ECG Challenge: A 73-year-old woman is admitted to the hospital for a community-acquired pneumonia, which was confirmed on a chest x-ray. Her temperature is 102°F, and on physical examination, she has rales and rhonchous sounds at the left base. A routine ECG is obtained (ECG A). After admission, the ECG is examined by the hospitalist, who is uncertain about the rhythm. He therefore repeats the ECG (ECG B).
ECG A shows a regular rhythm at a rate of 100 bpm. The QRS complex duration is normal (0.10 second), and there is a normal morphology. The axis is extremely leftward between −30° and −90° (positive QRS complex in lead I and negative complex in leads II and aVF). There are 2 causes of an extreme left axis: an old inferior wall infarction in which the negative QRS complex results from a deep Q wave and a left anterior fascicular block in which there are a normal initial R wave and a deep S wave. The leftward axis in this case is attributable to a left anterior fascicular block. The QT/QTc intervals are normal (340/440 milliseconds). No obvious P waves are seen before or after any of the QRS complexes, suggesting a junctional tachycardia. However, there are subtle irregularities of the T wave, especially obvious in leads II and aVL (^), and a negative deflection after the T wave (^) in V1. These abnormalities are suggestive of a P wave superimposed at the end of the T wave.
The subsequent ECG B shows a regular rhythm at a rate of 96 bpm. The QRS complex duration, axis, and morphology are the same as seen in ECG A. The QT/QTc intervals are also the same. One premature complex is seen (^). It is wider and has a different morphology than the other QRS complexes; hence, this is a premature ventricular complex. After this complex, there is a pause, and as a result, a clear P wave can be seen (+), after which there is the usual QRS complex. The P wave is positive in leads II, aVF, and V5; hence, this is a sinus P wave. The PR interval associated with this QRS complex is prolonged (0.40 second; ┌┐); that is, there is prolonged atrioventricular conduction or a first-degree atrioventricular block. Using this PR interval, it can be seen that the negative deflection after the T wave (↑) is indeed the P wave. Hence, the rhythm is a sinus tachycardia, and as a result of the prolonged PR interval, the P and T waves merge, making it difficult to identify the P wave.
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.