Pain during dialysis in artery leg graft

My father has pain in just the artery during dialysis. It doesn’t happen until about 1/2 way through the treatment (about 1 hour). The pain comes and goes. It’s always in the artery. He’s had the graft in his leg since Feb. 4, 2008. We had to start using it after only 3 weeks from surgery due the graft in his arm failing.

Any ideas on what would be causing the pain or what would help eliminate it?