This series of rhythm strips demonstrates a supraventricular tachycardia with a wide QRS. The SVT stops abruptly after a 6 mg dose of adenosine is given IVP. This confirms the diagnosis of SVT vs sinus tachycardia. When the tachycardia stops, a sinus rhythm resumes, and the QRS is now narrow. This confirms that the bundle branch block was rate-dependent. This is caused by differing refractory periods in the fasicles - the right bundle branch was able to recover its function quickly during the tachycardia, but the left bundle was not able to keep up at that rate.