Continuous renal replacement therapy: opinions and evidence

Resistance of vascular access catheters for continuous renal replacement therapy: An ex vivo evaluation

Aim

To assess the resistance posed by double-lumen vascular access dialysis catheters at low and high blood flow.

Design

Controlled ex vivo study

Setting

ICU Laboratory of tertiary hospital.

Subjects

Eleven proprietary vascular access catheters for continuous renal replacement therapy. Methods: Heparinized spent red cells diluted in polygeline solution were pumped using the Aquarius hemofiltration machine (Edwards Life Sciences, Sydney, NSW, Australia) and its standard circuit through several vascular access catheters. Blood flow was increased and then decreased in steps of 50 ml/min (50, 150, 200, 250 and 300 ml/min) while catheter outflow and inflow pressures were recorded. The pressure-flow relationship (hydraulic resistance) of each catheter was then calculated. Study catheters were divided into two groups according to their internal diameter (large gauge vs. smaller gauge) or length (long or short). Hydraulic resistances were compared between the groups. Results: Different double lumen catheters posed clearly different resistances to flow. For all groups of catheters, there was a linear relationship between pressure and flow. No statistically significant difference between short and long catheters could be demonstrated (p=0.715). On the other hand, larger gauge catheters (13 Fr or greater) had significantly lower resistances than smaller gauge (<13 Fr) catheters (p=0.0062). Furthermore, all larger gauge catheters had resistances lower than 0.430 mmHg/ml/min, while all smaller gauge catheters had resistances greater than 0.490 mmHg/ml/min. Conclusions: Commercial double-lumen dialysis catheters have variable resistance to blood flow under standard ex vivo conditions. Although both length and internal diameter varied, internal diameter had a dominant effect on resistance. This information might be useful to clinicians in guiding their choice of catheters for clinical use. (Int J Artif Organs 2008; 31: 905–9)

Metabolic and Nutritional Aspects of Acute Renal Failure in Critically Ill Patients Requiring Continuous Renal Replacement Therapy

Acute renal failure (ARF) is rarely an isolated process but is often a complication of underlying conditions such as sepsis, trauma, and multiple-organ failure in critically ill patients. As such, concomitant clinical conditions significantly affect patient outcome. Poor nutritional status is a major factor in increasing patients' morbidity and mortality. Malnutrition in ARF patients is caused by hypercatabolism and hypermetabolism that parallel the severity of illness. When dialytic intervention is indicated, continuous renal replacement therapy (CRRT) is a commonly used alternative to intermittent hemodialysis because it is well tolerated by hemodynamically unstable patients. This paper reviews the metabolic and nutritional alterations associated with ARF and provides recommendations regarding the nutritional, fluid, electrolyte, micronutrient, and acid-base management of these patients. The basic principles of CRRT are addressed, along with their nutritional implications in critically ill patients. A patient case is presented to illustrate the clinical application of topics covered within the paper.

Comparative Quantitative Acid-Base Analysis in Coronary Artery Bypass, Severe Sepsis, and Diabetic Ketoacidosis

The main objective of this study was to assess the relationship of standard base excess (SBE) to delta strong ion difference effective (ΔSIDe) in critical illness. Critical illness is characterized by variable plasma nonvolatile weak acid components (ΔA-), and SBE becomes discordant with ΔSIDe. The author hypothesized that both acid-base models are equivalent when SBE and ΔSIDe are corrected for ΔA-. A retrospective chart review was performed to assess this hypothesis by looking at changes in SBE, ΔSIDe, and ΔA-in 30 coronary artery bypass graft surgery patients, 30 severe sepsis patients, and 15 diabetic ketoacidosis patients. SBE equals the sum of the ΔSIDe and ΔA-. The SBE quantifies the magnitude of the metabolic acid-base derangement, the ΔSIDe quantifies the plasma strong cation/anion imbalance, and the ΔA-quantifies the magnitude of the hypoalbuminemic alkalosis. The partitioning of SBE into physicochemical components can facilitate analyses of complex acid-base disorders in critical illness.

Analyzing Chinese patients with post-operative acute kidney injury

BACKGROUND

To investigate clinical characteristics and risk factors of Chinese patients with post-operative acute kidney injury (PO-AKI).

METHODS

Patients with PO-AKI in Ruijin Hospital from December 1997 to December 2005 were retrospectively studied.

