Improvement in sodium cromoglycate delivery from a spacer device.

The study by Dr O'Callaghan et al (June 1993;48:603-6) is an interesting attempt to study some of the variables affecting drug delivery from holding chambers. Such studies are essential if we are to use these devices optimally, and it could be argued that it is unethical to proceed to radioisotope deposition studies without such basic data. It would, however, be wrong to make general recommendations based on a single study, particularly as previous studies assessing various chambers have shown that a number of factors, including chamber volume and valve design, can alter the dose delivered from a chamber.' 2 Among other variables, this study attempts indirectly to assess the possible effect of static on the dose available from a holding chamber by using an "antistatic" spray. It should be noted, however, that the dose < 5 gm available at one second with this spray is very similar to previously published results with a large volume chamber in which an antistatic spray was not used,' and the dose in particles < 5 gm available at 20 seconds in this study when using the spray is very similar to the dose obtained in the previous study at 30 seconds without a spray. The half life of the aerosol in the previous study was in the region of 15-20 seconds, which is significantly greater than that obtained in this study with the spray. Inspection of images produced with labelled aerosols would also suggest that impaction, influenced by gravity, is the principal cause of drug loss in large volume spacers.3 Similarly, the results with multiple actuations differ in magnitude from previously reported results. In a previous study' the "respirable" dose delivered per actuation when using four actuations in a large volume chamber was reduced, but only to that delivered from a metered dose inhaler alone, suggesting that, for certain chambers, two actuations can be used with little effect on efficiency. It is possible that these discrepancies are due to the choice of chamber or formulation of metered dose inhaler, although my previous unpublished work suggests that the Fisonair performs at least as well as the other large volume spacers available. The formulation of the metered dose inhaler used delivers a relatively high dose and hence generates a denser aerosol than will be the case for most therapeutic metered dose inhalers; this may be relevant to the results. Alternatively, some other unrecognised factor may be influencing these results. Preliminary results with an electrometer suggest that variable static charge can be induced on a chamber by the way in which it is handled, but the relevance of this to clinical practice needs to determined. Reviewing the available evidence, further work is required to clarify these and other related issues. MARK L EVERARD Princess Margaret Hospitalfor Children, Perth, Western Australia 6001