Fluid resuscitation and hyperchloremic acidosis in experimental sepsis: improved short-term survival and acid-base balance with Hextend compared with saline

Stewart's physicochemical approach in neurosurgical patients with hyperchloremic metabolic acidosis during propofol anesthesia

There is both in vitro and clinical evidence that high-dose propofol can inhibit mitochondrial respiration, resulting in metabolic acidosis. The purpose of this study was to evaluate the effects of propofol anesthesia on the acid-base status in neurosurgical patients with large amount of normal saline administration. Thirty patients undergoing clipping of cerebral aneurysm were randomly assigned to receive propofol (n=15) or isoflurane (n=15). Propofol dose (mean+/-standard error) infused for maintenance was 5.7+/-0.2 mg/kg/h in propofol group. Acid-base parameters such as PaCO2, pH, serum bicarbonate concentration, standard base excess, serum electrolyte concentration, total protein, albumin, lactate, and phosphate were measured before and 4 hours after the induction of anesthesia, and after surgery. The apparent strong ion difference (SIDa), the effective SID (SIDe), and the amount of weak plasma acid were calculated using the Stewart equation. There were no significant differences in pH, PaCO2, bicarbonate, and lactate between 2 groups throughout the whole investigation period. After surgery, standard base excess significantly decreased in both groups without intergroup difference. SIDa and SIDe significantly decreased in both groups, and lactate and strong ion gap significantly increased after surgery in propofol group, but there were no significant differences between 2 groups. Both propofol and isoflurane were associated with hyperchloremic metabolic acidosis in neurosurgical patients with large amount of normal saline administration. The acid-base balance between the 2 anesthetics was similar using Stewart's physicochemical approach.

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.

Optimizing intraoperative fluid therapy

PURPOSE OF REVIEW

Correcting the fluid status of the surgical patient is an integral part of good anaesthetic practice. There have been few areas in anaesthesia and perioperative medicine as controversial as fluid resuscitation. Uncertainties still exist as to what the best solution to give is, whether it be a colloid or a crystalloid, and how and when to give it. As well as increasing awareness of the different properties of various colloids, there has been interest in the nature of the carrier solutions, essentially a choice between saline or Ringer's lactate (compound sodium lactate or Hartmann's solution). In this article we review recent studies involving crystalloids, the 'new colloids', and on the amount and timing of fluid therapy.

RECENT FINDINGS

Saline based fluids (including most colloids) are associated with a hyperchloremic metabolic acidosis, and a hypocoagulable state, although these may not necessarily harm the patient. Saline may have deleterious effects on renal function. Colloids in solutions similar to Ringer's lactate ('balanced solutions') may avoid these effects although few are currently available. Several studies that have used fluids (along with other therapies) to improve organ perfusion around the time of surgery have been associated with a better outcome.

SUMMARY

Compared with Ringer's lactate, saline, and saline-based colloids are associated with a hyperchloremic metabolic acidosis, and a hypocoagulable state although they may not be associated with adverse patient outcomes. Increasing awareness of the 'Stewart hypothesis' has led to new ways of managing hyperchloremic metabolic acidosis. The 'crystalloid-colloid debate' continues, and has led to an awareness that these different fluids, along with their carrier solutions are drugs with different effects. Several studies, in which patients have received more fluid in the protocol group, have found better clinical outcomes in the 'optimized' patients.

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.

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.

A total balanced volume replacement strategy using a new balanced hydoxyethyl starch preparation (6% HES 130/0.42) in patients undergoing major abdominal surgery

BACKGROUND AND OBJECTIVE

The kind of fluid for correcting hypovolaemia is still a focus of debate. In a prospective, randomized, controlled and double-blind study in patients undergoing major abdominal surgery, a total balanced volume replacement strategy including a new balanced hydroxyethyl starch (HES) solution was compared with a conventional, non-balanced fluid regimen.

METHODS

In Group A (n = 15), a new balanced 6% HES 130/0.42 was given along with a balanced crystalloid solution; in Group B (n = 15), an unbalanced conventional HES 130/0.42 plus an unbalanced crystalloid (saline solution) were administered. Volume was given when mean arterial pressure (MAP) was <65 mmHg and central venous pressure (CVP) minus positive end-expiratoric pressure (PEEP) level was <10 mmHg. Haemodynamics, acid-base status, coagulation (thrombelastography (TEG)) and kidney function (including kidney-specific proteins, N-acetyl-beta-d-glucosaminidase (beta-NAG) and alpha-1-microglobulin) were measured after induction of anaesthesia, at the end of surgery, 5 and 24 h after surgery.

RESULTS

Group A received 3533 +/- 1302 mL of HES and 5333 +/- 1063 mL of crystalloids, in Group B, 3866 +/- 1674 mL of HES and 5966 +/- 1202 mL of crystalloids were given. Haemodynamics, laboratory data, TEG data and kidney function were without significant differences between the groups. Cl- concentration and base excess (-5 +/- 2.4 mmol L-1 vs. 0.4 +/- 2.4 mmol L-1) were significantly higher in patients of Group B than of Group A.

CONCLUSIONS

A complete balanced volume replacement strategy including a new balanced HES preparation resulted in significantly less derangement in acid-base status compared with a non-balanced volume replacement regimen. The new HES preparation showed no negative effects on coagulation and kidney function.

