Paced Rhythms
The Pacemaker
 FIGURE 13.1 A. Anatomy of a dual-chamber pacemaker. B. ECG appearance of atrial and ventricular pacing. |
Electrode Types
Unipolar
Cathode is on the lead tip; anode is located proximally within the pulse generator.
Pacemaker artifacts are generally more conspicuous.
Bipolar
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.
Indications
Sinus node dysfunction in patients with normal AV node function.
Ventricular Pacemakers
Fixed-Rate Ventricular Pacer
VOO Mode
The ventricle is paced.
Ventricular impulses not sensed.
Ventricular pacing not triggered or inhibited.
Indications
Complete AV block.
Related Complications
This pacer fires in an uninhibited fashion such that paced impulses are asynchronous with a patient’s native rhythm. Paced impulses may arrive at the end of the T wave and precipitate an arrhythmia.
Pacemaker Syndrome
Loss of atrial-ventricular synchrony can lead to a decrease in cardiac output. Common symptoms of asynchrony include fatigue, shortness of breath, and light-headedness. This can also occur in VVI mode.
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.
Indications
Complete AV block in the setting of permanent atrial fibrillation. This is the mode of a temporary transvenous pacing wire.
Related Complications
Oversensing
A VVI pacemaker may erroneously interpret extracardiac artifacts as the patient’s native ventricular impulses. Artifact can be produced by muscle tremors and electromagnetic interference. The device’s response is to inhibit the delivery of impulses. This can result in syncope.
Pacemaker Syndrome
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 | ||||||||||||||||||||
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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.
Class I Indications4
Ejection Fraction ≤ 35%
LBBB with QRS ≥ 150 msec
Sinus Rhythm
NYHA Class II, III, or IV Heart Failure
Placement of CRT in patients with a non-LBBB pattern and a QRS duration ≥150 seconds, patients with an LBBB pattern but a QRS 120-149 seconds in duration, or patients with atrial fibrillation is considered a class IIa recommendation.
Anatomy
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.
Mechanism
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. |
Rate
The ventricular rate should approximate the programmed upper limit of the pacemaker.
Mistaken For:
Sinus Tachycardia
Ventricular tachycardia
(if pacer artifacts from bipolar leads are barely visible)
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