Medicine chapters
Dialysis and renal replacement therapy (indications AEIOU)
What the kidney does and which of its jobs dialysis can and cannot replace, how haemodialysis, haemofiltration and peritoneal dialysis work, the AEIOU indications, how often each is done, their complications, and transplantation.
- Dialysis replaces the kidney's excretory jobs (removing urea, potassium, acid and water) but not its endocrine jobs (erythropoietin, calcitriol, renin), so dialysis patients still need ESAs and active vitamin D.
- Urgent indications are AEIOU: refractory Acidosis, Electrolytes (hyperkalaemia), Intoxication, fluid Overload, Uraemic complications (pericarditis, encephalopathy).
- Haemodialysis is usually 4–5 hours, three times a week via an AV fistula; CAPD uses 3–5 exchanges a day; its main complication is peritonitis (Staph. epidermidis). Transplant is the best RRT.
On this page
Overview
Renal replacement therapy (RRT) means treatments that take over the work of failed kidneys: haemodialysis, haemofiltration, peritoneal dialysis and kidney transplantation. It is used for AKI with dangerous complications and for end-stage kidney disease (CKD stage G5).
To understand what RRT can and cannot do, start with what a healthy kidney does.
Functions of the kidney
Non-endocrine (excretory and regulatory) functions
| Function | What failure causes | Replaced by dialysis? |
|---|---|---|
| Excretion of nitrogenous waste (urea, creatinine, urate) and drugs | Uraemia, drug toxicity | Yes, partly |
| Water and sodium balance | Oedema, hypertension, pulmonary oedema | Yes (ultrafiltration) |
| Potassium balance | Hyperkalaemia | Yes |
| Acid–base balance (H+ and ammonium excretion, bicarbonate regeneration) | Metabolic acidosis | Yes (bicarbonate or lactate in dialysate) |
| Phosphate excretion, calcium balance | Hyperphosphataemia | Partly (binders still needed) |
| Gluconeogenesis; breakdown of insulin and other peptide hormones | Hypoglycaemia risk, lower insulin needs in diabetics | Not really |
Endocrine functions
| Hormone | Made where / how | Effect of loss | Replacement |
|---|---|---|---|
| Erythropoietin | Peritubular interstitial cells, in response to hypoxia | Normocytic anaemia | ESA (epoetin, darbepoetin) + iron |
| Calcitriol (1,25-dihydroxyvitamin D) | 1α-hydroxylase in the proximal tubule | Low calcium, secondary hyperparathyroidism, renal bone disease | Alfacalcidol or calcitriol |
| Renin | Juxtaglomerular cells (afferent arteriole) | Overactivity contributes to hypertension in CKD | ACE inhibitors/ARBs to block it |
| Prostaglandins, kinins | Locally | Loss of protection of renal blood flow | — |
Dialysis replaces only the excretory and regulatory functions. A dialysis patient still needs erythropoietin, active vitamin D, phosphate binders and BP control. A successful transplant replaces all functions, endocrine included.
Managing the symptoms of CKD (before and alongside dialysis)
| Problem | Management |
|---|---|
| Anaemia | Correct iron (IV iron), then ESA; target Hb 100–120 g/L |
| Bone disease / hyperphosphataemia | Low-phosphate diet, phosphate binders with meals, alfacalcidol, cinacalcet, parathyroidectomy if refractory |
| Hyperkalaemia | Low-potassium diet, stop K+-retaining drugs, binders, correct acidosis |
| Acidosis | Oral sodium bicarbonate |
| Fluid overload, hypertension | Salt and fluid restriction, loop diuretics, antihypertensives (ACE inhibitor/ARB) |
| Nausea, anorexia | Antiemetics, dietitian review; persistent symptoms suggest the need for dialysis |
| Pruritus | Control phosphate and PTH, emollients for dry skin, antihistamines, adequate dialysis |
| Restless legs, cramps | Correct iron deficiency, adequate dialysis |
| Malnutrition | Normal protein intake, dietitian, avoid excessive protein restriction |
| Dyslipidaemia, CV risk | Statin, stop smoking |
| Gout | Colchicine for attacks; allopurinol at reduced dose (avoid NSAIDs) |
| Psychological and end-of-life care | Shared decisions; conservative care is a valid choice for frail patients |
Rationale of dialysis: how it works
- Diffusion: blood and dialysate flow on opposite sides of a semipermeable membrane (in the dialyser or the peritoneum). Small solutes (urea, creatinine, potassium) move from high to low concentration. Countercurrent flow keeps the gradient steep. Bicarbonate moves from dialysate into blood to correct acidosis.
