P waves “dancing in & out”. Should I be worried?

Answer: Isorhythmic AV dissociation. Generally benign, self-limiting & not cause for concern.

Overview

• Isorhythmic AV dissociation is a rhythm I have also heard referred to as“Accrochage”.

• It refers to the phenomenon of competing Junctional & Sinus Rhythm where both the AV node & SA Node produce impulses at similar cycle lengths (heart rates).

• On the ECG, this produces a mixture of Junctional & Sinus Rhythm at very similar rates.

• Since both rhythms usually produce narrow QRS complexes, the RR interval will often appear roughly regular with a stable morphology; however, the P waves will appear to “dance in and out” of the QRS complex, sometimes appearing before the QRS, sometimes buried within the QRS, and sometimes appearing just after the QRS complex.

Cause

• The AV Node & Sinus node firing at nearly identical rates (hence the term “iso-rhythmic”).

• This usually occurs in the context of high vagal tone (or anti-arrhythmic medication suppressing SA node action), such as in a sleeping patient, where the SA nodal activity is depressed enough to allow the AV junction to compete with the SA node to control Heart Rate.

Peeking below the ECG

Prognosis

• Typically benign & caused by reversible high vagal tone/antiarrhythmic medication.

• Sometimes this rhythm produces symptoms which may warrant a review of their medication or consideration of device implantation in rare circumstances.

• I distinctly remember one patient presenting with symptoms akin to “pacemaker syndrome” despite not having a pacemaker implanted. The patient felt episodic bouts of palpitations, lightheadedness, fatigue & nausea at night when they were watching TV or going to bed. Their cardiac loop recorder demonstrated Isorhythmic AV dissociation occurring at every time they felt symptoms.

Common Mistakes

• This benign rhythm is commonly mistaken for complete heart block due to the presence of apparent AV dissociation.

• However, unlike most forms of complete heart block, this AV dissociation is usually reversible with increasing Sympathetic tone, whereas the AV relationship in CHB or Mobitz-II usually worsens with greater sympathetic tone since these rhythms are often driven by the presence of underlying His-Purkinjie disease (with some exceptions such as the context of a vasovagal episode or inferior MI related bradyarrhythmias as discussed in a previous post).

My Two Cents:

• Isorhythmic AV dissociation (Accrochage) is a common rhythm observed in a resting ECG

• It is ususally caused by high vagal tone & is therefore reversible & benign unless there is some underlying SA nodal dysfunction.

• Occasionally, it causes symptoms of poor AV association such as palpitations, light-headedness & nausea, warrenting more serious investigation.

Thanks for tuning in :)
Cheers
Mitch & CPP Team

Learn more about the ECG features & Clinical implications of bradyarrhythmias in our ECG course here