Adding lactate to the prime solution during hypothermic cardiopulmonary bypass: a quantitative acid-base analysis

Comparison of the trometamol-balanced solution with two other crystalloid solutions for fluid resuscitation of a rat hemorrhagic model

Currently, the optimal resuscitation fluid remains debatable. Therefore, in the present study, we designed a trometamol-balanced solution (TBS) for use as a resuscitation fluid for hemorrhagic shock. Hemorrhagic shock was induced in 18 male Wistar-Kyoto rats, which were assigned to normal saline (NS), Ringer's solution (RS), and TBS groups. During the hemorrhagic state, their hemodynamic parameters were recorded using an Abbott i-STAT analyzer with the CG4+ cartridge (for pH, pressure of carbon dioxide, pressure of oxygen, total carbon dioxide, bicarbonate, base excess, oxygen saturation, and lactate), the CG6+ cartridge (for sodium, potassium, chloride, blood glucose, blood urea nitrogen, hematocrit, and hemoglobin), and enzyme-linked immunosorbent assay kits (calcium, magnesium, creatinine, aspartate aminotransferase, alanine aminotransferase, bilirubin, and albumin). Similar trends were found for the parameters of biochemistries, electrolytes, and blood gas, and they revealed no significant changes after blood withdrawal-induced hemorrhagic shock. However, the TBS group showed more effective ability to correct metabolic acidosis than the NS and RS groups. TBS was a feasible and safe resuscitation solution in this study and may be an alternative to NS and RS for resuscitation in hemorrhagic shock patients without liver damage.

Renal hemodynamics and oxygenation during experimental cardiopulmonary bypass in sheep under total intravenous anesthesia

Renal medullary hypoxia may contribute to the pathophysiology of acute kidney injury, including that associated with cardiac surgery requiring cardiopulmonary bypass (CPB). When performed under volatile (isoflurane) anesthesia in sheep, CPB causes renal medullary hypoxia. There is evidence that total intravenous anesthesia (TIVA) may preserve renal perfusion and renal oxygen delivery better than volatile anesthesia. Therefore, we assessed the effects of CPB on renal perfusion and oxygenation in sheep under propofol/fentanyl-based TIVA. Sheep (n = 5) were chronically instrumented for measurement of whole renal blood flow and cortical and medullary perfusion and oxygenation. Five days later, these variables were monitored under TIVA using propofol and fentanyl and then on CPB at a pump flow of 80 mL·kg^-1·min^-1 and target mean arterial pressure of 70 mmHg. Under anesthesia, before CPB, renal blood flow was preserved under TIVA (mean difference ± SD from conscious state: -16 ± 14%). However, during CPB renal blood flow was reduced (-55 ± 13%) and renal medullary tissue became hypoxic (-20 ± 13 mmHg versus conscious sheep). We conclude that renal perfusion and medullary oxygenation are well preserved during TIVA before CPB. However, CPB under TIVA leads to renal medullary hypoxia, of a similar magnitude to that we observed previously under volatile (isoflurane) anesthesia. Thus use of propofol/fentanyl-based TIVA may not be a useful strategy to avoid renal medullary hypoxia during CPB.

Plasma-Lyte 148 vs. Hartmann's solution for cardiopulmonary bypass pump prime: a prospective double-blind randomized trial

BACKGROUND

The mechanisms of acid-base changes during cardiopulmonary bypass (CPB) remain unclear. We tested the hypothesis that, when used as CPB pump prime solutions, Plasma-Lyte 148 (PL) and Hartmann's solution (HS) have differential mechanisms of action in their contribution to acid-base changes.

METHODS

We performed a prospective, double-blind, randomized trial in adult patients undergoing elective cardiac surgery with CPB. Participants received a CPB prime solution of 2000 mL, with either PL or HS. The primary endpoint was the standard base excess (SBE) value measured at 60 minutes after full CPB flows (SBE60min). Secondary outcomes included changes in SBE, pH, chloride, sodium, lactate, gluconate, acetate, strong ion difference and strong ion gap at two (T2min), five (T5min), ten (T10min), thirty (T30min) and sixty (T60min) minutes on CPB. The primary outcome was measured using a two-tailed Welch's t-test. Repeated measures ANOVA was used to test for differences between time points.

