The disclosed embodiments relate to an apparatus and method for providing a warning. In one example, an apparatus includes a sensor, which is configured to be coupled to a body of a patient and to output a photoplethysmograph signal, which is indicative of pulse waveforms in the body. The apparatus also includes a processor, which is coupled to process the photoplethysmograph signal so as to identify sequential pulse waveforms in the signal, the processor detecting a cardiac arrhythmia based on identifying a shape feature of the pulse waveform occurring simultaneously with a change in rate or rhythm of the pulse waveforms or an electrocardiographic waveform, and to output a warning responsive to the simultaneous occurrence.
