By Ken Grauer, MD
Figure. 12-lead ECG obtained from a 61-year-old woman diagnosed
as having sinus tachycardia and 1° AV block.
Clinical Scenario: The ECG in the Figure was obtained from a 61-year-old woman who was being treated with flecainide for "arrhythmia." Her tracing was interpreted as showing sinus tachycardia with 1° AV block, with the conduction disturbance being seen best in lead V1. How would you interpret this ECG?
Interpretation: The key to interpreting the rhythm in this 12-lead ECG lies with recognizing that the very beginning of this tracing is irregular. Specifically, the first R-R interval is longer than all others on the tracing. After the first few beats, the rhythm becomes regular at a rate of about 110 beats/minute. The QRS complex is obviously widened. However, despite the seemingly upright "P" wave in lead V1, the mechanism of this rhythm is not sinus. Sinus rhythm is defined by the presence of an upright P wave in lead II. A look at lead II in this tracing fails to show an upright P wave. Instead, there appear to be a number of sharp negative deflections in the baseline. Careful inspection of these negative deflections with calipers reveals that they are regularly occurring at a rate of about 220/minute (see the dots below leads II, III, and aVF in the Figure). Attention to the baseline in leads II and III shows the characteristic sawtooth pattern of atrial flutter in the longer initial R-R interval, which is not nearly as apparent in the rest of the tracing. Thus, the rhythm is atrial flutter, initially with a variable ventricular response with 2:1 AV conduction becoming established by the third beat on the tracing. QRS widening is due to bifascicular block (right bundle branch block and left anterior hemiblock), and was not a new finding for this patient.
Several important points are highlighted by this tracing. First is the fact that 1° AV block is rarely seen with sinus tachycardia. This is because rapid heart rates usually result in shortening of the PR interval. Awareness of this point should be the first clue that the small upright deflection seen midway between QRS complexes in lead V1 is unlikely to be a sinus P wave conducting with 1° AV block. The second important point about rhythm interpretation that is illustrated by this tracing is in showing how helpful it may be to always look for the pause (even if brief) in the rhythm. Except for the very first R-R interval on this tracing (which is longer than all other R-R intervals), flutter waves are nearly completely hidden by the QRS complex. Finally, note should be made that the atrial rate of flutter in this rhythm (220/minute) is slower than the usual range of 250-350 minute. This most probably is the result of antiarrhythmic treatment with flecainide.
Dr. Grauer, Professor and Associate Director, Family Practice Residency Program, Department of Community Health and Family Medicine, College of Medicine, University of Florida, Gainesville, is on the editorial board of Emergency Medicine Alert.