Methods and systems of catheterization include a flexible catheter adapted for insertion into a heart of a living subject. The catheter has a lumen for passing an electrically conductive fluid therethrough, which is propelled by a peristaltic pump. A fluid reservoir connected to the lumen supplies the fluid to the catheter. Electrocardiogram circuitry is connectable to the subject for monitoring electrical activity in the heart. An electrically conductive cable diverts induced charges in the fluid from the catheter electrodes, for example by shorting to a rotating element in the peristaltic pump.
