ECG tutorial: Pacemakers

INTRODUCTION — Atrial and ventricular pacing can be seen on the electrocardiogram (ECG) as a pacing stimulus (spike) followed by a P wave or QRS complex, respectively. The ECG has the ability to show normal and abnormal pacemaker function.

ATRIAL PACING ONLY — Atrial pacing appears on the ECG as a single pacemaker stimulus followed by a P wave (waveform 1) (see "Modes of cardiac pacing: Nomenclature and selection") The morphology of the P wave depends upon the location of the atrial lead; it may be normal, diminutive, biphasic, or negative. The PR interval and configuration of the QRS complex are similar to those seen in sinus rhythm. They are independent of atrial pacing, and the duration and configuration are determined by the intrinsic characteristics of the patient's conduction system.

There is 100 percent capture if the rhythm is entirely paced. There may be intermittent pacing, however, if the sinus rate is faster than the programmed lower rate limit. In these cases, a paced beat will be seen after a pause that is equal to this lower rate. For example, if the pacemaker is set at 60 beats per minute (bpm), the pacemaker will only pace if the rate falls below 60 bpm or if there is a pause of one second.

VENTRICULAR PACING ONLY — Ventricular demand pacing appears on the ECG as a single pacemaker spike followed by a QRS complex that is wide, bizarre, and resembles a ventricular waveform (waveform 2). (See "Modes of cardiac pacing: Nomenclature and selection".)

●LBBB – The pacemaker lead is usually in the right ventricular apex so the paced QRS complex has a left bundle branch block (LBBB) configuration since right ventricular activation occurs before activation of the left ventricle. It is also negative in the inferior leads. Sometimes, the lead may be placed higher up in the right ventricular septum or outflow tract, and, while there is still an LBBB pattern, the inferior leads may have variable axis.

●RBBB – Rarely, there may be a right bundle branch block (RBBB) pattern with pacing in the right ventricular apex, though there is the possibility the lead was inadvertently placed in the left ventricle if this is seen.

●His bundle pacing – Another option is that the lead is placed to pace the His bundle. In this location, the paced QRS complex is narrow and looks similar to a native beat. (See 'His bundle pacing' below.)

With ventricular pacing, there may or may not be atrial activity noted, depending upon the nature of the patient's underlying rhythm, the atrial rate, and the occurrence of ventriculoatrial conduction. If intrinsic or native atrial activity is present with a single chamber ventricular-only pacemaker, it occurs at a rate that differs from the ventricular rate since it is dissociated from the QRS complex. Frequently, ventricular-only pacing is used in association with atrial fibrillation.

There may be episodic pacing in patients who have a ventricular demand pacemaker. The pacemaker is activated and delivers a stimulus only when the intrinsic ventricular rate falls below a predetermined lower limit. Pacemaker activity is suppressed when the intrinsic heart rate is faster. The escape interval (the time between the last intrinsic beat and the paced beat) is equivalent to the rate at which the pacemaker is set to activate. Similar to atrial pacing, if the pacemaker is set at 60 bpm, the pacemaker will only pace the ventricle if the rate falls below 60 bpm or there is a pause of one second. If the native rate is slow, there will be 100 percent ventricular pacing (figure 1). The ECG may have evidence of fusion or pseudofusion beats if the pacemaker rate and intrinsic heart rate are nearly identical and the native and paced QRS complex occur simultaneously.

Fusion beats show a QRS morphology that is a mixture of the native and paced QRS complex. Pseudofusion beats show a native QRS with a pacing spike just after the beginning of the QRS complex, giving the appearance of pacing, but since the pacing spike is after the ventricle has started to depolarize, the pacing stimulus is delivered during the refractory period and does not capture the heart. Pseudopseudofusion is when a pacing stimulus from the atrium occurs at the onset of a premature ventricular complex/contraction (also referred to as premature ventricular beats or premature ventricular depolarizations), creating the appearance of ventricular pacing though this has not actually occurred.

