Dialysis is the process of removing excess water, solutes, and toxins from the blood in people whose kidneys can no longer perform these functions naturally. This is referred to as renal replacement therapy. Dialysis is primarily used to provide an artificial replacement for lost kidney function (renal replacement therapy) due to renal failure
- Dialysis works on the principles of diffusion of solute through a semipermeable membrane that separates two solutions.
- The direction of diffusion depends on the concentration of solute in each solution.
- Rate and efficiency depend on the concentration gradient, the temperature of the solution, pore size of the membrane, and molecular size.
Indications
The decision to initiate dialysis or hemofiltration in patients with renal failure depends on several factors. These can be divided into acute or chronic indications.
- Indications for dialysis in the patient with acute kidney injury are:
- Metabolic acidosis in situations where correction with sodium bicarbonate is impractical or may result in fluid overload.
- Electrolyte abnormality, such as severe hyperkalemia, especially when combined with AKI.
- Intoxication, that is, acute poisoning with a dialysable drug, such as lithium, or aspirin.
- Fluid overload not expected to respond to treatment with diuretics.
- Complications of uremia, such as pericarditis or encephalopathy.
- Chronic indications for dialysis:
- Symptomatic renal failure
- Low glomerular filtration rate (GFR) (RRT often recommended to commence at a GFR of less than 10-15 mls/min/1.73m2). In diabetics dialysis is started earlier.
- Difficulty in medically controlling fluid overload, serum potassium, and/or serum phosphorus when the GFR is very low
Goals
- Reduce level of nitrogenous waste.
- Correct acidosis, reverse electrolyte imbalances, remove excess fluid.
Two main types of dialysis
A. Hemodialysis
- Hemodialysis removes wastes and water by circulating blood outside the body through an external filter, called a dialyzer, that contains a semipermeable membrane
- In hemodialysis, the patient’s blood is pumped through the blood compartment of a dialyzer, exposing it to a partially permeable membrane.
- The dialyzer is composed of thousands of tiny synthetic hollow fibers.
- The fiber wall acts as the semipermeable membrane. Blood flows through the fibers, dialysis solution flows around the outside the fibers, and water and wastes move between these two solutions.
- The cleansed blood is then returned via the circuit back to the body.
- Ultrafiltration occurs by increasing the hydrostatic pressure across the dialyzer membrane.
- This usually is done by applying a negative pressure to the dialysate compartment of the dialyzer.
- This pressure gradient causes water and dissolved solutes to move from blood to dialysate, and allows the removal of several litres of excess fluid during a typical 3 to 5-hour treatment.
- Studies have demonstrated the clinical benefits of dialyzing 5 to 7 times a week, for 6 to 8 hours.
- These frequent long treatments are often done at home while sleeping but home dialysis is a flexible modality and schedules can be changed day to day, week to week.
Types of venous access for hemodialysis
- External shunt
- The cannula is placed in a large vein and a large artery that approximate each other.
- External shunts, which provide easy and painless access to the bloodstream, are prone to infection and clotting and cause erosion of the skin around the insertion area.
- Arteriovenous fistulas or graft
- Large artery and vein are sewn together (anastomosed) below the surface of the skin (fistula) or subcutaneous graft using the saphenous vein, synthetic prosthesis, or bovine xenograft to connect artery and vein.
- The purpose is to create one blood vessel for withdrawing and returning blood.
- The advantage is greater activity range than AV shunt and no protective asepsis.
- The disadvantage is the necessity of two venipunctures with each dialysis.
- Vein catheterization
- Femoral or subclavian vein access is immediate
- May be the short or long-term duration.
Complications during hemodialysis
- Dysequilibrium syndrome
- A rapid removal of urea from the blood.
- Reverse osmosis, with water moving into brain cells.
- Cerebral edema
- Possible headache, nausea, vomiting, confusion, and convulsions; usually occurs with initial dialysis treatments
- Shorter dialysis time and slower rate minimize.
- Hypotension
- Results from excessive ultrafiltration or excessive antihypertensive medications.
- Hypertension
- Results from fluid volume overload (water and/or sodium), causing disequilibrium syndrome or anxiety.
- Arrhythmias
- Due to hypotension and rapid removal of potassium.
B. Peritoneal dialysis
- Peritoneal dialysis is a treatment for patients with severe chronic kidney failure
- Wastes and water are removed from the blood inside the body using the peritoneal membrane as a natural semipermeable membrane.
- Wastes and excess water move from the blood, across the peritoneal membrane, and into a special dialysis solution, called dialysate, in the abdominal cavity which has a composition similar to the fluid portion of blood.
- In peritoneal dialysis, a sterile solution containing minerals and glucose is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine, where the peritoneal membrane acts as a semipermeable membrane.
- The peritoneal membrane or peritoneum is a layer of tissue containing blood vessels that lines and surrounds the peritoneal, or abdominal, cavity and the internal abdominal organs (stomach, spleen, liver, and intestines).
- The dialysate is left there for a period of time to absorb waste products, and then it is drained out through the tube and discarded. This cycle or “exchange” is normally repeated 4-5 times during the day, (sometimes more often overnight with an automated system).
- Ultrafiltration occurs via osmosis; the dialysis solution used contains a high concentration of glucose, and the resulting osmotic pressure causes fluid to move from the blood into the dialysate. As a result, more fluid is drained than was instilled.
- Peritoneal dialysis is less efficient than hemodialysis, but because it is carried out for a longer period of time the net effect in terms of removal of waste products and of salt and water are similar to hemodialysis.
- Peritoneal dialysis is carried out at home by the patient.
Complications of Peritoneal Dialysis
- Excessive loss of fluid can result in hypovolemic shock or hypotension while excessive fluid retention can result in hypertension and edema.
- The volume of dialysate removed and weight of the patient are normally monitored; if more than 500ml of fluid are retained or a litre of fluid is lost across three consecutive treatments, the patient’s physician is generally notified.
- Also monitored is the color of the fluid removed: normally it is pink-tinged for the initial four cycles and clear or pale yellow afterwards. The presence of pink or bloody effluent suggests bleeding inside the abdomen while feces indicates a .
- The patient may also experience pain or discomfort if the dialysate is too acidic, too cold or introduced too quickly, while diffuse pain with cloudy discharge may indicate an infection. Severe pain in the rectum or perinium can be the result of an improperly placed catheter. The dwell can also increase pressure on the diaphragm causing impaired breathing, and constipation can interfere with the ability of fluid to flow through the catheter.