- Ultrafiltration: water is pushed or pulled across the membrane by pressure (haemodialysis) or osmosis (glucose in peritoneal dialysis fluid).
- Convection: solutes are dragged along with the water ("solvent drag"); this is the main mechanism of haemofiltration and clears larger "middle molecules" such as β2-microglobulin better.
- Cells and large proteins do not cross the membrane.
Dialysis removes small solutes well but middle molecules poorly; build-up of β2-microglobulin causes dialysis amyloidosis and carpal tunnel syndrome after years of treatment.
The aims are to remove nitrogenous waste, keep electrolytes normal, correct acidosis and keep the patient at a normal fluid volume ("dry weight").
Indications for dialysis
Urgent indications (mainly in AKI): AEIOU
| Letter | Indication | Examples |
|---|---|---|
| A | Acidosis | Severe metabolic acidosis not corrected by medical treatment |
| E | Electrolytes | Hyperkalaemia that persists despite medical treatment, especially with ECG changes |
| I | Intoxication | Dialysable poisons and drugs: lithium, salicylate, methanol, ethylene glycol, theophylline; drugs causing AKI such as gentamicin |
| O | Overload | Pulmonary oedema not responding to diuretics |
| U | Uraemia | Uraemic pericarditis or tamponade, encephalopathy (confusion, fits), uraemic bleeding, symptomatic uraemia |
AEIOU – the five vowels: Acid, Electrolytes, Intoxication, Overload, Uraemia.
A low eGFR alone (for example 20 mL/min) is not an indication. Dialysis is started for symptoms or complications, or when eGFR is very low (around 10).
Chronic dialysis in ESKD
- Usually started when eGFR is around 10 mL/min/1.73 m² or earlier if there are uraemic symptoms, refractory fluid overload, hyperkalaemia, acidosis or malnutrition.
- Starting early has not been shown to improve survival; the timing is a shared decision.
- Some frail patients choose conservative care without dialysis.
Haemodialysis
Access
| Type | Notes |
|---|---|
| Arteriovenous fistula (AVF) | Best: radial or brachial artery joined to the cephalic vein (radiocephalic at the wrist), in the non-dominant arm; needs several weeks to mature; lowest infection and clotting rates |
| AV graft | Synthetic tube; when veins are poor (e.g. diabetes, vascular disease) |
| Tunnelled central venous catheter | Internal jugular (or femoral); usable at once; high risk of bacteraemia, thrombosis and central venous stenosis |
| Temporary non-tunnelled catheter | For urgent dialysis (jugular, femoral, subclavian) |
In a patient with CKD who may need dialysis, do not cannulate or take blood from the non-dominant forearm veins. They are needed for a future fistula. Do not take BP or compress the fistula arm (risk of thrombosis).
Frequency and adequacy
- Standard: 4–5 hours, three times a week. Blood flows at about 250–450 mL/min.
- More frequent or longer sessions improve wellbeing and BP but are harder to sustain.
- Fluid gained between sessions = intake minus losses. Patients limit fluid to keep interdialytic weight gain low.
- Adequacy: urea reduction ratio (URR) above 65% per session, or Kt/V (K = dialyser clearance, t = time, V = urea distribution volume).
- Signs of under-dialysis: itch, fatigue, poor sleep, restless legs, neuropathy, pericarditis.
