Artificial pacemakers

Artificial pacemakers (APs) are electrical devices that generate electrical impulses sent to the heart. Permanent pacemaker leads are implanted via thoracotomy or transvenous access, but some temporary emergency pacemakers can have leads placed on the chest.

There are several indications for the use of artificial pacemakers, but they generally include clinically significant bradycardia or high-grade AV block. Some tachyarrhythmias can be terminated by overdrive signals that capture the ventricles by creating short, higher-frequency shocks; the artificial pacemaker then slows to the chosen rate. In any case, ventricular arrhythmias are more amenable to instrumental treatment with devices that can perform cardioversion, defibrillation, and serve as a rhythm source (implantable cardioverter-defibrillators). Types of artificial pacemakers are designated by three to five letters denoting the following parameters:

• which chambers of the heart are stimulated; which chambers receive the impulse;
• how the artificial pacemaker responds to its own impulse (maintains or suppresses excitation);
• can it increase heart rate during exercise (HR-modifying);
• whether the stimulation is multi-chamber (in both atria, both ventricles, or more than one electrode in one chamber).

Indications for implantation

Arrhythmia	Shown (confirmed by research)	Possibly shown and supported by research or experience
Sinus node dysfunction	Bradycardia with clinical manifestations, including frequent, symptomatic sinus node skipping and bradycardia while taking appropriate medications (alternative approaches are contraindicated). Symptomatic chronotropic insufficiency (heart rate cannot meet physiological needs, i.e. it is too low to perform physical activity)	Heart rate < 40 beats per minute, when clinical manifestations are reliably associated with bradycardia. Syncope of unclear origin with pronounced dysfunction of the sinus node, recorded on the electrocardiogram or evoked during electrophysiological study
Tachyarrhythmia	Continuous pause-dependent VT with or without QT prolongation when pacemaker effectiveness is documented	High-risk patients with congenital long QT syndrome
After acute MI	Permanent second-degree AV block in the His-Purkinje system with bifascicular block or third-degree block at the level of the His-Purkinje system or below. Transient AV block of the second or third degree at the level of the AV node, combined with block of the branches of the His bundle. Permanent AV block of the second or third degree, accompanied by clinical symptoms	No
Multifascicular block	Intermittent third degree AV block. Type II AV block Alternating bifascicular block	There is no evidence that syncope is due to AV block, but other possible causes (especially VT) have been excluded. A greatly prolonged HF interval* (>100 ms) in asymptomatic patients, detected incidentally during electrophysiological examination. Nonphysiologic pacemaker-induced intraventricular block detected incidentally during electrophysiologic testing
Hypersensitive carotid sinus syndrome and neurocardiogenic syncope	Recurrent syncope with carotid sinus stimulation. Ventricular asystole lasting > 3 s with carotid sinus compression in patients not taking drugs that suppress the sinus node or AV conduction	Recurrent syncope without obvious triggering events and with a marked decrease in heart rate. Recurrent neurocardiogenic syncope with significant clinical manifestations associated with bradycardia, as confirmed clinically or by tilt table testing
After heart transplant	Bradyarrhythmias with clinical symptoms, suspected chronotropic insufficiency or other established indications for permanent cardiac pacing	No
Hypertrophic cardiomyopathy	Indications are the same as in case of sinus node dysfunction or AV block	No
Dilated cardiomyopathy	Indications are the same as in case of sinus node dysfunction or AV block	Refractory to drug therapy, accompanied by clinical symptoms, idiopathic dilated or ischemic cardiomyopathy with III or IV functional class of heart failure according to NYHA and prolonged QRS complex (130 ms), LV end-diastolic diameter of 55 mm and LV ejection fraction < 35% (biventricular pacing)
AV block	Any type of second-degree AV block associated with clinically evident bradycardia. Third-degree AV block or high-grade second-degree AV block at any anatomical level if associated with the following: Bradycardia with clinical symptoms (including heart failure), if it is believed to be associated with blockade; Arrhythmias and other conditions requiring the use of drugs that cause bradycardia; Documented asystole ≥3.0 sec or any rhythm <40 bpm in awake, asymptomatic patients; Catheter ablation of the AV junction; Postoperative block that did not resolve after the intervention; Neuromuscular diseases in which uncontrolled progression of conduction disturbances is possible (eg, myotonic muscular dystrophy, Cairns-Sayre syndrome, Erb's dystrophy, Charcot-Marie-Tooth disease with or without clinical manifestations)	Asymptomatic third-degree AV block at any anatomical level when the ventricular rate during walking is 40 beats per minute, especially with cardiomegaly or LV dysfunction. Asymptomatic second-degree block type 2 with a narrow QRS complex (pacemaker indicated for a wide complex). Asymptomatic second-degree block type 1 at or below the bundle branch detected during an electrophysiological study performed for other indications. First- or second-degree AV block with clinical manifestations suggestive of pacemaker syndrome.

