ECG Review: An Interpolated Tracing
ECG Review
An Interpolated Tracing
By Ken Grauer, MD, Professor, Department of Community Health and Family Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.
Scenario: The ECG shown above was obtained from a 50-year-old woman with "skipped beats." Is there anything unusual about these ectopic beats and the effect that they have on the normal beats in this tracing? What happens at the onset of the last lead change (i.e., at the onset where we see leads V4,V5,V6)?
Interpretation: The underlying rhythm in this tracing is sinus, as determined by the upright P wave with normal PR interval seen for the 3rd complex in lead II. There are frequent premature ventricular contractions (PVCs) on this tracing, which total four in number. The first three PVCs are said to be "interpolated" because they occur sandwiched in between two normal sinus beats without the compensatory pause that typically follows a PVC. Most PVCs are associated with retrograde conduction of the impulse back toward the AV node. If this retrograde conduction extends far enough to reach the atria, a negative P wave may be seen in lead II after the PVC. Even when retrograde conduction from a PVC does not extend all the way back to the atria, retrograde conduction usually lasts long enough to prevent forward conduction of the next sinus P wave. This accounts for the "compensatory pause" that is usually seen following a PVC (as occurs for the last PVC on this tracing). However, if the timing is just right, a PVC may occur at a moment when enough recovery of the conduction system has occurred to allow forward conduction of this next sinus beat. This is what happens with interpolated PVCs. Of interest, we do not see a P wave in front of the 2nd, 5th, and 9th beats on this tracing, even though these are sinus beats. This phenomenon is known as "concealed conduction," in which retrograde conduction from the preceding PVC, while not enough to block forward conduction of the next sinus beat, is enough to prolong the ensuing PR interval. The sinus P waves preceding the 2nd, 5th, and 9th beats on this tracing are almost certainly "concealed" within the T wave of the PVC that precedes these beats. The final intriguing complexity manifest on this tracing relates to the bizarre occurrence at the onset of the last lead change. The last PVC on this tracing occurs just before this lead change. The discontinuity produced in the T wave of this last PVC results from the fact that the leads change before the T wave is completely recorded.
The ECG shown above was obtained from a 50-year-old woman with "skipped beats." Is there anything unusual about these ectopic beats and the effect that they have on the normal beats in this tracing? What happens at the onset of the last lead change (i.e., at the onset where we see leads V4,V5,V6)?Subscribe Now for Access
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