RESULTS

Twenty-five participants were randomized to PL and 25 to HS. Baseline characteristics, EURO and APACHE scores, biochemistry, hematology and volumes of cardioplegia were similar. Mean (SD) SBE at T60min was -1.3 (1.4) in the PL group and -0.1 (2.7) in the HS group; p=0.55. No significant differences in SBE between the groups was observed during the first 60 minutes (p=0.48). During CPB, there was hyperacetatemia and hypergluconatemia in the PL group and hyperlactatemia and hyperchloremia in the HS group. No significant difference between the groups in plasma bicarbonate levels and total weak acid levels were found. Complications and intensive care unit and hospital length of stays were similar.

CONCLUSIONS

During CPB, PL and HS did not cause a significant metabolic acidosis. There was hyperacetatemia and hypergluconatemia with PL and hyperchloremia and hyperlactatemia with HS. These physiochemical effects appear clinically innocuous.

The ideal crystalloid - what is 'balanced'?

PURPOSE OF REVIEW

This review explores the contemporary definition of the term 'balanced crystalloid' and outlines optimal design features and their underlying rationale.

RECENT FINDINGS

Crystalloid interstitial expansion is unavoidable, but also occurs with colloids when there is endothelial glycocalyx dysfunction. Reduced chloride exposure may lessen kidney dysfunction and injury with a possible mortality benefit. Exact balance from an acid-base perspective is achieved with a crystalloid strong ion difference of 24 mEq/l. This can be done simply by replacing 24 mEq/l of chloride in 0.9% sodium chloride with bicarbonate or organic anion bicarbonate substitutes. Potassium, calcium and magnesium additives are probably unnecessary. Large volumes of mildly hypotonic crystalloids such as lactated Ringer's solution reduce extracellular tonicity in volunteers and increase intracranial pressure in nonbrain-injured experimental animals. A total cation concentration of 154 mmol/l with accompanying anions provides isotonicity. Of the commercial crystalloids, Ringer's acetate solution is close to balanced from both acid-base and tonicity perspectives, and there is little current evidence of acetate toxicity in the context of volume loading, in contrast to renal replacement.

SUMMARY

The case for balanced crystalloids is growing but unproven. A large randomized controlled trial of balanced crystalloids versus 0.9% sodium chloride is the next step.

Effect of hyperosmolar sodium lactate infusion on haemodynamic status and fluid balance compared with hydroxyethyl starch 6% during the cardiac surgery

Background and Aim:

No solution has been determined ideal for fluid therapy during cardiac surgery. Previous studies have shown that hyperosmolar sodium lactate (HSL) infusion has improved cardiac performance with smaller volume infusion, which resulted in negative fluid balance. This study compared the effects between a patent-protected HSL infusion and hydroxyethyl starch (HES) 6% on haemodynamic status of the patients undergoing cardiac surgery.

Methods:

In this open-label prospective controlled randomized study, patients were randomly assigned to receive loading dose of either HSL or HES 6%, at 3 mL/kgBW within 15 min, at the beginning of surgery. Haemodynamic parameters and fluid balance were evaluated, while biochemical parameters and any adverse effect were also recorded. Haemodynamic and laboratory parameters were analyzed through repeated measures analysis of variance. Statistical assessment of fluid management was carried out through Student t-test. All statistical analyses were performed using the statistical package for the social sciences^® version 15, 2006 (SPSS Inc., Chicago, IL).

Results:

Out of 100 enrolled patients in this study (50 patients in each arm), 98 patients were included in analysis (50 in HSL group; 48 in HES group). Cardiac index increased higher in HSL group (P = 0.01), whereas systemic vascular resistance index decreased more in HSL than HES group (P = 0.002). Other haemodynamic parameters were comparable between HSL and HES group. Fluid balance was negative in HSL group, but it was positive in HES group (−445.94 ± 815.30 mL vs. +108.479 ± 1219.91 mL, P < 0.009).

Conclusion:

Administration of HSL solution during the cardiac surgery improved cardiac performance and haemodynamic status better than HES did.

Hydrogen ion concentration and coronary artery bypass graft surgery with and without cardiopulmonary bypass

Background

Acidosis during cardiopulmonary bypass (CPB) has been related to the strong ion difference (SID) and the composition of intravascular fluids that are administered. Less intravascular fluids tend to be administered during off- than on-pump CABG and should influence the degree of acidosis that develops. This study aimed to explore the role of CPB in the development of acidosis by comparing changes in hydrogen ion concentration ([H⁺]) and electrolytes in patients undergoing on- and off-pump coronary artery bypass graft (CABG) surgery.

