An ultrasonic implantable device includes an ultrasonic sensor having a plurality of transducers. The sensor is configured for mounting to a vessel wall. A first of the transducers directs sound waves in a direction at least partially upstream or downstream in the vessel. A second of the transducers directs sound waves in a radial direction through an interior of the vessel against a sidewall of the vessel. The sensor monitors a change in frequency of the sound waves from the first transducer to determine a fluid velocity in the vessel. The sensor also monitors a reflection time of the sound waves from the second transducer that return from the sidewall to determine an internal diameter of the vessel. The determined fluid velocity and vessel diameter can be used to determine a volumetric flow rate of the fluid in the vessel.