By Ken Grauer, MD
Professor Emeritus in Family Medicine, College of Medicine, University of Florida
Dr. Grauer reports no financial relationships relevant to this field of study.
The ECG in the figure below was obtained from a young adult who presented to the emergency department with dizziness, a near syncopal episode, and chest discomfort. Does this ECG suggest AV block is the cause of his symptoms?
The rhythm in the long lead II at the bottom of the tracing shows an underlying sinus mechanism, with variation in the P-P interval consistent with sinus arrhythmia. There are four premature ventricular contractions (PVCs). The PR interval does not remain constant throughout the tracing. Instead, the PR interval appears to be shorter before beats 5 and 8, and longer before beats 3 and 13. Considering the short pauses following beats 4 and 7, the obvious concern in this patient with near syncope is the rhythm might represent second-degree AV block of the Mobitz I type.
There is no AV block on this tracing. Beats 4 and 7 are PVCs that are followed by a compensatory pause. It is extremely likely that an on-time sinus P wave is hidden within the ST-T wave of beats 4 and 7. Retrograde conduction from these PVCs prevents conduction of these hidden P waves to the ventricles. This results in the brief pause after these PVCs.
Beats 2 and 12 also are PVCs, but they are “interpolated” PVCs, because they are sandwiched between two sinus-conducted beats without a compensatory pause. Because the P waves after these PVCs occur a little bit later in the cardiac cycle, forward conduction to the ventricles is possible, albeit with slight delay. This phenomenon is known as “concealed conduction” since prolongation of the PR interval preceding beats 3 and 13 can be explained only by postulating events not seen on the actual ECG. The point to emphasize is prolongation of the PR interval of the P wave following an interpolated PVC is common, and it is not the result of AV block
Note the PR interval preceding beats 9, 10, and 11 remains the same. This supports our presumption that we are not dealing with Mobitz I second-degree AV block. I suspect the reason for the slightly shorter PR interval preceding beats 5 and 8 is the result of either junctional escape or ectopic atrial escape beats, but not AV block.
Finally, there are no acute ST-T wave changes on this ECG. Therefore, this patient has underlying sinus arrhythmia with frequent uniform PVCs, but no clear evidence for AV block, and no reason forthcoming from this ECG to explain his symptoms of dizziness, near syncope, and chest discomfort.
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