Intravenous fluids - Fluid and electrolyte balance

Intravenous fluids

Intravenous fluids may be necessary for rapid fluid replacement, e.g. in a shocked patient, or for maintenance in patients who are unable to eat and drink or who are unable to maintain adequate intake in the face of large losses, e.g. due to diarrhoea. When prescribing intravenous fluids certain points should be remembered:

·        Are intravenous fluids the best form of fluid replacement? If possible, oral fluids are preferable or if swal-low is impaired consider nasogastric administration, which has the advantage of allowing nasogastric feed to be given to provide nutrition.

·        Which intravenous fluid should be given? Ideally this should be the one that matches any fluid and electrolyte deficit or losses most closely. For example, blood loss should be replaced with a blood transfusion and salt and water loss (e.g. vomiting, diarrhoea) with normal saline. Additional potassium replacement is often needed in bowel obstruction, but may be dangerous in renal failure.

·        In calculating the volume required for maintenance check if is there increased insensible loss, e.g. due to sweating in pyrexial patients, or are there other fluids being administered which need to be taken into account? For example, some patients are on intravenous drugs or intravenous nutritional supplements (total parenteral nutrition).

·        Patients at risk of cardiac failure (elderly, cardiac disease, liver or renal impairment) require special caution as they are more prone to develop fluid overload.

There is no universally applicable fluid regimen. The choice of fluid given and the rate of administration depend on the patient, any continued losses and all patients must have continued assessment of their fluid balance using fluid balance charts, observations and clinical examination as well as monitoring of serum electrolytes by serial blood tests.

Fluid preparations: Intravenous fluid has to be isotonic to lysis of red blood cells. The administration of water alone would lead to water moving across cell membranes by osmosis, such that the cells would swell up and burst. Giving hypertonic fluid is equally dangerous, as it causes water to move out of cells.

·        Most intravenous fluids used are crystalloids (saline, dextrose, combined dextrose/saline, Hartmann’s solution). It should be remembered that dextrose is rapidly metabolised by the liver; hence giving dextrose solution is the equivalent of giving water to the extra-cellular fluid compartment. If insufficient sodium is given in conjunction, or the kidneys do not excrete the free water, hyponatraemia results. This is a common problem, often because of inappropriate use of dextrose or dextrosaline and because stress from trauma or surgery as well as diseases such as cardiac failure promote antidiuretic hormone (ADH) release. This leads to a mild form of syndrome of inappropriate antidiuretic hormone (SIADH;) where there is water retention by the kidneys with resulting hyponatraemia.

·        Colloids (albumin, dextran or gelatin-based fluids) contain high-molecular-weight components that tend to be retained in the intravascular compartment. This increases the colloid osmotic pressure (oncotic pressure) of the circulation and draws fluid back into the vascular compartment from the extracellular space. A smaller volume of colloid compared to crystalloid is needed to have the same haemodynamic effect. Theoretically they are of benefit for rapid expansion of the intravascular compartment; however, they have anti-coagulant, antiplatelet and fibrinolytic effects, which may be undesirable. There has been no consistent demonstrable benefit of using colloid over crystalloid in most circumstances. In addition, the use of albumin solution in hypoalbuminaemic patients (which seems logical) has been associated with increased pulmonary oedema, possibly due to rapid haemodynamic changes or capillary leakage of albumin.

Fluid regimens: These should consist of maintenance fluids (which covers normal urinary, stool and insensible losses) and replacement fluids for additional losses and to correct any pre-existing dehydration. Fluid regimens must also take into account that patients of differing

weight have different fluid and electrolyte requirements (see Table 1.4). Potassium is added to intravenous fluids in patients who are not being fed, although this should be done with care. Both hypokalaemia and hyper-kalaemia  are potentially life-threatening and serum potassium must be checked daily in patients who are given potassium replacement. Patients with acute or chronic renal failure should not have potassium added routinely to fluid replacement (although hypokalaemia should of course be treated). Rapid administration of potassium is dangerous, so even in hypokalaemia no more than 10 mmol/h is recommended (except in severe hypokalaemia within an intensive care setting) and the potassium must be uniformly mixed in the bag.

A typical daily maintenance regime for a 70 kg man with normal cardiac and renal function consists of 8 hourly bags of:

·        1 L of 0.9% saline with 20 mmol KCl added,

·        1 L of 5% dextrose with 20 mmol KCl added and

·        1 L of 5% dextrose with 20 mmol KCl added.

In general, dextrosaline is not suitable for maintenance, as it provides insufficient sodium and tends to cause hyponatraemia. Postoperative patients are also more prone to hyponatraemia due to mild SIADH, so may require proportionally more sodium, e.g. 2 L of 0.9% saline to 1 L of 5% dextrose. Replacement fluids generally need to be 0.9% saline, as losses tend to have a high sodium concentration, e.g. drain fluid, blood, vomitus and diarrhoea.

Fluids should not be prescribed without taking into account the patient’s current fluid balance, continued losses and underlying coexistent diseases. It should also be remembered that intravenous fluids do not provide any significant nutrition.