Severity of underlying disease, affecting likelihood of recovery of kidney function
Toxicity from dialyzable drug/toxin
Timing of RRT initiation
The optimal time to initiate RRT in critically ill patients remains uncertain. There are different guidelines giving recommendations for the timing of initiation of RRT in the ICU (Table 51.1).
Table 51.1Guidelines in the USA and UK on when to initiate RRT in the ICU.
Kidney Disease Improving Global Outcomes (KDIGO) Consortium, USA
National Institute for Health and Care Excellence (NICE), UK
Initiate renal replacement therapy (RRT) emergently when life‐threatening changes in fluid, electrolyte, and acid–base balance exist
Discuss any potential indications for RRT with a nephrologist, pediatric nephrologist, and/or critical care specialist immediately to ensure that the therapy is started as soon as needed
Consider the broader clinical context, the presence of conditions that can be modified with RRT, and trends of laboratory tests – rather than single BUN and creatinine thresholds alone – when making the decision to start RRT
Refer adults, children, and young people immediately for RRT if any of the following are not responding to medical management:
Complications of uremia (e.g. pericarditis or encephalopathy)
Base the decision to start RRT on the condition of the adult, child, or young person as a whole and not on an isolated urea, creatinine, or potassium value
Benefits of earlier dialysis
Drawbacks of earlier dialysis
Earlier control of metabolic derangements
Iatrogenic episodes of hemodynamic instability that may impede kidney recovery
Primarily used for removal of drugs in acute poisoning.
Modes of RRT
Intermittent hemodialysis (IHD)
IHD is a diffusion‐based therapy. Blood is pumped through the compartment of the filter at a higher flow rate than with CRRT techniques, and dialysate is pumped in a counter‐current direction at very high flow rates to encourage solute exchange.
In IHD, solute clearance occurs mainly by diffusion, whereas volume is removed by ultrafiltration.
Traditionally, intensivists have managed AKI with IHD empirically delivered 3–4 times a week, lasting 3–4 hours per session. The main disadvantage of IHD is the risk of systemic hypotension caused by rapid electrolyte and fluid removal.
Slow low efficiency daily dialysis is a variant of IHD that is associated with less hypotension; compared with IHD, both blood flow and dialysate rates are substantially slower (100–200 mL/min).
Continuous veno‐venous hemodialysis (CVVHD)
CVVHD is a diffusion‐based therapy. Blood is pumped through the blood compartment of the filter and dialysate flows counter‐currently (Figure 51.1). The counter‐current flow optimizes the diffusion gradient and thus the resulting clearances.
With CVVHD, dialysate flow is less than the blood flow, corresponding to clearances closely related to dialysate flow.
Continuous veno‐venous hemofiltration (CVVH)
CVVH is a convection‐based therapy. Blood is pumped through the blood compartment of the filter and a significant filtrate flow is produced by action of the filtrate pump.
Filtrate flow requires compensation by infusion of a substitution fluid to the blood flow pre‐ or post‐filter. This way, high filtrate flows can be generated that enhance solute removal.
Continuous veno‐venous hemodiafiltration (CVVHDF)
CVVHDF combines the use of both diffusion and convection therapies. Blood is pumped through the blood compartment of the filter and dialysate flows counter‐currently.
The counter‐current flow optimizes the diffusion gradient.
In addition, a substitution fluid is infused into the blood flow either pre‐ or post‐filter. This is paralleled by filtration of plasma water across the membrane resulting in convective clearance.
Pros and cons of intermittent versus continuous RRT
There are no definitive data supporting one technique over another, although meta‐analyses tend to support potential survival and renal recovery benefit with continuous treatments.
The optimal mode of RRT depends on the therapeutic aim.
Continuous therapies may be associated with less hypotension and disequilibrium syndromes (Table 51.2).
Intermittent therapies mainly rely on diffusion, thus necessitating high dialysate flow rates to maintain high concentration gradients.
Continuous therapies mainly rely on convection, performed as a low efficiency technique.
Table 51.2Differences between intermittent and continuous renal replacement therapy.
Arteriovenous fistula results in reduced blood flow to hand
Severe symptoms: surgical or radiologic revision Mild symptoms: improve with time
Dialysis associated (different from uremic pericarditis)
Intensification of dialysis to 6–7 times/week Minimize or discontinue anticoagulation Treatment failure or evidence of tamponade: pericardiectomy
Occurs in first few treatments More common in profoundly uremic patients Due to CNS edema from rapid osmolar shifts Symptoms: nausea, emesis, headache, confusion, seizures
Lower blood flows and shorter treatment duration during initial sessions
Anaphylactic and anaphylactoid reactions
Anaphylaxis: IgE mediated Anaphylactoid: release of mast cell mediators Usually ~5–20 minutes into hemodialysis Drug induced (e.g. iron dextran) Bradykinin‐mediated reactions
Stop hemodialysis without return of extracorporeal blood to patient Epinephrine, antihistamines, corticosteroids, respiratory support Using gamma ray or steamed filters may prevent hypotension (first use) Mild symptoms (e.g. chest/back pain) (20–40 minutes into hemodialysis) may improve over time and hemodialysis does not need to be stopped
Fever and pyrogenic reactions
Water or bicarbonate dialysate Improperly sterilized dialyzers Use of central venous dialysis catheters Cannulation of infected arteriovenous grafts or fistulae
If hemodynamically unstable, hold dialysis and initiate supportive measures (vasopressor, fluid bolus) Infectious work up (e.g. catheter sites or arteriovenous graft) Prompt use of antibiotics
Follow‐up tests and monitoring
Drug clearance increases with RRT. Monitor drug levels (e.g. antibiotics, anticonvulsants) to ensure adequate therapeutic levels.
Continuous modalities require the patient to be bedbound, and thus will need vigilant nursing protocols to prevent pressure ulcers.
Reasons for discontinuation of RRT in the ICU
There are a paucity of data on optimal timing for discontinuation but the following are reasons for stopping RRT in the ICU:
Increase in urine output is the most common determinant of kidney function recovery and thus successful weaning from dialysis.
Decrease in BUN and creatinine.
Improved metabolic state.
Improved fluid overload.
Withdrawal of therapy.
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