My drug book (Taber's) says "Administer over at LEAST 30 minutes"....so I guess, you could give it over 4 hours...I've never heard of it.
The peak is "end of infusion", duration is 4-6 hrs. So if you did give it over 4 hours, it would, seemingly, have a higher serum concentration. But I'd think it could be renal toxic, since it seems to be fairly toxic to the kidneys at the recommended doses and times.
PDR 2008: "Zosyn should be administered by IV infusion over 30 minutes. The usual total dose of Zosyn for adults is 3.375g every 6 hrs totaing 13.5g."
I think it's a fairly new evidence-based practice. New, as in fall 2009. I think it originated per pharmacy recommendations. I'm just happy my facility hasn't created this a mandatory practice.
My old facility had us start a dedicated IV for the Zosyn infusions- especially if there were a bunch of other IVPBs. Practically NOTHING is compatible with Zosyn, and having it hung for 4hrs, they're practically like a continuous infusion. It definitely caused quite a bit of drama.