An implantable cardiac pacer is instructable to vary the stimulation pulse interval, the stimulation pulse width, the stimulation pulse amplitude, the pacer sensitivity, the refractory period and the mode (demand or asynchronous).
A master parameter control circuit is provided to vary selected parameters of the heart pacer in accordance with an externally applied control signal.
An external control unit produces two sequential series of pulses, the first series being an access comprising at least one digital "0" state which must be detected by an access code detecting circuit within the master parameter control before the following parameter code will be accepted to vary the pacer parameters. The parameter code, generated subsequently to the access code, determines the selected set of pacer parameters, and can be selected from external controls upon the external control unit.
The master parameter control circuit includes means for recognizing the access code to enable the master parameter control circuit to only thereafter accept the selected parameter code. The entire parameter code must then be entirely received prior to changing the pacer parameters.