Diagnosis of metabolic acid-base disturbances in critically ill patients

Acid-base balance in peritoneal dialysis patients: a Stewart-Fencl analysis

BACKGROUND

Evaluation of acid-base disorders using the Stewart-Fencl principle is based on assessment of independent factors: strong ion difference (SID) and the total concentration of non-volatile weak acids (Atot). This approach allows for a more detailed evaluation of the cause of acid-base imbalance than the conventional bicarbonate-centered approach based on the Henderson-Hasselbalch principle, which is a necessary yet insufficient condition to describe the state of the system. The aim of our study was to assess acid-base disorders in peritoneal dialysis (PD) patients using both of these principles.

METHODS

A total of 17 patients with chronic renal failure (10 men), aged 60.7 (22-84) years, treated by PD for 25.7 (1-147) months were examined. A control group included 17 healthy volunteers (HV) (8 males), with a mean age of 42.7 (22-77) years and normal renal function. Patients were treated with a solution containing bicarbonate (25 mmol/L) and lactate (15 mmol/L) as buffers; eleven of them used, during the nighttime dwell, a solution with icodextrin buffered by lactate at a concentration of 40 mmol/L. The following equations were employed for calculations of acid-base parameters according to the Stewart-Fencl principle. The first is SID = [Na+] + [K+] + 2[Ca(2+)] + 2[Mg(2+)] - [Cl-] - [UA-], where SID is the strong ion difference and [UA-] is the concentration of undetermined anions. For practical calculation of SID, the second equation, SID = [HCO3-] + [Alb-] + [Pi-], was used, where [Alb-] and [Pi-] are the charges carried by albumin and phosphates. The third is Atot, the total concentration of weak non-volatile acids, albumin [Alb] and phosphates [Pi].

RESULTS

The capillary blood pH in PD group was 7.41 (7.27-7.48), [HCO3-] levels 23.7 (17.6-29.5) mmol/L, SID 36.3 (29.5-41.3) mmol/L, sodium-chloride difference 39.0 (31.0-44.0) mmol/L, [Pi] 1.60 (0.83-2.54) mmol/L, and [Alb] 39.7 (28.8-43.4) g/L (median, min-max). Bicarbonate in blood correlated positively with SID (Rho = 0.823; p < 0.001), with the sodium-chloride difference (Rho = 0.649; p < 0.01) and pH (Rho = 0.754; p < 0.001), and negatively with residual renal function (Rho = -0.517; p < 0.05). Moreover, the sodium-chloride difference was also found to correlate with SID (Rho = 0.653; p < 0.01). While the groups of PD and HV patients did not differ in median bicarbonate levels, significantly lower median value of SID were observed in PD patients, 36.3 vs. 39.3 mmol/L (p < 0.01); additionally, PD patients were shown to have significantly lower mean value of serum sodium levels, 138 vs. 141 mmol/L (p < 0.01), and serum chlorides levels, 100 vs. 104 mmol/L (p < 0.001). Despite the higher [UA-] levels in PD patients, 9.1 vs. 5.4 mmol/L (p < 0.001), this parameter was not found to correlate with bicarbonate levels.

CONCLUSIONS

The results suggest that the decreased bicarbonate in PD patients results from a combination of decreased sodium-chloride difference and mildly increased unmeasured anions.

The reproducibility of Stewart parameters for acid-base diagnosis using two central laboratory analyzers

BACKGROUND

Acid-base derangements can be interpreted using the Stewart-Fencl approach, which includes calculation of the apparent strong ion difference (SID(app)), the effective SID (SID(eff)), and the strong ion gap (SIG). These calculations require the measurement of several variables. We hypothesized that the SID and SIG calculated by different analyzers would not be reproducible because of variability in the measured values.

METHODS

In this prospective observational study conducted in a biochemistry laboratory, we analyzed 179 routine blood samples from consecutive patients over a 3-mo period using two automated blood chemistry analyzers, the LX20 (Beckman) and the Modular (Roche). Measured and calculated parameters from the two analyzers were compared.

RESULTS

Although the correlation between measured values was satisfactory, there were large differences in the limits of agreement for calculated values (SID(app): 9.6 mEq/L, SID(eff): 6.4 mEq/L, and SIG: 11.7 mEq/L) and a weak correlation (SID(app): r(2) = 0.54 and SIG: r(2) = 0.12) between the analyzers.

CONCLUSIONS

The results of the Stewart-Fencl approach for interpretation of acid-base status can vary according to the analyzer used. These differences may have important clinical and research implications..