Pre-dialysis urea 25 mmol/L; post-dialysis urea 8 mmol/L
URR = (25 − 8) ÷ 25 × 100 = 68%
Above 65% → adequate session
Anuric patient drinks 2 L a day; 2 days between sessions
Gain ≈ 2 L × 2 days = 4 L ≈ 4 kg to remove at the next session
Complications of haemodialysis
- Intradialytic hypotension (fluid removed faster than it refills the circulation): cramps, dizziness.
- Dialysis disequilibrium syndrome: at the first sessions in very uraemic patients, blood urea falls faster than brain urea, water moves into the brain → headache, vomiting, confusion, fits (cerebral oedema). Prevent with short, gentle, repeated sessions.
- Access problems: fistula thrombosis, stenosis, bleeding, aneurysm; steal syndrome.
- Infection: catheter bacteraemia (Staph. aureus), endocarditis, septic arthritis, vertebral osteomyelitis; blood-borne viruses (hepatitis B and C).
- Dialysis-related amyloidosis (β2-microglobulin) after years: carpal tunnel syndrome, arthropathy, bone cysts.
- Others: air embolism, haemolysis, reactions to the dialyser, anticoagulation bleeding, hyperkalaemia between sessions.
Haemofiltration and haemodiafiltration
- Haemofiltration: no dialysate; plasma water and solutes are removed by convection through a high-flux membrane, and a replacement fluid is infused.
- Continuous venovenous haemofiltration (CVVHF): runs all day through a dual-lumen central catheter; gentle, so preferred in haemodynamically unstable ITU patients with AKI.
- Haemodiafiltration combines diffusion and convection to clear middle molecules (β2-microglobulin).
Peritoneal dialysis (PD)
How it works
- A soft Tenckhoff catheter is tunnelled into the peritoneal cavity.
- Dialysate (containing glucose or icodextrin as the osmotic agent, and lactate as buffer) is run in, left to dwell, then drained.
- The peritoneal membrane acts as the semipermeable membrane; urea, creatinine and potassium diffuse into the fluid, and water follows the glucose by osmosis (stronger glucose bags remove more water).
Types and frequency
- CAPD (continuous ambulatory PD): fluid in the abdomen at all times; 3–5 exchanges a day of 1.5–3 L, each taking about 20–40 minutes, done by the patient at home with a sterile technique.
- APD (automated PD): a machine performs exchanges overnight while the patient sleeps.
Advantages and drawbacks
| PD advantages | PD drawbacks / contraindications |
|---|---|
| Done at home; independence, travel | Needs a motivated patient (or carer) and clean home |
| No vascular access or anticoagulation | Unsuitable with major abdominal surgery, adhesions, hernias, stomas |
| Gentle, steady removal; better preserves residual renal function | Less efficient; fails when residual function is lost |
| Useful in children and where machines are scarce | Glucose absorption causes weight gain and hyperglycaemia |
Complications of PD
A cloudy bag with abdominal pain in a CAPD patient is peritonitis until proved otherwise; the usual organism is the skin commensal Staph. epidermidis.
- Peritonitis (about one episode every 2 patient-years): cloudy effluent (>100 white cells/mm³), abdominal pain, fever. Commonest organism: Staphylococcus epidermidis (coagulase-negative staph), then Gram-negatives, Staph. aureus, rarely TB or fungi. Treat with intraperitoneal antibiotics; fungal peritonitis needs catheter removal.
- Exit site and tunnel infections.
- Catheter problems: poor drainage, often from constipation; displacement.
- Raised intra-abdominal pressure: hernias, fluid leak, hydrothorax (pleural effusion, usually right-sided), scrotal swelling.
- Metabolic: hyperglycaemia, weight gain, hypertriglyceridaemia (glucose load); protein loss into the dialysate causing hypoalbuminaemia; hypokalaemia.
- Encapsulating sclerosing peritonitis: rare, serious late complication with thickened peritoneum and recurrent bowel obstruction.
