What Do You Think 34

There is a lot to report in this ECG.

What do you think?

There are two major features seen in this 12-lead ECG.

Dual chamber pacing (red highlight) and ventricular ectopy (yellow highlight).

There is atrial (Ap) and ventricular (Vp) pacing. There is some concern regarding the different sizes of the atrial stimulus artefacts with the first being very small (red arrow), even though a paced P waves follows (P).

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Why the difference in stimulus artefact size?

This is very common and normal in 12 lead ECGs with bipolar atrial pacing.

ECG atrial pacing. The atrial stimulus artefacts vary considerably in size depending on the respiratory cycle. The ability of current to reach the skin surface from the cathode in the right atrium depends on the amount of air in the lungs and the resistance to current flow called transthoracic impedance. The impedance rises with inspiration (small arrows), which in turn reduces the amplitude of the artefact, and the opposite occurs with expiration (large arrows).

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In our case study, ventricular pacing results in a right bundle branch block configuration in V1 (red highlight).

This suggests left ventricular pacing. We see two ventricular stimulus artefacts due to biventricular pacing with the left stimulus artefact (LV) occurring about 40 ms before the right(RV).

Here is another example of biventricular pacing with two stimulus artefacts (red highlight) and a right bundle branch block configuration (yellow highlight).

For optimal cardiac resynchronization therapy, there should be ventricular pacing 90 or maybe 95%of the time. With native rhythms and particularly ventricular ectopy, this maybe impossible to achieve without electro physiologic intervention, such as His bundle or ventricular ectopy ablation. There are also two programmable pacemaker options that are available, both of which have limited value.

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Negative hysteresis.

Hysteresis has been available at least for ventricular pacing since the dawn of cardiac pacing, even as anon-programmable option in VVI generators. In summary, the pacemaker will extend the sensing period by a set amount in order to encourage native ventricular rhythms and thus avoid unnecessary ventricular pacing.  The appearance on the ECG can be confusing as the pacing rhythm appears to violate the programmed low rate.

Atrial fibrillation. Hysteresis allows the sensing window of the pacemaker to be extended (1500 ms yellow highlight) although the pacing rate (red highlight) without sensing is not altered (1200 ms red highlight). Not surprisingly, there is ventricular fusion(red stippled arrow, V fusion) after 1200 ms (blue highlight).

Negative hysteresis has the opposite effect and shortens the sensing period to encourage ventricular pacing. Once again it appears to violate the programmed lower rate limit. This can be seen in our case study.

The atrial pacing repetition rate at rest is 1000 ms / 60 bpm. However the distance from the sensing of the second ventricular ectopic to atrial pacing is only 720 ms / 83 bpm. The red arrow indicates where atrial pacing would occur if there was no negative hysteresis.

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Ventricular triggered pacing.

This is an unproven pacemaker algorithm which attempts to pace both ventricles on sensing of the native ventricular rhythm. This is effectively ventricular triggered pacing (VVT or maybe DDT)which attempts to synchronize ventricular contraction during the native beat. It is present as a programmable function in at least two manufacturer’s products (VentricularSense Response – Medtronic Inc, Minneapolis, MN andBiV Trigger – Boston Scientific, St Paul, MN). Ventricular sensing of the native rhythm is variable and may be late and therefore biventricular pacing is unlikely to contribute significantly to ventricular depolarization and hence resynchronization.

My concern is that the default setting is ON and if not required, must be programmed OFF. Hence it is frequently seen with biventricular pacing as it is not programmed OFF.

The rhythm strips show biventricular pacing with a very short LV-RV latency and a right bundle branch block configuration (red highlight). There is frequent multifocal  ventricular ectopy with the triggered stimulus artefacts lying midway through the native QRS(yellow highlight). The appearances are confusing and suggest ventricular under sensing although the timings are not correct. However, not all ectopics have embedded stimulus artefacts (blue highlight), but the timing of the next paced beat suggests that the ectopic is actually sensed. As with VVT pacing, there is a programmable upper rate limit for biventricular triggering with the default being about 130 bpm. The non-triggered ventricular ectopic is earlier than the triggered ones and thus violates the upper rate sensing limit.

This is also demonstrated in the next ECG:        

Triggered single chamber biventricular pacing with a biphasic stimulus artefact (red highlight). This isa characteristic appearance of two stimulus artefacts, each with an opposite vector. A ventricular triplet is sensed as suggested by the timing of the next paced beat, The first ventricular ectopic demonstrates triggering with embedded stimulus artefacts (yellow highlight), whereas the next two are also sensed(blue highlight), but do not trigger an output as the upper rate sensing limit has been violated. Once again, the ECG is confusing.

Finally there are the ventricular couplets (red highlight) in our case study.

These occur in a trigeminal pattern and even though there is no name for these, my physiologists refer to them as “trigemilets”, the amalgam of trigeminy and couplets.

Here is another example.

Ventricular couplets (red highlight) are separated by two sinus beats with first degree AV and right bundle branch block.

What does our case study ECG show?

• Atrial pacing with stimulus artefact respiratory effect.
• Biventricular pacing with a right bundle branch block configuration.
• Negative hysteresis.
• Trigeminal ventricular couplets.

Harry Mond