Methods

Eighty two patients had arterial blood gas measurements pre-operatively, following CABG and at approximately 0600 h the morning after surgery. Carbon dioxide tension (PaCO₂) and concentrations of sodium, potassium, chloride, [H⁺], bicarbonate and haemoglobin were measured and strong ion difference calculated. Data was analysed using mixed repeated-measures analysis of variance.

Results

Intra-operatively, mean SID decreased more in the on- compared to the off-pump group (4.0 mmol/L, 95% confidence interval 2.8-5.3 mmol/L, p < 0.001). Neither [H⁺] or PaCO₂ changed significantly and there were no significant difference between the groups. By the morning following surgery, [H⁺] and PaCO₂ had both increased (p < 0.001) and difference in SID had disappeared (p = 0.17).

Conclusion

Despite significant differences in changes in SID, there were no differences in [H⁺] between patients during or after CABG surgery whether performed on- or off-pump. This may be have been the result of greater haemodilution in the on- compared to the off-pump group, compensating for change in SID by reducing the concentration of weak acids. Although it was associated with significantly greater decrease in SID, CPB was not associated with any significant increased risk of acidosis.

Plasma acetate, gluconate and interleukin-6 profiles during and after cardiopulmonary bypass: a comparison of Plasma-Lyte 148 with a bicarbonate-balanced solution

INTRODUCTION

As even small concentrations of acetate in the plasma result in pro-inflammatory and cardiotoxic effects, it has been removed from renal replacement fluids. However, Plasma-Lyte 148 (Plasma-Lyte), an electrolyte replacement solution containing acetate plus gluconate is a common circuit prime for cardio-pulmonary bypass (CPB). No published data exist on the peak plasma acetate and gluconate concentrations resulting from the use of Plasma-Lyte 148 during CPB.

METHODS

Thirty adult patients were systematically allocated 1:1 to CPB prime with either bicarbonate-balanced fluid (24 mmol/L bicarbonate) or Plasma-Lyte 148. Arterial blood acetate, gluconate and interleukin-6 (IL-6) levels were measured immediately before CPB (T1), three minutes after CPB commencement (T2), immediately before CPB separation (T3), and four hours post separation (T4).

RESULTS

Acetate concentrations (normal 0.04 to 0.07 mmol/L) became markedly elevated at T2, where the Plasma-Lyte group (median 3.69, range (2.46 to 8.55)) exceeded the bicarbonate group (0.16 (0.02 to 3.49), P < 0.0005). At T3, levels had declined but the differential pattern remained apparent (Plasma-Lyte 0.35 (0.00 to 1.84) versus bicarbonate 0.17 (0.00 to 0.81)). Normal circulating acetate concentrations were not restored until T4. Similar gluconate concentration profiles and inter-group differences were seen, with a slower T3 decay. IL-6 increased across CPB, peaking at T4, with no clear difference between groups.

CONCLUSIONS

Use of acetate containing prime solutions result in supraphysiological plasma concentrations of acetate. The use of acetate-free prime fluid in CPB significantly reduced but did not eliminate large acetate surges in cardiac surgical patients. Complete elimination of acetate surges would require the use of acetate free bolus fluids and cardioplegia solutions.

TRIAL REGISTRATION

Australia and New Zealand Clinical Trials Register (ANZCTR): ACTRN12610000267055.

Effect of pump prime on acidosis, strong-ion-difference and unmeasured ions during cardiopulmonary bypass

We tested the hypothesis that a cardiopulmonary bypass prime with lactate would be associated with less acidosis than a prime with only chloride anions because of differences in the measured strong-ion-difference. We randomised 20 patients to a 1500 ml bypass prime with either a chloride-only solution (Ringer's Injection; anions: chloride 152 mmol/l) or a lactated solution (Hartmann's solution; anions: chloride 109 mmol/l, lactate 29 mmol/l). Arterial blood was sampled before bypass and then two, five, 15 and 30 minutes after initiating bypass. We used repeated measures analysis of variance to compare groups. In both groups, the base-excess and measured strong-ion-difference decreased markedly from baseline after two minutes of bypass. The chloride-only group had greater acidosis with lower base-excess and pH (P < 0.05), greatest after five minutes of bypass (C5). Contrary to our hypothesis, however, the difference between the groups was not due to a difference in the measured strong-ion-difference, P = 0.88. At C5 when the difference in standard base-excess between the groups was greatest, 1.9 mmol/l (95% confidence interval: 0.1 to 3.6 mmol/l, P < 0.05), the difference in the measured strong-ion-difference was only 0.2 mmol/l (95% confidence interval: -2.4 to 2.7 mmol/l, P > 0.05). There was, however a difference in the net-unmeasured-ions (strong-ion-gap). We conclude that acid-base changes with cardiopulmonary bypass may differ with the prime but that the early differences between chloride-only and lactated primes appear not to be due to differences in the measured strong-ion-difference. We suggest future studies examine other possible mechanisms including unmeasured ions.

