The correct interpretation of this patient’s ECG is atrial flutter with variable atrioventricular block. Atrial flutter is a macro re-entrant supraventricular arrhythmia arising in the right atrium and usually (but not always!) identified by saw-tooth–appearing flutter waves.
The atrial rate in atrial flutter typically ranges from 200 to 350 beats/min. The QRS appearance will be narrow and similar to that of sinus rhythm, because conduction occurs normally down the atrioventricular node unless there is aberrant conduction.
The ventricular rate is dependent on the ability of the node to control rapid conduction. In this case, there appear to be three flutter waves for each QRS complex (3:1 flutter). If the ventricular rate is 80 beats/min, the rate in the atrium is approximately 240 beats/min. A regular ventricular rate of 150 beats/min should make you suspicious for atrial flutter (2:1 flutter).
The variable atrioventricular block on this ECG is evidenced by the presence of two, rather than three, flutter waves per QRS complex (seen after the fourth, fifth, and 10th QRS complexes on the rhythm strip). This case illustrates that flutter may be present with a ventricular rate of less than 100 beats/min.