This tracing was recorded a minute later, following the intravenous administration of diltiazem. The first four beats, show the atrial flutter with 1:1 conduction and RR intervals of 0.22 seconds (rate= 272). These are followed by three beats with RR intervals of 0.44 seconds, indicating 2:1 AV conduction. Then, there is a 2.7 second period of high grade AV block during which there are obvious flutter waves having a rate close to 300 but no QRS complexes.