The present invention relates to a method and apparatus for delivering two or more individual low-level pulses to correct certain cardiac arrhythmias such as high-rate ventricular tachycardia and ventricular fibrillation; at least the latter shocks are delivered in synchronism with a repeatable characteristic of the heart's electrical activity. Basically, the apparatus comprises an electrode sensor placed in the right ventricle of the heart or in the coronary sinus to act as a detector for the electrical activity of the heart. Connected to the electrode sensor is an arrhythmia detector chosen to detect such arrhythmias as ventricular tachycardia and ventricular fibrillation. Upon detection of one of these arrhythmias, the detector issues a signal which is interpreted by a programmable logic device to activate an energy storage device for delivering a first low-level shock to the heart through a shock delivering electrode. A slew rate detector is then activated by the logic device. The output of the slew rate detector is fed to a comparator to be compared with a continually updated reference signal supplied by the logic device. When the slew rate output equals or exceeds the value of the reference signal, the comparator issues a signal which activates the energy storage device to issue a second low-level shock to the heart through the shock delivering electrode. Additional shocks may be delivered in a manner similar to the delivery of the second shock under the control of the logic device.