Ventricular couplets but not isolated extrasystoles induce a "female" sustained, supraventricular tachycardia. What is the mechanism?

This is the case of a 28-year-old woman with sustained supraventricular tachycardia induced only by monomorphic ventricular couplets but not by isolated extrasystoles. The careful examination of electrocardiogram (ECG) was able to identify the mechanism of the tachycardia in the absence of electrophysiologic investigation. The tachycardia resulted sustained and symptomatic only during periods.