A cardiac assist system which provides simultaneous cardiac assistance during a confirmed cardiac arrythmia, but which provides for the inhibition of the cardiac assistance in the event battery voltage drops below a predefined point. In particular the present invention features a battery voltage safety limit during the deliver VF therapy state. During VF therapy state the system provides skeletal muscle stimulation while charging a defibrillation output capacitor. The battery voltage safety limit is set above a threshold which will insure proper device functionality, i.e. above the power-on-reset threshold (hereafter "POR"). If the battery voltage safety limit is reached the system inhibits the skeletal muscle stimulation to thereby avoid reaching the POR threshold, but yet still maintain the charging of the defibrillation output capacitor. Once charging of the defibrillation output capacitor is completed a synchronization routine is begun and defibrillation therapy is delivered. In such a manner the present invention prevents the dual demands of skeletal muscle stimulation along with charging of the defibrillation output capacitor to cause the battery voltage to collapse and drop below the power-on-reset threshold.