Understanding the different types of intravenous fluids is a key area of perioperative practice. Whilst there is a choice of crystalloid and colloid solution, the majority of anaesthetists will administer balanced crystalloid solutions which contains electrolytes and pH close to that of plasma, such as Hartmann’s solution or Plasma-Lyte 148, avoiding the routine administration of 0.9% Sodium chloride 5% Glucose, and colloids such as starches an gelatins.

Fluids are given intraoperatively for three reasons: firstly, to replace existing deficits (which can be substantial in emergencies, bowel obstruction, prolonged starvation etc), secondly to maintain fluid balance and finally to replace surgical losses (both blood/fluid loss and insensible losses from evaporation).

In practice, most anaesthetists start with 1–2 l of crystalloid, although the volume of fluid administered can be guided by clinical assessment (arterial blood pressure and heart rate). In major cases a fluid challenge (e.g. 250 mls of crystalloid) is administered and the response of central venous pressure or stroke volume are assessed. This will help to minimise the risk of both hypervolaemia and hypovolaemia, both of which will impair tissue oxygenation.