A cardiac monitoring device comprising a wearable unit comprising: a substrate; means for measuring ECG signals; data processing means comprising a data processor; and a wireless transmitter; wherein the data processing means is sealed within the substrate and arranged to receive the measured ECG signals, to sample each ECG signal associated with a heartbeat, to process the sampled signal data in the data processor to extract key data relating to that heartbeat, and to provide the key data to the wireless transmitter for transmission; and wherein the processor is programmed to apply a time window to the sampled data to isolate at least a portion of the data associated with a given heartbeat and measure the amplitude changes in that window, and to use the amplitude changes to identify the key data, characterised in that the key data comprise a time stamp of an R-peak in a QRS complex in the ECG signal and at least one further time or amplitude value corresponding to one or more pre-defined features of the ECG signal; wherein the at least one further time or amplitude value corresponding to the one or more pre-defined features of the ECG signal is chosen from one or more of: (i) R-S amplitude; (ii) temporal position of a P-wave; (iii) amplitude of a P-wave; (iv) amplitude variations in a P-wave; (v) temporal position of a T-wave; (vi) amplitude of a T-wave; (vii) amplitude of an S-T segment; and (viii) temporal extent of a Q-T interval; and wherein the data processor can be re-programmed during use by an instruction received by the wireless transmitter to change the one or more pre-defined features and the key data identified and provided to the wireless transmitter for transmission. a substrate; means for measuring ECG signals; data processing means comprising a data processor; and a wireless transmitter; wherein the data processing means is sealed within the substrate and arranged to receive the measured ECG signals, to sample each ECG signal associated with a heartbeat, to process the sampled signal data in the data processor to extract key data relating to that heartbeat, and to provide the key data to the wireless transmitter for transmission; and wherein the processor is programmed to apply a time window to the sampled data to isolate at least a portion of the data associated with a given heartbeat and measure the amplitude changes in that window, and to use the amplitude changes to identify the key data, characterised in that the key data comprise a time stamp of an R-peak in a QRS complex in the ECG signal and at least one further time or amplitude value corresponding to one or more pre-defined features of the ECG signal; wherein the at least one further time or amplitude value corresponding to the one or more pre-defined features of the ECG signal is chosen from one or more of: (i) R-S amplitude; (ii) temporal position of a P-wave; (iii) amplitude of a P-wave; (iv) amplitude variations in a P-wave; (v) temporal position of a T-wave; (vi) amplitude of a T-wave; (vii) amplitude of an S-T segment; and (viii) temporal extent of a Q-T interval; and wherein the data processor can be re-programmed during use by an instruction received by the wireless transmitter to change the one or more pre-defined features and the key data identified and provided to the wireless transmitter for transmission.