Paced Rhythms

Paced Rhythms

The Pacemaker

FIGURE 13.1 A. Anatomy of a dual-chamber pacemaker. B. ECG appearance of atrial and ventricular pacing.

Electrode Types


Cathode is on the lead tip; anode is located proximally within the pulse generator.

Pacemaker artifacts are generally more conspicuous.


Cathode and anode are located 1 cm apart at the distal end of the lead.

Pacemaker artifacts are smaller and may be difficult to appreciate.

ECG Appearance

Atrial Pacing and Capture

An atrial artifact (vertical line) is followed by a P wave.

Ventricular Pacing and Capture

A ventricular artifact (vertical line) will be followed by a wide QRS complex with a left bundle-branch block morphology.

Pacemaker Nomenclature

A five-letter code has been adopted to identify different types of pacemakers. The first letter corresponds to lead location. The second letter corresponds to the presence or absence of sensing capability. The third letter indicates the response of the pacemaker to a sensed impulse. The fourth letter indicates the pacemaker’s programmability and capacity for rate modulation. A pacemaker capable of rate modulation (R) is also capable of communicating (C) with external telemetry and multiprogrammability (M) in which three or more variables can be modified. Pacemakers capable of rate modulation use sensors for minute ventilation, QT interval, or stroke volume to detect changes in physiology and mimic the rate response of a normal sinus node. The code has revised its fifth letter to indicate whether multisite pacing is present in the atrium, ventricle, or both.1 Pacemakers are typically referred to by the first three letters of the NBG code. Common pacemaker modes are VVI and DDD.

FIGURE 13.2 North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiology Group (NBG Code) System of Categorizing Pacemakers.

Single-Chamber Pacing

Atrial Pacemakers

Atrial Demand Pacer

AAI Mode

The atrium is paced.

Atrial impulses are sensed.

When a native atrial impulse is sensed, the device will be inhibited from delivering a stimulus at that time.

Ventricular Pacemakers

Fixed-Rate Ventricular Pacer

VOO Mode

The ventricle is paced.

Ventricular impulses not sensed.

Ventricular pacing not triggered or inhibited.

Ventricular Demand Pacer

VVI Mode

The ventricle is paced.

Ventricular impulses are sensed.

When a native ventricular impulse is sensed, the device will be inhibited from delivering a stimulus at that time.

Dual-Chamber Pacing

Dual-chamber pacemakers consist of both atrial and ventricular leads capable of pacing and sensing. The principal advantage of DDD pacers is preservation of atrial-ventricular synchrony.

TABLE 13.1 Different Patterns of Sensing and Pacing Seen in Dual-Chamber Pacemakers

Atrial Complex

Ventricular Complex


ECG Appearance



The atrial channel senses a native atrial impulse and is inhibited from firing. This atrial impulse may successfully conduct to the ventricle (sensed by the ventricular channel) and result in a native QRS complex.




The atrial impulse is inhibited by sensing a native atrial impulse. The atrial impulse fails to conduct to the ventricles. The atrial channel triggers the ventricular channel to fire, resulting in a paced ventricular complex.




Atrial conduction depends on the firing of the atrial channel resulting in a paced atrial complex. This paced atrial impulse can conduct successfully to the ventricles and result in a native QRS complex.




Both atrial and ventricular conduction depend on the pacemaker. A paced ventricular impulse is delivered after a programmed interval following the paced atrial impulse.


Biventricular Pacing—Cardiac Resynchronization Therapy

Desynchrony in Heart Failure

Ventricular conduction delay is a common abnormality in patients with heart failure. Delay in ventricular activation renders the mechanics of ventricular contraction suboptimal.

Synchronizing left and right ventricular contraction through left and right ventricular pacemaker leads has been shown to improve clinical outcomes in randomized controlled trials.2,3


The left ventricular lead enters the ostium of the coronary sinus after passing through the right atrium. The lead passes through the coronary sinus and enters a cardiac vein and a tributary that leads to the ventricular wall. The electrode is often passively fixed in the midlateral aspect of the left ventricular wall. The final location of the left ventricular lead is variable and depends on the patient’s venous anatomy.

FIGURE 13.3 Anatomical positioning of the leads in a biventricular pacer.

ECG Appearance

Relative Narrowing of QRS

Right Bundle-Branch Block (RBBB) Morphology

The QRS morphology is variable and depends on the location of the left ventricular lead. Often, the morphology of the QRS complex changes to that of an RBBB.

Pacemaker-Mediated Tachycardia

Pacemaker-mediated tachycardia (PMT) is a form of tachycardia that involves the pacemaker itself as a part of the conduction pathway. Endless loop tachycardia is a form of PMT in which retrograde ventricular-atrial conduction results in repetitive atrial sensing and triggering of a ventricular impulse.


A premature ventricular contraction occurring after the atrial refractory period may be conducted in retrograde fashion past the AV node and result in atrial depolarization. Atrial depolarization then triggers ventricular pacing.

After a pacer stimulus is delivered to the ventricle, conduction can travel in a retrograde fashion to the atria through the normal conduction system.

The atrial electrode senses the retrograde atrial conduction. This triggers the ventricular electrode to pace the ventricle, and the cycle repeats until the retrograde ventricular-atrial conduction is interrupted.

ECG Appearance

Inverted P Waves

Retrograde conduction from ventricle to atria will result in inverted P waves in II, III, and aVF.

FIGURE 13.4 Conduction pathway of PMT.

FIGURE 13.5 ECG appearance of PMT in an inferior limb lead.


The ventricular rate should approximate the programmed upper limit of the pacemaker.

Nov 17, 2018 | Posted by in CARDIOLOGY | Comments Off on Paced Rhythms
Premium Wordpress Themes by UFO Themes