Pacemaker Rhythm on ECG

ECG can actually show atrial and ventricular pacing as a pacing stimulus, which is seen as a spike which precises P wave or QRS complex, respectively.

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Atrial pacing only

ECG will show a pacing stimulus (spike) which is followed by a P wave.

• P wave morphology will depend on the location of the atrial leads; it can be expected, biphasic, or negative.
• PR = similar to sinus rhythm
• QRS complex = similar to sinus rhythm
• The rhythm is called entirely paced when it is 100% captured.
  • But it is called intermittent captured when atrial pacemakers is in demand mode, in which it only paces the rhythm when the intrinsic atrial rate falls below the preset level.
    • For example, if the pacemaker is set at 60 beats/min, the pacemaker will only pace if the rate falls below 60 beats/min or if there is a pause of one second (60 beats/min ÷ 60 sec/min).
  • Intermittently paced rhythm – the paced beat will be seen after a pause which predicts low HR.

Ventricular Pacing only

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• ECG shows a single pacemaker impulse or spike which is before a QRS complex.
• LBBB is seen in the QRS configuration since the pacemaker lead is usually placed in the RV apex. Therefore RV activation occurs before LV activation. Due to the same reason, QRS can be negative in Inferior Leads (II, III, aVF)
• If the pacemaker lead is placed in an unusual LV apex, then RBBB is observed in the QRS configuration.
• Another place for a pacemaker lead placement is a bundle of His, in which QRS configuration is usually narrow or it is similar to normal QRS configuration.

Lead Placement	QRS configuration
RV Apex	LBBB & Negative in inferior leads (II, III, aVF)
LV Apex	RBBB
Bundle of His	Narrow / normal

• There may or may not be atrial activity in only ventricular-paced rhythm on ECG. Even it is present, then it is due to conduction from the ventricles to the atrial via the AV node.
• Like Atrial paced rhythm ventricular-paced rhythm can be entirely paced or intermittently paced.
• There is the possibility of getting some fusion or pseudo-fusion beats if the pacemaker and naive heart impulse rate are similar & the QRS recording on ECG from the intrinsic and pacemaker occurs at the same time.
  • Fusion beat – a mix of intrinsic & pacemaker impulses.
  • Pseudofusion – intrinsic QRS with a pacemaker impulse spike just after the beginning of the QRS complex.

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Dual chamber – Atrioventricular sequential pacing

AV sequential pacing appears on ECG as a pacemaker impulse spike before P wave and QRS Complex.

Biventricular Pacing

Standard ventricular pacing is from the right ventricle. Hence, the QRS complex has a left bundle branch block (LBBB) morphology as the impulse originates from the right ventricle and is conducted to the left ventricle (a right to left direction). This produces a tall and broad R wave in leads I, V5, and V6, and a deep QS complex in lead V1.

With biventricular pacing, also known as cardiac resynchronization therapy, there are pacing leads in the right atrium, right ventricle, and the coronary sinus, which results in stimulation of the left ventricle.

HIS BUNDLE PACING

There is renewed interest in physiologic pacing with direct pacing of the His bundle. Pacing in this location causes ventricular activation to occur through the normal His-Purkinje system, often leading to a normal QRS complex and normal ventricular synchrony. This method can be used for those needing a pacemaker for native conduction system disease, for those undergoing atrioventricular node ablation and pacing for rapid atrial fibrillation, and as an alternative to cardiac resynchronization therapy.

In addition, it is possible for a bundle branch block to be corrected with His bundle pacing if the level of block is in the fibers of the proximal His bundle destined to become the right or left bundle and pacing is in the distal His bundle.

Selective His bundle pacing — Selective His bundle pacing is when there is only pacing of the His bundle. There will be a paced QRS morphology similar to the native QRS complex. If the native QRS is normal, then selective His bundle pacing will also have a normal QRS, since conduction is through the same pathway. However, if the baseline QRS has a fascicular or bundle branch block, the paced QRS complex may also have the fascicular or bundle branch block or may narrow the QRS complex. The pacing stimulus is not coincident with the onset of the QRS, but is usually >35 msec, which is equal to the native His-bundle-to-QRS-complex time.

Nonselective His bundle pacing — With nonselective His bundle pacing, the His bundle and the local myocardium are both captured with pacing. The QRS duration is longer than the native QRS duration with the appearance of a pseudo-delta wave. The stimulus-to-QRS onset is zero since the myocardium at the tip of the pacemaker lead on the antero-septum is also captured. The axis of the QRS complex matches the axis of the native QRS. However, it is possible for nonselective His bundle pacing to narrow the QRS complex if the native QRS has underlying fascicular or bundle branch block. This is manifested if the paced QRS has a shorter duration QRS than native QRS, there is a pseudo-delta wave, and the stimulus-to-QRS onset is zero.

Noncapture of the His bundle — Noncapture of the His bundle is not a desired outcome, but if the His bundle is not captured due to inadequate pacing output, lead dislodgement, or failure to locate the His bundle for pacing, then there is right ventricular pacing only. The QRS will be widened with a short stimulus to QRS duration, similar to pacing from anywhere else in the right ventricle.

PACER MALFUNCTION

Pacemakers may malfunction either by failure of capture or sensing.

Loss of capture — Pacemaker malfunction with inconsistent capturing (atrium or ventricle) can be diagnosed from the electrocardiogram (ECG) when there are pacemaker spikes that are not followed immediately by either a P wave or QRS complex.

Non-capture may be intermittent, so that only occasional non-captured pacemaker stimuli are seen, or persistent, where no native complexes follow pacing spikes. In the latter cases, if intrinsic cardiac activity is present, the pacemaker stimuli are dissociated from the native P waves or QRS complexes. There may be no underlying cardiac activity in severe cases of loss of capture and asystole is seen. Loss of capture may be due to lead dislodgement or malposition, inflammation or fibrosis at the lead/tissue interface, low pacemaker output, lead failure, or battery depletion.

Failure of sensing — Pacemakers may have undersensing of native cardiac activity or oversensing of non-physiologic signal.

In undersensing, the pacemaker does not see the native electrical signal in the chamber of interest and will deliver a pacing stimulus at the lower rate for the atrium or after the programmed atrioventricular delay in the ventricle. Therefore, there will be a pacing spike in the middle of or after the beginning of the native P wave or QRS or have no relation to the underlying cardiac activity. Depending on the refractoriness of the tissue, this pacing spike may or may not capture. The interval between the native and paced complexes is variable. Note that when pacemakers are turned to asynchronous modes (eg, AOO, VOO, DOO), they are programmed not to sense intrinsic cardiac signal and will have the appearance of undersensing.

It may seem that there is failure to sense a premature atrial or ventricular premature beat in some cases that are not actually representative of pacemaker malfunction. It may be dependent instead upon the timing of the premature beat, which may not be sensed by the pacemaker if the signal falls within a refractory period.