DUAL-CHAMBER ATRIOVENTRICULAR SEQUENTIAL PACING — Atrioventricular (AV) sequential pacing appears on the electrocardiogram as pacemaker spikes before the P wave and QRS complex (waveform 3) (see "Modes of cardiac pacing: Nomenclature and selection"). If the native atrial rate is faster than the programmed lower rate of the pacemaker, there will be no pacing spike prior to the P wave, since the P wave originates from the sinus node (or another atrial location). Depending on device programming and AV conduction, there is either a paced or a native QRS. The paced QRS is widened and resembles a left bundle branch block (LBBB) premature ventricular complex/contraction. (waveform 4). If there is a native QRS, the morphology is generally normal and narrow, but may be wide if there is an underlying bundle branch block.

The morphology of the P wave depends upon the location of the lead within the right atrium. It may be upright, biphasic, or negative and the amplitude may be normal, increased, or diminutive. The PR interval is determined by the pacemaker and represents the delay between the atrial and ventricular stimuli. The pacemaker is essentially functioning as the AV node. The second or ventricular spike is followed by a QRS complex that is bizarre and usually has an LBBB morphology since the ventricular lead is typically in the right ventricle. If the ventricular lead is pacing the His bundle, it will look like an intrinsic narrow QRS. (See 'His bundle pacing' below.)

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 (waveform 5). (See "Cardiac resynchronization therapy and conduction system pacing in heart failure: System implantation and programming".)

Sometimes the coronary sinus lead is replaced by an epicardial or endocardial left ventricular lead. Ventricular pacing occurs from both the right ventricular and coronary sinus/left ventricular leads. The timing between the leads can be changed so that one lead is paced prior to the other or they can be paced simultaneously. Depending on the timing of pacing between the two leads, the QRS axis and morphology may change significantly. It is also possible to do left ventricular-only pacing.

Assuming left ventricular pacing occurs simultaneously or prior to right ventricular pacing, activation of the left ventricle precedes right ventricular activation. Therefore, the initial impulse is directed left to right, producing a large Q wave or QS complex in lead I. In addition, leads V5-V6 also frequently demonstrate a QS complex. However, this is not definitive for biventricular pacing, as an LBBB may be associated with a QS complex in these leads also. Biventricular pacing will also produce a tall R wave in lead V1, as the impulses are directed toward this lead. This QRS complex morphology may also be seen with a right ventricular pacing electrode located at the interventricular septum. Therefore, while a QS complex in leads V5-V6 and a tall R wave in lead V1 are strongly suggestive of a biventricular pacemaker, an initial Q wave or a QS complex in lead I is more definitive for left or biventricular pacing. In addition, if there is a significant difference between right and left ventricular pacing stimuli, two ventricular pacing spikes may be seen, with the second one occurring after the start of the QRS. This may be confused with pacemaker malfunction.

HIS BUNDLE PACING — Direct pacing of the His bundle causes ventricular activation to occur through the normal His-Purkinje system, often leading to a normal QRS complex and normal ventricular synchrony. (See "Atrial fibrillation: Atrioventricular node ablation" and "Permanent cardiac pacing: Overview of devices and indications".)

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 [1].

The ECG in His bundle pacing can be variable depending on whether there is selective, nonselective, or noncapture of the His bundle [2].

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 ms, which is equal to the native His-bundle-to-QRS-complex time (waveform 6).

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 (waveform 7).

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.

LEFT BUNDLE BRANCH PACING — As a physiologic alternative to His bundle pacing, left bundle branch (LBB) pacing is an option for patients with depressed or preserved left ventricular ejection fraction. It is postulated to be hemodynamically equivalent to His bundle pacing in electrically activating the heart more physiologically compared with right ventricular apical pacing and offers better long-term thresholds than His bundle pacing, and thus can prolong battery life.

The LBB itself (or one of the fascicles) is captured by the pacing stimulus by screwing the pacemaker lead deep into the interventricular septum. It can used for standard pacing indications but is also being considered as an alternative to biventricular pacing in ongoing studies [3,4].

