Renal replacement therapy (RRT) is a term used to refer to modalities of treatment that are used to replace the waste filtering functions of a normal kidney. Acute kidney insufficiency (AKI) causes progressive deterioration in the regulation of body acid-base balance, as well as electrolyte and fluid balance. In such a setting, RRT is essential to assist the body to survive ensuing metabolic derangements.
Researchers have noted that dialysis, which is one of the most common RRT used, cannot compensate for all the tasks performed by a kidney, and thus the term ‘renal support therapy’ has been suggested to be a better name.
Patients receiving hemodialysis in a hospital
Indications for RRT
RRT is used whenever the kidney functions fail to achieve homeostasis. This includes situations such as:
Acute kidney insufficiency
Chronic renal failure
Myasthenia gravis
Septic shock
Acute hepatic failure superimposed on chronic failure
Modes of RRT
Different techniques used for RRT include:
Intermittent Hemodialysis (IHD) – often associated with hypotension, allows only intermittent removal of wastes
Continuous RRT – more costlier and complex, requires continuous anticoagulation therapy, avoids hypotensive episodes
Slow continuous ultrafiltration (SCUF)
Continuous venovenous hemodialysis (CVVHD)
Continuous venovenous hemofiltration (CVVHF)
Continuous venovenous hemodiafiltration (CVVHDF)
Continuous arterio-venous hemofiltration (CAVHF)
Slow low-efficiency daily dialysis (SLEDD)
Peritoneal dialysis
Plasmapheresis or plasma exchange
Criteria for RRT
Initiation of RRT is based on criteria such as:
Oliguria (insufficient urine output)
Anuria (the absence of urine production)
High serum creatinine and urea levels above fixed criteria
Metabolic acidosis without compensation
Dangerous fluctuations in serum sodium and potassium
Pulmonary edema which does not respond to diuretic therapy
Temperature soaring above 40 0C
High blood urea with complications such as encephalopathy, neuropathy or pericarditis
Cardiac failure
Lithium overdose
Patients who need to be infused with large amounts of fluid or blood, but cannot tolerate volume infusions without risk of pulmonary edema
Hypothermia
Chronic renal failure with refractory volume expansion, acidosis or hyperkalemia, a bleeding diathesis or uremic symptoms
The principle of RRT
RRT is based on semi-permeable membrane use to filter out wastes in the form of solutes, by diffusion or convection, and excessive water, in a process called ultrafiltration. The rate of ultrafiltration depends upon transmembrane pressure and the membrane permeability.
Moreover, the diffusion of the solute depends upon factors such as:
The inter-compartment concentration gradient separated by the membrane
Temperature
Diffusion coefficient
Surface area of the membrane
Solute characteristics such as molecular weight, protein-binding and the flow rate of blood as well as dialysate flow
Basic mechanism of different modes of RRT
Hemofiltration depends on water removal by a transmembrane pressure which is above the oncotic pressure of the plasma. In peritoneal dialysis water is removed by increasing the osmolarity of the dialysate.
Intermittent hemodialysis depends on the same mechanism as hemofiltration to remove excess water. It is made up of three parts:
The blood delivery system
The dialysate
The dialyzer
Newer breakthroughs in extracorporeal blood purification methods may result in effects that transcend basic replacement of renal function, such as the modulation of the sepsis cascade.
Complications of RRT
The main disadvantages of RRT relate to:
Catheter-related complications – blood loss, disconnection, infection or failure of access
Mechanical complications of the extracorporeal circuit
Fluctuations in the salt-water balance
Activation of the coagulation cascade
Complications specific to hemodialysis include:
Hypotension
Complications that can occur during transport
Accidental removal of tubes, including connections to a ventilator, dialysis catheters or airway tubes
Deterioration of hemodynamic status due to cardiac arrhythmias, removal of drugs used in treating underlying conditions
Cardiac arrest
Infections
Coagulopathy
Venous thrombosis
Obstruction of dialysis catheters or blood vessels
Air embolism
Muscle cramps
Anaphylactoid reactions to the dialyzer
Specific complications of peritoneal dialysis include:
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