*HB - interval from the beginning of the signal appearance in the His system to the beginning of the first ventricular signal. Source: Gregoratos G. et al. ACC/AHA/NASPE 2002 Guideline update for implantation of cardiac pac Vol. 106. -Suppl. 16. - P. 2145-2161.

For example, the IVR, encoded by WIR, generates (V) and conducts (V) an impulse in the ventricle, suppresses its own excitation (I), and can increase the frequency during physical exertion (R).

WI and DDD pacemakers are used most often. They have the same effect on survival, but physiological pacemakers (AAI, DDD, VDD) in comparison with WI reduce the risk of atrial fibrillation and heart failure and slightly improve the quality of life.

Advances in pacemakers include devices with lower power consumption, new batteries, and micro-glucocorticoid-release electrodes, which lower the pacing threshold, all of which increase the longevity of the pacemaker. The power-on option affects the automatic change of pacing type in response to the impulses sent (e.g., changing from DDDR to WIR during atrial fibrillation).

Pacemaker malfunction may include increased or decreased threshold for sensing the impulse being sensed, no stimulus or capture, or abnormal pacing rates. The most common abnormality is tachycardia. Rate-adjustable pacemakers may generate impulses in response to vibration, muscle activity, or when exposed to a magnetic field during MRI. In pacemaker-dependent tachycardia, a normally functioning dual-chamber pacemaker senses a premature ventricular impulse or sends an impulse that is conducted to the atrium via the AV node or back along the accessory pathway, resulting in pacing of the ventricles at a high rate, cyclically. Another complication associated with a normally functioning pacemaker is crossover inhibition, in which the ventricular pathway senses the atrial pacing impulse when using a dual-chamber pacemaker. This leads to inhibition of ventricular stimulation and the development of "pacemaker syndrome", in which disruption of conduction through the AV node due to ventricular stimulation leads to the appearance of dizziness, unsteadiness of gait, cerebral, cervical (swelling of the jugular veins) or respiratory (dyspnea) symptoms.

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Coding of artificial pacemakers

1	II	III	IV	V
Stimulated	Perceiving	Response to event	Changing frequency	Multi-chamber stimulation
A - atrium V - ventricle D - both cameras	A - atrium V - ventricle D - both cameras	0 - no 1 - inhibits the pacemaker T- stimulates the pacemaker to excite the ventricles D - both chambers: stimuli perceived in the ventricle inhibit; enhances stimuli perceived in the ventricle Perceived in the atrium	0 - non-programmable R - with the ability to change heart rate	0 - no A - atrium V - ventricle D - both cameras

Environmental exposure includes exposure to electromagnetic radiation sources such as a surgical knife or MRI, although MRI may be safe if the pacemaker and electrodes are not inside the magnet. Cell phones and electronic security systems are potential sources of exposure; phones should not be placed near the pacemaker, but talking on them is safe. Walking through metal detectors does not interfere with the pacemaker unless the patient lingers in them.

Complications from implantation of artificial pacemakers are rare, but myocardial perforation, bleeding, and pneumothorax are possible. Postoperative complications include infection, displacement of the electrodes, and the pacemaker itself.

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