This is a good strip to demonstrate the change in the appearance of a T wave when a premature P wave occurs on the preceding T wave. The PACs found the atria ready to depolarize and produced a P wave that landed on top of the preceding T wave, making it appear taller than the others. The PACs also reset the sinus node, causing a slight delay before the next sinus discharge. The PACs occurred while the ventricles were still refractory, so no QRS complexes followed.
Premature atrial contraction
This ECG is from an 88-year-old man with congestive heart failure. No other clinical information is known. It shows an underlying sinus rhythm with atrial bigeminy - every other beat is a premature atrial contraction. There is very little, if any, difference in the morphology of the sinus P waves and the ectopic P waves, indicating that the ectopic focus is in the vicinity of the sinus node. There is no "compensatory" pause, because PACs penetrate the sinus node, resetting it. So, the underlying sinus rate here is about 72 beats per minute. There are several mechanisms for bigeminy to occur, but ectopic bigeminy is the most common.
This strip shows an underlying sinus bradycardia with a rate less than 40/min. There is one "premature" beat, which can be considered to be ectopic, because it interrupts an otherwise regular rhythm. The interesting thing is that the premature beat is not terribly early - it is about 740 ms from the previous beat. If all the beats were spaced like this, the heart rate would be about 84/min. There is probably an element of "escape" here, in that the ectopic beat is able to express itself due to the slow rate. A faster sinus rate would override this ectopic focus. So, we could view this early beat as a help, rather than a problem. The most important consideration here is to address the cause of the bradycardia, and treat appropriately.
This strip offers something interesting for both your basic-level students and for your more advanced students. First, it is a good example of sinus rhythm with a premature beat. The PR interval was measured by the machine at .21 sec (218 ms). The premature beat is supraventricular - that is, it is not a PVC. Because of the slightly long PRI in this strip, it's P wave COULD be buried in the preceding T wave. That would make this a premature atrial contraction (PAC).
For discussion with your more advanced students, the P wave could, instead, be retrograde, and occurring during the QRS or slightly after it. That would make the premature beat junctional, or an atrial echo beat. The origin of the premature beat is mostly academic - there is likely no clinical need to determine the origin.
In looking for clues as to the origin of the premature beat, we would scrutinize the premature beats for "hidden" P waves. Upright and before the premature beat would indicate a PAC. Negative P waves before, during, or after the premature QRS would indicate PJCs. In this strip, the T waves just before the premature beats are slightly deeper than the other T waves. This could indicate atrial "echo", or reciprocal beats, which requires the presence of dual junctional pathways, in which the impulse turns around, reenters the atria, and causes a new beat. It can be helpful to look at multiple leads (the more the better) in your search for P waves. For a look at this patient's 12-lead ECG, go to this link.
The P wave of a premature beat often penetrates the SA node and "resets" it, causing the next normal beat to occur after a "normal" R-to-R interval from the premature beat. This fact can help us find "hidden" P waves, as well.
Another interesting feature of this strip for your students who are interpreting 12-Lead ECGs, is that this ECG shows the criteria for left ventricular hypertrophy. See the link above for the 12-lead and discussion.
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