Diagnosing metabolic acidosis in the critically ill: bridging the anion gap, Stewart, and base excess methods

Elevated strong ion gap: A predictor of the initiation of continuous renal replacement therapy in acute kidney injury

BACKGROUND

There is no optimal timing for continuous renal replacement therapy (CRRT) in acute kidney injury (AKI). AKI is a reason for the increased unmeasured anions, which refers to the increased organic acids in the blood, and they can be detected by calculating strong ion gap (SIG). SIG level at the moment of the AKI diagnosis may be a predictor for the initiation of CRRT.

METHODS

Patients who were diagnosed with AKI in the first week of the intensive care unit (ICU) period were included in this prospective observational study. At the moment of the AKI diagnosis, blood gas samples were recorded, and SIG was calculated.

RESULTS

The median level of SIG at the moment of the AKI diagnosis of CRRT (+) patients was significantly higher than CRRT (-) patients (7.4 and 3.2 mmol L-1, respectively). In the multivariate Cox regression analysis, the likelihood of the initiation of CRRT was increased 1.16-fold (1.01-1.33) and 4.0-fold (1.9-8.7) by only 1 mmol L-1 increases in SIG and SIG ≥6 mmol L-1 at the moment of AKI diagnosis, respectively (p = 0.035 and p < 0.001).

CONCLUSIONS

Increased SIG at the moment of the AKI diagnosis in patients with AKI may be a predictive marker to initiate CRRT.

Prognostic Significance of the Strong Ion Gap in Patients in Medical and Surgical Intensive Care Units

Background This study aimed to analyze acid-base imbalance by assessing the arterial blood gas (ABG) samples of the medical and surgical intensive care unit (ICU) patients by the Stewart approach and demonstrate the advantages of this method in delineating the acid-base status in cases where Henderson-Hasselbalch, anion gap, and base excess cannot optimally depict the imbalance and create recognition in the clinicians in this regard.  Methodology Adult (i.e., age > 18 years) patients admitted to the ICU of our institution during a one-year study period were included in this study. The patients were divided into two groups based on the indication of admission to the ICU as medical or surgical. The ABG, sodium, potassium, calcium, magnesium, phosphate, chloride, albumin, lactate, hemoglobin, hematocrit, leukocyte, blood urea nitrogen, and creatinine values determined during the first 24-hour period were used for calculating the Acute Physiologic Assessment and Chronic Health Evaluation (APACHE II), strong ion difference apparent (SIDa), and SID effective (SIDe) scores, which were subsequently compared between the groups.  Results Overall, 220 (110 medical and 110 surgical) patients were included. The mean patient age was 63.56 ± 18.08 years. The mean APACHE II scores were 21.99 and 19.63 in the medical and surgical groups, respectively. Overall, 110 patients died, while 110 were referred to the regular patient floor. The mean APACHE II score of the patients who died was 28.3, and the latter group had a mean APACHE II score of 13.57. There was a significant difference between the surgical and medical patient groups regarding mean values of APACHE II, SIDa, and SIDe scores. Also, the differences were significant between the patients who died and were discharged. There was a significant difference between the patients who died and were discharged regarding the strong ion gap (SIG); however, the medical and surgical patient groups were not different concerning the SIG values.  Conclusions We conclude that SIDa, SIDe, and SIG can be used in medical and surgical ICU patients to predict prognosis.

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.

Effects of the Type of Intraoperative Fluid in Living Donor Kidney Transplantation: A Single-Center Retrospective Cohort Study

PURPOSE

Perioperative fluid management in kidney transplant recipients is crucial to supporting the fluid, acid-base, and electrolyte balance required for graft perfusion. However, the choice of intraoperative crystalloids in kidney transplantation remains controversial. We conducted a single-center retrospective cohort study to evaluate the impact of intraoperative fluids on acid-base and electrolyte balance and graft outcomes.

MATERIALS AND METHODS

We included 282 living donor kidney transplant recipients from January 2010 to December 2017. Patients were classified into two groups based on the type of intraoperative crystalloids used (157 patients in the half saline group and 125 patients in the balanced crystalloid solutions group, Plasma-lyte).

