A mathematical model of aerosol holding chambers

A mathematical model of aerosol delivery from holding chambers (spacers) wa s developed incorporating tidal volume (V-T), chamber volume (V-ch), appara tus dead space (V-D), effect of valve insufficiency and other leaks, loss o f aerosol by immediate impact on the chamber wall, and fallout of aerosol i n the chamber with time. Four different spacers were connected via filters to a mechanical lung model, and aerosol delivery during "breathing" was det ermined from drug recovery from the filters. The formula correctly predicte d the delivery of budesonide aerosol from the AeroChamber (Trudell Medical, London, Ontario, Canada), NebuChamber (Astra, Sodirtalje, Sweden) and Nebu haler (Astra) adapted for babies. The dose of fluticasone proprionate deliv ered by the Babyhaler (Glaxco Wellcome, Oxbridge, Middlesex, UK) was 80% of that predicted, probably because of incomplete priming of this spacer. Of the above-mentioned factors, initial loss of aerosol by impact on the chamb er wall is most important for the efficiency of a spacer. With a V-T of 195 mt, the AeroChamber and Babyhaler were emptied in two breaths, the NebuCha mber in four breaths, and the Nebuhaler in six breaths. Insufficiencies of the expiratory valves were demonstrated by comparison of pressure flow curv es during "inspiratory" flow with and without occluded expiratory openings. Insufficient inspiratory valves were demonstrated by comparison of "expira tory" pressure flow curves with and without occluded inspiratory openings. With children breathing through the spacers, mask pressure variations were generally on the same order as that seen with the mechanical respirator, su pporting the clinical relevance of the in vitro findings.