What Do You Think 62

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What do you think?

I have divided the tracing into sections.

The tracing commences with sinus tachycardia 100 bpm (red arrows), a broad QRS (160 ms) and anon-conducted P wave confirming second degree AV block. Although the P waves are not clear, it is most likely Wenckebach AV block as the PR interval before the non-conducted P wave is longer than the PR interval after it.

Then follows an extended period of ventricular standstill (yellow highlight) without an ventricular escape rhythm. The sinus rate increases from 100 bpm to 120 bpm.

This is paroxysmal AV block which is a run of dropped beats or ventricular standstill often resulting in syncope. The term paroxysmal refers to “unexpected clusters of ventricular standstill”. There are two major types, extrinsic and intrinsic which are very different both clinically and in selection of therapy (see 'What do you think 59').

As with many of the ECG tracings of paroxysmal AV block, it is difficult to be sure of the significance of the findings without a detailed clinical history. In our case, by ECG criteria, the bundle branch block and increasing sinus rate during the standstill would suggest an intrinsic aetiology, whereas the Wenckebach AV block favours extrinsic.

Unlike most cases of intrinsic paroxysmal AV block, the ventricular standstill does not terminate with junctional or ventricular ectopy (see 59. What do you think?).    

Rather in this case, a regular ventricular escape rhythm of 50 bpm (green highlight) arises from a site in the ventricle distal to the bundle branch block but able to conduct retrograde bypassing the block so as to depolarize the heart with a narrow QRS. This is most likely from a site high in the fascicles. The atrial rate is even faster (130 bpm) excluding heightened vagal tone. The AV block terminates abruptly with a marked sinus tachycardia with 1:1 conduction (purple highlight).

Although the combination of paroxysmal AV block and complete heart block is rare, I was sent another example one week later (thanks Gareth).

Despite the absence of Wenckebach AV block, this example fits more an extrinsic paroxysmal AV block.  

The combination of paroxysmal AV block and complete heart block is a conundrum.

Let me explain.

Ask any cardiologist to explain this ECG and a majority will call this complete heart (AV) block rather than paroxysmal AV block.

Here is an example of complete heart (AV) block.

The first tracing has abrupt ventricular standstill and no escape rhythm, whereas the second has a regular, albeit slow, ventricular subsidiary pacemaker focus and there is no relationship with the sinus pacemaker i.e. complete AV block.  

Although I have no strong dislike for the term complete heart block and still use it, I prefer complete AV block which is technically more correct and avoids the use of complete heart block when talking to patients who become frightened when the heart is “physically” blocked.  

With our tracings we have two conditions:Paroxysmal AV block and complete AV block within the single tracing. Are they related and can they occur together?

The answer is yes. In order to explain this I have created a flow chart.  

In every ECG textbook, AV blocks are defined as first degree, second degree and third degree (complete)AV block. Both first and second degree AV block have slowing of AV conduction and/or intermittent dropped P waves. With complete AV block, there is no AV conduction with an escape subsidiary pacemaker focus.

Between second degree and complete AV block, there are a large number of conduction abnormalities which are usually not grouped and do not fit into the traditional “degrees” of block.They can be listed into two groups.

The first is a group of broadQRS complexes without non-conducted P waves and  are the “bundle branch blocks”. I have included six recognised ECG abnormalities. The second group encompasses a wide range of conduction abnormalities with or without a broad QRS or dropped P waves. Most are named by the ECG appearance. A common thread is that AV conduction although fragile remains at least partially intact. The blocks are neither second degree nor complete AV block and can progress to complete heat block. This second group, I refer to as high degree AV blocks recognising that there is a high degree AV block (blue writing) which is atypical Wenckebach AV block with two or more non-conducted consecutive P waves.

This classification allows the transition of paroxysmal AV block to complete AV block and thus although rare, we can visualize both paroxysmal AV block and complete AV block on the same tracing. With the examples shown, the paroxysmal AV block may be either intrinsic and extrinsic.

There is another feature on the ECG which may cause confusion.

Although complete AV block is clearly defined as no conducted P waves, there is a period where the timing of the P wave in relation to the previous T wave allows AV conduction (blue highlight). This short critical zone after the T wave is called the supernormal phase of excitability or conductivity. When the propagating wave of depolarization arrives at the block during this critical point in the timing, there is enhanced conduction, despite the underlying complete AV block. This narrow zone lies at the end of the relative refractory period, where phase 3 merges into phase 4 (See What do you think 60). Despite the rare AV conduction, we still prefer to call this complete AV block.

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Harry Mond