HEMOFILTRATION Development
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REVIEW Hernofiltration: treating and preventing infection Paul C. Turner' and Hilary Humphreys2 'Public Health Laboratory and 'Division of Microbiology, University Hospital, Queen's Medical Centre, Nottingham, UK Key words: Hernofiltration, infection, intensive care, kidney failure, catheterization GENERAL PRINCIPLES OF HEMOFILTRATION development. Furthermore, fluid balance can be accurately controlled and regulated, allowing other Renal failure is a recognized feature of the acutely ill, support measures such as total parenteral nutrition to intensive care patient, often in association with multi- be used [3]. Unlike CH, peritoneal dialysis is less suited organ failure. Henlofiltration is now routinely used for to the ICU as there is a delay before beneficial effects treating acute renal failure in the intensive care unit are seen and it cannot be used in the presence of intra- (ICU). Unlike hemodialysis and peritoneal dialysis, abdominal wounds, infection, ileus, or with a vascular both of which rely upon diffusion of molecules prosthesis [4]. across a concentration gradient, hemofiltration utilizes Although the underlying principle remains the convective solute transport (known as solvent drag) same, several modifications of hernofiltration are in whereby molecules are swept along by a moving stream use [5]. The two commonest variations are continuous of solvent [l]. This is analogous to the filtration that venovenous hernofiltration (CVVH) and continuous occurs naturally through the glomerulus. A special arteriovenous hemofiltration (CAVH). CAVH, which filter is used to remove plasma water and unbound requires arterial access, is circulation driven, resulting in particles weighing between 500 and 10 000 Da. The lower flow rates and filtrate volumes than CVHH. For nitrogenous substances traditionally associated with it to be effective, a mean arterial pressure of 60 mmHg uremia are removed, although urea itself is not is required, making it less useful in the septic, hypo- efficiently filtered. The process results in the produc- tensive patient. CAVH has now been supplanted tion of a large volume of filtrate containing an excess by CVVH, which is pump driven, achieving both of waste particles. The deficit of water and electrolytes higher flow rates and filtrate volumes, resulting in are made good by infusing fluid back into the patient's better clearance of waste products. CH can also be blood, usually after it has passed through the filter supplemented by hemodialysis, particularly in the (Figure 1). presence of a hypercatabolic state, to give continuous Continuous hemofiltration (CH) was first used or intermittent hemodiafiltration [5-71. CVVH is to treat a patient with renal failure in 1977 [2]. The performed via a dual lumen catheter inserted centrally, shortage of technical equipment and specialist staff preferably into the jugular or subclavian vein, but required to provide hemodialysis, combined with femoral catheterization may be necessary when access excellent tolerance even in hemodynanlically compro- at other sites is compromised [7]. mised patients, have been the main reasons for its Kierdorf has reviewed the advantages and dis- advantages of CH versus intermittent dialysis treatment Corresponding author and reprint requests: and the problems that accompany hemodialysis in the Dr Hilary Humphreys ICU patient [XI. CH decreases APACHE I1 scores, Division of Microbiology, Department of Clinical Laboratory significantly increases arterial blood pressure, avoids Sciences, University Hospital, Queen's Medical Centre, major fluctuations in electrolyte imbalances (which Nottingham NG7 2UH, UK. may improve cerebral function), and increases the Tel: +44 (0)115-9709162 Fax: t44 (0)115-9709233 survival rate for patients with multiple organ failure. Email [email protected] These benefits are partly offset by the requirement Accepted 30 June 1996 for continuous anticoagulation, hyperlactatemia and 80 Turner and Hurnphreys: Hernofiltration and Infection 81 reduced efficiency in the control of azoteniia per unit catheter-related infections include the distinction tinie [S]. Indications for CH include an increasing between colonizationhfection and contamination by variety of other conditions (Table 1) [9-111. Although semiquantitation of catheter tip cultures, or defining there are no absolute contraindications to hemo- catheter-related bactereniia as organisni(s) recovered filtration, extra care is needed for patients at increased from a catheter segment or tip with the same organ- risk of hemorrhage (e.g. in severe thrombocyto- ism(s) recovered froin blood cultures without clinical penia), when the anticoagulation required may be evidence of another source of infection [14-161. As problematic [9]. patients on CH are likely to have more than one catheter, confirming a diagnosis of catheter-related bacteremia and determining its source is more difficult PREVENTION AND DIAGNOSIS OF than in other patient groups. Diagnostic options HEMOFILTRATION-RELATED INFECTION include semiquantitative blood cultures taken through the hemofiltration catheter. However, this is tinie- As the procedure involves cannulation of a large consuming and cumbersome to perform. Alternatively, vein, and with CAVH an artery, insertion should be removal of the catheter for culture of the tip may be regarded as a sterile procedure and subject to the same performed, but this is usually a difficult decision in precautions as central venous catheters. This has been patients with limited vascular access [17,1X]. Further- reviewed recently and recommendations emphasize more, patients on CH may already be receiving that insertion should be considered a minor operation, treatment with multiple antimicrobial agents, reducing but because of the condition of patients requiring CH the chance of successfully culturing the organism(s) and the need for other organ support (e.g. ventilation), responsible. it is rarely possible to perform the procedure in In a series of 17 patients receiving continuous an operating theater [12]. As with central venous arteriovenous hemodiafiltration (CAVHD) for renal catheters, it is difficult to justify the routine use of prophylactic antibiotics to reduce the risk of infection. Handling of the cannulae should be kept to a mini- Venous (afferent) Venous (efferent) mum, apart from during use and flushing with heparin. circuit circuit It is advisable not to use the catheter for obtaining routine blood samples unless there is limited vascular access or infection at that site is suspected. Unfortunately, there are no generally agreed clinical or microbiological definitions for diagnosing heniofiltration-related infections. Furthermore, infec- tive coniplications vary from center to center according to the techniques used for catheter insertion, culture protocols, type of patient and duration of treatment. Attempts to define hemofiltration-related infection should be based on guidelines similar to those of tliiid catheter-related infection [13-151. This would allow a Heparin 1 (infusate) more accurate assessment of infection risk among different patient groups. Definitions used to diagnose Table 1 Indications for hemofiltration Acute renal failure Fluid and electrolyte imbalancc ____- Myoglobinuric acute renal failure r- --1 Total parenteral nutrition with fluid restriction I Filter I Ccrcbral edema and increased intracranial pressure Severe burns Septic shock with multiple organ failure - - - r]Ultrafiltrate - - - Hepatorenal syndrome (waste products)5- Hepatic encephalopathy Cori-ection of acid-base dmturbancet Figure 1 Schematic representation of continuouz Pulmonary edema venovenous hernofiltration (CWH). 82 Clinical Microbiology and Infection, Volume 2 Number 2, October 1996 failure (mean duration 15 days), 35% of the catheters on sepsis, it seems likely that CH will remain the removed for suspected infection were colonized, but treatment of choice for renal support in the ICU catheter-related infection was confirmed in only one patient with severe hemodynamic instability, particu- patient [6]. No mention of the criteria used to larly in the absence of local facilities for hemodialysis distinguish colonization from infection was made. [33,34]. Yet another dificulty that may obscure clinical diagnosis, although less common with modern systems, is the effect of hemofiltration on core temperature, CLEARANCE OF ANTIMICROBIAL AGENTS which may mask a pyrexial response. The reduction in core temperature observed can be explained in The effect of hemofiltration on the clearance of drugs part by the extracorporeal circulation required for will influence the dose, dosage interval and require- hernofiltration and the removal of pyrogens in the ment for measurement of antimicrobial agents. The filtrate [19]. efficiency of antimicrobial removal or filtration is related to the sieving coefficient (SC), which is a mathematical expression of the ability of a solute to THERAPEUTIC POTENTIAL OF HEMOFllTRATlON cross a membrane by convection. The SC is also FOR SEPSIS dependent on the degree of protein binding. An SC of close to 1 (e.g. amikacin, imipenem, ciprofloxacin, Hemofiltration has been advocated as a useful adjunct vancomycin) implies the agent crosses the membrane in the treatment of the sepsis syndrome both with easily, whereas when the SC approaches zero (e.g. and without multiple organ failure [20]. Experimental amphotericin B, oxacillin), there will be little removal studies in animals have shown increased survival times [35].