In Europe (except for Cyprus, Gibraltar & Malta), the voltage for the nebuliser is the same as the UK (220v) and a standard travel plug adapter is all that is needed. If travelling to the USA, South America, the Caribbean, Cyprus, Gibraltar & Malta, you will need a 110v nebuliser. A plug adapter is not enough.
Discuss this with your Physiotherapy Department well in advance of the holiday and they may be able to lend you one. Alternatively, they are hireable in the USA from medical equipment companies.