ECG Challenge: A 76-year-old man with a history of hypertension for which he is taking lisinopril, metoprolol, and diltiazem presents to his primary care physician for a routine physical examination. He reports fatigue and occasional lightheadedness. His physical examination is unremarkable except for a slow pulse, which prompts an ECG.
The rhythm is regular at a rate of 40 bpm. The QRS complexes have a normal duration (0.12 s). There is an RSR′ morphology in leads V1 (←) and a terminal S wave in leads I and V5 through V6 (↑). This is a morphology of a right bundle-branch block. The axis is normal between 0° and +90° (positive QRS complex after accounting for the terminal S wave that reflects delayed right ventricular activation and a positive QRS complex in lead aVF). There is low QRS complex voltage in the precordial leads, defined as a QRS complex amplitude <10 mm in each lead. The QT/QTc intervals are normal (520/425 ms and 500/410 ms when the prolonged QRS complex duration is considered). There are P waves seen (+), but there is no relationship between the P waves and QRS complexes (ie, there is atrioventricular dissociation). There are 2 causes for atrioventricular dissociation, including complete heart block in which the atrial rate is faster than the rate of the QRS complex and an accelerated lower pacemaker in which the atrial rate is slower than the rate of the QRS complexes. In this case, the atrial rate is faster (90 bpm) than the ventricular rate (40 bpm). Hence, there is complete heart block and the escape rhythm is junctional. The first 6 P waves have a constant PP interval (└┘) and a rate of 90 bpm. However, the 7th through 9th P waves have a longer PP interval (↔). It is also noted that QRS complexes 4 to 6 have a negative deflection following them (^). These are retrograde P waves that reset the sinus node, accounting for the longer PP interval. In ≈20% to 30% of antegrade complete heart block, there may be retrograde or ventriculoatrial conduction. This may be attributable to:
the fact that antegrade and retrograde atrioventricular nodal conduction properties are different;
the presence of dual atrioventricular nodal pathways;
a concealed bypass tract that generally only conducts in a retrograde direction.
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- © 2015 American Heart Association, Inc.