Renal - Dialysis Flashcards
At what stage of CKD should patients be offered renal replacement therapy?
CKD 5.
Renal replacement therapy includes dialysis (haemodialysis, peritoneal dialysis), haemofiltration or transplantation. Transplantation is better, but shortage of kidneys means patients start dialysis whilst waiting transplant.
In CKD 5 patients cannot:
- maintain electrolyte balance (e.g. hyperkalaemia)
- maintain water balance (e.g. odema)
- acid base balance (e.g. metabolic acidosis)
- excrete metabolic waste products (e.g. uraemia)
These functions are taken over by RRT
Haemodialysis
Most common form of RRT used for ESRD and is also used in AKI. There is diffusion of solutes from blood to dialysate across a semi-permeable membrane down a concentration gradient. Utilises 2 processes - dialysis and haemofiltration:
- dialysis: removes unwanted solutes by diffusion
- haemofiltration: removes water, which carries with it unwanted soluble substances
Haemofiltration can be performed on its own (e.g. AKI)
Principle of dialysis
If blood is separated from a suitable fluid by a semipermeable membrane, electrolytes and other substances diffuse across the membrane until equilibrium is reached. In haemodialysis, a synthetic membrane is used, whereas in peritoneal dialysis, the peritoneal membrane is used. Blood solute concentration falls as a result.
Principle of haemofiltration
Haemofiltration is similar to glomerular filtration. If blood is pumped at a higher hydrostatic pressure than the fluid on the other side of the membrane, then water in the blood is forced through the membrane by ultrafiltration with its dissolved elctrolytes and other substances.
When is haemofiltration alone used?
Haemofiltration is most often used in AKI. It may be either continuous or intermittent, and also allows control of intravascular volume by adjustment of the rate of fluid replacement.
Practical aspects of haemodialysis
Blood is pumped past one side of a semi-permeable membrane while dialysate fluid is pumped past the other side in the opposite direction. Membranes usually arranged within a cartridge as hollow fibres.
Amount of fluid removed by ultrafiltration is controlled by altering hydrostatic pressure of the blood compared with that of the dialysate fluid. The dialysate is made up of the essential constituents of plasma - sodium, potassium, chloride, calcium, magnesium, glucose - and a buffer (e.g. bicarbonate).
Blood and dialysate equilibrate across membrane. Plasma concentration, can therefore, be controlled by altering the dialysate composition. Concentration of K+ in the dialysate is usually lower than that in plasma to promote potassium movement out of the blood.
Heparin used in circuit to preven clotting. In patients at risk of bleeding, prostacyclin can be used instead although this can cause hypotension by vasodilatation.
Buffers in haemodialysis
Bicarbonate is preferred, but precipitates with calcium or magnesium and must be made up just before dialysis
- useful in unstable patients and when liver disease impairs lactate or acetate metabolism (other forms of buffer)
Lactate and acetate metabolised by the liver to produce bicarbonate. But until this occurs, the removal of bicarbonate by dialysis lowers the PCO2 and this can inhibit ventilation, contributing to hypoxaemia. Acetate is also a vasodilator so can cause hypotension.
How is dialysis access achieve for haemodialysis?
Haemodialysis requires 2 points of vascular access - one to remove blood and one to return it from the dialyzer
- short term can be achieved with large bore duel lumen central venous catheter
- tunneled through the skin to reduce risk of infection
Long term access requires artifical arteriovenous fistula
- usually created in the arm by joining radial or brachial artery to a vein in a side to side, or side to end manner
- fistula dilates over several months and high flow through it allows two large bore needles to be placed in it for dialysis
Fistula can also be constructed by joining an artery and vein with a synthetic Goretex graft. Occasionally external shunt is used.
Acute complications of haemodialysis
1) Hypotension - movement of blood out of the circulation into dialysis circuit
2) Dialysis dysequilibrium - over aggressive initial dialysis causes osmotic changes in the brain as the plasma urea falls
- nausea and headache –> seizures and coma
3) Headache - during dialysis, vasodilatory effect of acetate
4) Itch - during or after haemodialysis may be itch of chronic renal failure, exacerbated by histamine released caused by a mild allergic reaction to the dialysis membrane
- rarely exposure of blood to dialysis membrane –> generalised allergic response
5) Cramps - electrolyte shifts across muscle cell membranes
6) Hypoxaemia - during dialysis, hypoventilation caused by removal of bicarbonate or pulmonary shunting as a result of vasomotor changes that are induced by substances activated by the dialysis membrane
7) Hypokalaemia - removing potassium levels excessively –> dysrhythmias
8) Air embolism - problems within the dialysis circuit, should be treated by placing the patient head down onto their left side with 100% oxygen
Chronic complications of haemodialysis
Access MC problem - fistula thrombosis, aneurysm formation and infection, especially with synthetic grafts or temporary central venous access
Systemic infection introduced at the access site or acquired from dialysis circuit. Transmission of blood borne viruses such as viral hepatitis and HIV is a potential hazard.