RESULTS

Patients' mean age was 62.2 ± 18.1 years. There were 111 males and 57 females. The mean serum creatinine at diagnosis was 370.41 ± 320.92 μmol/L and the mean estimated glomerular filtration rate was 33.56 ± 24.24 mL/min. For the outcome of the patients, 38 died and the mortality rate was 22.6%. There were 17 patients (10.1%) with Acute Dialysis Quality Initiative-RIFLE (risk-injury-failure-loss-end classification) phase R, 21 (12.5%) with phase I, and 130 (77.4%) with phase F. There was no significant difference in mortality regarding patients who underwent different types of surgeries. For the risk factors related to PO-AKI, acute tubular necrosis (ATN) increased relative risk of mortality PO-AKI (odds ratio = 7.089, 95% confidence interval = 2.069-24.288, p < 0.001). Multivariate regression models showed that ATN had a positive correlation with mortality of PO-AKI.

CONCLUSIONS

PO-AKI is one of the most common causes of AKI in patients who underwent operations. Special attention should be paid to risk factors related to PO-AKI in order to improve prognosis.

Antibiotic dosing in critical illness

Early and effective antibiotic therapy is essential in the management of infection in critical illness. The loading dose is probably the most important dose and is a function of the volume of distribution of the drug and the desired plasma concentration but independent of renal function. Antibiotics are classified in a number of ways that have implications for dosing. Doses of hydrophilic agents such as β-lactams should be increased in the early stages of sepsis as the extravascular space increases. For lipophilic agents such as macrolides, the inflammatory process is less important, although factors such as obesity will affect dosing. Classification can also be based on pharmacodynamic properties. Concentration-dependent antibiotics such as aminoglycosides should be administered by extended interval regimens, which maximize bactericidal effect, minimize nephrotoxicity and allow time between doses for the post-antibiotic effect. The critical factor for time-dependent agents, such as β-lactams, is time above the MIC. Ideally administration of these agents should be continuous, although vascular access availability can restrict infusion time to between 4 and 6 h, which is probably adequate. As well as antibiotic factors, patient factors such as hepatic and renal failure will affect dosing. Hepatic failure will affect antibiotic metabolism, although it is most important in end-stage failure. Renal failure and support will affect drug elimination. Knowledge of these factors is essential. Patient safety and prevention of unnecessary harm is a weighty consideration in critical illness. To ensure effective treatment and minimize adverse effects, therapy should be reviewed daily and adjusted in the light of changes in patient organ function and underlying pathology.

Clinical review: Optimal dose of continuous renal replacement therapy in acute kidney injury

Continuous renal replacement therapy (CRRT) is the preferred treatment for acute kidney injury in intensive care units (ICUs) throughout much of the world. Despite the widespread use of CRRT, controversy and center-specific practice variation in the clinical application of CRRT continue. In particular, whereas two single-center studies have suggested survival benefit from delivery of higher-intensity CRRT to patients with acute kidney injury in the ICU, other studies have been inconsistent in their results. Now, however, two large multi-center randomized controlled trials - the Veterans Affairs/National Institutes of Health Acute Renal Failure Trial Network (ATN) study and the Randomized Evaluation of Normal versus Augmented Level (RENAL) Replacement Therapy Study - have provided level 1 evidence that effluent flow rates above 25 mL/kg per hour do not improve outcomes in patients in the ICU. In this review, we discuss the concept of dose of CRRT, its relationship with clinical outcomes, and what target optimal dose of CRRT should be pursued in light of the high-quality evidence now available.

Continuous renal replacement therapies: a brief primer for the neurointensivist

Continuous renal replacement therapy (CRRT) is a renal replacement modality that is often used in the ICU setting, including the neuro-ICU. This form of renal replacement therapy has been used classically for acute renal failure in patients with hemodynamic compromise, but is gaining acceptance as a method to control vascular and extra-vascular volume and mediate cytokines in non-renal diseases. Although these uses are briefly discussed, this review concentrates on the different forms of continuous renal replacement, mainly focusing on the technology of convective versus diffusive modalities and briefly on filter technology. There is also discussion on the various anticoagulation regimes used in CRRT including data on performing CRRT without anticoagulation. This review is not meant to be a discussion on the pros and cons of CRRT versus intermittent dialysis, but rather a primer on the technology of CRRT and how this therapy may affect general care of the ICU patient.

Renal recovery after severe acute renal failure

BACKGROUND

There is limited information about renal recovery to independence from renal replacement therapy (RRT) and about factors associated with its occurrence after severe acute renal failure (ARF).

