Abstract: A pacemaker is disclosed that includes circuitry for monitoring the current and/or voltage delivered to the heart during a pacing pulse. A microprocessor connects to the monitoring circuitry and analyzes the current (and/or voltage) to determine when capture occurs. When capture occurs, the microprocessor terminates the pacing pulse to save energy. Conversely, if capture does not occur within a predetermined maximum time period, or if the pacing pulse amplitude falls below a predetermined threshold value, the processor immediately causes a safety pulse to fire to insure a regular beating of the heart. By monitoring the current and/or voltage delivered, and by permitting the pacing pulse width to be variable, the pacemaker of the present invention can closely track the pacing threshold of the patient with a minimum expenditure of energy.