RESULTS

Compared with the half saline group, the Plasma-lyte group showed less metabolic acidosis and hyponatremia during surgery. Hyperkalemia incidence was not significantly different between the two groups. Changes in postoperative graft function assessed by blood urea nitrogen and creatinine were significantly different between the two groups. Patients in the Plasma-lyte group exhibited consistently higher glomerular filtration rates than those in the half saline group at 1 month and 1 year after transplantation after adjusting for demographic differences.

CONCLUSION

Intraoperative Plasma-lyte can lead to more favorable results in terms of acid-base balance during kidney transplantation. Patients who received Plasma-lyte showed superior postoperative graft function at 1 month and 1 year after transplantation. Further studies are needed to evaluate the superiority of intraoperative Plasma-lyte over other types of crystalloids in relation to graft outcomes.

Semiquantitative acid-base analysis in dogs with typical hypoadrenocorticism

OBJECTIVE

To describe the semiquantitative acid-base status of dogs with untreated naturally occurring typical hypoadrenocorticism and to compare this to the status determined by traditional acid-base analysis.

DESIGN

Retrospective study.

SETTING

University teaching hospital.

ANIMALS

Thirty-three dogs with newly diagnosed typical hypoadrenocorticism between 2000 and 2017.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Dogs were included if they had newly diagnosed hypoadrenocorticism, post-ACTH stimulation serum cortisol concentration <2 μg/dL, and blood collected within 6 hours of presentation for acid-base, electrolyte, and serum biochemical assays. Dogs were excluded if the Na⁺ :K⁺ ratio was ≥28 or the dog had received a mineralocorticoid-containing corticosteroid medication within the preceding month. Traditional acid-base analysis identified normal acid-base status in 1 dog, simple respiratory acid-base abnormalities in 2 of 33 dogs, and simple metabolic acidosis in 14 of 33 dogs. A mixed disorder was most common, noted in 16 of 33 dogs. The semiquantitative approach identified metabolic abnormalities in all cases. All dogs had ≥1 acidifying process, and 29 of 33 had both acidifying and alkalinizing processes. Acidosis attributable to excess free water was present in all dogs, and an acidifying phosphate effect was present in 27 of 33. Hyperlactatemia contributed to the acidosis in 8 of 33 dogs, with a median (range) lactate concentration of 1.5 mmol/L (13.5 mg/dL) (0.3-4.2 mmol/L [2.7-37.8 mg/dL]).

CONCLUSIONS

Dogs with untreated Addison's disease have complex acid-base derangements. The semiquantitative approach to acid-base analysis provides greater insight into the underlying mechanisms of metabolic acid-base abnormalities in these dogs, particularly because lactic acidosis appears to be a minor influence in most cases.

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

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

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

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

The effects of the chloride:sodium ratio on acid-base statusand mortality in septic patients

BACKGROUND/AIM

Calculation of the chloride:sodium (Cl^-:Na⁺) ratio is proposed to enable a quick evaluation of the effect of Cl^- and Na⁺ on the acid-base balance in critically ill patients. In the present study, the relationship of the Cl^-:Na⁺ ratio of septic patients with acid-base status and ICU mortality were investigated.

MATERIALS AND METHODS

In our two-center study, 434 patients who were diagnosed with sepsis were included. The patients were divided into three groups: low (<0.75), normal (≥0.75, <0.80), and high (≥0.80) Cl^-:Na⁺ ratio groups. Patients' demographic data, blood gas values, length of ICU stay, and ICU mortality were recorded.

RESULTS

In the low and high groups, ICU mortality was significantly higher than in the normal group (29.3%, 37.1%) (P = 0.005). There was a negative correlation between the Cl:Na⁺ ratio and each of HCO₃^-, standard base excess, and PaCO₂ (r² = 0.21, r² = 0.19, and r² = 0.17) (P < 0.001 for each). In the multivariate analysis, the ICU mortality was increased 2.6-fold (1.2-5.8) by low Cl^-:Na⁺ ratio (P = 0.019).