METHODS

We conducted a population-based surveillance among all adult residents of the Calgary Health Region surviving ICU admission from May 1, 1999 to April 30, 2002. The primary objective was to determine the rate of and the factors associated with 90-day survival and recovery to independence from RRT in critically ill patients with severe ARF.

RESULTS

At 90 days, 96 patients (40%) were alive. Of these, 72% were RRT independent with most (87%) requiring <4 weeks to recover. Prior to RRT, the median (IQR) serum creatinine and mean (SD) serum urea were 395 (252-517) micromol/L and 29.2 (18) mmol/L, respectively. Oliguria was present in 76%. Intermittent hemodialysis was the initial modality in 46% and continuous renal replacement therapy (CRRT) in 54%. By multivariate analysis, male sex (odds ratio (OR) 7.6, 95% CI, 2.2-27, p=0.01) and a diagnosis of septic shock (OR 3.9, 95% CI 1.02-14.5, p=0.05) were associated with an increased odds of recovery. Conversely, a higher Charlson co-morbidity index score (OR 0.71, 95% CI, 0.6-0.85, p=0.04) and a higher pre-RRT serum creatinine (OR 0.20, 95% CI, 0.05-0.80, p=0.02, p=0.02) were associated with reduced odds of recovery. Chronic kidney disease or the initial modality of RRT were not associated with recovery.

CONCLUSIONS

The majority of severe ARF patients who survive their acute illness are independent of RRT by 90 days. Male sex and a diagnosis of septic shock are independently associated with recovery while a greater co-morbidity score and a higher serum creatinine prior to RRT are predictive of non-recovery.

Continuous arteriovenous hemodialysis and continuous venovenous hemofiltration in burn patients with acute renal failure

Acute renal failure (ARF) is a very common condition that may occur in patients with major burn injuries. The majority of burn patients with ARF have a high mortality rate, ranging from 73% to 100%. There are several ways to treat ARF in burn patients, including peritoneal dialysis (PD), intermittent hemodialysis, and continuous renal replacement therapy (CRRT). CRRT is generally used in patients in whom intermittent hemodialysis has failed to control hypovolemia, as well as in patients who cannot tolerate intermittent hemodialysis. Additionally, PD is not suitable for patients with burns within the abdominal area. For these reasons, most patients with unstable hemodynamic conditions receive CRRT. In this study (conducted in our burn unit between 1997 and 2004), six burn patients received CRRT: three received continuous arteriovenous hemodialysis (CAVHD) and the other three received continuous venovenous hemofiltration (CVVH). The patients were all males, with a mean age of 49.8 years (range, 27-80 years), and a mean burnt surface area of 65.1% (range, 30-95%). Four patients died due to multiple organ failure, and two patients recovered from severe ARF. CRRT has been proven safe and useful for burn patients with ARF. According to this study, we conclude that CVVH is an appropriate tool for treating ARF, with a lower incidence of vascular complications than CAVHD.

Three-year survival after four major post–cardiac operative complications*

OBJECTIVE

After cardiac surgery, major morbidities known to be primary contributors to perioperative mortality are cardiac failure, respiratory failure, renal failure, and the need for mediastinal exploration. The first aim of this study was to ascertain long-term survival in cardiac surgery patients with and without the occurrence of major morbidities to investigate if long-term survival was comparable. The second aim of this study was to evaluate the prevalences and risk factors related to the four major morbidities in this patient population.

DESIGN

Retrospective observational outcome study.

SETTING

Cardiothoracic intensive care unit at a university hospital.

PATIENTS

We included 2,683 of 3,253 consecutive cardiac surgery patients cared for in a uniform fashion.

METHODS AND MAIN RESULTS

Perioperative mortality was significantly increased by the occurrence of major morbidity. In-hospital mortality was 0.7% in the absence of major morbidity compared with 72% when all major morbidities occurred. Three-year mortality for the entire study population was 15%, whereas the 3-yr long-term survival was significantly less for patients with morbidities compared with those without. Various independent perioperative risk factors were found for perioperative major morbidity and mortality.

CONCLUSIONS

Successful acute treatment and measures to identify and reduce the risk of major morbidities are necessary to improve outcome. In addition, long-term follow-up and management of morbidities are necessary to possibly improve long-term survival.

