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.
Premature atrial contraction
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.
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).
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