CONCLUSION

The Cl^-:Na⁺ ratio is a useful parameter for showing the relationship between Cl^- and Na⁺ and their impact on acid-base status. Low Cl^-:Na⁺ ratio at ICU admission can be used as a prognostic indicator for increased ICU mortality in septic patients.

Unmeasured anions and mortality in critically ill patients in 2016

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

Acid-base disorders in calves with chronic diarrhea

The aim of this study was to analyze disorders of acid-base balance in calves with chronic diarrhea caused by mixed, viral, bacterial and Cryptosporydium parvum infection. We compared results ob- tained with the classic model (Henderson-Hasselbalch) and strong ion approach (the Steward model). The study included 36 calves aged between 14 and 21 days. The calves were allocated to three groups: I - (control) non-diarrheic calves, group II - animals with compensated acid-base imbalance and group III calves with compensated acid-base disorders and hypoalbuminemia. Plasma concentrations of Na+, K+, Cl-, C12+, Mg2+, P, albumin and lactate were measured. In the classic model, acid-base balance was determined on the basis of blood pH, pCO2, HCO3-, BE and anion gap. In the strong ion model, strong ion difference (SID), effective strong anion difference, total plasma concentration of nonvolatile buffers (A(Tot)) and strong ion gap (SIG) were measured. The control calves and the animals from groups II and III did not differ significantly in terms of their blood pH. The plasma concentration of HCO3-, BE and partial pressure of CO2 in animals from the two groups with chronic diarrhea were significantly higher than those found in the controls. The highest BE (6.03 mmol/l) was documented in calves from group II. The animals from this group presented compensation resulted from activation of metabolic mechanisms. The calves with hypoal- buminemia (group III) showed lower plasma concentrations of albumin (15.37 g/L), Cl (74.94 mmol/L), Mg2+ (0.53 mmol/L), P (1.41 mmol/L) and higher value of anion gap (39.03 mmol/L). This group III presented significantly higher SID3 (71.89 mmol/L), SID7 (72.92 mmol/L) and SIG (43.53 mmol/L) values than animals from the remaining groups (P < 0.01), whereas A(Tot) (6.82 mmol/L) were significantly lower. The main finding of the correlation study was the excellent relationship between the AGcorr and SID3, SID7, SIG. In conclusion, chronic diarrhea leads to numerous water-electrolyte disorders. Characterization of acid-base disturbance in these cases suggests that classic model have some limitations. This model can not be recommended for use whenever serum albumin or phosphate concentrations are markedly abnormal.

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

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

Comparison of the effects of normal saline versus Plasmalyte on acid-base balance during living donor kidney transplantation using the Stewart and base excess methods

BACKGROUND

Ischemia-reperfusion injury is an inevitable consequence of kidney transplantation, leading to metabolic acidosis. This study compared the effects of normal saline (NS) and Plasmalyte on acid-base balance and electrolytes during living donor kidney transplantation using the Stewart and base excess (BE) methods.

METHODS

Patients were randomized to an NS group (n = 30) or a Plasmalyte group (n = 30). Arterial blood samples were collected for acid-base analysis after induction of anesthesia (T0), prior to clamping the iliac vein (T1), 10 minutes after reperfusion of the donated kidney (T2), and at the end of surgery (T3). In addition serum creatinine and 24-hour urine output were recorded on postoperative days 1,2, and 7. Over the first postoperative 7 days we recorded episodes of graft failure requiring dialysis.

RESULTS

Compared with the Plasmalyte group, the NS group showed significantly lower values of pH, BE, and effective strong ion differences during the postreperfusion period (T2 and T3). Chloride-related values (chloride [Cl(-)], free-water corrected Cl(-), BEcl) were significantly higher at T1, T2, and T3, indicating hyperchloremic rather than dilutional metabolic acidosis. Early postoperative graft functions in terms of serum creatinine, urine output, and graft failure requiring dialysis were not significantly different between the groups.