Acid-base variables in patients with acute kidney injury requiring peritoneal dialysis in the pediatric cardiac care unit

PURPOSE

We aimed to clarify the acid-base abnormalities of patients with acute kidney injury (AKI) requiring peritoneal dialysis (PD) in pediatric cardiac care units.

METHODS

A retrospective observational study was conducted in a pediatric cardiac care unit in a tertiary care university hospital. The subjects were 40 patients with AKI requiring PD between 2003 and 2005, and controls matched by type of surgery and body weight. Acid-base variables, including blood gas data and electrolytes, were assessed. The Stewart-Figge variables, including strong ion difference apparent (SIDa), strong ion difference effective (SIDe), and strong ion gap (SIG), were calculated.

RESULTS

Blood gas analyses showed that the PD group was more acidemic, with a lower mean bicarbonate and a lower mean base excess, typical features of metabolic acidosis. The strong ion analyses revealed that the PD group had lower mean sodium and albumin concentrations. Based on the Stewart-Figge methodology, SIDa was smaller in the PD group than in the control group, but SIG was similar in the two groups. Receiver-operating characteristic curve analyses showed that serum albumin was the only prognostic factor associated with PCCU mortality, even after adjustment for PD treatment.

CONCLUSION

Patients with AKI requiring PD in a pediatric cardiac care unit had significant metabolic acidosis compared to controls matched by the type of surgery and body weight. Hyponatremia and hypoalbuminemia were characteristics of these patients. The calculated SIDa was smaller in the PD than in the control group. Only the serum albumin had a significant prognostic value.

Clinical utility of Stewart's method in diagnosis and management of acid-base disorders

For the past 5 decades, a bicarbonate-based approach has been the dominant method used for the diagnosis and treatment of acid-base disorders. This approach, however, has been criticized by some as (1) qualitative and not quantitative in nature and (2) incapable of detecting important diagnoses. Stewart, using principals of electroneutrality and conservation of mass, developed a "new" approach to the diagnosis and management of these disorders. The proponents of Stewart's approach believe that it not only offers a mechanistic explanation for the disorders but also provides the tool to make a more accurate diagnosis. Although Stewart's approach has been largely ignored by nephrologists and renal physiologists, it is increasingly used by anesthesiologists and intensivists. This review discusses the clinical utility of Stewart's method compared with the traditional bicarbonate-based approach. Although Stewart's method proposes a different, however not new, approach, it does not improve our ability to diagnose more accurately or manage these disorders. Stewart's method also does not provide the tool to prognosticate any better than the traditional method.

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.

Metabolic effects of citrate- vs bicarbonate-based substitution fluid in continuous venovenous hemofiltration: a prospective sequential cohort study

BACKGROUND

Studies investigating the metabolic effects of citrate-based substitution fluids are lacking. This study aims to compare the effect of citrate- vs bicarbonate-based substitution fluid used during continuous venovenous hemofiltration (CVVH) for acute kidney injury on acid-base balance and electrolytes in critically ill patients.

METHODS

This was a prospective sequential cohort study in patients with a contraindication for systemic anticoagulation. The first cohort was treated by bicarbonate-based CVVH (n = 10) and the second cohort was treated by CVVH with citrate-based substitution fluid (n = 19). Flow of the latter was coupled to blood flow, and ionized calcium concentrations were monitored and kept constant by calcium-glubionate infusion.

RESULTS

No major differences between the 2 groups were found in baseline acid-base parameters. In both groups, arterial pH increased after initiation of treatment and normalized on the average within 18 hours in either group. No differences were found in bicarbonate concentrations. Electrolyte control was comparable for the groups.

CONCLUSION

Citrate-based substitution fluid is comparable to bicarbonate-based substitution fluid during CVVH in critically ill patients with acute kidney injury, concerning acid-base balance and electrolyte control. This implies complete conversion of citrate to bicarbonate in the patients studied.

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.

Differences in acid-base behavior between intensive care unit survivors and nonsurvivors using both a physicochemical and a standard base excess approach: a prospective, observational study

PURPOSE

This study aimed to test the hypothesis that intensive care unit survivors and nonsurvivors differ with regard to type and severity of acid-base disorders.

MATERIALS AND METHODS

Prospective, observational, cohort study of 107 consecutive patients admitted in a 7-bed intensive care unit during a 6-month period that stayed at least 4 days. All acid-base variables for the first 3 days and the day of discharge were analyzed.

