What Do You Think 59

Narelle from CardioScan sent me this ECG.

What do you think?

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We only have a short rhythm strip, but it contains a wealth of information.

• The rhythm is sinus (red arrows).
• When the P waves conduct (red full arrows), thePR is normal as is the QRS width and morphology (red highlight).
• There is a run of non-conducting P waves (red stippled arrows).
• There are two escape beats with a different QRS morphology (yellow highlight) to the conducted QRS complexes. They are unlikely to be conducted despite the apparent fixed relationship to the previous P waves. The rhythm is too slow for aberrant ventricular conduction and although bradycardia dependent bundle branch block is possible, it is rare. These are likely to be fascicular escape beats.‍
• These escape beats conduct retrograde (VA)despite anterograde block (blue stippled arrows). However, there are no echo beats because of AV block.‍
• During AV block, the sinus rhythm is slower than when it conducts. This is a critical finding as it suggests that the AV block is vagal mediated with the sinus rate increasing as the vagal tone wanes and AV conduction is restored.

 The diagnosis is extrinsic paroxysmal AV block.

Paroxysmal AV block.

Paroxysmal AV block is part of a broad group of AV conduction abnormalities which have no accepted name, and I call them high degree AV block. Although most are associated with advanced conduction abnormalities which we call intrinsic, there are some that occur with a normal conducting system but influenced by heightened vagal tone and are extrinsic.

Unlike complete AV block, high degree AV block borders complete AV block where AV conduction, although fragile, remains at least partially intact. The blocks are neither second degree AV block nor complete heart block but rather somewhere between and include bilateral bundle branch block, advanced AV block, trifascicular block, alternating AV block, multilevel AV block, paroxysmal AV block and pan-conduction defect. The ECG appearances may differ, but with considerable overlay.

On the ECG, paroxysmal AV block is a run of non-conducted P waves with ventricular standstill, often resulting in syncope, with the term “paroxysmal”meaning they occur in unexpected clusters. There are four sub-groups which can be differentiated by age, syncope triggers, prodroma, length of history, associated heart disease and ECG appearances.

These sub-groups are:

• Extrinsic paroxysmal AV block.
• Intrinsic paroxysmal AV block.
• Hybrid paroxysmal AV block.
• Idiopathic paroxysmal AV block.

Extrinsic paroxysmal AV block.

The addition of “extrinsic” to the name suggests that there is no conduction tissue disease, but rather the block is vagal mediated. Think Wenckebach with high degree AV block. In the original description of Wenckebach AV block, there was only one non-conducted P wave. With two or more non-conducted P waves even with a narrowQRS, the rhythm is high degree AV block and longer a systolic runs fit the definition of extrinsic paroxysmal AV block.

Slow sinus rhythm (60 bpm)with PR prolongation (red highlight) and a narrow QRS. There is further minimal slowing of the sinus rate during ventricular standstill followed by recovery ofAV conduction and a shorter PR interval (yellow highlight). This would be consistent with heightened vagal tone.

Like sinus arrest and neuro-cardiogenic syncope, extrinsic paroxysmal AV block is more frequent in the young and there are well known parasympathetic provoking triggers, that precipitate clinical episodes. These include emotional stress, dehydration, drawing blood, a dental visit and prolonged standing. Prodromal symptoms include warmth, an empty feeling in the abdomen, dizziness, nausea and sweating. The prodrome is dependent on the developing bradycardia which may differentiate extrinsic from the other forms which tend to be abrupt in onset.

Wenckebach AV block may precede the ventricular standstill.

Above: Wenckebach AV block.

‍Below: The sinus rate increases once AV conduction recommences(green stippled arrows). The PR interval prior to ventricular standstill is marginally longer than after it (red highlight).

To help differentiate extrinsic paroxysmal AV block from other types, it is helpful to document the progress from Wenckebach AV block to ventricular standstill.

Sinus rhythm (red arrows),narrow QRS with progression from Wenckebach AV block (red highlight) to 2:1 AV block (yellow highlight), prior to ventricular standstill.

Because heightened vagal tone is common to both sinus and AV nodal blocks, it is not surprising they may occur together.  

Sinus rhythm with marked first degree AV block, degenerating into sinus arrest rather than extrinsic paroxysmal AV block. Although this ECG is consistent with neuro-cardiogenic syncope it also requires peripheral vaso-dilatation and hypotension to make a clinical diagnosis.

Intrinsic paroxysmal AV block.