Patients cannot excrete fluid between dialysis sessions so they must limit their fluid intake. If fluid is excessive it can be retained and contribute to hypertension and oedema
- thirst triggered by high plasma osmolality, plasma sodium is major determinant of this
- patients advised to reduce sodium intake to reduce thirst
Deposition of dialysis amyloid protein containing beta 2 microglobulin can cause carpel tunnel syndrome and a destructive arthropathy with cystic bone lesions.
Neurological complications due to phosphate binding compounds that contain aluminium and aluminium contamination of dialysate fluid can cause aluminium toxicity - dementia, myoclonus, seizures, and bone disease
- condition improves with deferoximine treatment
How is haemodialysis used in poisoning?
Dialysis can be used to remove water soluble drugs or their metabolites
- rapid removal may be necessary to reduce exposure of the patient to toxic effects of these compounds
Dialysis useful if renal function is impaired. Haemofiltration is also effective if dialysis not available.
Compounds include:
- toxic alcohols (methanol, ethylene glycol and isopropanol)
- lithium
- metformin
- salicylates
- sodium valproate, barbiturates or theophylline (rare)
What is peritoneal dialysis?
Fluid is infused through a tube into the peritoneal cavity. Water and solutes then move across the semi-permeable peritoneal membrane. The membrane consists of three layers: mesothelium, interstitium and the peritoneal capillary wall. Water moves from the plasma to a dialysate solution with a high glucose (amino acids can be used instead) content by osmosis.
Solutes move with the water and also move by diffusion into the dialysis fluid. Peritoneal dialysis is slower than haemodialysis so acute problems such as hypotension, hypoxia, dysrhythmias and disequilibrium are uncommon.
Limit to how much dialysis that can take place - large patients may not be able to get enough renal replacement using peritoneal dialysis.
Technical aspects of peritoneal dialysis
Catheter tunneled through the skin and placed in the peritoneal cavity (provides permanent access). Bags of sterile dialysate fluids are attached to catheter and drain into peritoneal cavity by gravity. Cather clamped with the empty bag connected, and when dialysis is finished, the catheter is unclamped and the fluid drained by gravity into the bag which is then disconnected and discarded.
Technique is continued ambulatory peritoneal dialysis (CPAD) because patients can go about their normal daily activities with the fluid in the abdomen. Around 4 fluid “exchanges” are required each day. Patients instil a fresh 2L of dialysate every 4h. Common to instil a “strong bag” of high osmotic strength over night to remove water.
Intermittent peritoneal dialysis (IPD) is a less common approach. Machine pumps fluid into the peritoneal cavity every 20min for a 12 to 48 h period. Some patients use this system every night to avoid changing bags during the day.
Any residual renal function contributes significantly to overall efficacy of the dialysis. Must start with small volumes and only when catheter is secured and not infected. May not be possible if abdominal surgery or sepsis has caused fibrosis, adhesions, or loss of a peritoneal surface.
What infective complications can occur in peritoneal dialysis?
Infection –> peritonitis
Most infection from skin derived gram positive staphylococci (coagulase negative staph - e.g. S.epidermidis most common), or gut derived gram negative organisms (e.g. E.coli)
Infection –> fever, abdominal pain and tenderness. Dialysate fluid is cloudy when it is removed from the abdomen and contains excess white blood cells (>100 cells/mm3 with >50% neutrophils)
Rx is a few fluid exchanges to wash out the peritoneum and then normal dialysis continues, but antibiotics are added to the dialysis bag or given systemically. Usually vancomycin is used to cover gram positive infection and an aminoglycoside, ciprofloxacin or ceftazidime is used to cover gram negatives. If infection is severe or fungal, the catheter should be removed and systemic antibiotics used. Repeated peritonitis reduces permeability of the peritoneal membrane.
Other than infection, what other complications can occur in peritoneal dialysis?
Protein loss (around 5-10g per day is lost into the dialysate fluid)
- use of dialysate fluid containing amino acids helps
- protein loss increases during peritonitis
Glucose loss
- large amount of glucose absorbed from glucose containing bags
- problematic in diabetes
- contributes to hypertriglyceridaemia
Others:
- hernias
- impaired ventilatory capacity
- back pain (increased intraabdominal pressure)
- beta 2 microglobulin is cleared better in peritoneal dialysis cf. haemodialysis so dialysis amyloid is rare.
