An apparatus and method for catheter-based sensing and injection at a target site within a patient's body. An injection catheter is equipped with electrodes at its distal end which contact the surface of a target site, such as an AV node of the heart. Electrical signals detected at the target site by the electrodes are fed via leads to the proximal end of the catheter, where they are received by a monitor, such as an EKG monitor. If the electrical signals satisfy predetermined criteria, a needle within the catheter is extended into the target site, and a therapeutic agent is injected into the target site.