A medical procedure for treatment of tachycardia (rapid heartbeat) or cardiac disrhythmia uses a catheter which includes a flexible coaxial transmission line (coax) terminated by an antenna. The antenna and coax are introduced into a chamber of the heart. The antenna is brought into contact with a wall of the heart. Action potentials generated by the heart are coupled through the antenna and the coaxial cable to a standard electrocardiograph apparatus for display. Other electrodes placed about the body also produce action potentials which are displayed by the electrocardiograph. The position of the antenna in the chamber of the heart is adjusted with the aid of the displayed action potentials until the antenna is in contact with the region to be ablated or injured as indicated by its characteristic electrical signature. When the antenna is adjacent to or in contact with the desired location, radio frequency or microwave frequency electrical energy is applied to the proximal end of the coax and through the coax to the antenna. The action potentials may be viewed while the electrical energy is applied. The power of the electrical energy is slowly increased until the desired amount of blockage of the bundle of His or damage to the ectopic focus has been achieved.