A system for monitoring blood flow in a patient comprises a first unit having an ultrasound transducer and a fastener for fastening the unit to the patient. A controller subsystem comprises the first unit and a separate second unit. The controller subsystem is configured to: control the ultrasound transducer to transmit plane-wave pulses into the patient in a propagation direction; sample reflections of the plane-wave pulses, received at the ultrasound transducer, from a region within the patient, to generate pulse-Doppler response signals; and process the pulse-Doppler response signals to estimate a series of values, over time, of a measure proportional, but not equal, to the total blood volume flow passing through the region. A monitoring subsystem is configured to monitor the series of values over time and to generate a signal if a set of one or more of the values satisfies a predetermined criterion.