Introduction
Fluid management is an essential part of a junior doctor’s practice and hence knowledge of the composition for each intravenous fluid prescribed is essential.
This article discusses the composition of commonly prescribed crystalloids and colloids. More detail on fluid management and prescribing in clinical practice can be found here.
| Na+ (mmol) | K+ (mmol) | Ca2+ (mmol) | Cl- (mmol) | HCO3- (mmol) | pH | Osmolality (mOsm) | |
| 5% Dextrose | 0 | 0 | 0 | 0 | 0 | 4.1 | 278 |
| 0.9% Saline | 154 | 0 | 0 | 154 | 0 | 5.5 | 300 |
| Hartmann’s | 131 | 5 | 4 | 112 | 29* | 6.5 | 281 |
| Normal Plasma | 135-145 | 3.5-5.0 | 2.2-2.6 | 100-110 | 22-26 | 7.35-7.45 | 275-300 |
Table 1 – Compositions of commonly prescribed crystalloids; *present as lactate, converted to bicarbonate by the liver
Crystalloids
Dextrose
5% dextrose solution is a hypotonic (and isosmotic) fluid containing only dextrose and water. Dextrose, the D-isomer of glucose, is rapidly taken up into cells to be metabolised, leaving the remaining free water component to equilibrate across all the body compartments.
Only 7% of the fluid therefore stays in the intra-vascular space. This means that 5% dextrose has no role in fluid resuscitation of a patient, only in fluid maintenance regimes.
The main advantage* of dextrose is being able to maintain hydration without administering an excess of electrolytes, and it can also be prescribed with supplementary potassium if required.
*The energy produced by the metabolism of the dextrose is relatively negligible and should not be considered to have any substantial calorific or nutritional value, dextrose used only as a means of hydration.
Normal Saline
0.9% sodium chloride solution (commonly termed “Normal Saline”) is an isotonic solution containing Na+, Cl–, and water. It equilibrates throughout both the intra-vascular and interstitial spaces (approximately 25% volume within the intra-vascular space) and this makes it useful in both resuscitation and maintenance regimes.
Potassium can be added to the solution too, aiding in electrolyte management. It should not be used as a lone fluid maintenance however, as excessive saline replacement can result in a hyperchloraemic acidosis.
Hartmann’s Solution
Hartmann’s solution is a balanced isotonic solution containing Na+, Cl–, K+, HCO3– (as lactate), Ca2+, and water. Similar to Normal Saline, it distributes in the intra-vascular and interstitial spaces, making it useful for both resuscitation and fluid maintenance.
Hartmann’s solution is considered to be more “physiological” than Normal Saline as it contains other electrolytes in concentrations similar to plasma (see Table). It also contains lactate, which it uses to generate alkalising HCO3– ions.
Hartmann’s Solution and Acidosis
A common misconception is that Hartmann’s solution cannot be used for patients in severe acidosis due to its “lactate”, however it is important to make the distinction between lactic acid (an acid) and lactate (its conjugate base).
In patients in lactic acidosis, lactic acid dissociates to form H+ ions and lactate anions, making serum lactate level useful as an index of the severity of acidosis (as each lactate anion implies the production of a H+ ion).
Hartmann’s solution contains lactate to generate alkalising HCO3– ions and thus, as an alkalising solution, it does not cause an acidosis. Indeed, it can aid in reducing the risk of acidosis with the administration of large volumes in such patients.
However, the presence of lactate anions can interfere with the usefulness of serial lactate measurements as a marker for the treatment of lactic acidosis, therefore there should be a guarded use of lactate as a marker of acidosis if patients have received Hartmann’s solution.
Colloids
Colloids are rarely used in most routine surgical practice. Colloids are solutions containing proteins with large molecular weights, aiming to maintain a high plasma oncotic pressure to keep fluid within the intravascular compartment (in theory an advantage during fluid resuscitation)
However, clinical trials have shown their limited benefit in resuscitation*, and they also come with a small risk of anaphylaxis.
*This is likely due to loss of tight endothelial junctions in critically ill patients, with the proteins subsequently leaching into the interstitium
Human albumin solution (HAS) is still routinely used in patients who are unable to produce sufficient protein (such as decompensating liver disease). By temporarily increasing the plasma oncotic pressures, HAS allows intravascular volumes to be maintained.
Blood products and their prescribing can be found in more detail here.
Key Points
- Intravenous fluids can be divided into crystalloids and colloids
- Crystalloids form the mainstay of fluid prescribing, the most common fluids prescribed being 5% dextrose, Hartmann’s solution, and 0.9% sodium
- Colloids have limited use in resuscitation, showing no advantage over crystalloid