Effects of levosimendan and dobutamine in experimental acute endotoxemia: a preliminary controlled study

OBJECTIVE

To test the hypothesis that levosimendan increases systemic and intestinal oxygen delivery (DO(2)) and prevents intramucosal acidosis in septic shock.

DESIGN

Prospective, controlled experimental study.

SETTING

University-based research laboratory.

SUBJECTS

Nineteen anesthetized, mechanically ventilated sheep.

INTERVENTIONS

Endotoxin-treated sheep were randomly assigned to three groups: control (n=7), dobutamine (10 microg/kg/min, n=6) and levosimendan (100 microg/kg over 10 min followed by 100 microg/kg/h, n=6) and treated for 120 min.

MEASUREMENTS AND MAIN RESULTS

After endotoxin administration, systemic and intestinal DO(2) decreased (24.6+/-5.2 vs 15.3+/-3.4 ml/kg/min and 105.0+/-28.1 vs 55.8+/-25.9 ml/kg/min, respectively; p<0.05 for both). Arterial lactate and the intramucosal-arterial PCO(2) difference (DeltaPCO(2)) increased (1.4+/-0.3 vs 3.1+/-1.5 mmHg and 9+/-6 vs 23+/-6 mmHg mmol/l, respectively; p<0.05). Systemic DO(2) was preserved in the dobutamine-treated group (22.3+/-4.7 vs 26.8+/-7.0 ml/min/kg, p=NS) but intestinal DO(2) decreased (98.9+/-0.2 vs 68.0+/-22.9 ml/min/kg, p<0.05) and DeltaPCO(2) increased (12+/-5 vs 25+/-11 mmHg, p<0.05). The administration of levosimendan prevented declines in systemic and intestinal DO(2) (25.1+/-3.0 vs 24.0+/-6.3 ml/min/kg and 111.1+/-18.0 vs 98.2+/-23.1 ml/min/kg, p=NS for both) or increases in DeltaPCO(2) (7+/-7 vs 10+/-8, p=NS). Arterial lactate increased in both the dobutamine and levosimendan groups (1.6+/-0.3 vs 2.5+/-0.7 and 1.4+/-0.4 vs. 2.9+/-1.1 mmol/l, p=NS between groups).

CONCLUSIONS

Compared with dobutamine, levosimendan increased intestinal blood flow and diminished intramucosal acidosis in this experimental model of sepsis.

Lactate versus non-lactate metabolic acidosis: a retrospective outcome evaluation of critically ill patients

Introduction

Acid–base abnormalities are common in the intensive care unit (ICU). Differences in outcome exist between respiratory and metabolic acidosis in similar pH ranges. Some forms of metabolic acidosis (for example, lactate) seem to have worse outcomes than others (for example, chloride). The relative incidence of each type of disorder is unknown. We therefore designed this study to determine the nature and clinical significance of metabolic acidosis in critically ill patients.

Methods

An observational, cohort study of critically ill patients was performed in a tertiary care hospital. Critically ill patients were selected on the clinical suspicion of the presence of lactic acidosis. The inpatient mortality of the entire group was 14%, with a length of stay in hospital of 12 days and a length of stay in the ICU of 5.8 days.

Results

We reviewed records of 9,799 patients admitted to the ICUs at our institution between 1 January 2001 and 30 June 2002. We selected a cohort in which clinicians caring for patients ordered a measurement of arterial lactate level. We excluded patients in which any necessary variable required to characterize an acid–base disorder was absent. A total of 851 patients (9% of ICU admissions) met our criteria. Of these, 548 patients (64%) had a metabolic acidosis (standard base excess < -2 mEq/l) and these patients had a 45% mortality, compared with 25% for those with no metabolic acidosis (p < 0.001). We then subclassified metabolic acidosis cases on the basis of the predominant anion present (lactate, chloride, or all other anions). The mortality rate was highest for lactic acidosis (56%); for strong ion gap (SIG) acidosis it was 39% and for hyperchloremic acidosis 29% (p < 0.001). A stepwise logistic regression model identified serum lactate, SIG, phosphate, and age as independent predictors of mortality.

Conclusion

In critically ill patients in which a measurement of lactate level was ordered, lactate and SIG were strong independent predictors of mortality when they were the major source of metabolic acidosis. Overall, patients with metabolic acidosis were nearly twice as likely to die as patients without metabolic acidosis.

Management of sepsis during the early "golden hours"

Severe sepsis and septic shock are common causes of morbidity and mortality. Interventions directed at specific endpoints, when initiated early in the "golden hours" of patient arrival at the hospital, seem to be promising. Early hemodynamic optimization, administration of appropriate antimicrobial therapy, and effective source control of infection are the cornerstones of successful management. In patients with vasopressor-dependent septic shock, provision of physiologic doses of replacement steroids may result in improved survival. Administration of drotrecogin alfa (activated), (activated protein C) has been shown to improve survival in patients with severe sepsis and septic shock who have a high risk of mortality. In this article we review the multi-modality approach to early diagnosis and intervention in the therapy of patients with severe sepsis and septic shock.

Hyperchloremic Acidosis Increases Circulating Inflammatory Molecules in Experimental Sepsis

RATIONALE

Hyperchloremic acidosis is common in the critically ill and is often iatrogenic. We have previously shown that hyperchloremic acidosis increases nuclear factor-kappaB DNA binding in lipopolysaccharide-stimulated RAW 264.7 cells. However, evidence that hyperchloremic acidosis leads to increased inflammation in vivo has been limited to nitric oxide.

