Definition/Introduction

Pacemakers are electric activity-generating devices used to treat patients with slow heart rates 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 1 or 2 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]

Issues of Concern

Common Indications for Pacemaker Placement

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

Less Common Indications

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

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 blocks, such as those with electrolyte abnormalities, Lyme disease, sleep apnea

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

Clinical Significance

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.

Transvenous Systems: Most cardiac pacing systems use transvenous electrodes to transmit electrical impulses from the pulse generator to the heart musculature.

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

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

Types of Pacemakers

There are 3 basic kinds of pacemakers:

• Single chamber. One lead attaches to the upper or lower heart chamber.
• Dual-chamber. Uses 2 leads, 1 for the upper and 1 for the lower chamber
• Biventricular pacemakers (used in cardiac resynchronization therapy). 

Modes of Cardiac Pacing

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

• Letter 1. In the area being paced, A stands for atria, V stands for Ventricle, D stands for Dual, O stands for none
• Letter 2. For the area that 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, and R stands for rate adaptiveness.

The modes are explainable by dividing them into categories of a 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, the pacemaker paces at a programmed rate regardless of the heart's intrinsic electrical activity.

• 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, the pacemaker paces at a programmed rate regardless of the heart's intrinsic electrical activity.

• AAI

           A- Pacing in the atrium

           A- Sensing in the atrium

           I- Inhibit

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 fully adapts to intrinsic heart rhythm and mimics normal conduction as much as possible.

DDD has 4 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 poor sinus and AV node function.

• 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 the sinus node function drops below the pacemaker's programmed lower rate, it causes AV dissociation due to the inability to pace in the atrium. VDD mode should only be used in patients with good SA node function. It might be used when the patient has a high pacing threshold in the atrium. This way, the pacemaker can 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 be to either pace or inhibit

This mode's primary use is in patients with atrial tachyarrhythmias and mode switch algorithms. DDI mode results in AV dissociation if the atrial rate exceeds the set rate. P wave tracking is excellent for AV synchrony; however, if the patient goes into atrial fibrillation with a rapid heart rate, one does not want to track the 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 intrinsic activity. DOO mode is asynchronous pacing and is 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 heart's inability to appropriately increase its rate with increased activity or metabolic demand, leading to exercise intolerance. Usually, there is a problem with the SA node function. The pacemaker utilizes its sensing ability with 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 wants to ensure that the patient has a viable atrial activity. If the atrium is healthy, the clinician wants to maintain AV synchrony as much as possible. The first important question is whether 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 is VVIR.
• If the patient is chronotropically competent, the mode of choice is VVI.

 Paroxysmal Atrial Arrhythmias:

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

AV Node conduction not Intact: 

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

Nursing, Allied Health, and Interprofessional Team Interventions

Pacemakers are implanted when the intrinsic electrical activity of the heart is either dyssynchronous or the patient has symptomatic episodes resulting from the asynchronous electrical impulse stimulated from the SA node. As mentioned above, different pacemaker types and modes are available, and the interprofessional team's closed-loop monitoring and deep and close monitoring of these modes are of the utmost importance in managing patients with cardiac pacemakers.