syrup or elixir form when the infant is at risk from necrotising enterocolitis. In some cases the osmolality of the elixir is so high that even mixing with infant formula does not reduce the osmolality to a tolerable level. For example, when a clinically appropriate dose of dexa-methasone elixir was mixed in volumes of formula appropriate for a single feeding for a 1500 g infant, the osmolalities of the mixes increased by at least 300% compared to formula alone (see Table 3.3).

Volatile anaesthetics

The aqueous solubilities of several volatile anaesthetics can be related to the osmolarity of the solution.10 The inverse relationship between solubility (expressed as the liquid/gas partition coefficient) of those anaesthetics and the osmolarity is shown in Table 3.4.

These findings have practical applications for the clinician. Although changes in serum osmolarity within the physiological range (209-305 mosmol dm have only a small effect on the liquid/gas partition coefficient, changes in the serum osmolarity and the concentration of serum constituents at the extremes of the physiological range may significantly decrease the liquid/gas partition coefficient. For example, the blood/gas partition coefficient of isoflurane decreases significantly after an infusion of mannitol. This may be attributed to both a transient increase in the osmolarity of the blood and a more prolonged decrease in the concentration of serum constituents caused by the influx of water due to the osmotic gradient.

Rehydration solutions

An interesting application of the osmotic effect has been in the design of rehydration solutions. During the day the body moves many litres of fluid from the blood into the

Table 3.3 Osmolalities of drug-infant formula mixtures"

Drug (dose)

Volume of drug (cm3)

Mean measured osmolality

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