Acid-base effects of a bicarbonate-balanced priming fluid during cardiopulmonary bypass: comparison with Plasma-Lyte 148. A randomised single-blinded study

Fluid-induced metabolic acidosis can be harmful and can complicate cardiopulmonary bypass. In an attempt to prevent this disturbance, we designed a bicarbonate-based crystalloid circuit prime balanced on physico-chemical principles with a strong ion difference of 24 mEq/l and compared its acid-base effects with those of Plasma-Lyte 148, a multiple electrolyte replacement solution containing acetate plus gluconate totalling 50 mEq/l. Twenty patients with normal acid-base status undergoing elective cardiac surgery were randomised 1:1 to a 2 litre prime of either bicarbonate-balanced fluid or Plasma-Lyte 148. With the trial fluid, metabolic acid-base status was normal following bypass initiation (standard base excess 0.1 (1.3) mEq/l, mean, SD), whereas Plasma-Lyte 148 produced a slight metabolic acidosis (standard base excess -2.2 (2.1) mEq/l). Estimated group difference after baseline adjustment was 3.6 mEq/l (95% confidence interval 2.1 to 5.1 mEq/l, P=0.0001). By late bypass, mean standard base excess in both groups was normal (0.8 (2.2) mEq/l vs. -0.8 (1.3) mEq/l, P=0.5). Strong ion gap values were unaltered with the trial fluid, but with Plasma-Lyte 148 increased significantly on bypass initiation (15.2 (2.5) mEq/l vs. 2.5 (1.5) mEq/l, P < 0.0001), remaining elevated in late bypass (8.4 (3.4) mEq/l vs. 5.8 (2.4) mEq/l, P < 0.05). We conclude that a bicarbonate-based crystalloid with a strong ion difference of 24 mEq/l is balanced for cardiopulmonary bypass in patients with normal acid-base status, whereas Plasma-Lyte 148 triggers a surge of unmeasured anions, persisting throughout bypass. These are likely to be gluconate and/or acetate. Whether surges of exogenous anions during bypass can be harmful requires further study.

Changing the priming solution from Ringer's to Hartmann's solution is associated with less metabolic acidosis during cardiopulmonary bypass

BACKGROUND AND OBJECTIVE

Previously, it was noted that changing the solutions used for priming and intravascular volume replacement from Hartmann's to Ringer's resulted in a more profound metabolic acidosis developing during cardiopulmonary bypass (CPB). The aim of this study was to examine the effects of changing the solutions back to Hartmann's on metabolic acidosis that develops during CPB in patients undergoing heart surgery.

METHODS

Two groups of patients were studied sequentially: the first received Ringer's (n = 63) and the second Hartmann's solution (n = 66). Arterial blood samples were taken before induction of anaesthesia and towards the end of CPB. Samples were analysed in a blood gas analyser.

RESULTS

Hydrogen ion concentration increased from 38 (4) to 41 (7)mmol/L in the Ringer's group, but decreased from 38 (5) to 36 (6) mmol L(-1) in the Hartmann's group. Changes in PaCO2 (0.77, p < 0.001) and volume of fluid administered (r= 0.23, p <0.01) were significant univariate correlates of change in hydrogen ion concentration, but haemoglobin concentration was not (r < 0.01, p = 0.97). Analysis of variance for repeated measures found significant between subject effects on the change in hydrogen ion concentration during CPB caused by the choice of intravascular solution used (p < 0.001) and PaCO2 (p = 0.001), but not as a result of the volume of solution administered (p > 0.10).

CONCLUSIONS

Changing the solutions used for priming and intravascular volume replacement from Ringer's to Hartmann's was associated with a reduction in metabolic acidosis that developed during CPB.