OBJECTIVES

To determine if acidosis, induced by dilute hydrochloric acid (HCl) infusion, will increase circulating inflammatory mediator levels in an experimental model of severe sepsis in rats.

METHODS

Eighteen hours after inducing lethal sepsis by cecal ligation and puncture in 20 adult, male, Sprague-Dawley rats, we randomized animals into three groups. In groups 2 and 3, we began an IV infusion of 0.1 N HCl to reduce the standard base excess (SBE) by 5 to 10 mEq/L and 10 to 15 mEq/L, respectively. In group 1, we infused a similar volume of lactated Ringer solution. In all groups infusion continued 8 h or until the animal died.

MEASUREMENTS AND MAIN RESULTS

We measured arterial blood gases, whole-blood lactate, and chloride, tumor necrosis factor (TNF), interleukin (IL)-6, and IL-10 levels at 0 h, 4 h, and 8 h. All measured cytokines increased over time. Compared to group 1, animals in groups 2 and 3 exhibited greater increase in all three cytokines, with the greatest increases seen with severe acidosis.

CONCLUSION

Moderate (SBE, - 5 to - 10) and severe (SBE, - 10 to - 15) acidosis, induced by HCl infusion, increases circulating levels of IL-6, IL-10, and TNF in normotensive septic rats.

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.

Resuscitation from Hemorrhagic Shock: Fluid Selection and Infusion Strategy Drives Unmeasured Ion Genesis

BACKGROUND

This study compares unmeasured ion generation by different resuscitation fluids and strategies for hemorrhagic shock (HS).

METHODS

A rectus crush injury and 40% hemorrhage initiated controlled HS in 24 minipigs. Pigs (n = 8/gp) were untreated (NON) or bolused (10 cc/kg bw) with HBOC-201 (HBOC), or 6% Hetastarch (HEX) after 20 minutes of HS. Additional boluses occurred for hypotension (mean arterial pressure [MAP] <60 mm Hg) or tachycardia (heart rate [HR] >baseline) for 4 hours; other therapy was withheld, simulating delayed evacuation. Hemodynamics, acid-base parameters, and strong ion difference (SID) and strong ion gap (SIG), were assessed. Data are means +/- SD or percent; significance for p < 0.05.

RESULTS

Initial MAP was similar (p > 0.05) as was ultimate survival (p > 0.05). By 30 minutes, MAP was higher with HBOC (63 +/- 8; p = 0.01) versus HEX (37 +/- 5) or NON (35 +/- 4). HBOC required less fluid than HEX (515 +/- 58 versus 830 +/- 45 mL, p = 0.019). Lactate was similar between groups (p > 0.05). pH was highest in HBOC by 180 minutes (p < 0.05). SID was constant in NON, decreased in HEX, but increased in HBOC (p < 0.05 by 60 minutes). SIG remained unchanged in NON and HEX, but declined in HBOC (p < 0.05 by 30 minutes).

CONCLUSIONS

HBOC resuscitation required the least fluid. Unmeasured anions were prevalent in HEX and NON (+ SIG), whereas HBOC liberated unmeasured cations (- SIG); differences were inapparent when only evaluating pH. Only HBOC increased the SID, electrochemically promoting alkalosis. Resuscitation fluid differences may aid care in high lactate conditions where an induced counterbalancing metabolic alkalosis may be beneficial.

Calcium supplementation of saline-based colloids does not produce equivalent coagulation profiles to similarly balanced salt preparations

OBJECTIVES

The primary objective of this study was to test the hypothesis that calcium alone does not account for the observed coagulation differences between saline-based and balanced electrolyte IV fluid preparations.

DESIGN

Controlled, nonblinded, in vitro observational study.

SETTING

University-based anesthesia research laboratory.

PARTICIPANTS

Ten healthy volunteers.

INTERVENTIONS

The volunteers donated fresh blood for in vitro 40% and 60% dilution with 6 intravenous fluid preparations (lactated Ringer's solution, human albumin solution, and 4 hydroxyethyl starch preparations). All saline-based fluids were supplemented with calcium chloride to obtain ionized concentrations >or=1.0 mmol/L.

MEASUREMENTS AND MAIN RESULTS

After dilution of the fresh blood with the study fluids, samples were analyzed by using the Thrombelastograph. Three colloid preparations produced minimal coagulation derangement, even at 60% dilution (human albumin solution, tetrastarch in saline, and pentastarch in balanced electrolyte solution), whereas pentastarch in saline and hetastarch in balanced electrolyte produced a mildly hypocoagulable state at 60% dilution.

CONCLUSIONS

The different coagulation profiles between the 2 pentastarch preparations, as well as similar profiles of pentastarch in saline and hetastarch in balanced electrolyte solution, suggest that calcium is not solely responsible for previously observed effects.

