In a comparative trial of seven different spacer devices published in Thorax (1996;51:835-40) researchers found that whereas one device could deliver 124% of the dose recovered using a metered dose inhaler alone, another may deliver as little as 21% of the same drug.

Overall, large volume spacers performed appreciably better than the smaller spacers tested with sodium cromoglycate and salbutamol.

A valved chamber with exhalation ports is useful to prevent exhalation into the chamber and allow inhalation of drug remaining in the chamber after the first breath in.
A space mask that fits snugly on the nose is what is preferred be it a round mask or anatomical mask.

Ref: BMJ 1996;313:320 (10 August)