Hi Ian,
I had a long telephone conversation with the nurse when I was in the car last week,sorry can't remember his name .
My suggestion, given that I'm not aware of anything which would detect the VND, was to ensure the needle wasn't dislodged using a system that is in use and documented on a published paper by the Renal Unit at Nottingham using a Rotoclix system which we used to sell and still have a few bits left which I'm happy to send FOC to try for him.
Basically it's a velcro wrist/armstrap with a 5mm diameter plastic clip to hold the needle/s tubing securely in place whilst he's asleep.
https://www.nuh.nhs.uk/download.cfm?doc=docm93jijm4n547Let me know if they want to try it.
BR
Chris
www.healthtec.co.uk