RESULTS

Survivors had significant metabolic acidosis upon admission, which was due to hyperlactatemia, an excess of unmeasured anions, and principally, hyperchloremia. A progressive decrease in these anions in the presence of constant hypoalbuminemia led to normal standard base excess at discharge. Nonsurvivors had greater metabolic acidosis upon admission with acidifying variables in similar proportions to that of the survivors. On the day of death, nonsurvivors had a similar degree of metabolic acidosis but a different proportion of the anions (less chloride and more lactate) compared with the day of admission. Unmeasured anions were greater in nonsurvivors both on the day of admission and on the day of death.

CONCLUSIONS

Intensive care unit survivors and nonsurvivors differed in the severity of metabolic acidosis; however, the proportion of the different anions causing the acidosis on admission was similar between these 2 groups.

[Metabolic alkalosis despite hyperlactatemia and hypercapnia. Interpretation and therapy with help of the Stewart concept]

Acid-base disturbances are commonly found in critically ill patients and are often associated with fatal complications. The basis of a successful treatment is a thorough understanding of the causes of these disorders. The "classical methods" to explain acid-base disorders--pH, base excess and bicarbonate concentration--mostly do not provide a causal correlation to the underlying pathology. An unusual case of a combined respiratory-metabolic disorder with hyperlactatemia and hypercapnia is presented. An acidosis masked by hypochloremic and hypoalbuminemic alkalosis was identified with the help of Stewart's concept and finally permitted a successful therapy. The modern Stewart concept provides enhanced information, enabling an exact diagnosis and causal therapy even in complex cases.

Metabolic acid-base disorders in the critical care unit

The recognition and management of acid-base disorders is a commonplace activity in the critical care unit, and the role of weak and strong acids in the genesis of metabolic acid-base disorders is reviewed. The clinical approach to patients with metabolic alkalosis and metabolic acidosis is discussed in this article.

Severe hyperlactatemia with normal base excess: a quantitative analysis using conventional and Stewart approaches

INTRODUCTION

Critically ill patients might present complex acid-base disorders, even when the pH, PCO2, [HCO3-], and base excess ([BE]) levels are normal. Our hypothesis was that the acidifying effect of severe hyperlactatemia is frequently masked by alkalinizing processes that normalize the [BE]. The goal of the present study was therefore to quantify these disorders using both Stewart and conventional approaches.

METHODS

A total of 1,592 consecutive patients were prospectively evaluated on intensive care unit admission. Patients with severe hyperlactatemia (lactate level > or = 4.0 mmol/l) were grouped according to low or normal [BE] values (<-3 mmol/l or >-3 mmol/l).

RESULTS

Severe hyperlactatemia was present in 168 of the patients (11%). One hundred and thirty-four (80%) patients had low [BE] levels while 34 (20%) patients did not. Shock was more frequently present in the low [BE] group (46% versus 24%, P = 0.02) and chronic obstructive pulmonary disease in the normal [BE] group (38% versus 4%, P < 0.0001). Levels of lactate were slightly higher in patients with low [BE] (6.4 +/- 2.4 mmol/l versus 5.6 +/- 2.1 mmol/l, P = 0.08). According to our study design, the pH, [HCO3-], and strong-ion difference values were lower in patients with low [BE]. Patients with normal [BE] had lower plasma [Cl-] (100 +/- 6 mmol/l versus 107 +/- 5 mmol/l, P < 0.0001) and higher differences between the changes in anion gap and [HCO3-] (5 +/- 6 mmol/l versus 1 +/- 4 mmol/l, P < 0.0001).

CONCLUSION

Critically ill patients may present severe hyperlactatemia with normal values of pH, [HCO3-], and [BE] as a result of associated hypochloremic alkalosis.

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.

Causes of metabolic acidosis in canine hemorrhagic shock: role of unmeasured ions

Introduction

Metabolic acidosis during hemorrhagic shock is common and conventionally considered to be due to hyperlactatemia. There is increasing awareness, however, that other nonlactate, unmeasured anions contribute to this type of acidosis.

Methods

Eleven anesthetized dogs were hemorrhaged to a mean arterial pressure of 45 mm Hg and were kept at this level until a metabolic oxygen debt of 120 mLO₂/kg body weight had evolved. Blood pH, partial pressure of carbon dioxide, and concentrations of sodium, potassium, magnesium, calcium, chloride, lactate, albumin, and phosphate were measured at baseline, in shock, and during 3 hours post-therapy. Strong ion difference and the amount of weak plasma acid were calculated. To detect the presence of unmeasured anions, anion gap and strong ion gap were determined. Capillary electrophoresis was used to identify potential contributors to unmeasured anions.

