A6. Acute management of fluid imbalance
Fluid therapy refers to the use of intravenous (or rarely, oral) fluids in the management of illness.
Types of fluid
“Pure water” infusions are not actually pure water, but 5% dextrose (glucose) in water (D5W) infusions. Pure water has 0 osmolarity and so would cause haemolysis, but D5W has near-physiological osmolarity (250 mOsm/L). The glucose is rapidly metabolised, yielding pure water. D5W is used in pure water deficit, like dehydration or in patients unable to drink.
Crystalloid solutions are those which contain electrolytes. No crystalloid infusion contains the exact same electrolyte composition as plasma, but those which are similar to plasma are called “balanced” crystalloids while those which aren’t are called “unbalanced” crystalloids. Crystalloids are the most widely used infusions and can be used to replace any fluid loss. 1 L of blood loss can be replaced by 4 L of crystalloids.
“Physiological” saline/0,9% NaCl solution, also called normal saline (NS) is widely known and widely used. However, it’s an unbalanced solution because it contains more sodium and chloride than plasma, and none of the other electrolytes. As such, it should not be used to replace lots of fluid, because the high chloride content can cause hyperchloraemic metabolic acidosis.
Balanced solutions like Ringer-lactate (also called lactated Ringer), Sterofundin® and Isolyte® are more similar to plasma and are better suited for replacing large volumes.
Colloid solutions contain large solute molecules which can not leave the blood vessels. In theory this allows the solution to stay in the vessels rather than entering the interstitial and intracellular space, contributing more to circulating volume. These solutions contain albumin, gelatine, dextrans, etc. Some argue that colloids should be used in shock, but there’s little evidence that colloids are better than crystalloids.
Maintenance fluid requirement
For calculating maintenance fluid requirement per hour of a patient, regardless of age, do this: (4:2:1 rule)
- 4 mL of fluid per kg in 0 – 10 kg
- + 2 mL of fluid per kg in 10 – 20 kg
- + 1 mL of fluid per kg above 20 kg
Indications and choice of fluid
| Indication | Choice of fluid |
|---|---|
| Exsiccosis/dehydration | D5W |
| Shock | Crystalloid |
| Long-term infusion | Balanced crystalloid |
| Haemorrhage | Blood + crystalloid. Coagulation factors, thrombocytes if necessary |
| Hypoalbuminaemia | Albumin |
| Severe hyponatraemia | Hypertonic saline |
| Cerebral oedema | Hypertonic saline |
| Emergency | Physiological saline |
Complications
Complications of fluid therapy are rare. In a person with heart failure or kidney failure, it can cause hypervolaemia and worsening organ failure, possibly even pulmonary oedema.
Excessive infusion of normal saline may lead to hyperchloraemic metabolic acidosis due to the excessive chloride ions forcing bicarbonate to move intracellularly.