The diagnostic unit 1 consists of a microprocessor-based control section 5, a data memory 6 and a pacemaker 7. The control section 5 monitors heart action using the intracardiac electrogram and a second signal representative of intracardiac pressure. It detects events of interest to clinicians including bradycardia, tachycardia, cardiac pauses, pressure pauses, interference and pacing pauses, and records the numbers of each type of event. Selective recordings of the waveforms of the sensed signals are made in memory within the unit whenever an event of interest is detected. The unit is worn by the patient for periods of up to three weeks, and the parameters of the events to be sensed can be programmed into the unit by the clinician using a computer 4. The same computer 4 is used to extract and display the recorded data.