The effect of continuous veno-venous hemofiltration or direct hemoperfusion with polymyxin B-immobilized fiber on neutrophil respiratory oxidative burst in patients with sepsis and septic shock

Neutrophil activates and injures tissues and organs during sepsis or septic shock. Blood purification therapies such as continuous veno-venous hemofiltration (CVVH) and direct hemoperfusion with polymyxin-immobilized fiber (PMX-DHP) have been used for the treatment of sepsis and septic shock, however, the effects of such therapies on neutrophil activation have previously been poorly understood. We sought to evaluate neutrophil reactive oxygen species (ROS), especially H2O2 production, in the pathophysiology of sepsis or septic shock and the effect of CVVH or PMX-DHP on neutrophil ROS. Seven critically ill septic patients requiring CVVH (and 12 matched septic patients who did not require CVVH as control) and seven septic shock patients treated with PMX-DHP were studied. We found that patients with sepsis or septic shock had significantly higher levels of neutrophil ROS compared with normal volunteers (183 +/- 42, 292 +/- 90, and 103 +/- 30) (P < 0.05, and < 0.005). Neutrophil ROS did not change over time in patients treated either with CVVH or without CVVH. In contrast, neutrophil ROS significantly inhibited PMX-DHP treatment in patients with septic shock (pretreatment; 292 +/- 88 vs. post-treatment; 205 +/- 93, P < 0.05). In conclusion, neutrophil ROS was significantly enhanced in the sepsis or septic shock affected patients. CVVH did not affect neutrophil ROS while PMX-DHP significant inhibited neutrophil ROS.

Removal of inflammatory cytokines and endotoxin by veno-venous continuous renal replacement therapy for burned patients with sepsis

OBJECTIVE

To evaluate the effect of veno-venous continuous renal replacement therapy (CRRT) on the plasma levels of endotoxin and cytokines in severely burned patients with sepsis.

METHODS

Twenty adult severely burned patients with sepsis were studied. For the diagnosis of sepsis, patients were randomly divided into CRRT (n=10) and Control (n=10). Both groups received conventional therapy after admission. Veno-venous CRRT was administered to 10 patients in the CRRT group whenever patients were determined to be septic. The plasma level of endotoxin, TNF-alpha, IL-1 beta, IL-6 and IL-8 were measured at 0, 1, 2, 6, 12, 36 and 60 h after CRRT initiation, and at 0, 12, 36 and 60 h after the patients were diagnosed as having sepsis in the Control group.

MAIN RESULTS

Plasma level of endotoxin and all the cytokines after CRRT initiation were significantly lower than those before the treatment (P<0.01). The serial change of endotoxin, IL-1 beta, IL-6 and IL-8 was significantly lower at 12, 36 and 60 h after treatment compared with Control groups (P<0.01). A significant decrease in plasma TNF-alpha levels was seen at 36 and 60 h after treatment compared with Control groups (P<0.01).

CONCLUSION

Plasma endotoxin and cytokines (TNF-alpha, IL-1 beta, IL-6 and IL-8) can be removed effectively with CRRT in severely burned patients with sepsis.

Influence of continuous veno-venous hemofiltration on the course of acute pancreatitis

AIM: To investigate whether continuous veno-venous hemofiltration (CVVH) in different filtration rate to eliminate cytokines would result in different efficiency in acute pancreatitis, whether the saturation time of filter membrane was related to different filtration rate, and whether the onset time of CVVH could influence the survival of acute pancreatitis.

METHODS: Thirty-seven patients were classified into four groups randomly. Group 1 underwent low-volume CVVH within 48 h of the onset of abdominal pain (early CVVH, n = 9). Group 2 received low-volume CVVH after 96 h of the onset of abdominal pain (late CVVH, n = 10). Group 3 underwent high-volume CVVH within 48 h of the onset of abdominal pain (early CVVH, n = 9). Group 4 received high-volume CVVH after 96 h of the onset of abdominal pain (late CVVH, n = 9). CVVH was sustained for at least 72 h. Blood was taken before hemofiltration, and ultrafiltrate was collected at the start of CVVH and every 12 h during CVVH period for the purpose of measuring the concentrations of TNF-α, IL-1β and IL-6. The concentrations of TNF-α, IL-1β and IL-6 were measured by swine-specific ELISA. The Solartron 1 255 B frequency response analyzer (British) was used to observe the resistance of filter membrane.

