a cardiac electrotherapy system has flow velocity measurement capability for measuring a velocity of blood flow at a region of a tricuspid valve of a heart . a pacing system is provided which outputs a pacing electrical signal to the heart . a control unit which is responsive to a measured flow velocity from the flow velocity measurement unit detects heart irregularities and controls electrical pacing signals to the heart . in detecting heart irregularities , peak flow velocity waveforms are detected and analyzed . the flow velocity measurement employs a doppler ultrasonic transducer which is mounted on a cardiac pacing lead in spaced relation to a pacing electrode at a distal end of the lead . when the pacing lead is inserted in the heart , the pacing electrode is placed at an apex of the right ventricle and the piezoelectric doppler transducer is positioned at or near the tricuspid valve . the flow velocity transducer is preferably formed as an annular piezo body having associated electrodes . an ultrasonic lens directs ultrasonic rays from the transducer and an ultrasonic wave inhibitor structure prevents transmission of ultrasonic waves in an undesired direction . with the cardiac electrotherapy system disclosed , improved and more reliable monitoring of cardiac activity is achieved and a corresponding improved pacing results .