Pacemakers are electric activity generating devices which are used for the treatment of patients with slow heart rate, or symptomatic heart blocks and in patients with heart failure.[1] All cardiac pacemakers are generally composed of a pulse generator that generates the electrical current required for stimulation of heart musculature and one or two electrodes (also referred to as leads), which are responsible for transmitting the electrical activity generated by the pulse generator to the heart musculature.[2][3]

Common indications for pacemaker placement include:

1. Sinus node dysfunction ( Class I indication)[4]
2. Acquired AV Block
3. Post myocardial infarction[5]

Other less common indications include:

• Congenital complete heart block
• Long QT syndrome
• Hypertrophic cardiomyopathy[6][7]
• Heart failure

Following are the conditions in which cardiac pacing is not indicated:

• Syncope of undetermined etiology
• Sinus bradycardia without significant symptoms
• Sinoatrial block or sinus arrest without significant symptoms
• Asymptomatic prolonged PR intervals with atrial fibrillation
• Asymptomatic bradycardia during sleep
• Asymptomatic 2nd-degree Mobitz I (Wenckebach ) AV block
• Right bundle branch block with left axis deviation without syncope or other symptoms compatible with intermittent AV block
• Reversible AV block such as those with electrolyte abnormalities, Lyme disease, sleep apnea.

PACEMAKER TYPES AND SYSTEMS:

Pulse Generators:  These are the "battery" component of the pacemaker, which normally produces the electrical activity required to transmit to the heart musculature. Pulse generators are currently placed most commonly in the infraclavicular region of the anterior chest wall.

Trans venous Systems: Most of the cardiac pacing systems make use of the Transvenous electrodes for transmitting electrical impulses from the pulse generator to the heart musculature.

Epicardial systems: These work by the direct stimulation through the pulse generator by attaching directly to the surface of the heart. They are less common use nowadays, and transvenous pacing has completely replaced them.

Leadless systems: There have been some newer innovations to develop leadless systems due to some limitations with transvenous and epicardial pacing systems.[8][9][10] 

Types of Pacemakers:

There are three basic kinds of pacemakers:

• Single chamber. One lead attaches to the upper or lower heart chamber
• Dual-chamber. Uses two leads, one for the upper and one for the lower chamber
• Biventricular pacemakers (used in cardiac resynchronization therapy). 

Modes of Cardiac Pacing:

The modes of pacemakers are based on generic code known as NBG ( combined from NASPE/BPEG) and typically consist of 5 letters.[11]

• Letter 1. The area being paced, A stands for atria, V stands for Ventricle, D stands for Dual, O stands for none
• Letter 2. The area which is sensed, A stands for atria, V stands for Ventricle, D stands for Dual, O stands for none
• Letter 3.  The response of the pacemaker to sensing: O stands for none, I stands for inhibiting, T stands for triggering, D stands for dual
• Letter 4. Rate adaptiveness. O stands for none, R stands for rate adaptiveness

The modes are explainable by dividing them into categories of single chamber or dual chamber:

Single Chamber Modes

• VOO

V- Pacing in the ventricle

O- Sensing is OFF

O- Response to sensing is OFF

In this mode, pacemaker paces at a programmed rate regardless of the intrinsic electrical activity of the heart.

• VVI

V- Pacing in the ventricle

V- Sensing in the ventricle

I-Inhibit

In this mode, the pacemaker can sense the electrical activity and withhold pacing when not required.

• AOO  

A- Pacing in the atrium

O- Sensing is OFF

O- Response to sensing is OFF

In this mode, pacemaker paces at a programmed rate regardless of the intrinsic electrical activity of the heart.

• AAI

           A- Pacing in the atrium

           A- Sensing in the atrium

           I- Inhibit

If the patient is chronotropically incompetent, the mode of choice will be DDDR

If the patient is chronotropically competent, the mode of choice will be DDD.

In this mode, the pacemaker can adapt to the intrinsic atrial rate and should be able to pace when needed and inhibit when not required.

Dual Chamber Modes

Dual Chamber Modes can further subdivide into Tracking Modes and Non-Tracking modes.

Tracking Modes:

• DDD

D- Pacing in the atrium and ventricle

D- Sensing in the atrium and ventricle

D- Inhibit and or trigger

Intrinsic P-wave and QRS can inhibit pacing, and intrinsic P-wave or atrial pace can trigger an AV delay.

This mode is fully capable of adapting to intrinsic heart rhythm and mimicking normal conduction as much as possible.

DDD has four distinct pacing patterns

1. AsVs (Atrial sensed ventricle sensed): Used when the patient has good sinus node function and good AV node function
2. AsVp (Atrial sensed Ventricular paced): Used when the patient has a good sinus node function but poor AV node conduction
3. ApVs (Atrial paced Ventricular sensed): Used when the patient has poor sinus node function but has intact AV node conduction
4. ApVp (Atrial paced Ventricular paced): Used when the patient has a poor function in both the sinus node and AV node.

