ECG Challenge: A 65-year-old man with a history of hypertension being treated with a β-blocker presents to his physician for a routine physical examination. His pulse is noted to be slower than usual; hence, an ECG is obtained.
There is a regular rhythm at a rate of 48 bpm. The QRS complex duration is normal (0.08 second), and there is a normal morphology, as well as an axis between 0° and 90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (440/390 milliseconds). P waves are seen (+), but there is no relationship to the QRS complexes; that is, there are variable PR intervals. This represents atrioventricular dissociation. The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, there is an underlying sinus rhythm. Most of the PP intervals are constant (└┘) with a rate of 75 bpm. There are 2 causes of atrioventricular dissociation: complete (third-degree) heart block in which the atrial rate is faster than the rate of the QRS complexes or an accelerated lower pacemaker (ie, junctional or ventricular) in which the atrial rate is slower than the rate of the QRS complexes. This is therefore complete heart block, and the escape rhythm is junctional. The etiology of the escape rhythm is not based on the rate of the escape rhythm but it established by the QRS morphology, which in this case is normal. There are 2 longer PP intervals (↔). Each of these longer PP intervals is associated with a QRS complex (third, fourth, and sixth; [v]) that has a slightly different morphology as a result of a negative deflection that is seen after these QRS complexes (^). This represents a retrograde P wave, a result of ventriculoatrial conduction associated with the junctional complex. As a result of retrograde atrial activation, the sinus node is reset, accounting for the long PP interval. Although there is complete atrioventricular block, ventriculoatrial conduction may be seen in up to 40% of cases. This is attributable to different antegrade and retrograde atrioventricular nodal conduction parameters; dual atrioventricular nodal pathways, 1 of which is capable of ventriculoatrial conduction; or the presence of a concealed bypass tract that conducts only in a retrograde fashion.
In addition, 2 premature P wave (*) have a morphology that is different from the morphology of the sinus P waves. These are premature atrial complexes that are not conducted, that is, blocked premature atrial complexes. These non conducted P waves can also reset the sinus node automaticity.
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- © 2015 American Heart Association, Inc.