Crystalloids are substances that form a true solution and pass freely through semipermeable membranes. They contain water, dextrose, and electrolytes, and stay in the intravascular compartment for about 45 minutes.

Crystalloids pass easily through capillary and glomerular membranes but, although they do not diffuse through cell membranes, membrane pumps and metabolism soon alter their distribution. Their composition varies depending on the type of solution. Sodium is the particle responsible for plasma volume expansion and this determines the initial distribution of the crystalloid (isotonic solutions will distribute throughout the ECF where sodium is found, but hypotonic solutions, which contain more water, will distribute in cells as well). Dextrose in water is basically free water as it does not contain sodium, and distributes to both the intracellular fluid and extracellular fluid. Dextrose is not an effective plasma expander as inadequate amounts reach the intravascular compartment, and it should never be used in volume resuscitation.

Infusion of 09% saline or colloids suspended in 09% saline can produce a dose-dependent hyperchloraemic metabolic acidosis. The serum sodium is often normal. Calculation of the anion gap is useful as it is normal in a hyperchloraemic metabolic acidosis but raised in lactic acidosis. The urinary chloride will be high in the former and low in the latter. The clinical significance of saline-induced hyperchloraemic metabolic acidosis is unknown.

Crystalloids are used to restore extracellular electrolyte and volume deficits, are well tolerated, and are usually administered peripherally. Other advantages of crystalloids include low cost, safety, easy storage, and availability. The primary limitation of crystalloids as replacement solutions is the amount of fluid needed to replace plasma volume. It is estimated that 5 litres of crystalloid would replace 1 litre of blood because only one-fourth of the volume reaches the circulation. Using crystalloids as volume expanders has theoretical disadvantages. The subsequent increase in interstitial volume could impair wound healing and gas exchange. The most commonly used crystalloids are:

• Hartmann's solution - resembles the extracellular fluid. (Alexis Hartmann was a US paediatrician. Hartmann's solution was used in sick children with a metabolic acidosis because the lactate is metabolised to bicarbonate within a few hours. It is avoided in certain patients: renal failure because of the risk of hyperkalaemia, liver failure because of the risk of lactic acidosis, and diabetes because of lactate metabolism to glucose. The calcium content of Hartmann's means that it may form clots if mixed with stored blood (which contains citrate) in the same intravenous line.)

The electrolyte content of common crystalloids in mmol litre-1 (mg dL-1) is shown in Table 5.5.

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