This disclosure is directed toward techniques for classifying a tachycardia as supraventricular tachycardia or ventricular tachycardia. A method comprises detecting a tachycardia based on at least one value of a cardiac interval, delivering vagal stimulation in response to the detection of the tachycardia, sensing a physiological parameter other than the cardiac interval during or subsequent to delivering the vagal stimulation, and classifying the tachycardia as supraventricular or ventricular based on the sensed physiological parameter. In some examples, the method includes sensing a response of a physiological parameter other than cardiac rate to the vagal stimulation, such as pressure or a morphological characteristic of the cardiac electrical waveform. The method may include providing an indication to a user based on the classification of supraventricular tachycardia, or delivery of appropriate electrical therapy based on the classification of ventricular tachycardia or ventricular fibrillation.