• VDD

V- Pacing in the ventricle

D- Sensing in the atrium and ventricle

D- Inhibit and or trigger

Intrinsic QRS can inhibit ventricular pacing, and Intrinsic P-wave can trigger an AV delay.

In this mode, one cannot pace the atrium, but an intrinsic atrial activity can trigger an AV delay resulting in P-wave tracking and possibly maintaining AV synchrony.

The primary issue with VDD programming mode is that if sinus node function drops below the pacemaker programmed lower rate, then it will cause AV dissociation due to the inability to pace in the atrium.

VDD mode should only be used in patients with good SA node function. Might be used in a situation where the patient has a high pacing threshold in the atrium. This way, the pacemaker will be able to sense in the atrium, maintain AV synchrony, and not waste battery life by pacing in the high threshold atrium.

 Non-Tracking Modes

• DDI

D - Pacing in the atrium and ventricle

D - Sensing in the atrium and ventricle

I - Response to that sensing will be to either pace or inhibit

This mode's primary use is in patients with atrial tachyarrhythmias and mode switch algorithms. DDI mode will result in AV dissociation if the atrial rate goes high than the set rate. P wave tracking is excellent for AV synchrony; however, if the patient goes into atrial fibrillation with rapid heart rate, one does not want to track atrium and pace the ventricle at a high rate.

• DOO

D- Pacing in the atrium and ventricle

O- Sensing is OFF

O- Response to that sensing is OFF

This mode results in AV sequential pacing at the lower rate limit regardless of the heart's own intrinsic activity. DOO mode is asynchronous pacing and usually used only in certain situations, such as when a magnet is placed over a pacemaker or sometimes when a patient is having surgery.

R- Rate Response

Rate Response or Rate Adaptive Pacing is used in patients with chronotropic incompetence. Chronotropic incompetence is defined as the inability of the heart to appropriately increase its rate with increased activity or metabolic demand that leads to exercise intolerance. Usually, there is a problem in SA node function.

The pacemaker utilizes its sensing ability with the aid of sensors that can sense motion or minute ventilation changes according to the activity and pace the heart at a required rate.

Choosing a Pacing Mode:

While selecting a pacemaker mode, the clinician will want to make sure that the patient has a viable atrial activity. If the atrium is healthy, the clinician will want to maintain AV synchrony as much as possible.

The first important question is if the sinus node function is intact?

If sinus node function is not intact and the patient has atrial arrhythmias:

 Chronic Atrial Arrhythmias, e.g., Atrial Fibrillation or Atrial Flutter:

• If the patient is chronotropically incompetent, the mode of choice will be VVIR
• If the patient is chronotropically competent, the mode of choice will be VVI.

 Paroxysmal Atrial Arrhythmias:

• If the patient is chronotropically incompetent, the mode of choice will be DDDR
• If the patient is chronotropically competent, the mode of choice will be DDD.

If the sinus node is intact and the patient has normal sinus rhythm or sinus bradycardia:

Intact AV Node conduction

• If the patient is chronotropically incompetent, the mode of choice will be AAIR.
• If the patient is chronotropically competent, the mode of choice will be AAI.
• If the patient is chrontropically incompetent, the mode of choice will be DDDR
• If the patient is chronotropically competent, the mode of choice will be DDD.

AV Node conduction not Intact: 

• If the patient is chrontropically incompetent, the mode of choice will be DDDR.
• If the patient is chronotropically competent, the mode of choice will be DDD.

Pacemakers are implanted in situations when the intrinsic electrical activity of the heart is either dyssynchronous, and the patient is having symptomatic episodes resulting from the asynchronous electrical impulse stimulated from the SA node. Different pacemaker types and modes as mentioned above are available, and a deep and close monitoring and understanding of these modes and close-loop monitoring by the inter-professional team is of utmost importance in the management of patients with cardiac pacemakers.

Pacemaker Related Complications:

Pacemaker Syndrome: Pacemaker syndrome is a disease that represents the clinical consequences of suboptimal atrioventricular (AV) synchrony or AV dyssynchrony, regardless of the pacing mode after pacemaker implantation.

Symptoms of Pacemaker Syndrome are:

• Cannon A-waves
• Chest pain
• Confusion
• Dizziness
• Fatigue
• Palpitations
• Shortness of breath
• Syncope

Other complications associated with Pacemakers include:

• Pneumothorax
• Cardiac perforation
• Significant pocket hematoma
• Lead dislodgement
• Venous thrombosis and obstruction
• Mechanical lead complications