- Failure of the membrane (ultrafiltration failure) over years; switch to haemodialysis.
Haemodialysis vs peritoneal dialysis
| Feature | Haemodialysis | Peritoneal dialysis |
|---|---|---|
| Membrane | Synthetic dialyser | Patient's peritoneum |
| Where | Hospital or unit (home possible) | Home |
| Frequency | 4–5 h, 3 times weekly | CAPD 3–5 exchanges daily; APD nightly |
| Access | AV fistula, graft or catheter | Tenckhoff catheter |
| Anticoagulation | Needed (heparin) | Not needed |
| Fluid and diet | Stricter fluid and potassium limits | More liberal |
| Main infections | Catheter bacteraemia (Staph. aureus) | Peritonitis (Staph. epidermidis) |
| Best for | Unstable fluid status, failed PD, abdominal problems | Independent patients, children, those with poor vascular access or heart disease |
Kidney transplantation
- The best form of RRT for suitable patients: better survival and quality of life, and it restores endocrine functions. Pre-emptive living donor transplantation (before dialysis) gives the best results.
- The graft is placed in the iliac fossa; native kidneys are usually left.
- Needs ABO compatibility, a negative cross-match, and good HLA matching (A, B, DR).
- Lifelong immunosuppression (tacrolimus, mycophenolate, steroids), with risks of infection (CMV, BK virus, Pneumocystis), skin cancer and post-transplant lymphoproliferative disease, diabetes and drug nephrotoxicity.
- Rejection: hyperacute (preformed antibodies, minutes), acute (weeks to months, treatable), chronic (gradual decline).
- Some diseases recur in the graft (FSGS, IgA nephropathy, MPGN).
Expect: the AEIOU list and what is not an indication (eGFR of 20 alone); which kidney functions dialysis cannot replace (erythropoietin, calcitriol) and the drugs that replace them; diffusion, ultrafiltration and convection; haemodialysis 4–5 hours three times a week with a radiocephalic fistula in the non-dominant arm; CVVHF in unstable ITU patients; CAPD exchanges and complications (peritonitis with Staph. epidermidis, hernias, pleural effusion, hyperglycaemia, protein loss, hypertriglyceridaemia); dialysis disequilibrium; β2-microglobulin amyloid and carpal tunnel; under-dialysis symptoms; and transplantation as the best RRT.
Quick revision
- Dialysis replaces excretory functions, not EPO or calcitriol.
- AEIOU: acidosis, electrolytes (K+), intoxication, overload, uraemia.
- Dialysable: lithium, salicylate, methanol, ethylene glycol.
- Chronic dialysis around eGFR 10 or for symptoms; eGFR alone is not an indication.
- HD: 4–5 h × 3/week; radiocephalic AVF, non-dominant arm; URR >65%.
- HD problems: hypotension, disequilibrium, access infection, β2-microglobulin amyloid.
- CAPD: 3–5 exchanges/day; peritonitis (Staph. epidermidis), hernia, hydrothorax, hyperglycaemia, protein loss.
- CVVHF for unstable ITU patients. Transplant is the best RRT.
Written by The House Job from standard references (Kumar and Clark's Clinical Medicine).
FAQ
Why do dialysis patients still need erythropoietin injections?
Dialysis only cleans the blood and removes water; it cannot make hormones, so the erythropoietin the kidney used to produce must be given as a drug.
What is the difference between dialysis and haemofiltration?
Dialysis clears solutes mainly by diffusion into a dialysate, while haemofiltration removes plasma water and dissolved solutes by convection and replaces it with clean fluid.
What does a cloudy peritoneal dialysis bag mean?
Peritonitis until proven otherwise; the fluid should be sent for cell count and culture and intraperitoneal antibiotics started.
Why is the first dialysis session kept short in a very uraemic patient?
Removing urea too quickly lowers blood urea faster than brain urea, drawing water into the brain and causing disequilibrium with headache, confusion or fits.