ECG Challenge: A 77-year-old man presents for a preoperative evaluation before a hernia repair. He has a history of a previous myocardial infarction (6 years ago), and 2 years ago he underwent cardiac catheterization and a percutaneous coronary intervention because of drug-refractory angina pectoris. Since that time, he has been free of cardiac symptoms, but the surgeon requested a preoperative cardiac evaluation because of his history. Physical examination demonstrates normal heart sounds, but there is a grade 2/6 harsh midsystolic murmur heard at the base with faint radiation to the carotids. It is felt to represent aortic sclerosis. Paradoxical splitting of S2 is also appreciated.
The ECG shows a slightly irregular rhythm. The first RR interval is slightly shorter (rate, 64 bpm) than the rest of the RR intervals, which are regular with a rate of 45 bpm. There are no P waves seen before the first 2 QRS complexes, but there are P waves present before all of the other QRS complexes (+) with a stable PR interval (0.20 s). In addition, there is a second P wave present after the T wave (*); this P wave is not followed by a QRS complex (ie, it is nonconducted). There is a stable PP interval (↔) and a regular atrial rate at 90 bpm. Based on the PP interval (↔), it can be seen that there are on-time P waves at the end of the first QRS complex and before and after the second QRS complex (↓). These P waves are unrelated to the QRS complexes. The P waves are positive in leads I, II, aVF, and V4 to V6. Hence, there is an underlying sinus rhythm with a second-degree atrioventricular block (AV) block (defined as an occasional nonconducted P wave) and a pattern of 2:1 conduction or 2:1 AV block. This may be attributable to either a Mobitz type I or type II. The QRS complex duration is increased (0.14 s) and there is a deep S wave in lead V1 and a broad R wave in leads I and V6. While this is consistent with a pattern of a left bundle branch block, there is a septal R wave in lead V1. As the septal (median) branch innervating the septum (in a left to right direction) comes from the left bundle, septal forces (ie, small Q wave in lead I, aVL, and V5-V6 and small R wave in lead V1) should not be present. In addition, there is a small S wave in lead V6, representing terminal forces directed left to right. Left to right forces should not be present with a left bundle branch block. Therefore, this is more consistent with an intraventricular conduction delay, ie, slow conduction through the normal His-Purkinje system. The QT/QTc intervals are slightly prolonged (520/450 ms) but are normal when the prolonged QRS complex duration is considered (460/400 ms). Because the QT interval includes the QRS complex, and the ST segment and T wave, as well, prolongation of the QRS complex duration needs to be considered when establishing the QTc interval. The amount of widening of the QRS complex that is above the normal width needs to be subtracted from the QT interval measurement before the QT is corrected for heart rate.
The initial P waves are not associated with the first 2 QRS complexes (ie, there is AV dissociation). Because the atrial rate is faster than the ventricular rate, this is a third-degree or complete AV (heart) block. It can be seen that the morphology of the first 2 QRS complexes is slightly different from the rest of the QRS complexes. They are also slightly wider (0.16 s). Hence, these QRS complexes are ventricular (ie, an escape ventricular rhythm), indicating that 2:1 AV block is a Mobitz type II or a conduction abnormality within the His-Purkinje system.
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- © 2014 American Heart Association, Inc.