A Head-to-Head Comparison of the In Vitro Coagulation Effects of Saline-Based and Balanced Electrolyte Crystalloid and Colloid Intravenous Fluids

Both fluid composition (e.g., type of hydroxyethyl starch) and formulation (e.g., saline or balanced salt carrier solution) may alter whole blood coagulation. We therefore enrolled 10 healthy volunteers to test ex vivo, thrombelastograph-based blood coagulation differences of eight crystalloid and colloid solutions at 20%, 40%, and 60% dilutions. Saline and lactated Ringer's solution produced a hypercoagulable state at 20%-40% dilutions. Saline, hetastarch in saline, pentastarch in saline, tetrastarch in saline, and human albumin solutions all produced a hypocoagulable state at 60% dilution. Hetastarch in saline also produced a hypocoagulable state at 40% dilution. The larger molecular weight starches produced more intense coagulation abnormalities than the medium molecular weight compounds formulated similarly (i.e., suspended in saline or balanced salt solution). The balanced salt solutions caused fewer coagulation abnormalities, especially pentastarch in balanced salt solution. This balanced salt pentastarch preparation produced the least derangement of coagulation of the colloid solutions at all dilutions, causing hypercoagulability at the lower dilutions and minimal coagulation derangement at 60% dilution. These data support the theory that smaller molecular weight hydroxyethyl starches and colloids suspended in balanced salt solutions preserve coagulation better than large molecular weight starches and saline-based colloids, as judged by thrombelastography.

Effect of increased cardiac output on hepatic and intestinal microcirculatory blood flow, oxygenation, and metabolism in hyperdynamic murine septic shock

OBJECTIVE

Septic shock-associated organ dysfunction is attributed to derangements of microcirculatory perfusion and/or impaired cellular oxygen utilization. The hepatosplanchnic organs are regarded to play a pivotal role in the pathophysiology of sepsis-related organ failure. In a murine model of septic shock, we tested the hypothesis whether achieving normotensive, hyperdynamic hemodynamics characterized by a sustained increase in cardiac output would allow maintenance of regional microvascular perfusion and oxygenation and, thus, hepatic metabolic capacity.

DESIGN

Prospective, controlled, randomized animal study.

SETTING

University animal research laboratory.

SUBJECTS

Male C57Bl/6 mice.

INTERVENTIONS

Fifteen hours after sham operation (n = 11) or cecal ligation and puncture (CLP) (n = 9), mice were anesthetized, mechanically ventilated, and instrumented (central venous and left ventricular pressure-conductance catheter, portal vein and superior mesenteric artery ultrasound flow probes). Animals received continuous intravenous hydroxyethylstarch and norepinephrine to achieve normotensive and hyperdynamic hemodynamics, and glucose was infused to maintain normoglycemia.

MEASUREMENTS AND MAIN RESULTS

Measurements were recorded 18, 21, and 24 hrs post-CLP. In CLP mice, titration of hemodynamic targets were affiliated superior mesenteric artery and portal vein flow. Using a combined laser-Doppler flowmetry and remission spectrophotometry probe, we found well-maintained gut and liver capillary perfusion as well as intestinal microcirculatory hemoglobin oxygen saturation, whereas hepatic microcirculatory hemoglobin oxygen saturation was even increased. At 24 hrs post-CLP, the rate of de novo gluconeogenesis as derived from hepatic C-glucose isotope enrichment after continuous intravenous 1,2,3,4,5,6-C6-glucose infusion (condensation biosynthesis modeling after gas chromatography-mass spectrometry isotope measurements) was similar in the two experimental groups.

CONCLUSIONS

During murine septic shock achieving normotensive hyperdynamic hemodynamics with fluid resuscitation and norepinephrine, exogenous glucose requirements together with the lack of norepinephrine-induced increase in the rate of gluconeogenesis mirror impaired metabolic capacity of the liver despite well-maintained hepatosplanchnic microvascular perfusion and oxygenation.

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.

Monitoring skeletal muscle and subcutaneous tissue acid-base status and oxygenation during hemorrhagic shock and resuscitation

Gastric tonometry correlates with the severity of blood loss during shock. However, tonometry is cumbersome, has a slow response time, and is not practical to apply in the acute resuscitation setting. We hypothesized that subcutaneous tissue (SC) and skeletal muscle (SM) pH, pCO2, and pO2 changes are comparable with changes seen in bowel tonometry during shock and resuscitation. Thirteen male mini-swine (25-35 kg; n = 4 control, n = 9 shock) underwent laparotomy and jejunal tonometry. A multisensor probe (Diametrics Medical, Roseville, MN) was placed in the carotid artery, the chest SC, and the adductor muscle of the leg (SM). PaCO2 was maintained between 40 and 45 mmHg. Shocked animals were hemorrhaged and kept at mean arterial pressure of 40 mmHg. Animals were bled until a reinfusion of >10% of the total shed blood was needed to maintain the mean arterial pressure at 40 mmHg. Animals were resuscitated with shed blood plus 2x shed volume in lactated Ringer's solution (20 min) and were observed for 3 h. The average blood loss was 47.2% +/- 8.7% of calculated blood volume. During the hemorrhagic phase, SM and SC displayed tissue acidosis (r2 = 0.951), tissue hypercapnea (r2 = 0.931), and tissue hypoxia (r2 = 0.748). Overall, pH displayed the best correlation between SM and SC during shock and resuscitation. PCO2 in the jejunum (tonometry), SM, and SC increased during decompensation. However, during resuscitation as tonometric pCO2 normalized, only SC pCO2 decreased to its baseline value, whereas the SM pCO2 decrease tended to lag behind. Bland-Altman analyses demonstrated that the variability of the tissue pH changes in SM and SC are predictable according to the phases of hemorrhage and resuscitation. Changes in tissue pH correlated during bleeding and during resuscitation among SC and SM, and these changes followed the trends in gut tonometry as well. Continuous pCO2 and pO2 monitoring in the SM and SC tissues had significant correlations during the induction of shock only. SM and SC continuous pH and pCO2 monitoring reflect bowel pCO2 values during hemorrhagic shock. The response of these indicators as potential surrogates of impaired tissue metabolism varies among tissues and according to the phases of hemorrhage or resuscitation.