CONCLUSIONS

Both NS and Plamalyte can be used safely during uncomplicated living donor kidney transplantation. However, Plasmalyte more stably maintains acid-base and electrolyte balance compared with NS especially during the postreperfusion period.

Pumpless extracorporeal CO(2) removal restores normocapnia and is associated with less regional perfusion in experimental acute lung injury

BACKGROUND

Lung protective ventilation may lead to hypoventilation with subsequent hypercapnic acidosis (HA). If HA cannot be tolerated or occurs despite increasing respiratory rate or buffering, extracorporeal CO2-removal using a percutaneous extracorporeal lung assist (pECLA) is an option. We hypothesised that compensation of HA using pECLA impairs regional perfusion. To test this hypothesis we determined organ blood flows in a lung-injury model with combined hypercapnic and metabolic acidosis.

METHODS

After induction of lung injury using hydrochloric acid (HCl) aspiration and metabolic acidosis by intravenous HCl infusion in nine pigs, an arterial-venous pECLA device was inserted. In randomised order, four treatments were tested: pECLA shunt (1) with and (2) without HA, and clamped pECLA shunt (3) with and (4) without HA. Regional blood flows were measured with the coloured microsphere technique.

RESULTS

HA resulted in higher perfusion in adrenal glands, spleen and parts of splanchnic area (P < 0.05) compared with normocapnia. During CO2-removal with pECLA, regional perfusion decreased to levels comparable with those without pECLA and normocapnia. Cardiac output (CO) increased during HA without a pECLA shunt and was highest during HA with a pECLA shunt compared with normocapnia. During CO2-removal with pECLA, this variable decreased but stayed higher than during normocapnia with clamped pECLA shunt (P < 0.05).

CONCLUSION

In our lung-injury model, HA was associated with increased systemic and regional blood flow in several organs. pECLA provides effective CO2 removal, requiring a higher CO for perfusion of the pECLA device without improvement of regional organ perfusion.

Advanced arterial blood gas analysis in septic shock: a Singaporean nursing case review

INTRODUCTION

The admission to the intensive care unit with a diagnosis of sepsis and/or septic shock is not uncommon. The aim of this article is to present a nursing case review of a patient admitted to the intensive care unit with a diagnosis of septic shock and the use of bedside acid-base formulae to inform clinical decision making.

METHOD

We chose to use a case review. This method is useful in reporting unusual or rare cases and is typically seen more in medicine than in nursing.

DISCUSSION

The gentleman in question was a self-presentation with a short history of fever and worsening shortness of breath. His condition worsened where he required admission to the intensive care unit. The use of 'advanced' acid-base interpretation to guide his nursing care provided a platform from which to advance a deeper understanding of the intricacies the critically ill patient often presents.

CONCLUSION

The use of case review is enlightening in understanding the disease process and the decision-making that accompanies this. The lessons learnt are applicable to a wider nursing audience because understanding acid-base physiology is beneficial in supporting and advancing critical care nursing practice.

The use of chloride-sodium ratio in the evaluation of metabolic acidosis in critically ill neonates

Acid-base disturbances have been usually evaluated with the traditional Henderson-Hasselbach method and Stewart's physiochemical approach by quantifying anions of tissue acids (TA). It is hypothesized that an increase in tissue acids during metabolic acidosis would cause a compensatory decrease in the plasma chloride (Cl) relative to sodium (Cl-Na ratio) in order to preserve electroneutral balance. Therefore, we aimed to investigate the use of Cl-Na ratio as a bedside tool to evaluate the identifying raised TA in neonates as an alternative to complex calculations of Stewart's physiochemical approach. This retrospective study was conducted between January 2008 and December 2009. Infants were included in the study when blood gas analysis reveals a metabolic acidosis; pH <7.25 and sHCO(3) concentration was <22 mEq/L. The Cl-Na ratio, sodium-chloride difference (Diff(NaCl)), anion gap (AG), albumin-corrected AG (AG(corr)), strong ion difference (SID), unmeasured anions (UMA), and TA were calculated at each episode of metabolic acidosis. A total of 105 metabolic acidosis episodes occurred in 59 infants during follow-up. Hypochloremic metabolic acidosis occurred in 17 (16%) of samples, and all had increased TA. The dominant component of TA was UMA rather than lactate. There was a negative correlation between the Cl-Na ratio and SID, AG(corr), UMA, and TA. Also, there was a positive correlation between Diff(NaCl) and SID, AG(corr), UMA, and TA. Base deficit and actual bicarbonate performed poorly in identifying the TA. In conclusion, our study suggested that Diff(NaCl) and Cl-Na ratio are simple and fast, and may be an alternative method to complex Stewart's physiochemical approach in identifying raised UMA and TA in critically ill neonates.

