESULTS

The predominant intraoperative fluid was either 0.9% saline or lactated Ringer's solution. The operation length, estimated blood loss, total fluid infused, total saline infused, lactate and corrected chloride were significantly higher in the acidosis group; however, central venous oxygen saturation was lower in the normal group. Among these factors, total infused saline and lactate level were independent factors related to metabolic acidosis. The comparison between the types of fluid revealed that the saline group had a significantly lower SBE, strong ion difference and higher corrected chloride. SBE was significantly correlated with lactate and total infused saline. ICU and hospital length of stay were significantly longer in the acidosis group.

CONCLUSIONS

Post-operative metabolic acidosis following major abdominal surgery was closely related to both hyperchloremic acidosis associated with large saline infusion and lactic acidosis caused by lactataemia.

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.

The Atlas of Physiology and Pathophysiology: Web-based multimedia enabled interactive simulations

The paper is a presentation of the current state of development for the Atlas of Physiology and Pathophysiology (Atlas). Our main aim is to provide a novel interactive multimedia application that can be used for biomedical education where (a) simulations are combined with tutorials and (b) the presentation layer is simplified while the underlying complexity of the model is retained. The development of the Atlas required the cooperation of many professionals including teachers, system analysts, artists, and programmers. During the design of the Atlas, tools were developed that allow for component-based creation of simulation models, creation of interactive multimedia and their final coordination into a compact unit based on the given design. The Atlas is a freely available online application, which can help to explain the function of individual physiological systems and the causes and symptoms of their disorders.

A mathematical model of blood-interstitial acid-base balance: application to dilution acidosis and acid-base status

We developed mathematical models that predict equilibrium distribution of water and electrolytes (proteins and simple ions), metabolites, and other species between plasma and erythrocyte fluids (blood) and interstitial fluid. The models use physicochemical principles of electroneutrality in a fluid compartment and osmotic equilibrium between compartments and transmembrane Donnan relationships for mobile species. Across the erythrocyte membrane, the significant mobile species Cl−is assumed to reach electrochemical equilibrium, whereas Na+and K+distributions are away from equilibrium because of the Na+/K+pump, but movement from this steady state is restricted because of their effective short-term impermeability. Across the capillary membrane separating plasma and interstitial fluid, Na+, K+, Ca2+, Mg2+, Cl−, and H+are mobile and establish Donnan equilibrium distribution ratios. In each compartment, attainment of equilibrium by carbonates, phosphates, proteins, and metabolites is determined by their reactions with H+. These relationships produce the recognized exchange of Cl−and bicarbonate across the erythrocyte membrane. The blood submodel was validated by its close predictions of in vitro experimental data, blood pH, pH-dependent ratio of H+, Cl−, and HCO3−concentrations in erythrocytes to that in plasma, and blood hematocrit. The blood-interstitial model was validated against available in vivo laboratory data from humans with respiratory acid-base disorders. Model predictions were used to gain understanding of the important acid-base disorder caused by addition of saline solutions. Blood model results were used as a basis for estimating errors in base excess predictions in blood by the traditional approach of Siggaard-Andersen (acid-base status) and more recent approaches by others using measured blood pH and Pco2values. Blood-interstitial model predictions were also used as a basis for assessing prediction errors of extracellular acid-base status values, such as by the standard base excess approach. Hence, these new models can give considerable insight into the physicochemical mechanisms producing acid-base disorders and aid in their diagnoses.

The acid-base effects of free water removal from and addition to oxygenated and deoxygenated whole blood: an in vitro model of contraction alkalosis and dilutional acidosis

This study was conducted to describe the acid-base effects of hydration and dehydration of oxygenated and deoxygenated whole blood. Whole blood samples from goats were equilibrated in a tonometer to a partial pressure of carbon dioxide of 40 mm Hg and oxygen (PO₂) of 100 mm Hg or 30 mm Hg. Contraction alkalosis was achieved by evaporating blood samples to 80% of the original volume. Dilutional acidosis was achieved by increasing the blood sample volume by 20% by addition of sterile water. Acid-base, electrolyte, hemoglobin, lactate, albumin, and phosphorus concentrations were measured at baseline and after dehydration or hydration. A 20% dehydration of whole blood caused a 22% increase in sodium concentration and a significant increase in base excess of +3 mEq/L (P < 0.01); bicarbonate concentration increased only 7% to 9%. A concurrent increase was found in phosphorus, albumin, hemoglobin, and lactate concentrations. A 20% dilution of whole blood caused a 21% decrease in sodium concentration and a significant decrease in base excess of -5 mEq/L (P < 0.01) with an 11% to 15% decrease in bicarbonate concentration. A concurrent decrease was found in phosphorus, albumin, and hemoglobin concentrations. No significant difference was observed between the acid-base effects on oxygenated versus deoxygenated blood in any experiment. Dilutional acidosis and contraction alkalosis of whole blood are complex acid-base disorders resulting from direct changes in bicarbonate concentration in combination with changes in the concentration of weak plasma acids and buffering reactions. Therefore, bicarbonate concentration does not change to the same degree as the magnitude of contraction or dilution.

A balanced view of balanced solutions

The present review of fluid therapy studies using balanced solutions versus isotonic saline fluids (both crystalloids and colloids) aims to address recent controversy in this topic. The change to the acid-base equilibrium based on fluid selection is described. Key terms such as dilutional-hyperchloraemic acidosis (correctly used instead of dilutional acidosis or hyperchloraemic metabolic acidosis to account for both the Henderson-Hasselbalch and Stewart equations), isotonic saline and balanced solutions are defined. The review concludes that dilutional-hyperchloraemic acidosis is a side effect, mainly observed after the administration of large volumes of isotonic saline as a crystalloid. Its effect is moderate and relatively transient, and is minimised by limiting crystalloid administration through the use of colloids (in any carrier). Convincing evidence for clinically relevant adverse effects of dilutional-hyperchloraemic acidosis on renal function, coagulation, blood loss, the need for transfusion, gastrointestinal function or mortality cannot be found. In view of the long-term use of isotonic saline either as a crystalloid or as a colloid carrier, the paucity of data documenting detrimental effects of dilutional-hyperchloraemic acidosis and the limited published information on the effects of balanced solutions on outcome, we cannot currently recommend changing fluid therapy to the use of a balanced colloid preparation.