An implantable demand-inhibit type cardiac pacer for producing artificial stimulating pulses of a desired repetition rate and of a desired pulse width in the absence of normal R-wave activity in the patient's heart. The pacer is implemented principally from integrated circuit digital logic components and includes programmable registers, which when addressed by an externally applied code word of a predetermined format, allows the pulse width as well as the pulse rate of the pacer pulses to be changed to different desired values. Also incorporated in the pacer of this invention is circuitry which allows testing of the implanted energy source so that a physician can readily determine whether the energy source has discharged to a point where replacement is indicated.
A dual refractory period for paced beats and normal beats is provided along with digital circuits for insuring proper operation even in the presence of extraneous noise signals. Furthermore, fail-safe circuitry is provided to insure that the patient's heart will be properly stimulated even where, through a programming error, an unacceptable pacer pulse width or rate is entered into the programmable registers.