ECG Challenge Response ECG Response: October 21, 2014 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).

l A

'"'A— l - A A —

aVR

LajV i-A y v .

VI

l ~ A A _ l —AA

Lav\— V -A y

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) ( u ). 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 1 and aVF). They have a normal morphology, but there is early transition with a tall R wave in lead V2 (