A method and apparatus for alleviating or preventing hemodynamic collapse by delivering ventricular synchronized atrial pacing under certain conditions precedent to or during delivery of an anti-tachyarrhythmia therapy by an implantable cardioverter/defibrillator (ICD) are disclosed. The atrial pacing mode is preferably the AVT pacing mode, wherein the atria (A) are paced, the ventricles (V) are sensed, and the atrial pacing is triggered (T) by the ventricular sense signals. Alternatively, the ADD pacing mode can be employed if the underlying sinus atrial depolarizations can be sensed occasionally as non-refractory, atrial sense signals and employed to inhibit the delivery of an atrial pace pulse. The VS-AP delay between a ventricular sense (VS) and the delivered atrial pace (AP) pulse synchronization of the atrial pacing pulses can be selected to be a function of the prevailing V-V interval of the VT episode. The prevention or alleviation is accomplished by delivering atrial pacing pulses to the atria in synchronization with detected ventricular sense signals, whereby the atrial depolarization rate is increased and synchronized to the ventricular depolarization rate sufficiently to eject blood from the atria into the ventricles to be ejected therefrom upon a subsequent ventricular contraction.