RESULTS: The survival rate had a significant difference (94.44% vs 68.42%, P<0.01) high-volume and low-volume CVVH patients. The survival rate had also a significant difference (88.89% vs 73.68%, P<0.05) between early and late CVVH patients. The hemodynamic deterioration (MAP, HR, CVP) was less severe in groups 4 and 1 than that in group 2, and in group 3 than in group 4. The adsorptive saturation time of filters membranes was 120-180 min if the filtration rate was 1 000-4 000 mL/h. After the first, second and third new hemofilters were changed, serum TNF-α concentrations had a negative correlation with resistance (r: -0.91, -0.89, and -0.86, respectively in group 1; -0.89, -0.85, and -0.76, respectively in group 2; -0.88, -0.92, and -0.82, respectively in group 3; -0.84, -0.87, and -0.79, respectively in group 4). The decreasing extent of TNF-α, IL-1β and IL-6 was significantly different between group 3 and group 1 (TNF-α P<0.05, IL-1β P<0.05, IL-6 P<0.01), between group 4 and group 2 (TNF-α P<0.05, IL-1β P<0.05, IL-6 P<0.01), between group 1 and group 2 (TNF-α P<0.05, IL-1β P<0.05, IL-6 P<0.05), and between group 3 and group 4 (TNF-α P<0.01, IL-1β P<0.01, IL-6 P<0.05), respectively during CVVH period. The decreasing extent of TNF-α and IL-1β was also significantly different between survival patients and dead patients (TNF-α P<0.05, IL-1β P<0.05). In survival patients, serum concentration of TNF-α and IL-1β decreased more significantly than that in dead patients.

CONCLUSION: High-volume and early CVVH improve hemodynamic deterioration and survival in acute pancreatitis patients. High-volume CVVH can eliminate cytokines more efficiently than low-volume CVVH. The survival rate is related to the decrease extent of TNF-α and IL-1β. The adsorptive saturation time of filter membranes are different under different filtration rate condition. The filter should be changed timely once filter membrane adsorption is saturated.

The clearance of protein-bound solutes by hemofiltration and hemodiafiltration

BACKGROUND

Hemofiltration in the form of continuous venovenous hemofiltration (CVVH) is increasingly used to treat acute renal failure. Compared to hemodialysis, hemofiltration provides high clearances for large solutes but its effect on protein-bound solutes has been largely ignored.

METHODS

Standard clinical systems were used to remove test solutes from a reservoir containing artificial plasma. Clearances of the protein-bound solutes phenol red (C(PR)) and indican (C(IN)) were compared to clearances of urea (C(UREA)) during hemofiltration and hemodiafiltration. A mathematical model was developed to predict clearances from values for plasma flow Q(p), dialysate flow Q(d), ultrafiltration rate Q(f), filter size and the extent of solute binding to albumin.

RESULTS

When hemofiltration was performed with Q(p) 150 mL/min and Q(f) 17 mL/min, clearance values were C(PR) 1.0 +/- 0.1 mL/min; C(IN) 3.7 +/- 0.5 mL/min; and C(UREA) 14 +/- 1 mL/min. The clearance of the protein-bound solutes was approximately equal to the solute-free fraction multiplied by the ultrafiltration rate corrected for the effect of predilution. Addition of Q(d) 42 mL/min to provide HDF while Q(p) remained 150 mL/min resulted in proportional increases in the clearance of protein-bound solutes and urea. In contrast, the clearance of protein-bound solutes relative to urea increased when hemodiafiltration was performed using a larger filter and increasing Q(d) to 300 mL/min while Q(p) was lowered to 50 mL/min. The pattern of observed results was accurately predicted by mathematical modeling.

CONCLUSION

In vitro measurements and mathematical modeling indicate that CVVH provides very limited clearance of protein-bound solutes. Continuous venous hemodiafiltration (CVVHDF) increases the clearance of protein-bound solutes relative to urea only when dialysate flow rate and filter size are increased above values now commonly employed.

Renal replacement therapy in pediatric critical care medicine

PURPOSE OF REVIEW

The use of continuous renal replacement therapy in pediatric critical care has increased in the last 10 years. Adaptive makeshift machinery has been replaced with dedicated volumetric and/or gravimetric devices that afford accurate blood flow and ultrafiltrate production rates. While renal dysfunction continues to be related to primary renal disease, the incidence of secondary causes of acute renal failure continue to grow, especially in patients following cardiothoracic surgery, bone marrow transplantation, respiratory failure and multi-organ dysfunction syndrome.

