Abstract: Cardiac monitoring apparatus comprises means for detecting normal and/or ectopic complexes in an electrocardiogram waveform, means for comparing the intervals between successive detected complexes, and means for detecting that successive said intervals differ by more than a predetermined amount. Reversible counters are used in pairs with different counting rates for the counters of each pair to perform the comparing and detecting. Preferably the monitoring apparatus further comprises shifting means arranged to be clocked once for each electrocardiogram complex, whether ectopic or not, and operative to store ectopic indications, and means for indicating that, at any one time, the shift register means contains more than a predetermined number of ectopic indications.