Types of fluid available
IV fluids can be categorised according to their physical composition:
Crystalloids are solutions of small molecules in water (eg, sodium chloride, glucose, Hartmann’s)
Colloids are dispersions of large organic molecules (eg, Gelofusin, Voluven)
Fluids can also be categorised according to their mechanism of distribution in the body or their electrolyte loads.
The different types of fluid distribute into the various fluid compartments in different ways (see Figure 1 in the feature “Intravenous therapy – what pharmacists need to monitor”). In general, colloids remain in the intravascular space, while crystalloids distribute more readily into other tissues.
Sodium chloride (NaCl) distributes into the extracellular space (intravascular and interstitial spaces). Glucose solutions distribute throughout the intravascular, interstitial and intracellular compartments.
Glucose solution, at a concentration of 5 per cent, has the same tonicity as plasma and is used for fluid therapy. Hypertonic solutions of glucose (10 per cent or 50 per cent) are used when glucose substitution is required (eg, to treat hypoglycaemia).
Hypo- and hypertonic solutions of NaCl are also available, but their use is limited. Hypotonic NaCl is used to treat hypernatraemia. Hypertonic NaCl is sometimes used to correct hyponatraemia, and very strong solutions are used to manage aspects of head injury. Careful monitoring is required for these uses.
Colloid solutions
The characteristics of colloid infusions depend mainly on their molecular size. Many modern colloid solutions are based on hydroxyethyl starches (HESs) which have high molecular weights (70,000–450,000 daltons) and can provide volume expansion for 6–24 hours. The solutions’ duration of action depends on its starch’s molecular size (larger molecules tend to have a longer duration), the rate of degradation and the permeability of the vascular endothelium.
Tetrastarch (40 per cent substituted HES), with a mean molecular weight of 130,000 daltons, exerts its effect for 4–6 hours. Modified fluid gelatine, which is derived from animal collagen, has a molecular weight of 30,000 daltons. Its effective half life is about four hours, but its volume-restoring effect may be shorter in patients with capillary leakage.
Choosing a fluid
Deciding which fluids are appropriate for each patient depends on the type of fluid that has been lost and the body compartment(s) that require additional volume. Patients’ renal function, cardiac function, blood gases and electrolyte levels also need to be considered, where available.
For a patient requiring fluid maintenance who has healthy kidneys and no co-morbidities that affect fluid homeostasis, a suitable regimen will be a combination of a glucose-based IV fluid and a second fluid to boost intravascular volume (usually a sodium-based fluid).
The latter will need to provide 1–1.5mmol/kg sodium and 1mmol/kg potassium per day. Calcium and magnesium supplementation should be considered if oral intake is interrupted for more than a few days and should be guided by plasma level measurements.
Often this will be provided as a combination of NaCl 0.9 per cent and glucose 5 per cent infusions, or as “dextrose-saline” (usually 2.5–3L of a combined infusion of glucose 4 per cent and NaCl 0.18 per cent over 24 hours).
This dextrose-saline solution is not recommended for long-term maintenance because it provides less than the required daily amount of sodium, unless excess volume is administered. Also, it is only slightly more efficient than plain glucose infusions at restoring intravascular volume, so the required extra volume would increase the risk of interstitial oedema.