Results

During induction of shock, pH decreased significantly from 7.41 to 7.19. The transient increase in lactate concentration from 1.5 to 5.5 mEq/L during shock was not sufficient to explain the transient increases in anion gap (+11.0 mEq/L) and strong ion gap (+7.1 mEq/L), suggesting that substantial amounts of unmeasured anions must have been generated. Capillary electrophoresis revealed increases in serum concentration of acetate (2.2 mEq/L), citrate (2.2 mEq/L), α-ketoglutarate (35.3 μEq/L), fumarate (6.2 μEq/L), sulfate (0.1 mEq/L), and urate (55.9 μEq/L) after shock induction.

Conclusion

Large amounts of unmeasured anions were generated after hemorrhage in this highly standardized model of hemorrhagic shock. Capillary electrophoresis suggested that the hitherto unmeasured anions citrate and acetate, but not sulfate, contributed significantly to the changes in strong ion gap associated with induction of shock.

Hyperchloremia is the dominant cause of metabolic acidosis in the postresuscitation phase of pediatric meningococcal sepsis

OBJECTIVE

Metabolic acidosis is common in septic shock, yet few data exist on its etiological temporal profile during resuscitation; this is partly due to limitations in bedside monitoring tools (base excess, anion gap). Accurate identification of the type of acidosis is vital, as many therapies used in resuscitation can themselves produce metabolic acidosis.

DESIGN

Retrospective, cohort study.

SETTING

Multidisciplinary pediatric intensive care unit with 20 beds.

PATIENTS

A total of 81 children with meningococcal septic shock.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Acid-base data were collected retrospectively on 81 children with meningococcal septic shock (mortality, 7.4%) for the 48 hrs after presentation to the hospital. Base excess was partitioned using abridged Stewart equations, thereby quantifying the three predominant influences on acid-base balance: sodium chloride, albumin, and unmeasured anions (including lactate). Metabolic acidosis was common at presentation (mean base excess, -9.7 mmol/L) and persisted for 48 hrs. However, the pathophysiology changed dramatically from one of unmeasured anions at admission (mean unmeasured anion base excess, -9.2 mmol/L) to predominant hyperchloremia by 8-12 hrs (mean sodium-chloride base excess, -10.0 mmol/L). Development of hyperchloremic acidosis was associated with the amount of chloride received during intravenous fluid resuscitation (r = .44), with the base excess changing, on average, by -0.4 mmol/L for each millimole per kilogram of chloride administered. Hyperchloremic acidosis resolved faster in patients who 1) manifested larger (more negative) sodium chloride-partitioned base excess, 2) maintained a greater urine output, and 3) received furosemide; and slower in those with high blood concentrations of unmeasured anions (all, p < .05).

CONCLUSIONS

Hyperchloremic acidosis is common and substantial after resuscitation for meningococcal septic shock. Recognition of this entity may prevent unnecessary and potentially harmful prolonged resuscitation.

Acute phase proteins do not account for unmeasured anions in critical illness

BACKGROUND

The increasingly recognized prognostic impact of the strong ion gap in critical illness is in contrast to its largely unknown chemical nature. Experimental and clinical evidence suggest that acute phase proteins might account for elevation of the strong ion gap. The hypothesis of this investigation was that acute phase proteins account for strong ion gap in critically ill patients.

MATERIALS AND METHODS

The charges of the two acute phase proteins C-reactive protein and fibrinogen were estimated by a computer model. Additionally, 142 patients admitted to a medical intensive care unit of a university hospital were studied prospectively during a six month period. Serial daily observations were recorded and classified according to the systemic inflammatory state. The acute phase proteins C-reactive protein and fibrinogen were measured and the strong ion gap was calculated from the measured acid-base variables.

RESULTS

The approximated mean charges of C-reactive protein and fibrinogen at a pH of 7.4 are -4.0 and -13.6 per molecule, respectively. Therefore, their negative charge is too small to explain the elevated strong ion gap even during a substantial increase of C-reactive protein and fibrinogen due to an acute-phase reaction. Moreover, C-reactive protein did not correlate with the strong ion gap when partialized for creatinine (R = 0.02, P = 0.567). Fibrinogen did not correlate with the strong ion gap. Creatinine correlated with the strong ion gap (R = 0.42, P < 0.001). Neither systemic inflammatory state nor increasing C-reactive protein levels were associated with an increasing strong ion gap.

