An algorithm for detection of tachycardias and for discriminating between supraventricular tachycardia (SVT) and ventricular tachycardia (VT) when a 1:1 tachycardia condition is present that can be implemented in an implantable cardiac rhythm management device. Variability measures of AV and VA intervals during the tachycardia are computed and used to distinguish between SVT and VT.