After this there is a short run of 2:1 block, when, because the atrial rate has slowed, every other sinus impulse arrives at the lower AV node after the refractory period has ended and proceeds to the ventricles. P7 is the first APB and it cannot conduct to the ventricles, but the atrial escape, P8, can. P6, however, conducts down to the lower AV node just after the refractory period has ended and therefore continues to the ventricles. The laddergram shows why the first five sinus impulses are not able to conduct to the ventricles: they all arrive at the lower AV node while it is till refractory and they cannot proceed any further. If any atrial impulse arrives at the lower AV node during the refractory period it cannot conduct. To illustrate the effect this has on atrioventricular conduction I have shown what I believe the refractory period of the lower AV node might be by a pink parallelogram on the laddergram. So, why is it that none of the first five P waves are conducted, whereas after that every other P wave is conducted? I think it’s all down to the slight drop in atrial rate. The atrial escape beats – complexes 4 and 7 – also conduct but with a slightly different PR interval. After this, though, every other P wave is followed by a QRS, and complexes 3, 5 and 6 have a very similar PR interval, so I think these must be conducted sinus beats. These two complexes bear no relation to the P waves, so must be junctional escape beats and they arise because none of the first 4 P waves conduct. The first 2 complexes are of a slightly different shape from the rest and are separated by a long RR interval of 1.66 s. I think these must represent atrial escape beats. Now for the QRS complexes. There are also two P waves – P8 and P14 – which end the post-APB pauses and which are of slightly different morphology to the sinus P waves. However, there are two P waves – P7 and P13 – which are premature, and which therefore probably represent atrial premature beats (APBs). The P waves are generally pretty regular with a rate just above 100/min, slowing down slightly to just under 100/min. We will sort out the atrial rhythm first. I’ve identified and numbered all the P waves, then I have numbered the QRS complexes and finally, I have constructed a laddergram to show what I think is happening. First, the ventricular rate is not completely regular and, second, there are two distinct QRS morphologies. It might be tempting after glancing at this one to say that it shows complete heart block with a junctional escape rhythm, but a couple of features should prevent us from jumping to this conclusion. What exactly do you think is going on here? I’ve numbered the QRS complexes to facilitate discussion. I don’t have any clinical details but this month’s ECG is an exercise in rhythm analysis.
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