Why is there such a big difference in the sizes.A Shiley size 6, seems huge compared to a size 7 Portex blue blue fenstrated trach. You almost need a size 8 Portex to replace a size 6 Shiley trach, if you don't want a lot of air leaks.
I don't know if it is because of the difference in the country of origin or what, but with the two trachs, (and all of them) you need to watch I.D./O.D. and length, more than the size if changing types of trachs. The size of the cuff is also quite different. the Shiley is longer and with a "square" structure, where the portex is smaller and has more of a round structure. Hope this helps.
With Shiley, the size 6 or 8 has often been just that a size, not the ID (depends on the model). With Portex the size listed is in mm, but is the ID without the inner cannula in place, decrease by 1 mm with IC in. Watching ID and OD factors are key to matching the trach with the established stoma. As to why the differneces...my guess is manufacturer's preference.