What are fixed pacemakers?

Fixed rate. I assume you mean fixed rate pacemakers. We don't use them anymore. The very earliest pacemakers just fired at whatever rate you set them, once a second if you set the rate to 60. That lead to chaos if the patient's inherent heart rate was not always real slow. So they made pacemaker's which sense normal beats and only fire if the rate gets slow. They are inhibited when a heartbeat is sensed.
Fixed rate pacemaker. These are very old style pacemakers that pace regardless of the intrinsic rate of the heart. Almost all pacemakers are demand pacemakers that only pace when the heart rate gets below a set threshold with sensing. Demand pacemakers can sometimes be temporarily programmed to fixed mode with a magnet applied. Pacers near eol may revert to fixed.