In order to ensure that medications for treating respiratory tract diseases reach the site where their action can be most effective (targeting), the properties of the aerosol must be correct. The sizes of the particles in the aerosol and the spectrum in which these sizes are distributed as well as the mass of the medication that is transported are all fundamental characteristics that determine the regions of the respiratory tract that can be reached and the therapeutic effects that can be achieved. The distribution of active agent quantity over the particle size is thus of primary interest. Two values that can be determined experimentally are useful here: the mass median diameter (MMD) and the mass median aerodynamic diameter (MMAD). Half of the mass of the particles has a smaller diameter than the MMD/MMAD, the other half of the mass consists of particles with a larger diameter than the MMD/MMAD.
The MMD refers to a static aerosol mist and can be determined relatively quickly and easily by laser diffraction. The mass median aerodynamic diameter (MMAD) refers to the behaviour of the particles in an air flow and is therefore an extremely significant value for the purposes of inhalation treatment. It is determined by cascade impaction, the experimental setup for which is considerably more complex than laser diffraction. Since the MMD and MMAD of aqueous solutions are usually identical (e.g. salbutamol), reliable values for MMAD can also be calculated with laser diffraction1. In many cases, a correlation can also be established between MMD and MMAD for suspensions with particles having a significantly different shape and density than water.
Aerosols are generally made up of droplets that have different diameters. One measurement of the distribution of particle sizes is the standard deviation from the MMD/MMAD (GSD = geometric standard deviation). The larger the GSD, the wider the distribution. Aerosols with a GSD up to 1.15 are called monodisperse, if their GSD is greater than 1.15 they are polydisperse. Most commercial nebulisers deliver aerosols with a GSD of about 2.
1) Bitterle et al. . International Society of Aerosols in Medicine (ISAM), 16th International Congress, Tours, France, June 16 – 20, 2007: Correlation of laser diffraction and cascade impaction data upon nebulisation of 1MIU colistimethate sodium/3ml by the eFlow®rapid nebulizer