Supraventricular Tachycardia With Wenckebach Conduction
This ECG was obtained from a patient in a walk-in health clinic. We do not have any other information on the patient. We thank Joe Kelly for donating this interesting ECG to the GURU.
IRREGULAR RHYTHM If you march out the P waves, you will see that they are regular, at a rate of approximately 130 bpm. But the QRS complexes are not regular, and there are fewer QRS complexes than P waves.
WENCKEBACH CONDUCTION Looking closely at the PR intervals, you will notice that they progressively prolong. This “pushes” the QRS complexes progressively toward the right. Eventually, the T wave – and the refractory period – will land on the next P wave. That P wave will be unable to conduct to the wave, and no T wave of course, so the next P wave will conduct with a shorter PR interval.
We are including a short rhythm strip from this patient, with conduction marked with a laddergram.
NOT A TYPICAL PRESENTATION If your students have always learned “Second-degree AV block, Type I” or “Wenckebach” on a rhythm generator, they will expect to see a normal sinus rhythm, not sinus or atrial tach. They will also expect to see clearly repeating cycles of progressively prolonging PR intervals, until one P wave is non-conducted, producing a slight pause. They may not recognize the cause of the irregularity in this ECG unless they systematically analyze the P wave rhythm and then the QRS complexes and PR intervals. In this patient, V1 probably has the clearest P waves for analysis, and there is a continuous V1 rhythm strip on the bottom of the page.
For those more advanced students, there are some “atypical” aspects to this ECG. You may notice that the FIRST P-QRST in each cycle has a PR interval of about .22 seconds, EXCEPT for the fourth beat on the 12-Lead ECG. This one appears to have a very SHORT PRI, but it is more likely that the PR interval was so LONG, the QRS appeared slightly AFTER the next P wave. What do you think?
BOTTOM LINE We are hoping some of our experts, including our Consulting Expert, Dr. Grauer, will add more detailed comments to this ECG. The main points we would like to make on the most basic level are:
Real ECG rhythms may vary quite a bit from the basic examples seen on electronic rhythm generators, and in some brief references.
The “AV block” in this case is not a worrisome condition – it is more an expected lack of conduction due to a P wave landing in a refractory period. This is called “physiological block”. I would be more concerned about why this patient has tachycardia, and the clinical approach would be to evaluate the patient’s heart rate in light of his or her presenting symptoms.
