What Do You Think 48

I was asked for an opinion regarding this ECG.

The reporting doctor had requested a repeat ECG.

‍

Patient has a pacemaker.

What do you think?

‍

Should the ECG be repeated?

Let us start by looking at V1.

The background rhythm is atrial fibrillation (AF).

There is ventricular pacing (Vp).

There are 4 other “spikes” or pulses with a similar voltage (red highlight), but not as intense (pulse width) as the Vp stimulus artefact.

These pulses are due to subthreshold (below voltage required to pace) stimuli delivered into the heart during systole and once delivered, the “resistance” is measured between the lead cathode and the pulse generator can. The measurement is called intracardiac impedance and changes in this impedance can be used to indirectly determine cardiac contractility which in turn can be used to change the pacemaker rate, according to physiologic demand. The pulses are unipolar (cathode to pulse generator can) and thus prominent on the surface ECG.

‍

The rate adaptive pacing system is called closed loop stimulation (CLS) and is sold by Biotronik, a German manufacturer of cardiac implantable electronic devices, including pacemakers. In situations where the heart rate cannot change with emotion or exertion, such as sick sinus syndrome and atrial fibrillation with a slow ventricular response, the system allows the pacing rate to rise and fall physiologically.

The CLS system has 16 subthreshold biphasic pulses delivered each 46 ms for 250 ms(yellow highlight).  The pulses commence50 ms after the onset of a paced or sensed QRS. (blue lines)

The ECG shown earlier only had 4 pulses. The difference lies in the filters in the ECG machine that reduce or block the pulses. With this ECG, there is no filtering of the pulses but following each pulse there is a blanking zone where the next oneor two pulses are blocked from being detected on the ECG.

Let us review a number of these ECGs to highlight the way ECG machines deal with such artefact.

Atrial pacing (Ap) and ventricular sensing. There is a 50 ms initial delay (red highlight) followed by a bunch of artefacts for 250 ms finishing in the T wave (yellow highlight). The artefactis seen differently in each lead. The filter deals differently with each pulse and there is no blanking.    

Here are examples of marked blanking with and without atrial (blue highlight) and ventricular paced rhythm.

How can you be sure this is CLS artefact?

• Evidence of paced rhythm.
• The pulses are similar or varying size.
• The pulses lie within the 250 ms window, 50 ms after the onset of the paced or sensed QRS.  

On occasion a patient with the CLS sensor programmed ON will demonstrate no artefact on an ECG. This is because of the excellent work of the filters.

In other situations, the pulses continually change.

‍

Maybe filter response is related to the size of the pulses which alter with respiration.

Although the atrial pacing stimulus artefact can be seen throughout the tracing (red arrows, Ap), at the end of the strip the atrial stimulus artefacts increase in size and the CLS artefact is pronounced. The filters have changed, or respiration allows recognition on the ECG.

‍

The minute ventilation sensor also uses impedance measurements but this time measures transthoracic impedance. Minute ventilation, the product of respiratory rate and tidal volume is a highly physiologic variable, which closely reflects the metabolic demands of exercise, including cardiac output and heart rate.

Transthoracic impedance varies with respiration, falling with inspiration and rising with expiration. The system uses a conventional bipolar pacing lead.

A: Low energy, subthreshold pulses every 50 ms. The pulse generator recognizes the changes in impedance (green arrow) as changes in tidal volume and the oscillations as respiratory rate, the product of which is minute ventilation.                                                                                                                               B: The ECG demonstrates the pulses as continual regular interference (red highlight).

The 12-lead ECG can be very dramatic and is usually diagnosed as extra-corporeal interference with the ECG repeated several times.

Note the absence of artefact in leadI (red highlight). The arm leads lie at a distance from the current and do not detect the pulses. The ECG machine also recognizes the pulses as interference(yellow highlight). The interference is present with both paced and sensed rhythms.

Again, because of filtering, a variety of appearances can be seen.

Hyperventilation will result in pacing at the programmed upper sensor rate, which may result in the diagnosis of ventricular tachycardia.

The patient suffered from severe anxiety. Following sedation, the rate fell to 90 bpm. The underlying rhythm is atrial fibrillation and there are no interference pulses.

The pulse generator may also detect extra-corporeal pulses resulting in inappropriate sensor upper rate pacing.

Patient in emergency room, awake and alert. On attachment of ECG monitoring electrodes, the heart rate rose to sensor upper rate. With this monitor, a small current is passed through the chest electrodes and the skin impedance measured. A marked rise occurs if a monitoring lead is detached from the skin. Despite the pacemaker stimulus artefacts (V5) on a 12-lead ECG (red highlight), ventricular tachycardia was diagnosed, and cardioversion unsuccessfully attempted many times.

It is important to recognize these artefacts to prevent continually repeating the ECG.

The answer to the initial question is don’t repeat the ECG.

‍

Harry Mond