Programmable multi-mode cardiac pacemaker

A programmable cardiac pacemaker that is selectively programmable via a radio frequency communications link into one of a plurality of a single and dual chamber pacing modes. Depending upon the mode selected, an associated plurality of pacing parameters may be programmed, each within a predetermined range of possible magnitudes. The pacemaker includes a rate smoothing function for preventing the rate interval from changing by more than a predetermined percentage on a cycle-to-cycle basis; and a graceful degradation function for causing the ventricular pacing rate to decay when the atrial rate exceeds a programmed upper rate limit for a predetermined length of time. A graceful degration onset time and rate of decay are also programmable. Additionally, the pacemaker contains a set of pre-programmed alternative pacing parameters for at least one mode and which can be independently reverted to. The pacemaker control circuitry is organized about a central bus into an I/O controller and a pacing controller. Pacing control is continuous and its I/O communications operate on a prioritized basis. I/O control permits the programming of any register and the interrogation of a variety of analog parameters associated therewith.