The only information I have about this case is that the ECG was recorded from a 10-year-old child, but my question is a simple one: what is the rhythm? Please describe exactly what you think is happening.
Well, this one elicited quite a few responses, with several different explanations proposed.
I have tried to illustrate what I think is happening in the below laddergram that accompanies the rhythm strip (RR intervals shown in milliseconds).
It’s often useful when trying to solve an arrhythmic puzzle to start with identifying all the atrial activity. Here we have P waves that are positive in leads I and II and that occur at a rate of about 50/minute but are slightly irregular. There is therefore an underlying sinus bradycardia/arrhythmia. Some of these P waves fall immediately before QRS complexes, perhaps creating the impression of delta waves; some occur immediately after, resembling J waves. But they are all P waves, and this brings us to one of key abnormalities of the rhythm: there is AV dissociation.
The ventricular rate is, for the most part, just above 100 bpm and the QRS is narrow. I think this is because there is a junctional ectopic, or non-reentrant junctional, tachycardia. I don’t think this is an AVNRT because of the relatively slow ventricular rate and the presence of AV dissociation: in AVNRT there is a common upper pathway that makes retrograde block, and therefore AV dissociation, very unlikely (1).
Although the tachycardia is almost perfectly regular, there is a pause in the ventricular rhythm that is just under two normal RR intervals in duration. A possible explanation for this is that there is Wenckebach exit block from the junctional focus. This phenomenon has been reported previously, although it seems that it is usually caused by digitalis toxicity (2).
An interesting side-issue is the sinus bradycardia in a 10-year-old child. As Scott ingeniously suggests, this may be an automatic reaction to the junctional tachycardia, a circulatory system reflex that aims to correct the abnormally high cardiac output. We often see the opposite of this in complete AV block, where there is a reflex sinus tachycardia in the presence of a slow escape rhythm.
None of the P waves in this ECG conduct to the ventricles, so is there any heart block? No, none at all. The reason that none of the sinus impulses conduct is simply because they always arrive at the AV node when it is in its refractory state due to the rapid junctional discharges. The failure to conduct in these circumstances is physiological rather than pathological.
Please feel free to continue to comment on the ECG and my explanation.
I am grateful to everyone who has contributed to the discussion and to Su Baxter for sending me the ECG.