 When ablating tissue near the AV node (a special conduction center that carries electrical impulses from the atria to the ventricles), there is a risk of producing heart block – that is, normal conduction from the atria cannot be transmitted to the ventricles.Freezing tissue near the AV node is less likely to provoke irreversible heart block than ablating it with heat.As in catheter-based procedures, techniques using heating or cooling temperatures may be used to treat arrhythmias during heart surgery.