The strong ion gap predicts mortality in children following cardiopulmonary bypass surgery

OBJECTIVE

Stewart's strong ion theory quantifies unmeasured tissue acids produced following hypoxia or hypoperfusion, by calculation of the strong ion gap. Our study objectives were as follows: a) to determine the 24-hr profile of the strong ion gap following cardiopulmonary bypass surgery; and b) to compare the prognostic value in terms of intensive care unit mortality of this variable with blood lactate.

DESIGN

Prospective, observational study.

SETTING

Tertiary pediatric intensive care unit.

PATIENTS

Eighty-five children following surgery for congenital heart disease.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Arterial blood samples for lactate and strong ion gap calculation were obtained at intensive care unit admission and at 24 hrs. A raised strong ion gap (>3 mEq/L) was present in 41.1% and 51.7% of admission and 24-hr samples, respectively, being elevated at both time points in 30.5%. Both the strong ion gap and lactate increased with surgical complexity, but neither was correlated with length of bypass (r = .13 and -.02) or aortic cross-clamp (r = .13 and .10). The crude mortality was 5.8% (5/85). Four of the five deaths were associated with a persistently elevated strong ion gap, in contrast to two with ongoing hyperlactatemia (>2 mmol/L). The admission strong ion gap (cutoff, >3.2 mEq/L) was superior to lactate (cutoff, >3.0 mmol/L) as a mortality predictor (area under receiver operating characteristic curve of 0.85 [95% confidence interval, 0.74-0.95] vs. 0.71 [95% confidence interval, 0.44-0.98], respectively).

CONCLUSIONS

An elevated strong ion gap occurs commonly following bypass surgery and appears to be superior to lactate as a mortality predictor.

Clinical review: Acid-base abnormalities in the intensive care unit -- part II

Acid-base abnormalities are common in the critically ill. The traditional classification of acid-base abnormalities and a modern physico-chemical method of categorizing them will be explored. Specific disorders relating to mortality prediction in the intensive care unit are examined in detail. Lactic acidosis, base excess, and a strong ion gap are highlighted as markers for increased risk of death.

Effect of hypertonic saline dextran on acid-base balance in patients undergoing surgery of abdominal aortic aneurysm*

OBJECTIVE

To evaluate the magnitude and cause of metabolic acidosis after infusion of 7.5% sodium chloride 6% dextran 70.

DESIGN

Randomized, prospective clinical study.

SETTING

University hospital.

PATIENTS

Two groups of 14 patients each, undergoing repair of abdominal aortic aneurysm.

INTERVENTIONS

Patients were randomly assigned to receive either 250 mL of hypertonic saline dextran (HSD) or a conventional fluid regimen with 250 mL of hydroxyethyl starch in normal saline solution (H-NS) during the period of aortic clamping. Additionally, normal saline was used in both groups to reach a target pulmonary artery occlusion pressure of 15-18 mmHg. pH, Paco2, and serum concentrations of sodium, potassium, magnesium, calcium, chloride, lactate, albumin, and phosphate were measured. Strong ion difference was calculated as (sodium + potassium + magnesium + calcium) - (chloride + lactate). The amount of weak plasma acid was calculated.

MEASUREMENTS AND MAIN RESULTS

The infusion of HSD resulted in an immediate large increase in serum sodium (19 mmol/L) and chloride (22 mmol/L), whereas the infusion of H-NS led only to mild increases in serum sodium (3 mmol/L) and chloride (6 mmol/L). Both HSD and H-NS caused concomitant and equal decreases in the amount of weak plasma acid, strong ion difference, and pH (7.28-7.30). The reduction of bicarbonate was also identical and proportional to the extent of dilution due to infusion of HSD and H-NS. This induced metabolic acidosis was corrected spontaneously in both groups 24 hrs after surgery.

CONCLUSION

Both the intravenous administration of 7.5% sodium chloride and the conventional fluid regimen with saline-based 6% hydroxyethyl starch solution resulted in a metabolic acidosis of equal extent. This suggests dilution of plasma buffers or a decrease in strong ion difference to be the primary cause of metabolic acidosis.

Comparison of lactated Ringer's solution and a physiologically balanced 6% hetastarch plasma expander for the treatment of hypotension induced via blood withdrawal in isoflurane-anesthetized dogs

OBJECTIVE

To compare the effects of lactated Ringer's solution (LRS) with those of a physiologically balanced 6% hetastarch plasma expander administered to isoflurane-anesthetized dogs with hypotension induced by blood withdrawal.

ANIMALS

12 healthy Beagles.

PROCEDURE

Blood was withdrawn from isoflurane-anesthetized dogs (volume withdrawn measured) to a systolic arterial blood pressure (SAP) of 80 mm Hg. Six dogs each received either LRS or hetastarch solution (90 mL/kg/h, i.v.). Hemodynamic variables, pH, blood gas concentrations, PCV, serum electrolyte and total protein concentrations, and colloid osmotic pressure (COP) were determined at baseline, while SAP was 80 mm Hg, and after fluid treatment. The volume of fluid administered and rate of return of SAP to within 10% of baseline values were recorded.

