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Interpreting chaos

Following atrial septal defect closure and partial anomalous pulmonary venous return repair, the following tracing was recorded in a 12 y/o female. She was on continuous positive airway pressure mask and hemodynamic support with Norepinephrine, Neosynephrine and Simdax. Ventricular temporary pacing wires were placed during surgery and VVI pacing was set to 60 bpm backup.

On first sight everything looks pretty chaotic. However, when looking closer you find some regularity as beats appear in a certain sequence (grouped beating).

Assessing this tracing you find clear P waves, upright in I/II, with a PR of 100ms in the 2nd, 4th and 6th beat. These are highly likely sinus beats (SB).

The 3rd and 5th beat are atrial ectopic beats (AEB) with different P wave morphology and prolonged PR interval (about 180ms). The PR prolongation seems to appear due to the short coupling interval of the AEB (200ms), which renders the fast pathway of the AV node (AVN) refractory. The slow pathway, however, remains excitable and propagates conduction through the AVN, resulting in PR prolongation. This proofs the presence of dual AVN physiology in this patient. More information on dual AVN physiology can be found in the post from 14th Nov. 2022.

The AEBs further show a different QRS morphology (left bundle branch block - LBBB, left axis deviation - LAD). This is due to prematurity of the beats, resulting in functional LBBB.

Leg.: LBBB and LAD = blue square, SB with normal axis = green square.

The 7th beat in the tracing looks different. There is a P wave at the beginning, subsequently followed by a sharp deflection and a QRS complex which does not fit in with the prior seen QRS complexes.

The sharp deflection is the ventricular pacing (Vp) spike applied by the temporary pacemaker. The backup is set to 60 bpm. Therefore, after an interval of 1000ms following the prior QRS complex the pacemaker delivers Vp (if no ventricular signal is sensed within this time window).

Now, the last AEB that appears right after the 6th beat falls in slightly earlier, resulting in AV block as it now meets the slow pathway refractory. This AEB "resets" sinus node depolarization, postponing the following SB by 50ms. The following SB (P wave on the 7th beat) falls together with Vp by the pacemaker because both, the sinus rhythm and the pacing interval, now equal 1000ms. Thus, the 7th beat in the tracing is a fusion beat (SB and Vp fuse to form a different QRS morphology).

Interpreting this tracing, we now can see there is sinus rhythm at a cycle lengths (CL; P to P interval on the ECG) of 920ms.

Also, there is an atrial ectopic rhythm present at a CL of 940ms. When the SR gets just a little bit slower than an ectopic rhythm, you may get "escape-capture-bigeminy", which seems to be the case here.

A third rhythm present here is the pacemaker VVI backup at a rate of 60 bpm (ie a CL of 1000ms). This results in the presented sequence of SB, AEB and Vp. One might speak of a "trifold rhythm" with grouped beating.

After adjusting the ventricular backup rate to 50 bpm the rhythm quickly stabilized to pure SR at 55 bpm. This might suggest that some of the AEBs were, at least in part, triggered by the ventricular pacing stimulus.

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