CONCLUSION

Acute phase proteins do not account for an elevated strong ion gap in critically ill patients.

Acid-base disturbances in critically ill patients with cirrhosis

BACKGROUND/AIMS

The equilibrium of offsetting metabolic acid-base disorders in stable cirrhosis might be lost during episodes of hepatic decompensation, haemorrhage or sepsis. The purpose of this study was to determine whether the acid-base state is destabilized in critically ill patients with cirrhosis and whether this is associated with mortality.

PATIENTS AND METHOD

One-hundred and eighty-one consecutive patients with cirrhosis were investigated in a prospective observational cohort study on admission to a medical intensive care unit (ICU) of a university hospital. Arterial acid-base state was assessed according to the Gilfix methodology. Clinical data, ICU mortality and hospital mortality were recorded.

MAIN RESULTS

Patients had net metabolic acidosis owing to unmeasured anions and owing to hyperchloraemic, dilutional and lactic acidosis. Lactic acidosis, acidemia and acute renal failure on ICU admission were associated with increased mortality. Lactate and pH discriminated survivors from non-survivors. The presence of lactic acidosis could not always be recognized by customary acid-base parameters.

CONCLUSION

The stable equilibrium of acid-base disorders is lost when patients with cirrhosis become critically ill. Lactic acidosis and acidaemia are associated with increased ICU mortality caused by severe underlying organ dysfunction.

Das Säure-Basen-Modell nach Stewart

In addition to paCO(2), Stewart's acid base model takes into account the influence of albumin, inorganic phosphate, electrolytes and lactate on acid-base equilibrium. It allows a comprehensive and quantitative analysis of acid-base disorders. Particularly simultaneous and mixed metabolic acid-base disorders, which are common in critically ill patients, can be assessed. Stewart's approach is therefore a valuable tool in addition to the customary acid-base approach based on bicarbonate or base excess. However, some chemical aspects of Stewart's approach remain controversial.

Clinical Significance of Strong Ion Gap: between ICU and Hemodialysis Patients with Metabolic Acidosis

Metabolic acidosis is the most frequent acid-base disorder in critically ill patients and dialysis patients. This study is to compare the conventional approach with the physicochemical approach between the intensive care unit (ICU) and hemodialysis (HD) patients. Fifty-seven ICU patients and 33 HD patients were enrolled. All data sets included simultaneous measurements of arterial blood gas with base deficit (BD), serum electrolytes, albumin, lactate, and calculated anion gap observed (AGobs). Physiochemical analysis was used to calculate the albumin and lactate-corrected anion gap (AGcorr), the base deficit corrected for unmeasured anions (BDua), the strong ion difference apparent (SIDa), the strong ion difference effective (SIDe), and the strong ion gap (SIG). The SIDa (37.5±5.3 vs 33.9±9.0, p=0.045) and SIG (12.3±5.3 vs 8.6±8.8, p=0.043) was significantly higher in the HD group than the ICU group. SIG in the ICU group showed the highest correlation coefficient with AGobs, whereas SIG in the HD group with AGcorr. Concerning the contributions of the three main causes of metabolic acidosis, increased SIG was comparable between the ICU and HD group (n=48, 90.6% vs n=30, 93.8%), whereas hyperlactatemia (n=9, 17.0% vs n=0, 0%) and hyperchloremia (n=20, 35.1% vs n=2, 6.1%) was significantly increased in the ICU group compared with the HD group. Multiple underlying mechanisms are present in most of the ICU patients with metabolic acidosis compared with the HD patients. In conclusion, the physicochemical approach can elucidate the detailed mechanisms of metabolic acidosis in ICU and HD patients compared with conventional measures.

Effects of hypercapnia on BP in hypoalbuminemic and Nagase analbuminemic rats

STUDY OBJECTIVE

To determine if animals with abnormally low albumin levels are more susceptible to the effects of hypercapnia on BP compared to normal animals.

DESIGN

Prospective, controlled laboratory experiment.

SETTING

University research laboratory.

ANIMALS

Eighteen male Sprague-Dawley rats: 6 rats 10 to 12 weeks old (young Sprague-Dawley [YSD]), 6 rats 6 to 9 months old (old Sprague-Dawley [OSD]), and 6 rats 10 to 12 weeks old (Nagase analbuminemic mutant Sprague-Dawley [NAR]).

METHODS

Under general anesthesia and paralysis, we varied the Paco(2) by changing the respiratory rate on mechanical ventilation. Mean arterial pressure (MAP) was monitored in a continuous fashion. We obtained arterial blood for blood gas and electrolyte analysis, and nitric oxide (NO) production.

