18 Intravenous Fluids

Solutions of electrolytes and water are given intravenously, to meet normal fluid and electrolyte requirements or to replenish substantial deficits or continuing losses, when the patient is unable to take adequate amounts by mouth, e.g. nauseous, vomiting, haemorrhaging, dehydrated or ketotic.

The causes and severity of the electrolyte imbalance must be assessed from the clinical history and biochemical investigations. Sodium, potassium, chloride, magnesium, phosphate and water depletion can occur singly and in combination with or without disturbances of the acid–base balance. Actions must be taken to ensure that circulatory overload does not occur as a result of intravenous fluid therapy, as overloading will result in pulmonary oedema and breathlessness, which may cause respiratory arrest. Hence a record of all fluids infused is essential.

Plasma volume expanders are used for the treatment of circulatory shock, e.g. massive obstetric haemorrhage or where there is a sudden acute blood or plasma loss leading to a fall in blood pressure and resulting in blood cells collapsing while trying to redress their balance. They restore vascular volume, stabilizing circulatory haemodynamics and maintaining tissue perfusion.

Drugs should NOT be added to infusions of sodium bicarbonate, amino acids, mannitol, blood products or specially prepared fat emulsions, such as those used in neonatal intensive care units for feeding neonates (total parenteral nutrition) via the intravenous route.

Instructions regarding storage and degradation of solutions should be noted, and any deviation either from instructions or within the solution should indicate that the infusion should not be commenced, or, if in progress, should be discontinued.

Additive labels should be used to indicate what has been added to the solution, time, strength and, if relevant, expiry time/date, as well as patient identity and the signature of the practitioners checking the infusion. The practitioner must also be aware of the suitability of the additive to the electrolyte solution and where to refer any enquiries.

NB: All fluids should accurately reflect daily requirements, and close monitoring is required to avoid fluid and electrolyte imbalance.

Intravenous fluids are commonly described as crystalloids and colloids.

Crystalloids

These are solutions with small molecules that flow easily from the bloodstream into the cells and tissues. They contain similar concentrations of osmotically active particles to extracellular fluid, so fluid does not shift between the extracellular and intracellular areas. Crystalloids can be described as:

 Isotonic – with a concentration of dissolved particles equal to that of intracellular fluid. Osmotic pressure is therefore the same inside and outside the cells, so they neither shrink nor swell with fluid movement.

 Hypotonic – less concentrated than extracellular fluid, so fluid moves from the bloodstream into the cells, causing cells to swell

 Hypertonic – more highly concentrated than extracellular fluid, so fluid is pulled into the bloodstream from the cells, causing cells to shrink.

Intravenous crystalloid fluids in common use

Sodium chloride 0.9%

This isotonic solution provides the most important extracellular ions in near physiological concentrations and is indicated in sodium depletion, used in fluid replacement and electrolyte balance, as an IV infusion, as a carrier for injections in which the prescribed drug requires reconstitution, or as a ‘flush’ for IV cannulae.

Cautions: impaired renal function, cardiac failure, hypertension, peripheral and pulmonary oedema, toxaemia of pregnancy.

Side effects: administration of large doses may give rise to sodium accumulation, oedema and hyperchloraemic acidosis.

NOTE: The term ‘normal saline’ should not be used to describe sodium chloride intravenous infusion 0.9%; the term ‘physiological saline’ is satisfactory, but it is preferable to state the actual composition of the fluid, i.e. sodium chloride intravenous infusion 0.9%.

Glucose solutions

These are used mostly to replace water deficit and should not be given alone as they may lead to hyponatraemia and other electrolyte disturbances.

Sodium chloride and glucose

These hypertonic solutions are indicated when there is combined water and sodium depletion. A 1 : 1 mixture of isotonic sodium chloride and 5% glucose allows some of the water (free of sodium) to enter body cells, which suffer most from dehydration, while the sodium salt with a volume of water determined by the normal plasma Na⁺ remains extracellular.

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