An implantable tachyarrhythmia control system includes a patch electrode having a sensor integrated therein, such as a piezoelectric sensor, which is capable of monitoring mechanical heart activity. When the patch electrode is sutured to the cardiac tissue, the piezoelectric sensor will be deformed due to the mechanical activity of the heart muscle, and will generate a corresponding electrical signal. The electrical signal from the sensor will exhibit relatively low-frequency periodicity and relatively low amplitude during normal heart activity. In the event of tachycardia or fibrillation, the signal will exhibit excursions beyond those occurring for normal heart activity, and will consequently have a higher energy content. The signal is thus an indicator for the onset of these cardiac events. The signal can be supplied to an implantable defibrillator and can be used as a primary or secondary trigger for initiating defibrillation therapy, such as one or more defibrillating pulses. The signal may alternatively be supplied to an implantable combination pacemaker/defibrillator, which also undertakes standard R-wave detection via a conventional pacing lead. The signal from the patch electrode sensor can be used as before as a primary or secondary trigger to initiate defibrillation therapy alone or in combination with the detected R-wave signal. The patch sensor signal may also be used, alone or in combination with the R-wave signal, to initiate a change in the pacing mode or to initiate a particular pacing therapy, such as an antitachycardia sequence.