Haemodialysis involves the blood of a patient being passed into a dialyser, where it interfaces with dialysate across a semi-permeable membrane. Dialysate has a similar content of salts (sodium, chloride, calcium, magnesium) to plasma, so most small molecules diffuse across the membrane into the dialysate. Fluid can be removed by applying a negative pressure on the dialysate side (ultrafiltration).

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Anticoagulation with heparin prevents the extracorporeal blood from clotting. Good vascular access (best of all, a subcutaneous arteriovenous fistula involving no synthetic material) is necessary for effective and safe treatment.

Most patients with end stage renal failure (ESRF) require three sessions of haemodialysis per week, each lasting 3-5 hours, depending on the patientâ€™s size, residual renal function, and the efficiency of the process. As this is an intermittent treatment, limitations on fluid, sodium, and potassium intake are often necessary. More frequent and/or longer treatments may be beneficial â€“ though more laborious for the patient.

Methods for assessing the adequacy of haemodialysis have focused on small molecule clearance, which can be predicted and measured relatively easily. Urea Reduction Ratio (ratio of blood urea before and after dialysis) and a calculated parameter Kt/V are commonly used (K is a dialyser-specific figure for rate of urea clearance; t is duration of dialysis; V is the volume of distribution of urea in a patient). There is continuing controversy over the best way of assessing adequacy of treatment; duration of dialysis also appears to exert a significant effect. (Further information on measuring haemodialysis adequacy, but use also haemodialysis specialist pathway).

Patients on dialysis remain at risk of most of the complications of renal failure plus some additional ones.

Routine (e.g. monthly) monitoring of biochemical parameters before and after dialysis is usual. In addition there will be monitoring of blood count, and usually liver function tests. Less frequently virology samples will be sent to screen for infections that may be transmitted by blood (e.g. hepatitis).

Prognosis depends on age and comorbid diseases (further info).

Problems associated with intermittent haemodialysis

Haemodialysis requires complex machinery and skills that are usually only available in a hospital or similar specialist setting â€“ although some relatively healthy patients are able to learn to dialyse themselves at home. The treatment is very time consuming, and travel is difficult. Post-dialysis symptoms, including fatigue, are common, sometimes leaving relatively few â€˜good daysâ€™ each week. These symptoms tend to be worse in patients with serious comorbid conditions.

During treatment, hypotension is common because of the need to remove fluid at each treatment. Muscle cramps have similar etiology; both tend to be worse in patients who do not comply with fluid and sodium restrictions and in whom it is therefore necessary to remove more fluid in each treatment. Dialysis-related symptoms are also more likely in patients with cardiovascular disease.

The sick dialysis patient

This information is relevant to all but is particularly aimed at those who may see patients away from a renal unit.

Seek help. It is always appropriate to consult the patient’s renal unit about their management if you are at all uncertain about the relevance of their renal failure. If it is a cardiovascular problem or possibly related to their dialysis treatment, emergency consultation is usually appropriate. All renal units will have a system for 24 hour availability of advice and assistance. Admission to hospital for non-renal indications should be immediately notified to their renal team.

Potassium and other electrolytes may be upset by coincident illnesses and should always be checked if a dialysis patient is unwell. Treat [K]>7.0 medically (further info) and consult renal unit urgently.

Fluid overload closely resembles heart failure, but may also be easily confused with pulmonary disease. There is elevated jugular venous pressure and/or may be clinical or radiological signs of pulmonary oedema. Typically a crisis occurs just before dialysis is due, or at a weekend when there is a longer interval between dialyses, or when they have missed treatment. It cannot usually be effectively treated with diuretics as these require functioning kidneys: dialysis is usually necessary.

Fluid management is critical in dialysis patients in hospital. Remember that their normal daily fluid requirements are only output plus 500mls, and that 1 litre of isotonic saline far exceeds their normal daily sodium restriction, if there are no deficits or increased losses. A fluid restriction of 1 litre daily is common.

Infection is a leading cause of death in patients on dialysis. Bacterial infections of all types seem to be increased in incidence, but there is a particular risk of infections related to vascular access sites or devices in patients on haemodialysis. Always send blood cultures from a sick dialysis patient. Consult the renal unit before giving blind antibiotic therapy; but if patient has artificial vascular access device, include cover for organisms likely to be associated with indwelling catheters (particularly coagulase negative and postive Staphylococci). First doses of many antibiotics are unchanged for patients with renal failure, but dosage interval may be very substantially prolonged.

Anticoagulation – residual unfractionated heparin is likely to be present for a few hours after haemodialysis, but patients with renal failure also have a bleeding tendency. If low molecular weight heparins or other anticoagulants have been used, their half lives may be much longer. Do not use full-dose low molecular weight heparin or other new anticoagulants without seeking advice first.

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