A body stimulation system including external components for generating and transmitting programming signals and implantable components including a signal generator with at least one alterable operating characteristic, a stimulation signal delivering system and circuit responsive to receive programming signals for establishing the operating characteristic in predetermined correspondence therewith. The stimulation signal delivering system is connected to the operating characteristic establishing circuitry for receiving the programming signals. In a preferred embodiment, the external components are prevented from transmitting programming signals during a stimulation signal and, more preferably, for a predetermined period following a stimulation signal. The predetermined period may be established such that the external system is activated during the refractory period of the tissue being stimulated. The external system may provide a programming signal and a second signal having characteristics discriminable from the characteristics of the programming signal such that the operating characteristic establishing circuitry is responsive to a received programming signal only during the occurrence of the second signal. The pulse generator of the implantable unit may also respond to the second signal to operate at a fixed rate during the second signal.

There is disclosed an external pacer for operating together with an improperly functioning implantable pacer. The external pacer operates in the demand mode; as in the case of an implantable demand pacer, when a heartbeat is detected the generation of a stimulating pulse is inhibited. However, the external pacer also includes a mechanism for sensing a stimulating pulse generated by the implantable pacer. When such a pacer pulse is detected, the external pacer immediately generates a pulse which reinforces the pulse generated by the implantable pacer.

A demand pacemaker having different standby and pacing rates and circuitry for selectively rendering the standby and pacing rates substantially identical. First and second capacitors control the standby and pacing rates, respectively, one capacitor being responsive to either a natural heartbeat or a pacemaker output pulse to alter the rate established by the other. In a preferred embodiment, the second capacitor is charged during a pacemaker output pulse and discharges through the first capacitor to decrease its charging time, the charging time of the first capacitor controlling the pacemaker output pulse frequency. Circuitry responsive to externally generated signals causes the second capacitor to charge during both a natural heartbeat or a pacemaker output pulse to render the pacemaker standby and pacing rates substantially identical. In this manner, the pacemaker of the present invention may be employed to monitor heart activity at a first, or standby, rate while providing a pacing function at a second, higher rate, on demand, until such time as the necessity for the pacing function is established with the standby rate then being modified to conform to the pacing rate.

An energy recovery system for an electrotherapy device, particularly for bone healing, which permits the device to be small, light and portable, and to have extended battery life. Included are a drive voltage source and a ground reference potential. Driving transistors are between an inductive load and, relatively, the voltage source and ground potential, with a storage capacitor connected between the latter two. The voltage source is commonly a battery, preferably a zinc-air battery. Induction load power is applied when both transistors turn on simultaneously. For energy recovery, diodes are connected between the first terminal of the inductive load and the ground potential and between the second terminal of the inductive load and the high voltage source, such that they are reversed biased when the drive transistors are conducting. When drive power is removed from the inductive load by switching off the drive transistors, a reverse EMF is established in the inductive load. As the inductive load magnetic field collapses, a reverse voltage is developed across the inductor higher than the applied voltage by an amount sufficient to forward bias the diodes. Thus, current flows from the inductive load through the diodes to the storage capacitor. Recovery of the energy stored in the inductor continues until the energy remaining is insufficient to maintain current flow to the storage capacitor.