What Do You Think 33

92-year old butcher’s apprentice with an implanted dual chamber pacemaker.

Urgently referred to pacemaker clinic with failure to pace in the atrium.

Well, but dizzy at times.

What do you think?

Let us review atrial pacing (red highlight).

Atrial stimulus artefact, P wave (Ap) followed by AV conduction (AV delay) and a native or intrinsic QRS/T (Vs).

Letus return to our case:

Stimulus artefact, no P wave is seen, but there is an AV delay and a native QRS/T confirming AV conduction. Despite the absence of a P wave, atrial depolarization is assumed and hence, there is no pacemaker or conduction malfunction .  

This scenario always causes confusion.

Some of you will insist there is a very small P wave (you may be correct).

However, this is more likely an “after potential”.

Let me explain further:  

The flow of energy from the pacemaker to the heart occurs via the negative pole or cathode, situated at the tip of the lead lying adjacent to myocytes. This energy flow from the lead is in the form of electrons, which at the electrode tissue interface are converted to negative ions, predominantly chloride (Cl^-). These ions then pass into the myocardium and if of sufficient strength will depolarize the relevant chambers before returning to the anode, where once again they are converted to electrons. During energy delivery, the sodium ions (Na⁺)are attracted to the negative cathode acting as a barrier to current flow, called polarization, which is important in cathode design. Once the the current flow ceases, the sodium ions no longer are attracted to the anode and they too dissipate, resulting in another voltage artefact on the ECG at the end of the stimulus artefact and of opposite polarity. Because current flow with unipolar pacing is between the cathode and the anode on the chest wall, the stimulus artefact and after potential may result in large deflections on the ECG.

Lead II, unipolar stimulus artefact.

Following the first stimulus artefact, there is a QRS/T indicating ventricular pacing(red highlight). However, following the second stimulus artefact, there is no ventricular capture, but rather an exponential decay curve consistent with an after potential (yellow highlight). With bipolar pacing, both the stimulus artefact and after potential are absent or very small.

Maybe this small wave at the end of the atrial stimulus artefact is an after potential(red arrow). If you disagree with me it doesn’t matter as I wanted the opportunity to discuss after potentials!

However, with atrial and ventricular pacing (Ap Vp), we cannot confirm atrial contraction as the AV delay is electronic and doesn’t require AV conduction.

Dual chamber pacing (Ap Vp). There is no P wave following the atrial stimulus artefact. This ECG demonstrates ventricular pacing, although no ventricular stimulus artefact is seen. In such cases there is a need to exclude atrial fibrillation and atrial exit block.

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Harry Mond