Haemodilution

Definition

This is increased plasma volume, leading to low haematocrit and hyponatraemia.

Pathogenesis

Water intoxication can arise from:

• Administration of too much intravenous fluid.

• Ingestion of too much water, especially if electrolyte levels are depleted.

Aetiology

Haemodilution occurs physiologically in pregnancy. This may result in lower haemoglobin concentrations than in the non-pregnant state. However, many women function well and do not require iron supplementation. It is common in chronic heart failure, where it may contribute to poor outcome.¹ At-risk groups for water intoxication include:

• Infants under 1 year old.

• Athletes - eg, marathon runners/extreme athletes and hikers who drink excessively during prolonged exertion.²³

• Also associated with the use of methylenedioxymethamfetamine (MDMA, or 'ecstasy') with prolonged dancing and high water intake at raves/events.⁴

• The mentally ill, in association with polydipsia.

Presentation

Symptoms

• Confusion

• Nausea/vomiting

• Seizures

• Coma

It may be fatal.

Signs

Brain oedema.

Management

Treat the underlying cause. Correct electrolyte balance in severe cases of water intoxication. Fkuid restriction may be considered in chronic heart failure.

Pharmacological

Diuretics.

Therapeutic role for haemodilution

• Acute normovolaemic haemodilution (ANH) may be used in operations where there is a large anticipated blood loss. It involves removing some of a patient's own blood and replacing it with fluids, to maintain normal volume. This process ensures that the blood that is lost during the operation is diluted, ie fewer red blood cells are in it. This reduces the load on the heart and allows the blood to flow more easily through the capillaries. The patient's own blood is returned after the operation.

• Haemodilution improves the flow properties of the blood so that, theoretically, oxygen and nutrient supply to the brain is improved (eg, after a cerebrovascular event) and damaged brain cells may survive. This treatment had been shown to reduce brain infarct size in animals with experimental stroke and it was first tried in the 1970s. However, a recent Cochrane review of the (considerable amount of) research showed that there were no benefits from this treatment.⁵⁶