Patient's clinical data:  47-year-old black man.  Another serial ECG (seen below) showed a supraventricular tachycardia (SVT) at a rate of 164/min that was diagnosed by an Electrophysiologist as atrioventricular nodal reentrant tachycardia (AVNRT).  How does knowing the mechanism of AVNRT help you in determining the nature of the mechanism in this week's ECG.