RESULTS

Mean +/- SD volume of blood withdrawn to decrease SAP to 80 mm Hg was 173 +/- 38 mL. Hemodynamic variables decreased after blood withdrawal but returned to baseline values more rapidly after infusion of a smaller volume of hetastarch solution, compared with the response to LRS infusion. Whereas PCV and serum total protein concentration decreased after administration of either solution, COP decreased only after administration of LRS. The total volume of hetastarch solution and LRS required to restore and maintain SAP to within 10% of baseline values was 1.1 +/- 0.9 and 4.4 +/- 1.7 times greater than the volume of blood removed, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Compared with LRS infusion, smaller volumes of hetastarch solution normalized and maintained SAP without lowering COP in isoflurane-anesthetized dogs after blood withdrawal.

Increased blood flow prevents intramucosal acidosis in sheep endotoxemia: a controlled study

Introduction

Increased intramucosal–arterial carbon dioxide tension (PCO₂) difference (ΔPCO₂) is common in experimental endotoxemia. However, its meaning remains controversial because it has been ascribed to hypoperfusion of intestinal villi or to cytopathic hypoxia. Our hypothesis was that increased blood flow could prevent the increase in ΔPCO₂.

Methods

In 19 anesthetized and mechanically ventilated sheep, we measured cardiac output, superior mesenteric blood flow, lactate, gases, hemoglobin and oxygen saturations in arterial, mixed venous and mesenteric venous blood, and ileal intramucosal PCO₂ by saline tonometry. Intestinal oxygen transport and consumption were calculated. After basal measurements, sheep were assigned to the following groups, for 120 min: (1) sham (n = 6), (2) normal blood flow (n = 7) and (3) increased blood flow (n = 6). Escherichia coli lipopolysaccharide (5 μg/kg) was injected in the last two groups. Saline solution was used to maintain blood flood at basal levels in the sham and normal blood flow groups, or to increase it to about 50% of basal in the increased blood flow group.

Results

In the normal blood flow group, systemic and intestinal oxygen transport and consumption were preserved, but ΔPCO₂ increased (basal versus 120 min endotoxemia, 7 ± 4 versus 19 ± 4 mmHg; P < 0.001) and metabolic acidosis with a high anion gap ensued (arterial pH 7.39 versus 7.35; anion gap 15 ± 3 versus 18 ± 2 mmol/l; P < 0.001 for both). Increased blood flow prevented the elevation in ΔPCO₂ (5 ± 7 versus 9 ± 6 mmHg; P = not significant). However, anion-gap metabolic acidosis was deeper (7.42 versus 7.25; 16 ± 3 versus 22 ± 3 mmol/l; P < 0.001 for both).

Conclusions

In this model of endotoxemia, intramucosal acidosis was corrected by increased blood flow and so might follow tissue hypoperfusion. In contrast, anion-gap metabolic acidosis was left uncorrected and even worsened with aggressive volume expansion. These results point to different mechanisms generating both alterations.

Science review: quantitative acid-base physiology using the Stewart model

There has been renewed interest in quantifying acid-base disorders in the intensive care unit. One of the methods that has become increasingly used to calculate acid-base balance is the Stewart model. This model is briefly discussed in terms of its origin, its relationship to other methods such as the base excess approach, and the information it provides for the assessment and treatment of acid-base disorders in critically ill patients.

Changes of serum chloride and metabolic acid-base state in critical illness

Alterations of electrolytes and albumin cause metabolic acid-base disorders. It is unclear, however, to what degree these plasma components affect the overall metabolic acid-base state in the course of critical illness. We performed serial analyses of the metabolic acid-base state in 30 critically ill patients over the course of 1 week. We applied a physical-chemical acid-base model and used a linear regression model to determine the influence of sodium, chloride, unmeasured anions and albumin on the net metabolic acid-base state. Progressive hypochloraemia was identified as the main cause of developing metabolic alkalosis. Changes in serum chloride and unmeasured anions were responsible for changes of 41% and 22% in the metabolic acid-base state, respectively. Sodium and albumin played a minor role. In conclusion, chloride is the major determinant of metabolic acid-base state in critical illness.

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.

HYPOTENSIVE RESUSCITATION OF MULTIPLE HEMORRHAGES USING CRYSTALLOID AND COLLOIDS

Hypotensive resuscitation has been advocated as a better means to perform field resuscitation of penetrating trauma. Our hypothesis is that hypotensive resuscitation using either crystalloid or colloid provides equivalent or improved metabolic function while reducing the overall fluid requirement for resuscitation of hemorrhage. We compared hypotensive and normotensive resuscitation of hemorrhage using lactated Ringer's (LR) with hypotensive resuscitation using Hextend (Hex), 6% hetastarch in isotonic buffered saline. Instrumented conscious sheep were hemorrhaged in three separate bleeds, 25 mL/kg at T0 and 5 mL/kg at both T50 and T70. Resuscitation was started at T30 and continued until T180. Hypotensive resuscitation to a mean arterial pressure (MAP) of 65 mmHg was performed with LR or Hex using a closed-loop resuscitation (CLR) system for a LR-65 and Hex-65 treatment protocol. A control treatment protocol was resuscitation with LR to a MAP target of 90 mmHg, LR-90. All treatment protocols were successfully resuscitated to near target levels. Two animals in the hypotensive treatment protocols died during the second and third bleedings, one in the LR-65 and one in the Hex-65 treatment protocol. Mean infused volumes were 61.4 +/- 11.3, 18.0 +/- 5.9, and 11.6 +/- 1.9 mL/kg in the LR-90, LR-65, and Hex-65 treatments, respectively (*P < 0.05 versus LR-90). Mean minimum base excess (BE) values were +1.9 +/- 1.4, -5.8 +/- 4.3, and -5.9 +/- 4.0 mEq/L in the LR-90, LR-65, and Hex-65 treatments, respectively. Hypotensive resuscitation with LR greatly reduced volume requirements as compared with normotensive resuscitation, and Hex achieved additional volume sparing. However, trends toward lower BE values and the occurrence of deaths only in the hypotensive treatment protocols suggest that resuscitation to a target MAP of 65 mmHg may be too low for optimal outcomes.

