Iatrogenic factors potentially contributing to fluid imbalance (circulating and tissue fluid volumes) in post‐operative patients are outlined in Box 16.12. Some patients require fluid replacement in the post‐operative period to ensure an adequate fluid balance, avoiding dehydration (NICE [148]).

Post‐operative fluid replacement should be based on the following considerations:

Daily maintenance fluids for sensible losses (i.e. measurable losses, e.g. urine output) and insensible losses (i.e. not measurable, e.g. sweating) will be dependent upon age, gender, weight and body surface area and will increase with pyrexia, hyperventilation and conditions that increase the catabolic rate.

Deciding on the optimal amount and composition of intravenous fluids to be administered and the best rate at which to give them can be a difficult and complex task, and decisions must be based on careful assessment of the patient's individual needs (NICE [148]). Where possible and clinically indicated, euvolaemic and haemodynamically stable patients should return to oral fluids as soon as possible (Powell‐Tuck et al. [165]). If intravenous fluids are required, the most commonly used replacement fluids are crystalloids and colloids, which have various effects on a range of important physiological parameters (Perel et al. [164]). All patients receiving intravenous fluids need regular monitoring. This should initially include at least daily reassessments of clinical fluid status, laboratory values (urea, creatinine and electrolytes) and fluid balance charts, along with weight measurement twice weekly (NICE [145]).

In December 2013, NICE published guidance outlining recommendations and algorithms covering the general principles for managing intravenous fluids for adults in hospital (see NICE [146]). See also Chapter c08: Nutrition and fluid balance.