Answer for February 5, 2013
This is atrial flutter with 4:1 (AV nodal) block, usually called just "4:1 block". The flutter waves are apparent at the typical rate of around 300. The AV junction does not normally conduct impulses arriving this fast. Typically, it conducts with 2 atrial impulses to one transmitted impulse (2:1), 4:1, and sometimes 3:1. The nomenclature describes the ratio of atrial:ventricular complexes. With an atrial rate of 300, 4:1 would be expected to to give a rate of 75 (300/4=75) as seen here. The "missing" flutter wave on the V1 strip is buried in the QRS complex, but don't forget to count it! AV nodal block with atrial flutter is physiologic - a ventricular rate of 300 would be a disaster for most!
NOTE: EKG's are for internal educational purposes of the University of Massachusetts Department of Family Medicine. Please do not forward without permission from Dr. Golding!