RESULTS

OSD rats had reduced serum albumin, while NAR rats were analbuminemic. Although NAR animals had a decreased buffer capacity compared to age-matched control animals (0.010 vs 0.013, p < 0.05), the MAP decreased in an identical fashion in all three groups. NO production increased with hypercapnia but was similar in all three groups. However, NAR rats had consistently higher plasma strong ion gap (2.8 to 4.1 mEq/L greater) compared to either YSD or OSD rats (p < 0.01), and baseline strong ion difference (mean +/- SD) was significantly lower in NAR rats (28.7 +/- 2.1 mEq/L) compared to either YSD rats (33.0 +/- 5.1 mEq/L) or OSD rats (31.2 +/- 5.1 mEq/L) [p < 0.05].

CONCLUSIONS

These findings suggest that analbuminemic or hypoalbuminemic rats are not more susceptible to hypercapnia-induced hemodynamic instability. Baseline values for apparent strong ion difference are lower in NA rats consistent with a reduced buffer base resulting from analbuminemia.

Trauma and aggressive homeostasis management

Three important issues concerning homeostasis in the acute care of trauma patients that are related directly to the stress response are hyperglycemia, lactic acidosis, and hypothermia. Recently, there has been a resurgence of interest in investigating the effects of aggressive thermal and glucose concentration and volume resuscitation on outcomes in critically ill and trauma patients. Significant reason exists to question the "conventional wisdom" relating to current approaches to restoring homeostasis in this patient population.

Comparison of three different methods of evaluation of metabolic acid-base disorders

OBJECTIVES

The Stewart approach states that pH is primarily determined by Pco2, strong ion difference (SID), and nonvolatile weak acids. This method might identify severe metabolic disturbances that go undetected by traditional analysis. Our goal was to compare diagnostic and prognostic performances of the Stewart approach with a) the traditional analysis based on bicarbonate (HCO3) and base excess (BE); and b) an approach relying on HCO3, BE, and albumin-corrected anion gap (AGcorrected).

DESIGN

Prospective observational study.

SETTING

A university-affiliated hospital intensive care unit (ICU).

PATIENTS

Nine hundred thirty-five patients admitted to the ICU.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The Stewart approach detected an arterial metabolic alteration in 131 (14%) of patients with normal HCO3- and BE, including 120 (92%) patients with metabolic acidosis. However, 108 (90%) of these patients had an increased AGcorrected. The Stewart approach permitted the additional diagnosis of metabolic acidosis in only 12 (1%) patients with normal HCO3, BE, and AGcorrected. On the other hand, the Stewart approach failed to identify 27 (3%) patients with alterations otherwise observed with the use of HCO3-, BE, and AGcorrected (16 cases of acidosis and 11 of alkalosis). SID and BE, and strong ion gap (SIG) and AGcorrected, were tightly correlated (R2 = .86 and .97, p < .0001 for both) with narrow 95% limits of agreement (8 and 3 mmol/L, respectively). Areas under receiver operating characteristic curves to predict 30-day mortality were 0.83, 0.62, 0.61, 0.60, 0.57, 0.56, and 0.67 for Sepsis-related Organ Failure Assessment (SOFA) score, SIG, AGcorrected, SID, BE, HCO3-, and lactates, respectively (SOFA vs. the rest, p < .0001).

CONCLUSIONS

In this large group of critically ill patients, diagnostic performance of the Stewart approach exceeded that of HCO3- and BE. However, when AGcorrected was included in the analysis, the Stewart approach did not offer any diagnostic or prognostic advantages.

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.

Treffpunkt Stewart

OBJECTIVE

Development of a two-buffer model which simulates the acid-base properties of blood and allows comparison of the different acidbase concepts according to Stewart and to Siggaard-Andersen.

METHODS

The two-buffer model consisted of different aqueous solutions of bicarbonate/CO(2) (pCO(2), sCO(2), pK(1)), HEPES buffer (A(tot), pK(a)) and electrolytes. These were used to calculate the pH from the independent variables according to Stewart - strong ion difference (SID), pCO(2) and total concentration of the weak acids (A(tot)) - from which all other dependent variables (cHCO(3)(-), cA(-), BB, BE) were obtained and compared with the measured values.

