Flagyl is not typically used for respiratory/lung infections.
It is for anaerobic bacteria which do not typically colonize
the lungs. (those would be air seeking--aerobic instead).
Some dramatic lung problems like collapsed lung which has an abscess might qualify.
But for bronchitis type things, Flagyl is not a typical choice.

Flagyl is used for infected jaw/tooth sockets, for some
hidden sinus ones (when sinuses are blocked no air gets in)
and some prostate problems, and some GI infections.

The fluoroquinolones however, are often commonly chosen, esp Levaquin. I'd ask for something else in this case.
Cipro and Avelox also belong to this family.

The higher dose Zithromax at 500mg a day is often used for bronchitis and especially for the somewhat resistant forms.

(do not use Dextromethophan products (DM) if you are on SSRIs or Cymbalta. You can use plain guaifenesin (Robitussin) or Mucinex safely however).

Flagyl is very bad for PN...but in a life threatening situation, there are not many antibiotics for anaerobic infections. So keep that in mind. In that event it would be a very tough decision.