Monophasic versus biphasic cardiac stimulation: mechanism of decreased energy requirements

The purpose of the present study was to examine the effects of monophasic and biphasic stimulation under conditions of full and incomplete repolarization in an in vivo dog model and in an in vitro rabbit ventricular single cell model. Strength-interval curves were constructed with monophasic cathodal stimulation and biphasic subthreshold anodal followed by cathodal stimulation in dogs prior to and late after left anterior descending coronary artery occlusion. At the monophasic absolute refractory period plus 10 msec, less cathodal current was required for biphasic compared to monophasic stimulation (P = 0.04). Moreover, the biphasic absolute ventricular refractory period (116 +/- 8 msec) was significantly shorter than the monophasic absolute ventricular refractory period (136 +/- 15 msec) (P less than 0.02). At coupling intervals greater than 30 msec after the monophasic absolute ventricular refractory period, there was no distinction between monophasic and biphasic stimuli. Similarly enhanced excitability was observed with biphasic stimuli in infarcted hearts. Voltage clamp measurements mimicking conditions of the in vivo studies demonstrated that when repolarization is incomplete, a hyperpolarizing prepulse reactivates additional sodium current resulting in enhanced excitability. In conclusion, biphasic stimulation consisting of a hyperpolarizing anodal prepulse followed by a cathodal pulse decreases the current required for excitation compared to cathodal monophasic stimulation in a critical zone near the ventricular absolute refractory period.