The authors report four observations: three are essentially clinical cases where sustained rate dependent left bundle branch block can induce syncope, where as there is no syncope when the same supraventricular tachycardia at the same frequency has narrow QRS complexes. The fourth case demonstrates the dramatic decrease of arterial electrophysiological slowly accelerated atrial pacing in a patient investigated for a loss of consciousness of unknown origin. The hemodynamic impairment due to intermittent left bundle branch block has been demonstrated even in patients with normal ventricular function. If there are critical hemodynamic events such as during fast supraventricular rhythms occurrence of a left bundle branch block may determinate a dramatic decrease of arterial pressure with syncope. Syncope of supraventricular tachycardias might be induced not only by very fast rate but also by functional left bundle branch block. It might have some interesting applications in the diagnosis of syncope when coexist electrophysiological data of supraventricular arrhythmia substrate and frequency dependent left bundle branch block.