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 with the fluid exiting the catheter at the distal portion. The lumen is connectable to an irrigation pump to form a fluid communication therewith. 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 connection to an isolated ground of the electrocardiogram.
