Isorhythmic AV Dissociation or Complete Heart Block?

Answer: Isorhythmic AV dissociation followed by restoration of sinus rhythm

• Generally benign, self limiting & not cause for concern.

What is isorhythmic AV dissociation?

Isorhythm dissociation is a rhythm where the SA node & AV node/junction spontaneously produce electrical impulses independently but at similar rates, often creating the illusion of complete heart block.

You will notice the P waves are all upright in lead II, indicating that this is likely a SA nodal rhythm rather than a junctional rhythm. However, for the first beat the PR interval is <100ms, which is generally considered non-physiologic.

PR intervals of <100ms can be a few things:

1. Junctional rhythm (usually producing a negative P wave in lead II).
2. Manifest pre-excitation from the presence of an Accessory Pathway (usually producing a delta wave on the inital aspect of the QRS)
3. Compelte Heart Block (usually persistent)
4. Isorhythm AV dissociation (usually transient and involves narrow QRS complexes and upright P waves in lead II)

In this case, we can deduce isorhythmic AV dissociation, where for the first two beats, the atrium is being depolarised by the SA node, and the ventricles are being depoalrised by the AV junction, producing AV dysynchrony, particularly in the first beat.

• From the 3rd beat onwards, we can see that the SA node has now overdrive suppressed the AV junction, and the patient is back in a true “Sinus Rhythm” with a stable physiologic PR interval.

Another example can be seen here:

What is the mechanism of AV dissociation?

• Technically, this rhythm strip may be described as a period of complete AV dissociation, however, the mechanism of complete AV dissociation is not a diseased AV node or His-Purkinjie system, but rather a tenacious AV node producing spontaneous electrical impulses that are competing with the impulses generated by the SA node.

• Usually this occurs in the setting of high vagal tone when the patient is in bedrest, or under the influence of sedation or anaesthesia. High parasympathetic tone asymetrically depresses the function of the SA node, more so than the AV node, slowing the SA nodal rate to become bradycardic, allowing the AV node to compete with the SA node.

• The other occasion you commonly see this rhythm is when a patient has been administered isoprenaline, since isoprenaline is a positive chronotropic agent, allowing the Junctional rate to compete with the SA node.

Symptoms & consequences:

• Occasionally, this can produce symptoms similar to so called  “pacemaker syndrome”. The AV dysynchrony can produce retrograde pulsations up the SVC which the patient may describe as “neck palpitations”.

• AV dysynchrony can also elicit a vasovagal response. I remember a patient with a loop recorder who’s symptoms of “dizzy spells” and “nausea” were reliably triggered by periods of isorhythmic AV dissociation.

• The vast majority of the time, isorhythmic AV dissociation is self-limiting, benign, asymptomatic and not generally a cause for concern if the clincial context is appropriate, such as the patient lying in bed.

Differentiating from junction rhythm:

• Junctional rhythm often produces inverted P waves in the inferior leads. Additionally, the PR interval is usually <120ms but stable, whereas isorhythmic AV dissociation produces upright p waves which “dance in and out” of the QRS complex with varying PR intervals.

Differentiating from Complete Heart Block:

• Isorhythmic AV dissociation is usually more transient and self limiting compared to complete heart block.
• Additionally, complete heart block due to His-Purkinjie disease will get worse with increasing sympathetic tone, whereas isorhythm AV dissociation will improve with increasing sympathetic tone.
• So, if you ask the patient to perform a straight leg raise in bed, or you agitate them in some way their sympathetic tone will improve resulting in an increase in Sinus Rate.
  • In the setting of isorhythmic AV dissociation, this will resolve and sinus rhythm will resume.
  • In the setting of complete heart block, the A:V ratio will worsen.

My two cents:

• Isorhythmic AV dissociation is a common and generally benign self-limiting rhythm. Often seen in patients with high vagal tone – think sedated cath lab patient, or EP lab, or bedresting patient.
• It generally goes unnoticed by patients, but can occasionally produce symptoms including neck pulsations and dizzy spells.
• Enhancing sympathetic tone usually improves the rhythm and restores sinus rhythm.

Thanks for tuning in

Cheers
Mitch