how to include inconsistent infusion rate into the model

Dear all, I ask to the experts here concerning my problem. I am beginner in using NONMEM and want to include an inconsistent infusion rate into the model. I collected sample from cancer patient. Baxter infusion pumps were used for 24 – 48 hour continuous venous infusion. I observed that the plasma concentrations varied especially at the end of infusion, probably due to inconsistent infusion rate. Could someone share his experiences with me? Any suggested code for it will be great help. All responses will be highly appreciated, thanks in advance and have a nice weekend ;-) regards, -Tomi Hendrayana Klinische Pharmazie Pharmazeutisches Institut Universitaet Bonn an der Immenburg 4 D-53121 Bonn