Selective left bundle pacing — Those with LBB capture will have an rSr (waveform 8), qR, or QR (waveform 9) in V1, similar to what is seen with a right bundle branch block pattern. Ideally, there is a proximal LBB potential recorded on the pacing electrode when sensing, with an LBB-to-local-electrogram time of 20 to 35 ms [5]. The best marker of LBB capture with pacing is when the time from the LBB potential to the R wave peak time (RWPT) in V6 equals the duration in native conduction (with a narrow QRS or right bundle branch block pattern).

If the LBB potential is not visible, the time from QRS onset to RWPT in V6 with pacing should equal the RWPT in native conduction V6 RWPT (waveform 10).

Nonselective left bundle branch pacing — The RWPT in V6 with nonselective LBB pacing is usually equivalent to the RWPT with selective LBB capture [6]. However, QRS duration is wide and typically the same as LV septal pacing.

Left ventricular septal pacing — With LV septal pacing, there is no capture of the left bundle conduction system. The QRS morphology does not change with changes in pacing output, which indicates only septal capture. There is no evidence of an LBB potential on the ECG lead, and there is a prolonged RWPT in V6, usually >80 ms QRS duration, especially when measured from QRS onset rather than the pacing stimulus, may still be narrow.

PACER MALFUNCTION — Pacemakers may malfunction either by failure of capture or sensing. (See "Pacing system malfunction: Evaluation and management".)

Loss of capture — Pacemaker malfunction with inconsistent capturing (atrium or ventricle) can be diagnosed from the ECG when there are pacemaker spikes that are not followed immediately by either a P wave or QRS complex (figure 2). 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 nonphysiologic signal (waveform 11). 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 AV 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 premature ventricular complex/contraction 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.

Oversensing may occur when the device sees a signal that does not originate from within the chamber of interest. This may be from electromagnetic interference, such as from electrocautery or other electrical interference, diaphragmatic myopotentials, pectoral muscle myopotentials if in a unipolar mode, lead fracture, or far field signals from another cardiac chamber. In oversensing, the device inhibits pacing. Therefore, no pacing spikes are seen at the expected time on the ECG, and unless there is intrinsic cardiac activity, there will be asystole.

SUMMARY

●Lead location – Pacemaker leads may be placed in the right atrium, right ventricle, and coronary sinus/left ventricle. (See 'Atrial pacing only' above and 'Ventricular pacing only' above and 'Biventricular pacing' above.)

●Dual-chamber atrioventricular pacing – If there is native cardiac activity faster than the programmed intervals, the P wave and QRS complex will resemble normal patterns. If there is slow cardiac activity, the ECG shows a pacing spike followed by a P wave or QRS complex. The morphology of a paced P wave may resemble or look different than the native P wave. The morphology of a paced QRS with the lead in the right ventricular apex has a left bundle branch block (LBBB) configuration and is usually negative in the inferior leads. (See 'Dual-chamber atrioventricular sequential pacing' above.)

●Biventricular pacing – The QRS complex with biventricular pacing will vary depending on right and left ventricular timing, but often has a tall R wave in V1 and an initial Q wave or QS complex in lead I. (See 'Biventricular pacing' above.)

●His bundle pacing – In selective His bundle pacing, the QRS duration and morphology match the native QRS with a stimulus-to-QRS time matching the underlying His-bundle-to-QRS time. With nonselective His bundle pacing, the QRS is longer than the native QRS with a pseudo-delta wave and pacing stimulus-to-QRS time of zero. (See 'His bundle pacing' above.)

●Left bundle branch pacing – With selective left bundle pacing, there is an rSR, qR, or QR in V1, similar to what is seen with a right bundle branch block (RBBB) pattern.

●Loss of capture – Loss of pacemaker capture is defined by a pacing spike with no P wave or QRS complex when the myocardium is physiologically capable of being depolarized. (See 'Loss of capture' above.)

●Failure of sensing

•Undersensing occurs when the pacemaker does not see native cardiac activity and paces inappropriately in the middle of or after a P wave or QRS complex, or has no relationship to the native cardiac signal. (See 'Failure of sensing' above.)

•Oversensing occurs when the pacemaker sees nonphysiologic activity and inhibits pacing. No pacing spikes are seen at the expected times.