RECENT FINDINGS

Although much of the outcome data for continuous renal replacement therapy has been retrospective in nature, these therapies are safe for use in the sickest of intensive care unit patients. Moreover, early data from the prospective pediatric continuous renal replacement therapy registry suggests that early intervention with continuous renal replacement therapy, as well as goal-directed fluid resuscitation may lead to improved survival in critically ill patients. In patients with sepsis and septic shock, continuous renal replacement therapy offers a means for blood purification.

SUMMARY

Though randomized placebo controlled trials are lacking at this time, center-based results suggest that continuous renal replacement therapy may prove beneficial to critically ill patients with sepsis and/or septic shock.

Multi-centre evaluation of anticoagulation in patients receiving continuous renal replacement therapy (CRRT)

BACKGROUND

Heparin (hepACG) and regional citrate anticoagulation (citACG) remain the most commonly reported continuous renal replacement therapy (CRRT) ACG methods employed. No prospective multi-centre published data exist that compare different ACG methods with respect to CRRT filter life span or patient complications.

METHODS

A total of 138 patients from seven US centres receiving 18 208 h of CRRT comprising a total of 442 CRRT circuits were utilized to assess filter life span and ACG-related complications in patients receiving CRRT with hepACG, citACG or no ACG (noACG).

RESULTS

Mean circuit life was 41.2+/-30.8 h. Mean circuit survival was no different for circuits receiving hepACG (42.1+/-27.1 h) and citACG (44.7+/-35.9 h), but was significantly lower for circuits with noACG (27.2+/-21.5 h, P<0.005). Kaplan-Meier analyses revealed no survival difference between hepACG and citACG circuits, but significantly lower survival for noACG circuits (P<0.001). Log-rank analysis showed that 69% of hepACG and citACG circuits whereas only 28% of noACG were functional at 60 h. Clotting rates were similar for hepACG circuits (58 out of 230, 25%) and citACG circuits (43 out of 158, 27%), but were significantly higher for noACG circuits (27 out of 54, 50%, P < 0.001). Life-threatening bleeding complications attributable to ACG were noted in the hepACG group but were absent in the citACG group.

CONCLUSIONS

The current analysis represents the largest evaluation of CRRT ACG methods to date. While the standard hepACG and citACG methods studied in the prospective paediatric CRRT registry led to similar filter life spans and were superior to noACG, our data suggest that citACG may result in less life-threatening complications.

Renal replacement therapy for acute renal failure in children: European guidelines

Acute renal failure (ARF) is uncommon in childhood and there is little consensus on the appropriate treatment modality when renal replacement therapy is required. Members of the European Pediatric Peritoneal Dialysis Working Group have produced the following guidelines in collaboration with nursing staff. Good practice requires early discussion of patients with ARF with pediatric nephrology staff and transfer for investigation and management in those with rapidly deteriorating renal function. Patients with ARF as part of multi-organ failure will be cared for in pediatric intensive care units where there should be access to pediatric nephrology support and advice. The choice of dialysis therapy will therefore depend upon the clinical circumstances, location of the patient, and expertise available. Peritoneal dialysis has generally been the preferred therapy for isolated failure of the kidney and is universally available. Intermittent hemodialysis is frequently used in renal units where nursing expertise is available and hemofiltration is increasingly employed in the intensive care situation. Practical guidelines for and the complications of each therapy are discussed.

Critical issues in nephrology

Renal and electrolyte problems are common in patients in the ICU. Several advances that occurred in the recent past have been incorporated in the diagnosis and management of these disorders and were reviewed in this article. Unfortunately, many important questions remain unanswered, especially in the area of ARF, where new therapies are anxiously awaited to make the transition from bench to bedside. Better studies are sorely needed to define the best approach to dialysis in patients who have ARF.

Stewart and beyond: new models of acid-base balance

The Henderson-Hasselbalch equation and the base excess have been used traditionally to describe the acid-base balance of the blood. In 1981, Stewart proposed a new model of acid-base balance based upon three variables, the "strong ion difference" (SID), the total weak acids (ATot), and the partial pressure of carbon dioxide (Pco2). Over 20 years later, Stewart's physiochemical model still remains largely unknown. In this review, we will present both the traditional and the Stewart models of acid-base balance and then derive each using an "ion equilibrium method." Modern theories of acid-base balance may be useful toward the understanding of complex acid-base disorders.