RESULTS

The normal pH (7.408) was calculated from the normal values for SID (48 mmol/l), pCO(2) (40 mmHg) and A(tot) (45.2 mmol/l) and agreed perfectly with the measured value (7.409+/-0.001). This was also valid for all calculated and measured pH values when the SID was varied: non-respiratory alkalosis ( upward arrow) or acidosis ( downward arrow), pCO(2):respiratory acidosis ( upward arrow) or alkalosis ( downward arrow) and A(tot):hyperproteinemic acidosis ( upward arrow) or hypoproteinemic alkalosis ( downward arrow) were varied and the sum of the buffer bases (BB) was always equal to the SID. All changes and hence BE were also equal, providing that A(tot) was normal. This was not the case, however, if A(tot) was outside the normal range, when BE was then the difference from the normal BB at the respective reference point. Whereas the deviation of the measured pCO(2) was acceptable (1.74+/-0.86 mmHg), this was not the case for the SID (-6.18+/-3.58 mmol/l) calculated from the measured ion concentrations (Na, K, Ca, Cl).

CONCLUSIONS

Despite controversial discussions, both concepts are much closer than might be expected. Whereas in the Stewart approach the focus of analysis is on plasma, with the Siggaard-Andersen approach it is on blood. Hence, a combined analysis of the blood gases (pH, pCO(2), pO(2), sO(2), cHb, BE) and of the strong ion gap (SIG) may be useful.

Correcting the anion gap for hypoalbuminaemia does not improve detection of hyperlactataemia

BACKGROUND

An elevated lactate level reflects impaired tissue oxygenation and is a predictor of mortality. Studies have shown that the anion gap is inadequate as a screen for hyperlactataemia, particularly in critically ill and trauma patients. A proposed explanation for the anion gap's poor sensitivity and specificity in detecting hyperlactataemia is that the serum albumin is frequently low. This study therefore, sought to compare the predictive values of the anion gap and the anion gap corrected for albumin (cAG) as an indicator of hyperlactataemia as defined by a lactate > or =2.5 mmol/l.

METHODS

A retrospective review of 639 sets of laboratory values from a tertiary care hospital. Patients' laboratory results were included in the study if serum chemistries and lactate were drawn consecutively. The sensitivity, specificity, and predictive values were obtained. A receiver operator characteristics curve (ROC) was drawn and the area under the curve (AUC) was calculated.

RESULTS

An anion gap > or =12 provided a sensitivity, specificity, positive predictive value, and negative predictive value of 39%, 89%, 79%, and 58%, respectively, and a cAG > or =12 provided a sensitivity, specificity, positive predictive value, and negative predictive value of 75%, 59%, 66%, and 69%, respectively. The ROC curves between anion gap and cAG as a predictor of hyperlactataemia were almost identical. The AUC was 0.757 and 0.750, respectively.

CONCLUSIONS

The sensitivities, specificities, and predictive values of the anion gap and cAG were inadequate in predicting the presence of hyperlactataemia. The cAG provides no additional advantage over the anion gap in the detection of hyperlactataemia.

Essentials of laboratory medicine for the nephrology clinician

The purpose of this review is to compile and bring to the attention of the pediatric nephrology community various aspects of laboratory medicine pertinent to nephrology. The review addresses different aspects in laboratory medicine that should be taken into account during interpretation of a test result, such as methodological and analytical issues, statistical considerations and the biological interpretation of a test result in the context of the clinical setting. An understanding of the considerations and limitations in laboratory medicine will be helpful to the pediatric nephrologist when ordering and interpreting biochemical tests.

Unmeasured anions in children after cardiac surgery

OBJECTIVES

Acidosis caused by increased unmeasured anion levels occurs frequently after cardiac surgery, with uncertain significance. We examined the ability of unmeasured anions and lactate to predict major events after cardiac surgery, in addition to lactate/increased unmeasured anion levels during low cardiac output states.

METHODS

In the initial 6 months, all patients admitted after cardiac surgery were enrolled. Arterial samples were taken at 0, 4, 8, 12, 24, and 36 hours postoperatively. The Stewart method was used to calculate excess acid and unmeasured anion levels. Major adverse events were defined as low cardiac output states requiring cardiac massage or mechanical support. In the second 6-month period, data were collected from a further 8 infants during cardiac arrest/extracorporeal membrane oxygenation cannulation.

