This EKG shows a sinus rhythm with ventricular bigeminy and retrograde conduction leading to retrograde depolarization of the sinus node, resulting in a longer pause (sinus node reset).
Then, a polymorphic ventricular tachycardia occurs over 7 beats. The QT interval of the sinus beats does not appear prolonged, thus ruling out Torsades de Pointes tachycardia. The most likely cause of this type of polymorphic ventricular tachycardia during a stress EKG is cardiac ischemia/coronary artery disease.