What Do You Think 42

I have presented this before, but it continues to bewilder people.

Is this heart block?

What do you think?

In each tracing, all recorded overnight, there is sinus bradycardia (red arrows), ventricular ectopics (red highlight) and non-conducted P waves (blue arrow) at the end of the ectopic T wave. This often called, in error, a non-conducted atrial ectopic.

These non-conducted beats are timed with the sinus cycle and are therefore, not ectopics, but rather sinus beats that do not conduct to the ventricle. For those fussy with measurements, there is also ventriculo-phasic sinus arrhythmia.

To explain the non-conducted P waves, we need to revisit the ventricular ectopic.

• Premature broad QRS (red highlight)
• No preceding P wave
• Full compensatory pause (green stippled arrow)
• Concealed within the ectopic QRS is the next sinus P wave (blue arrow), which does not conduct to the ventricle, because the conducting system is  refractory -referred to as concealed retrograde conduction.

As the sinus rate slows (red arrows), the non-conducted sinus P waves (blue arrows) creep beyond the QRS of the ventricular ectopic (red highlight) and are now “non-concealed” within the ST segment of the ventricular ectopic.The conducting system is still refractory, and the P waves do not conduct to the ventricle (blue arrows, yellow highlight).

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As the sinus rate (red arrows) slows further (blue highlight), the non-conducted sinus P waves becomes concealed within the ectopic T wave and eventually emerges once again at the end of the T wave (red highlight) or beyond it (blue arrows).

With marked sinus bradycardia, the ECG appearance is that of AV block referred to as “pseudo-AV block”.

Sinus bradycardia (red arrows) with first degree AV block and an early ventricular ectopic (red highlight).The next sinus P wave occurs well beyond the ectopic T wave (blue arrow) and would be expected to conduct to the ventricle. However, the conducting system is still refractory from the retrograde concealed conduction. This suggests that this is more than timing and there is also conducting tissue disease.

What happens if the AV conducting system is not refractory?                                            

AV conduction will occur and by definition there will be no compensatory pause.

This is called an interpolated ventricular ectopic.

Now you know the exciting next “What do you think?”

Or maybe not!!

It’s all in the timing.

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Harry Mond