Current concepts in resuscitation

Early recognition and differentiation of shock, as well as goal-directed resuscitation, are fundamental principles in the care of the critically ill or injured patient. Substantial progress has been made over the last decade in the understanding of both shock and resuscitation. Specific areas of advancement, particularly pertaining to hemorrhagic shock, include a heightened appreciation of dynamic measurements of preload responsiveness (e.g., respiratory-induced pulse pressure and venous diameter variability), an improved awareness of the detrimental effects of blood product transfusion, and better recognition of the complications of overzealous volume expansion. However, several areas of controversy remain regarding the optimal resuscitation strategy. These include the optimal targets for perfusion pressure and oxygen delivery, endpoints of resuscitation, resuscitative fluid, and transfusion strategies for packed red blood cells and blood products. This article reviews the diagnosis and differentiation of shock, measurements of tissue perfusion, current evidence regarding various resuscitative techniques, and complications of resuscitation.

Estimating the net effect of unmeasured ions in human extracellular fluid using a new mathematical model. Part I: Theoretical considerations

A theoretical framework for the formulation of a derived variable to be used for the prediction of the net effect of unmeasured charged species present in human extracellular fluid was explored. This new variable was based on contemporary strong ion and classical buffer base theories and tested against the standard base excess using simulation. It proved to be more accurate in predicting the existence of unmeasured charged species in the extracellular fluid when disturbances of either strong ions, weak acids or both were present.

Acid-base imbalance in uncomplicated ST-elevation myocardial infarction: the clinical role of tissue acidosis

Little information is available on acid-base imbalance in uncomplicated ST-elevation myocardial infarction (STEMI) submitted to primary percutaneous intervention (PCI). We therefore assessed acid-base imbalance in 257 consecutive uncomplicated STEMI patients submitted to PCI to determine whether its evaluation could help in identifying patients at higher risk for in-hospital complications (acute pulmonary edema and dysrhythmias). A basic metabolic profile was performed at hospital admission, that is before PCI. After PCI, we measured: creatinine, uric acid and NT-pro BNP and serum electrolytes. Peak troponin I was also considered. Acidemia was present in 11 patients (4.2%), HCO(3) < 22 in 62 (24.1%). Base excess < -3 was detectable in 70 patients (27.2%), anion gap > 12 in 13 (5.1%), Cl/Na < 0.79 in 93 patients (38.5%). Patients with a Cl/Na < 0.79 had a lower LVEF (p = 0.042) and higher values of NT-pro-BNP (p = 0.019) and of latency (p = 0.029) together with a higher length of stay (p = 0.017) and a higher incidence of in-hospital complications (p = 0.017). At backward stepwise regression analysis, the following variables resulted independent predictors of in-hospital complications: base excess OR 1.47 (95% CI 1.04-2.10) p = 0.031; Cl/Na ratio O.R. 1.85 (95% CI 1.05-3.27) p = 0.035. In STEMI patients submitted to mechanical revascularization the evaluation of acid-base status and, in particular the detection of even mild degrees of acidosis may help in risk stratification for in-hospital complications. A Cl/Na < 0.79 ratio and a base excess are independent predictors for in-hospital complications.