Science review: extracellular acidosis and the immune response: clinical and physiologic implications

Metabolic acidosis is among the most common abnormalities seen in patients suffering from critical illness. Its etiologies are multiple and treatment of the underlying condition is the mainstay of therapy. However, growing evidence suggests that acidosis itself has profound effects on the host, particularly in the area of immune function. Given the central importance of immune function to the outcome of critical illness, there is renewed interest in elucidating the effects of this all too common condition on the immune response. In this review we concentrate on the effects of extracellular acids on production and release of inflammatory mediators, and we demonstrate that different acids produce different effects despite similar extracellular pH. Finally, we discuss potential clinical implications.

Bench-to-bedside review: treating acid-base abnormalities in the intensive care unit - the role of buffers

The recognition and management of acid-base disorders is a commonplace activity for intensivists. Despite the frequency with which non-bicarbonate-losing forms of metabolic acidosis such as lactic acidosis occurs in critically ill patients, treatment is controversial. This article describes the properties of several buffering agents and reviews the evidence for their clinical efficacy. The evidence supporting and refuting attempts to correct arterial pH through the administration of currently available buffers is presented.

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.

Acid–base balance in acute renal failure and renal replacement therapy

The approach to acid-base balance based on the concept of strong ions, initially proposed by Stewart, is briefly overviewed. The anion gap and the strong anion gap are both discussed. Comments are made on the strong ion difference of fluids administered to patients and their impact on acid-base status will be commented. Renal failure patients have an altered acid-base balance; most commonly, a mixed type of metabolic acidosis (hyperchloraemic, and of a high anion gap) is observed. The consequences of renal metabolic acidosis are described. Finally, the impact of renal replacement therapy on acid-base balance is exposed; different modalities of renal replacement are considered in regard to their alkalinizing performance.

Effects of Hyperchloremic Acidosis on Arterial Pressure and Circulating Inflammatory Molecules in Experimental Sepsis

STUDY OBJECTIVE

To determine the effects of hyperchloremic acidosis, induced by dilute HCl infusion, on BP and circulating inflammatory mediators in an experimental model of severe sepsis in the rat.

DESIGN

Randomized, open-label, controlled experiment.

SETTING

University research laboratory.

PARTICIPANTS

Twenty-four adult, male, Sprague-Dawley rats.

INTERVENTION

Eighteen hours after inducing lethal sepsis by cecal ligation and puncture, animals were randomized and classified into three groups. In groups 2 and 3, we began an IV infusion of 0.1 N HCl to reduce the standard base excess (SBE) by 5 to 10 mEq/L and 10 to 15 mEq/L, respectively. In group 1, we infused a similar volume of lactated Ringer solution. In all groups, infusions were continued for 8 h or until the animals died.

MEASUREMENTS

We measured mean arterial pressure (MAP), arterial blood gases, electrolytes, plasma nitrate/nitrite, tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-10 levels at 0 h, 3 h, 6 h, and 8 h.

RESULTS

MAP remained stable in group 1 but decreased in groups 2 and 3 (p < 0.001), such that at 8 h MAP was much higher in group 1 (94 +/- 9.2 mm Hg) [+/- SD] compared to either group 2 (71.6 +/- 20.1 mm Hg) or group 3 (49.4 +/- 33.2 mm Hg) [p = 0.01]. This change in MAP correlated with the increase in plasma Cl(-) (R(2) = 0.50, p < 0.0001) and less well with the decrease in pH (R(2) = 0.24, p < 0.001). After 6 h of acidosis, plasma nitrite levels were significantly higher in group 2 animals compared to either group 1 or group 3 animals (p < 0.05). Plasma TNF-alpha, IL-6, or IL-10 levels were not significantly different from control animals.

CONCLUSION

Moderate acidosis (SBE of 5 to 10 mEq/L), induced by HCl infusion, worsened BP and increased plasma nitrate/nitrite levels but had no effect on circulating cytokines in septic rats. However, severe acidosis (SBE of 10 to 15 mEq/L), while still causing hypotension, did not affect plasma nitrate/nitrite levels.

Acid–base and electrolyte analysis in critically ill patients: are we ready for the new millennium?

PURPOSE OF REVIEW

Disorders of acid-base and electrolytes are commonly seen in critically ill patients. The presence of these disorders typically signals the development of an underlying pathology. These disturbances can be severe and are often associated with worse outcome. Indeed, metabolic acidosis is one of the ways we quantify organ failure. Although acid-base and electrolyte disorders may be a result of the underlying pathophysiology (eg, renal failure, respiratory failure, shock), they may also result from the way in which we manage critically ill patients.

