Despite the current availability of physiologic pacing systems, there remain distinct disadvantages associated with their operation. Although in theory the atrial synchronous pacemaker (VDD or DDD) is the ideal system, in practice it performs satisfactorily over a very limited range of heart rates in a limited number of individuals. We describe the TX system which incorporates ventricular pacing and is driven by T wave sensing determined by the QT interval. This pacemaker can distinguish between physiologic and pathologic tachycardias while providing an almost unlimited rate response even in the presence of an unresponsive atrium.