P Waves? U Waves? Or Both?

K. Wang, MD

Disclosures

December 16, 2013

Discussion

Judging from the timing, these waves could pass as P waves with first-degree atrioventricular (AV) block, U waves, or both, and it cannot be settled. In the rhythm strip of lead II in the bottom, 2 premature ventricular contractions (PVCs) are seen. The wave during the compensatory pause is too far out from the PVC for it to be a U wave. Therefore, it is a P wave with a long PR interval. As luck would have it, the P wave is isoelectric (flat) in V3 in this patient. Therefore, the wave indicated by the down-arrow (↓) is not a P wave but a U wave, proving that the waves in question are the summation of both. Without the help from the PVCs, this conclusion could not have been reached. Another good example of the usefulness of PVCs![1,2]

Why is it important to sort them out? If this patient has an aortic valve infective endocarditis, then first-degree AV block could be the first or only sign of paravalvular abscess, which is a surgical entity. If this is a patient who enjoys camping, it could be a sign of Lyme carditis. If this patient is taking digitalis, it could be a sign of digitalis toxicity. If you are seeing this patient for a preoperative consultation and the prominent U wave is a reflection of hypokalemia, it is important to correct the hypokalemia. Otherwise, the patient could have cardiac arrest during the induction of anesthesia the next day. If they are only U waves, then the patient is in an accelerated junctional rhythm, which should make one think of the following 3 conditions:

Digitalis intoxication;

Myocardial ischemia or infarction; or

An excess amount of catecholamines circulating, associated with any stressful condition.

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