ECG Challenge: A 76-year–old man presents to the emergency room 4 hours after the onset of substernal chest burning that radiated to his throat and jaw. He initially thought that this was gastrointestinal related, but it persisted despite therapy with antacids. He therefore presented to the emergency room.
There is a regular rhythm at a rate of 100 beats per minute. Although P waves are not obvious before the QRS complexes, there is a pause in the rhythm (↔) between the 12th and 13th (^) QRS complexes, and a P wave can be seen during this pause (+) with a PR interval of 0.36 seconds. Immediately after the 13th QRS complex (^), there is another P wave (*) present before the 14th QRS complex (ˇ), and the PR interval is longer (0.44 seconds). Using this PP interval (└┘), another P wave (•) can be seen before the 15th QRS complex with an even longer PR interval (0.48 seconds). Therefore, there is a stable PP interval (└┘) at a rate of 100 beats per minute. Based on this PP interval it can be seen that the P waves of the preceding QRS complexes are on the T wave or within the ST segment (↓), producing what appears to be a notching. The pause is the result of a nonconducted on-time P wave (▼), which is within the QRS complex. Importantly the following P wave (+) is on time as are all the subsequent P waves. Therefore there is a sinus tachycardia with a second-degree AV block (defined as occasional nonconducted on-time P wave), and the progressive lengthening of the PR interval is characteristic of Mobitz type I or Wenckebach. Because the P waves are not on the ST segments in complexes 13 (^) and 14 (ˇ), it can be seen that there is ST segment elevation in leads II, III, and aVF (↑). Therefore, there is an acute ST segment elevation inferior wall myocardial infarction.
The QRS complexes have a normal duration (0.08 seconds) and there is a normal axis between 0° and +90° (positive QRS complex in leads I and aVF). However, the QRS complex morphology is not normal, because there is loss of R wave amplitude in leads V3 to V6 with Q waves in leads V4 to V6. Although this looks like an old anterolateral myocardial infarction, the pattern of reverse R wave progression across the precordium and loss of R waves in the anterolateral leads is consisted with right-sided leads, which are often obtained in patients with an acute ST segment inferior wall myocardial infarction to evaluate for the presence of right ventricular involvement. As there is no ST segment elevation in the right sided leads V4-V6, the right ventricle is not involved.
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- © 2014 American Heart Association, Inc.