Sustained, paired electrical stimuli: Slowing of the ventricular rate and augmentation of contractile force

When the ventricle is electrically stimulated immediately after the termination of the absolute refractory period, a propagated depolarization occurs which does not result in a clearly defined secondary rise in intraventricular pressure. In 19 dogs, two fundamental consequences of such paired electrical stimuli were studied. Since the second depolarization is followed by a second refractory period, the time during which the ventricle is unresponsive to other electrical stimuli is effectively doubled. Rapid ventricular rates could thus be slowed by paired stimuli, regardless of the mechanism of tachycardia. In each of 16 dogs, spontaneously occurring sinus tachycardia was suppressed by paired electrical stimulation. In 8 animals, ventricular tachycardia was induced by digitalis intoxication and in 6 by means of exogenous stimulation with a separate electrical pacemaker; paired stimuli slowed the ventricular rate in every animal by an average of 57 beats per min. Striking augmentation of the ventricle's contractile response was found to be a second consequence of paired stimulation. This phenomenon was demonstrated in 8 dogs in which right ventricular contractile force was measured with a strain gauge arch. Paired electrical stimuli increased ventricular contractile force to an average of 275 per cent of the levels observed with single impulses delivered at the same rate. Preliminary studies in man indicate that slowing and augmentation of contractile force also occur when the human heart is stimulated in a similar manner. Appropriately timed, paired electrical stimuli to the ventricles result in profound alterations in both the electrical and contractile properties of the mammalian heart. It is possible that this technic will be applicable and beneficial in certain patients, when a slower heart rate and/or augmented myocardial contractile force is desirable.