[R] Expression for pedices

[Cressoni, Massimo (NIH/NHLBI) [F]]

I know that this maybe a trivial question. I am not able to plot pedices in graph axes.
Instead I am able to plot different math symbols :

XLABEL <- expression(paste(cmH,lim(f(x), x %->% 0),"O PEEP"))
works well

XLABEL <- expression(paste(cmH,[2],"O PEEP"))
is considered a wrong expression.

Thanks