The term “intrinsic” refers to disease within the infranodal conducting system and some authors limit the definition of paroxysmal AV block to this group. Whereas extrinsic paroxysmalAV block is “Wenckebach with high degree AV block”, intrinsic is Mobitz type II with high degree AV block. Because of prolonged ventricular stand still, the morbidity and mortality is very high unless appropriate intervention is undertaken. The asystolic pauses of intrinsic paroxysmal AV block maybe similar to extrinsic block but clinically the two are very different. Intrinsic paroxysmalAV block generally occurs in the elderly with advanced His-Purkinje disease often coupled to myocardial disease and on the ECG may have a bundle branch block. Other ECG features include ectopy which may precipitate and terminate ventricular standstill and there is no obvious sinus slowing (vagal) prior to or during a systole. Other descriptive names include a pause-dependent paroxysmal phase4 AV block and phase 4 AV block. Although technically correct, these names are confusing to the reader. The block occurs during the early spontaneous depolarization of phase 4 of the action potential in diseased cells, when the resting membrane potentials are very low. If an ectopic is generated or conducted to the ventricle during phase 4, the delay due to the compensatory pause may result in AV block and hence asystole.

Identifying ECG features include:

• A narrow QRS suggests intra-His pathology.
• A broad QRS suggests infra-His pathology, although because of diffuse advanced conduction tissue disease, it is not unusual to have concomitant intra-His pathology in the presence of a bundle branch block.
• Atrial or ventricular ectopy, initiating and terminating ventricular standstill.
• No obvious sinus slowing, but rather a modest increase in rate during or after ventricular standstill.

Features that precipitate ventricular standstill.

Non-conducted atrial ectopic.

Sinus bradycardia, bundle branch block and intrinsic paroxysmal AV block precipitated by a non-conducted atrial ectopic (blue stippled arrow).  

Conducted atrial ectopic.

Sinus rhythm with first degree AV block (red arrows). An atrial ectopic with minor aberrant ventricular conduction (blue arrow) results in a short run of ventricular standstill (red stippled arrows). Aberrant ventricular conduction is a physiologic phase 3 conduction block and can occur together with phase 4 block .

Atrial couplet.

Sinus rhythm (red arrows). A conducted atrial couplet (blue arrows) results in ventricular standstill (red stippled arrows).

Ventricular ectopic.

Sinus bradycardia 50bpm (red arrows) with a bundle branch block. A ventricular ectopic (yellow highlight) results in ventricular standstill (red stippled arrows).  

Non-conducted echo beat.  

Junctional rhythm with retrograde conduction (blue arrows) with either conducted (green stippled arrows) or non-conducted echo beats. Following a blocked echo beat (first blue stippled arrow), an escape sinus beat (red arrow) conducts with a prolonged PR interval allowing retrograde conduction (yellow highlight, second blue stippled arrow) to precipitate paroxysmal AV block.

Features that terminate ventricular standstill.

Once ventricular standstill occurs, AV conduction may not be re-established unless the transmembrane potential in that part of the diseased conducting tissue returns to normal. This is often achieved with an appropriately timed ectopic beat that resets the potential. On the surface ECG, this zone lies on or just beyond the terminal slope of the T wave and represents the supernormal phase (section 3).

Junctional ectopic.

Sinus rhythm. Ventricular standstill (red stippled arrows) terminated by a junctional ectopic (red highlight).

Ventricular ectopic.

Sinus rhythm, broad QRS complexes. A run of ventricular standstill is terminated by a ventricular ectopic (yellow highlight).

Hybrid paroxysmal AV block.

In clinical practice, it can be difficult to categorize the ECG of paroxysmal AV block into a specific grouping. Each case must be judged on both the clinical characteristics and ECG findings. The following ECG was obtained overnight from an asymptomatic male with marked first degree AV block.

Sinus rhythm (60 bpm) with marked first degree AV block (540 ms), a narrow QRS,  no precipitating ectopy and the sinus rate slow during ventricular standstill.

This is most likely nocturnal extrinsic paroxysmal AV block on a background of advanced first degree AV block and thus has features of intrinsic paroxysmal AV block as well. Despite being asymptomatic, permanent cardiac pacing will most likely be advised.

Idiopathic paroxysmal AV block.

In recent years, a rare group of paroxysmal AV blocks have emerged, which cannot be categorised as intrinsic or extrinsic, and for investigative and therapeutic reasons has been given its own classification as idiopathic. As with extrinsic paroxysmal AV block, there is no known structural cardiac pathology nor resting ECG changes prior to ventricular standstill. The cardio inhibitory sequences have no identifying features as seen with the other blocks. What is characteristic of this  block are low levels of endogenous adenosine and small observational studies reporting good therapeutic response to theophylline as well as dual chamber pacing.    

An ECG that fits the definition.

• Sinus rhythm with a narrow QRS.
• No ectopy.
• The sinus rhythm shows little change throughout the ventricular standstill.

The ECG diagnosis of all types of paroxysmal AV block are dependent on the underlying rhythm being sinus. However, the mechanism for the AV block is not dependent on sinus rhythm, but only requires it for ECG interpretation.                                                                                                            

What happens when the rhythm is not sinus?

The rhythm is probably very fine atrial fibrillation. During the night, there were abrupt pauses, suggestive of paroxysmal AV block.

Harry Mond