ECG Challenge: A 24-year-old woman is seen by her primary care physician for a physical examination before running a marathon. The results of her physical examination are normal. An ECG is obtained (ECG A). After the ECG is seen by the primary care physician, who notes a change from a previous ECG, he asks for it to be repeated (ECG B).
There is a regular rhythm at a rate of 80 bpm. There is a P wave before each QRS complex (+) with a constant, but long PR interval (0.40 s) (└┘). The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, this is a normal sinus rhythm with a first-degree atrioventricular block or prolonged AV conduction. The QRS complexes have a normal duration and axis between 0° and +90° (positive QRS complex in leads I and aVF). They have a normal morphology, but there is early transition with a tall R wave in lead V2 (←). This is the result of counterclockwise rotation of the electric axis in the horizontal plane, which is established by imagining the heart as if viewed from under the diaphragm. With counterclockwise rotation, left ventricular forces develop earlier and are seen in the right precordial leads. The QT/QTc intervals are normal (360/415 ms).
ECG B was obtained several minutes after ECG A. There is a regular rhythm at a rate of 72 bpm. The QRS complex duration, morphology, and axis and QT intervals are identical to those seen in ECG A. There is a P wave before each QRS complex, and the P wave is positive in leads I, II, aVF, and V4 through V6. Hence, this is a normal sinus rhythm and the P-wave morphology and axis are similar to the P wave in ECG A. However, the PR interval is now shorter (0.16 s; └┘). The presence of a normal sinus with 2 different PR durations (ie, normal and very long) is the result of changes in conduction through the atrioventricular node and reflects the presence of dual atrioventricular nodal pathways. There is a slow pathway, which has a short refractory period (recovers more quickly) and a fast pathway that has a long refractory period (recovers more slowly). These 2 pathways, which form a circuit within the atrioventricular node, are the substrate abnormality associated with the occurrence of an atrioventricular nodal reentrant tachycardia.
Please go to the journal’s Facebook page for more ECG Challenges: http://goo.gl/cm4K7. Challenges are posted on Tuesdays and Responses on Wednesdays.
- © 2014 American Heart Association, Inc.