Acid-base and bio-energetics during balanced versus unbalanced normovolaemic haemodilution

Fluids balanced to avoid acid-base disturbances may be preferable to saline, which causes metabolic acidosis in high volume. We evaluated acid-base and bio-energetic effects of haemodilution with a crystalloid balanced on physical chemical principles, versus crystalloids causing metabolic acidosis or metabolic alkalosis. Anaesthetised, mechanically ventilated Sprague-Dawley rats (n=32, allocated to four groups) underwent six exchanges of 9 ml crystalloid for 3 ml blood. Exchange was with one of three crystalloids with strong ion difference (SID) values of 0, 24 (balanced) and 40 mEq/l. Controls did not undergo haemodilution. Mean haemoglobin concentration fell to approximately 50 g/l after haemodilution. With SID 24 mEq/l fluid, metabolic acid-base remained unchanged. Dilution with SID 0 mEq/l and 40 mEq/l fluids caused a progressive metabolic acidosis and alkalosis respectively. Standard base excess (SBE) and haemoglobin concentration were directly correlated in the SID 0 mEq/l group (R2 = 0.61), indirectly correlated in the SBE 40 mEq/l group (R2 = 0.48) and showed no correlation in the SID 24 mEq/l group (R2 = 0.003). There were no significant differences between final ileal values of CO2 gap, nucleotides concentration, energy charge, or luminal lactate concentration. SID 40 mEq/l crystalloid dilution caused a significant rise in subcutaneous lactate. In this group mean kidney ATP concentration was significantly less than controls and renal energy charge significantly lower than SID 0 mEq/l and control groups. We conclude that a crystalloid SID of 24 mEq/l provides balanced haemodilution. Bio-energetic perturbations with higher SID haemodilution may be more severe and need further investigation.

What is a normal lactate level during cardiopulmonary bypass?

Blood lactate levels during cardiopulmonary bypass are often used to verify adequacy of perfusion. The present investigation aimed to propose a threshold for hyperlactatemia. Blood lactate levels in 5 121 cardiac surgical patients were retrospectively analysed by a review of database records. Hyperlactatemia was defined as a value equal to the 90th percentile of the identified lactate distribution at weaning from cardiopulmonary bypass. Patient demographics, background and outcome statistics were performed stratified on presence of hyperlactatemia. The threshold for hyperlactatemia was found to equal 2 mmol/l. Significant predictors of hyperlactatemia based on logistic regression modelling were age, complex surgery, duration of cardiopulmonary bypass, blood transfusion, acid base level, emergency operations, diabetes, vasoactive intervention, venous-blood-return to the heart-lung machine and renal function. Patients with hyperlactatemia required longer intensive care and postoperative ventilatory support. Complications were more frequent, especially: renal dysfunction, infections, respiratory and circulatory disorders. Hospital mortality was 13.3% compared to an overall level at 2.2%. The threshold for hyperlactatemia during cardiopulmonary bypass attained 2 mmol/l and predicted increased morbidity and mortality.

Increased concentrations of l -lactate in the rectal lumen in patients undergoing cardiopulmonary bypass †

BACKGROUND

Gut ischaemia may contribute to morbidity in patients after cardiopulmonary bypass (CPB), but little is known about the metabolic state of the large bowel in such patients. Therefore we estimated the concentrations of L-lactate and Pco(2) in rectal mucosa in patients undergoing cardiac surgery with or without the use of CPB.

METHODS

Patients undergoing coronary artery bypass grafting (CABG) (n=12) or off-pump CABG (n=10) were subjected to equilibrium dialysis of the rectal lumen during the procedure and in the first 4 h afterwards. Dialysate concentrations of L-lactate and Pco(2) were measured using an auto-analyser and compared with values obtained in healthy subjects (n=10).

RESULTS

During CPB, a 2- to 3-fold increase in luminal concentrations of L-lactate was observed (CABG vs off-pump CABG, P=0.05; CABG vs healthy subjects, P<0.01). The dialysate concentrations of L-lactate were higher than the mean systemic values (luminal-arterial gradient mean (sd) 0.9 (1.0) mmol litre(-1), P<0.05), and the two values were positively correlated (P<0.05). Luminal L-lactate concentrations remained elevated 4 h after the operation. In contrast, dialysate Pco(2) was equally high in patient and control groups and substantially higher than values observed in arterial blood.

