Does it matter if you use a different needle size for the Arterial flow than the venous flow (and visa versa). Deep in the back of my memory I think I read that if the arterial needle is bigger than the venous it can cause haemolysis. Is this true? Are there any other problems (presumably higher pressures)

Donna,
If you use a bigger arterial needle (15 ga) and (16 ga) for the venous- you might get the higher blood flow rate with pre pump arterial pressure below -200, but will have a higher venous pressure that will limit the blood flow rate used. If a smaller (16ga) arterial needle is used and a larger neelde is used for the venous (15 ga) then you will have the blood flow restricted by the too negative pre pump arterial pressure of >-200. Too negative of a pre pump arterial pressure can lead to hemolysis (-250). See reference in the Handbook of Dialysis (2nd edition) by John Daugirdas and Todd Ing pg 459-461. Causes are listed as Excessive suction at the arterial access site and High blood flow rates through a small needle.
hope that helps.