Fluid, electrolytes and nutrition: physiological and clinical aspects

Fluid and electrolyte balance is often poorly understood and inappropriate prescribing can cause increased post-operative morbidity and mortality. The efficiency of the physiological response to a salt or water deficit, developed through evolution, contrasts with the relatively inefficient mechanism for dealing with salt excess. Saline has a Na+:Cl- of 1:1 and can produce hyperchloraemic acidosis, renal vasoconstriction and reduced glomerular filtration rate. In contrast, the more physiological Hartmann's solution with a Na+:Cl- of 1.18:1 does not cause hyperchloraemia and Na excretion following infusion is more rapid. Salt and water overload causes not only peripheral and pulmonary oedema, but may also produce splanchnic oedema, resulting in ileus or acute intestinal failure. This overload may sometimes be an inevitable consequence of resuscitation, yet it may take 3 weeks to excrete this excess. It is important to avoid unnecessary additional overload by not prescribing excessive maintenance fluids after the need for resuscitation has passed. Most patients require 2-2.5 litres water and 60-100 mmol Na/d for maintenance in order to prevent a positive fluid balance. This requirement must not be confused with those for resuscitation of the hypovolaemic patient in whom the main aim of fluid therapy is repletion of the intravascular volume. Fluid and electrolyte balance is a vital component of the metabolic care of surgical and critically-ill patients, with important consequences for gastrointestinal function and hence nutrition. It is also of importance when prescribing artificial nutrition and should be given the same careful consideration as other nutritional and pharmacological needs.