Abstract: The invention provides a method of monitoring blood flow in a vertebrate animal subject. Unfocussed plane-wave ultrasound pulses are transmitted into the subject, along a transmission axis, from a single-element ultrasound transducer (2) fastened to the subject (5). Reflections of the ultrasound pulses are received, generating a succession of pulse-Doppler response signals over time. Each pulse-Doppler response signal is processed to determine a first respective spatial-maximum velocity value for blood flowing towards the single transducer element (2), and a second respective spatial-maximum velocity value for blood flowing away. Heartbeats are identified from said spatial-maximum velocity values and a quality metric is assigned to each identified heartbeat. A subset of the spatial-maximum velocity values is identified for which the assigned quality metric exceeds a threshold level.