RECENT FINDINGS

The application of the physical-chemical approach to acid-base analysis has led to recent developments in the identification and quantification and understanding of mechanisms for acid-base disorders commonly found in critically ill patients. Examples include a better understanding of the role of electrolytes (especially sodium and chloride) and weak acids in the pathophysiology of acid-base disorders, the implication of acid-base derangements on the inflammatory process and organ perfusion, and the importance of resuscitation fluid composition.

SUMMARY

By adopting a physical-chemical approach to acid-base analysis we are gaining insight to the complexities of acid-base disorders and how their treatments may affect outcome.

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

BACKGROUND

The effect of adding lactate to the cardiopulmonary bypass (CPB) prime was investigated using Stewart's quantitative acid-base approach. According to this quantitative model, serum pH and bicarbonate are determined by three independent factors: the partial pressure of carbon dioxide (PCO(2)), the total concentration of weak acids (e.g. albumin), and the strong ion difference. The apparent strong ion difference is calculated as the sum of sodium, potassium, magnesium and calcium minus chloride concentrations. The pH decreases with a smaller strong ion difference and vice versa.

METHODS

Twenty patients scheduled for coronary surgery were studied prospectively. All patients were treated identically, except for the prime, which either contained lactate or was lactate free. Just before bypass and before coming off bypass, haemoglobin, glucose, plasma osmolality and colloid osmotic pressure were determined; albumin, lactate, sodium, potassium, ionized calcium, magnesium, phosphate, arterial pH, PCO(2), bicarbonate, and base excess were measured for use in Stewart's analysis.

RESULTS

Metabolic acidosis had resolved by the end of bypass with the lactated prime. Although the strong ion gap (apparent minus effective strong ion difference) increased significantly in both groups, its composition differed significantly between the groups. The Stewart technique detected polyanionic gelatin as a weak acid component contributing to the unidentified anion fraction. Colloid osmotic pressure was maintained in both groups.

CONCLUSION

Exogenous lactate attenuates acidosis related to CPB. The oncotic and weak acid deficits produced by hypoalbuminaemia may be compensated for temporarily during CPB by polyanionic synthetic colloids such as succinylated gelatin.

Pro/con clinical debate: Hydroxyethylstarches should be avoided in septic patients

There are few issues in critical care medicine that have a less clearly defined standard of care than the intravenous fluid choice for resuscitation. Natural colloids (such as albumin) became popular during the Second World War when there was a need to develop a portable, easily stored, blood substitute. Early successes led to widespread use and a multibillion dollar industry. It is not surprising given the large demand, high costs and potential adverse effects of natural colloids that synthetic colloids have emerged. In the present article, two groups of clinical investigators remind us of the controversies surrounding the use of synthetic colloids.

Conventional or physicochemical approach in intensive care unit patients with metabolic acidosis

INTRODUCTION

Metabolic acidosis is the most frequent acid-base disorder in the intensive care unit. The optimal analysis of the underlying mechanisms is unknown.

AIM

To compare the conventional approach with the physicochemical approach in quantifying complicated metabolic acidosis in patients in the intensive care unit.

PATIENTS AND METHODS

We included 50 consecutive patients with a metabolic acidosis (standard base excess < or = -5). We measured sodium, potassium, calcium, magnesium, chloride, lactate, creatinine, urea, phosphate, albumin, pH, and arterial carbon dioxide and oxygen tensions in every patient. We then calculated HCO3-, the base excess, the anion gap, the albumin-corrected anion gap, the apparent strong ion difference, the effective strong ion difference and the strong ion gap.

RESULTS

Most patients had multiple underlying mechanisms explaining the metabolic acidosis. Unmeasured strong anions were present in 98%, hyperchloremia was present in 80% and elevated lactate levels were present in 62% of patients. Calculation of the anion gap was not useful for the detection of hyperlactatemia. There was an excellent relation between the strong ion gap and the albumin-corrected and lactate-corrected anion gap (r2 = 0.934), with a bias of 1.86 and a precision of 0.96.

CONCLUSION

Multiple underlying mechanisms are present in most intensive care unit patients with a metabolic acidosis. These mechanisms are reliably determined by measuring the lactate-corrected and albumin-corrected anion gap. Calculation of the more time-consuming strong ion gap according to Stewart is therefore unnecessary.

Techniques for assessing and achieving fluid balance in acute renal failure

Fluid therapy, together with attention to oxygen supply, is the cornerstone of resuscitation in all critically ill patients. Hypovolemia results in inadequate blood flow to meet the metabolic requirements of the tissues and must be treated urgently to avoid the complication of progressive organ failure, including acute renal failure. The kidney plays a critical role in body fluid homeostasis. Renal dysfunction disturbs this homeostasis and requires special attention to issues of fluid balance and fluid overload. In addition, fluid therapy is the only treatment that has been shown to be effective in the prevention of acute renal failure. Special attention to volume status is therefore required in patients at risk for acute renal failure. Hypovolemia is also a major causal factor of morbidity during hemodialysis and may contribute to further renal insults. Although the importance of fluid management is generally recognized, the choice of fluid, the amount, and assessment of fluid status are controversial. As the choice of fluids becomes wider and monitoring devices become more sophisticated, the controversy increases. This article provides an overview of the concept of fluid management in the critically ill patient with acute renal failure.