AV Dissociation and Complete AV Block: What’s the Difference?

When an ECG demonstrates P waves that are not consistently associated with a QRS, this does not always signify a complete AV block. As a primer: All cases of complete heart block involve AV dissociation, but not all cases of AV dissociation are due to complete heart block. Some definitions are in order. 

AV Dissociation occurs where there is independent activation of the atria and ventricles. An atrial pacemaker (usually the sinus node or ectopic atrial focus) triggers atrial activation, while a distal pacemaker (either the AV junction or ventricular focus with intrinsic or abnormal pacemaking ability) triggers ventricular activation.[2] 

1) P Waves: P waves may conduct to ventricles if given the chance

2) R-R Interval: Variable

Figure 2: AV Dissociation

Complete AV Block: No evidence of atrioventricular conduction (complete AV dissociation), with an atrial rhythm completely independent and faster than the ventricular (usually an escape) rhythm. Also referred to as a Third-degree AV block or Complete Heart Block. More examples are here.

1) Rule of Thumb: AV Dissociation + Atrial rate* > Ventricular rate

2) P Waves: P waves do not conduct to ventricles 

3) R-R Interval: Generally stable

*Theoretically, atrial rate may be slower than the ventricular rate as in Sinus Node Disease

Figure 3: 3rd degree AV Block

Isorhythmic AV Dissociation: When the atrial depolarization rate (from either sinus node or ectopic atrial focus) is similar to the ventricular depolarization rate (either the AV junction or ventricular focus with intrinsic pacemaking ability).[3]. For isorhythmic AV dissociation to occur there either needs to be slowing of the sinus rate, pauses in sinus rhythm, speeding of a subsidiary pacemaker, or a combination of these. 

1) Rule of Thumb: AV Dissociation + Ventricular Rate roughly the same as Atrial Rate 

2) P Waves: Variably before, within, or after QRS complex

3) R-R Interval: Stable

Figure 4: Isorhythmic AV Dissociation. Junctional Pacemaker Rate is Identical to Sinus Rate

AV dissociation can be independent of AV block. If atrial depolarization occurs when the AV node and ventricular myocardium have recovered from their refractory period, the atrial impulse can conduct to the ventricle, termed ventricular capture - this proves the absence of anterograde AV block. This is the physiologic explanation behind fusion and capture beats that may be observed in ventricular tachycardia (more discussion on this topic can be found here). If the focus distal to the atrium (the AV node, His bundle, or ventricle) reaches the atria when it has recovered from its refractory period, the impulse will conduct to the atria retrogradely, called atrial capture. This proves the absence of retrograde AV block.[2] 

Figure 5: AV Dissociation during Accelerated Junctional Rhythm. Examples of Atrial and Ventricular Capture

With the definitions in mind, we identify this ECG as probable complete AV block given the complete dissociation between P waves and QRS complexes and the atrial rate being faster than the ventricular rate.

Figure 6: Initial ECG zoomed in on V1 rhythm strip. Red arrow: p-p interval. Blue arrow: R-R interval. P: p waves

Why is This Important to Differentiate

The management of complete AV block and AV dissociation differs. For patients with complete heart block the more narrow questions are: why does this patient have a complete heart block and do they need a pacemaker? However, for patients with AV dissociation without complete heart block, the more broad question is: what is causing either a slowing of the sinus rate, a pause in the sinus rhythm, or the speeding up of the subsidiary pacemaker, and how can we treat this? Examples of each are shown below.  

Slowing of the Sinus Rate: Vagal hypertonia (coughing, vomiting, urinary retention, fecal impaction), drugs (beta-blockers, calcium channel blockers, clonidine, digoxin, etc), hypoxemia, metabolic derangements (hyperkalemia, hypothyroidism, hypothermia, hypoglycemia), infections (Chagas, Lyme, Syphilis), myocardial ischemia.[2,4,5]

Pause in Sinus Rhythm: Sinus Pauses or blocks, sinus arrest, sinus node dysfunction

Speeding up of a Subsidiary Pacemaker: AV junctional escape beat or rhythm, AV junctional premature complexes, accelerated AV junctional rhythm, AV junctional tachycardia, ventricular premature complexes, accelerated idioventricular rhythm, automatic ventricular tachycardia, ventricular paced rhythm

There is often an underlying reason behind these etiologies. For many of these etiologies, resolution of the inciting pathology will restore the underlying sinus rhythm. A patient with MI who has a post-reperfusion, accelerated idioventricular rhythm and AV dissociation will resolve as the myocytes are reperfused. A patient presenting with vomiting with AV dissociation has vagal hypertonia, and the etiology of vomiting needs treatment.[4] An asymptomatic young athlete found to have AV dissociation and an AV junctional rhythm likely has baseline, increased underlying vagal tone and needs no intervention. 

Back to the Patient 

The ED team consulted cardiology given the possible, complete AV block. Unfortunately, the patient suffered recurrent cardiac arrest and expired despite multiple rounds of ACLS.

1) All cases of complete AV block involve AV dissociation, but not all cases of AV dissociation are related to complete AV block

2) When you see AV dissociation, determine the cause of a slowing of the sinus rate, a pause in the sinus rhythm, or the speeding up of a subsidiary pacemaker

1) Bogossian H, Linz D, Heijman J, et al. QTc evaluation in patients with bundle branch block. Int J Cardiol Heart Vasc. 2020;30:100636. Published 2020 Sep 19. doi:10.1016/j.ijcha.2020.100636 

2) Wang K, Benditt DG. AV dissociation, an inevitable response. Ann Noninvasive Electrocardiol. 2011;16(3):227-231. doi:10.1111/j.1542-474X.2011.00436.x

3) Patel A, Pumill R, Goldman D, Damato AN. Isorhythmic atrioventricular dissociation revisited. Am Heart J. 1992;124(3):823-829. doi:10.1016/0002-8703(92)90306-g

4) deSouza IS, Dilip M. Fortuitous Identification of Fluctuating AV Block: A Case Report. J Emerg Med. 2019;57(1):e9-e12. doi:10.1016/j.jemermed.2019.03.033

5) Pavone C, Pelargonio G. Reversible Causes of Atrioventricular Block. Card Electrophysiol Clin. 2021;13(4):703-710. doi:10.1016/j.ccep.2021.07.004