CONCLUSIONS

Uncomplicated CPB is associated with moderate but sustained increases in luminal concentrations of L-lactate in the rectum, indicating metabolic dysfunction of the mucosa in the large bowel.

Hyperchloraemic metabolic acidosis following open cardiac surgery

AIMS

To describe acid-base derangements in children following open cardiac surgery on cardiopulmonary bypass (CPB), using the Fencl-Stewart strong ion approach.

METHODS

Prospective observational study set in the paediatric intensive care unit (PICU) of a university children's hospital. Arterial blood gas parameters, serum electrolytes, strong ion difference, strong ion gap (SIG), and partitioned base excess (BE) were measured and calculated on admission to PICU.

RESULTS

A total of 97 children, median age 57 months (range 0.03-166), median weight 14 kg (range 2.1-50), were studied. Median CPB time was 80 minutes (range 17-232). Predicted mortality was 2% and there was a single non-survivor. These children showed mild metabolic acidosis (median standard bicarbonate 20.1 mmol/l, BE -5.1 mEq/l) characterised by hyperchloraemia (median corrected Cl 113 mmol/l), and hypoalbuminaemia (median albumin 30 g/l), but no significant excess unmeasured anions or cations (median SIG 0.7 mEq/l). The major determinants of the net BE were the chloride and albumin components (chloride effect -4.8 mEq/l, albumin effect +3.4 mEq/l). Metabolic acidosis occurred in 72 children (74%) but was not associated with increased morbidity. Hyperchloraemia was a causative factor in 53 children (74%) with metabolic acidosis. Three (4%) hyperchloraemic children required adrenaline for inotropic support, compared to eight children (28%) without hyperchloraemia. Hypoalbuminaemia was associated with longer duration of inotropic support and PICU stay.

CONCLUSIONS

In these children with low mortality following open cardiac surgery, hypoalbuminaemia and hyperchloraemia were the predominant acid-base abnormalities. Hyperchloraemia was associated with reduced requirement for adrenaline therapy. It is suggested that hyperchloraemic metabolic acidosis is a benign phenomenon that should not prompt escalation of haemodynamic support. By contrast, hypoalbuminaemia, an alkalinising force, was associated with prolonged requirement for intensive care.

The meaning of acid-base abnormalities in the intensive care unit: part III -- effects of fluid administration

Stewart's quantitative physical chemical approach enables us to understand the acid–base properties of intravenous fluids. In Stewart's analysis, the three independent acid–base variables are partial CO₂ tension, the total concentration of nonvolatile weak acid (A_TOT), and the strong ion difference (SID). Raising and lowering A_TOT while holding SID constant cause metabolic acidosis and alkalosis, respectively. Lowering and raising plasma SID while clamping A_TOT cause metabolic acidosis and alkalosis, respectively. Fluid infusion causes acid–base effects by forcing extracellular SID and A_TOT toward the SID and A_TOT of the administered fluid. Thus, fluids with vastly differing pH can have the same acid–base effects. The stimulus is strongest when large volumes are administered, as in correction of hypovolaemia, acute normovolaemic haemodilution, and cardiopulmonary bypass. Zero SID crystalloids such as saline cause a 'dilutional' acidosis by lowering extracellular SID enough to overwhelm the metabolic alkalosis of A_TOT dilution. A balanced crystalloid must reduce extracellular SID at a rate that precisely counteracts the A_TOT dilutional alkalosis. Experimentally, the crystalloid SID required is 24 mEq/l. When organic anions such as L-lactate are added to fluids they can be regarded as weak ions that do not contribute to fluid SID, provided they are metabolized on infusion. With colloids the presence of A_TOT is an additional consideration. Albumin and gelatin preparations contain A_TOT, whereas starch preparations do not. Hextend is a hetastarch preparation balanced with L-lactate. It reduces or eliminates infusion related metabolic acidosis, may improve gastric mucosal blood flow, and increases survival in experimental endotoxaemia. Stored whole blood has a very high effective SID because of the added preservative. Large volume transfusion thus causes metabolic alkalosis after metabolism of contained citrate, a tendency that is reduced but not eliminated with packed red cells. Thus, Stewart's approach not only explains fluid induced acid–base phenomena but also provides a framework for the design of fluids for specific acid–base effects.

Das Stewart-Modell

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

New aspects of acid-base balance in intensive care

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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