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Letter to the Editor
Editor, – Many thanks for the excellent article about compatibilities of parenteral drug solutions (Aust Prescr 2008;31:98-101), written from a pharmacy point of view. It certainly contains much practical information for everyday clinical practice, but it might be helpful to add a few extra points from a clinical perspective.
Table 1 shows an incompatibility between lignocaine 2% and sodium bicarbonate solution. In practice, however, the two substances make an excellent marriage; the intense stinging of local anaesthetic injections is markedly reduced by mixing the two. The only problem (in practice) is that left to stand for a few minutes, crystals do form and can block fine needles. The practice is well known and has stood the test of decades.
It is also noted that diazepam precipitates in water. Is this really the case or could the cloudiness be an innocent emulsion? In any case, dilute diazepam (for example 10 mg in 10 mL saline) has been given intravenously for years and works very well. It is standard practice and certainly far easier to titrate than the 10 mg in 1 mL in the ampoule.
The article states that phenytoin must not be diluted as it will precipitate. With its extreme pH of 12, intravenous injection of phenytoin is made easier and less irritating by dilution in saline. Although not described in the product information, it is thankfully normal practice. 'Phenytoin - must be diluted in 0.9% saline (rather than dextrose) to avoid crystallization'.1