Extracellular water
Intravascular (blood) volumeProvision of metabolic substratesElectrolytes
Carbohydrate
Amino acids and protein
Fatty acidsManipulation of acid–base balanceForced acid or alkaline diuresisCoagulopathy correctionPlatelets
Clotting factorsImproving oxygen carriageDiluent for drugsIf caustic
If low solubilityOsmotic effectsManipulation of blood viscosity

Crystalloids

Crystalloids are relatively small molecules that dissociate into ions and form true solutions. In clinical terms, crystalloids pass through the capillary and glomerular membranes easily, but do not readily pass through cell membranes. This situation only applies immediately after administration, as metabolism and membrane pumps soon alter the distribution. The constituents of the commonly used IV crystalloid fluids are shown in Figure 40.2.

Figure 40.2 Constituents and physicochemical properties of crystalloids

	Na⁺	K⁺	Ca²⁺	Cl^–	HCO_3⁻	Osmolality	pH
Sodium chloride 0.9%	150	0	0	150	0	300	5
Dextrose 5%	0	0	0	0	0	280	4
Dextrose 10%	0	0	0	0	0	560	4
Dextrose 4% saline 0.18%	30	0	0	0	30	255	4.5
Hartmann’s solution	131	5	2	111	29	278	6
Sodium bicarbonate 8.4%	1000	0	0	0	1000	2000	8

Ionic concentrations in mmol L⁻¹, osmolality in mOsm kg⁻¹

Water

Water is the essential solvent of all the IV fluids. 1.5 mL kg^–1 h^–1 is a typical requirement for IV maintenance, and this will be the primary determinant of the volume given. The solutes determine how widely the water will be distributed in the body.

Electrolytes

Cations

Sodium is primarily an extracellular ion. Typical sodium requirement is 1 mmol kg^–1 day^–1. In contrast, potassium is primarily an intracellular ion. In concentrations similar to plasma levels, potassium solutions can be given rapidly, but this will have little effect on total body potassium. Replacement of potassium by the IV route should be done slowly and requires careful monitoring when more concentrated solutions are used, to avoid cardiac arrhythmias and cardiac arrest. The addition of potassium to crystalloid solutions such as Ringer’s lactate (Hartmann’s) is an attempt to mimic the electrolyte composition of plasma.

Calcium may be used to improve myocardial contractility when plasma levels are depleted. It is available as calcium chloride 10% (CaCl₂, which contains 0.68 mmol mL^–1 = 680 mmol L^–1 calcium ions). Note that using the combined atomic weight of calcium and two chlorine atoms (MW 111) does not provide the actual molarity of calcium. This is because calcium chloride (dihydrate) has two water molecules closely associated with it in the crystalline form in which it is weighed (CaCl₂.2H₂O). Calcium is also available as the gluconate 10% (0.22 mmol mL^–1). However, this preparation may be negatively inotropic, and can cause coronary vasoconstriction. Calcium may be useful when large amounts of blood and fresh frozen plasma have been rapidly transfused.

Magnesium is important in enzyme systems, and in muscle and neuronal function. Magnesium can be used therapeutically for arrhythmias (especially torsades de pointes), myocardial infarction, eclampsia and pre-eclampsia.

Anions

Chloride is the predominant anion in the body, and is a constituent to maintain the cationic/anionic balance of most IV fluids. Bicarbonate may be used to manipulate the pH of IV fluids. As heat sterilisation destroys the ion and thus produces CO₂, these solutions are sterilised by filtration and are relatively expensive. Phosphorus in the form of phosphate is the main intracellular anion. It is not routinely required in IV fluids but is important in long-term nutrition. Phosphate buffers are an important constituent of blood cell storage solutions.

Solutions

Sodium chloride 0.9% is the correct generic term for the solution isotonic with body fluids. It is also referred to as normal saline. ‘Normal’ in this sense is used to mean that it has the same tonicity as physiological fluids, and twice normal saline refers to sodium chloride 1.8%, which is used in cases of hyponatraemia. Unfortunately, ‘normal’ is an imprecise term that could also be interpreted in this case as 1 molar sodium chloride.

Compound sodium lactate (Hartmann’s) solution uses lactate (another base present in the body) in place of bicarbonate so it can be heat-sterilised. Lactate is readily metabolised in the liver.

Sodium bicarbonate 8.4% solution is used for the immediate correction of metabolic acidosis. Its alkalinity and high osmolality can easily cause tissue damage if small veins are used or if extravasation occurs. It contains 1000 mmol of sodium ions per litre, and this carries a risk of fluid retention, which may be a particular problem if the acidosis is the result of renal failure.

Bags of IV fluid may contain more volume than stated on the packaging. A litre bag of Hartmann’s solution, for example, may contain 1050 mL. This is to offset the priming volume of the giving set, but this clearly only applies to the first bag.

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