RESULTS

One hundred thirteen patients were analyzed. Major adverse events occurred in 8 (7.1%) of 113 patients. On admission, metabolic acidosis occurred in 94 of 113 samples: lactate alone (n = 25); mixed lactate and unmeasured anions (n = 44); and unmeasured anions alone (n = 25). All of the patients who experienced major adverse events had unmeasured anion levels of greater than 3 mEq/L on admission. Initial unmeasured anion levels were significantly higher in those infants with major adverse events (10.6 mEq/L [standard deviation, 8.2 mEq/L] vs 4.8 mEq/L [standard deviation, 6.6 mEq/L], P = .024). Lactate levels did not differ between the 2 groups. In the 16 patients sampled during major adverse events, metabolic acidosis occurred in 15 of 16, with a mean excess acid level of 14.9 mEq/L (standard deviation, 8.3 mEq/L). Although unmeasured anions made a significant contribution, lactate was the predominant acid.

CONCLUSIONS

After cardiac surgery, unmeasured anion levels were significantly higher in those children with major adverse events. The greatest risk of major adverse events was found in children with both increased lactate levels and increased unmeasured anion levels on admission.

The acidifying effect of lactate is neutralized by the alkalinizing effect of hypoalbuminemia in non-paracetamol-induced acute liver failure

BACKGROUND/AIMS

Hyperlactatemia with unexplained absence of metabolic acidosis is observed in acute liver failure. In chronic liver disease offsetting metabolic acid-base disorders could be revealed by means of physical-chemical acid-base analysis. The purpose of this study was to determine whether the acidifying effect of lactate is neutralized by the alkalinizing effect of hypoalbuminemia in acute liver failure.

METHODS

Serial arterial blood samples of 46 consecutive patients with non-paracetamol-induced acute liver failure were studied after admission to a medical ICU in a prospective investigation and compared to healthy controls. Acid-base state was assessed by quantitative physical-chemical analysis.

RESULTS

Lactate was increased and albumin was decreased in patients with acute liver failure compared to healthy controls resulting in normal net metabolic acid-base state. The alkalinizing effect of hypoalbuminemia was neutralized by the acidifying effect of elevated lactate. This observation was confirmed in serial analysis during 5 days after admission.

CONCLUSIONS

The acidifying effect of lactate is neutralized by the alkalinizing effect of hypoalbuminemia in non-paracetamol-induced acute liver failure. The absence of apparent metabolic acidosis in the presence of elevated lactate can be explained by means of the physical-chemical acid-base model.

Quantitative acid-base physiology using the Stewart model. Does it improve our understanding of what is really wrong?

Traditional theories of acid-base balance are based on the Henderson-Hasselbalch equation to calculate proton concentration. The recent revival of quantitative acid-base physiology using the Stewart model has increased our understanding of complicated acid-base disorders, but has also led to several new controversies. With the help of three patient histories, we show that the Henderson-Hasselbalch equation should be regarded as a simplified version of the more general Stewart model and not as something completely different. Therefore, both models may be useful at the bedside.

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.

Discordance between lactate and base deficit in the surgical intensive care unit: which one do you trust?

PURPOSE

Both lactate and base deficit (BD) are used as predictors of injury severity and mortality. We examined the significance of these measures when used in combination, and particularly when they provide conflicting data.

METHODS

We reviewed all intensive care unit patients with simultaneously obtained lactate and BD measurements. The ability to predict mortality and hospital stay was compared alone, in combination, and when there was disagreement between the measures. Receiver operating characteristic curves were generated to compare predictive abilities.

RESULTS

There were 1,298 patients with 12,197 sets of paired laboratory data; 1,026 trauma patients and 272 surgical patients. Lactic acidosis was present in 41% and a significant BD level (> 2) was found in 52%. Nonsurvivors had higher admission lactate (6.2 vs. 3.3) and base deficit (6.1 vs. 3.2) levels than survivors (both P < .01), with a modest correlation (r = .52) between the measures. The admission lactate and BD levels had similar predictive ability for mortality, with areas under the receiver operating characteristic curve of .7 and .66, respectively (both P < .01). However, the predictive ability of the BD level decreased significantly during the intensive care unit stay (area, .5) compared with lactate level (area, .68). Lactate and BD levels disagreed in 44% of all laboratory sets. In patients with a normal lactate level (< 2.2), the BD level had no predictive ability for mortality (area, .48; P = .26). However, in patients with a normal BD level (< 2.0), the lactate level retained its predictive ability for mortality (area, .67; P < .01). Lengths of stay were longer among patients with an increased lactate level, even when the BD level was normal. There was no improvement in predictive ability using a combination of the 2 measures.

CONCLUSIONS

Both lactate and BD levels may be used to identify lactic acidosis and predict mortality at admission. Increased lactate levels predict mortality and a prolonged course regardless of the associated BD level, whereas an increased BD level has no predictive value if the lactate level is normal.