ICD - Principles

Epicardium / visceral pericardium - the outer layer of the heart wall
Pericardial cavity - contains pericardial fluid
Parietal layer of serous pericardium - fused with fibrous pericardium
Outer fibrous pericardium - tough dense connective tissue that protects the heart and holds it in position.

Septum (Heart | Cleaveland Clinic, 2024) :
Layers of muscular tissue that separates heart walls into left and right sides.

2.1.2 Heart Chambers

Referring to Figure 16,
The heart consists of 4 chambers: (Heart | Cleaveland Clinic, 2024)

Right Atrium
Right Ventricle
Left Atrium
Left Ventricle

2.1.3 Heart Valves

Referring to Figure 16,
Heart Valves - open and close to allow blood to flow through the heart. It also prevents the blood from backflowing to the previous chamber or parts.

There are 2 types of Heart Valve: (Heart | Cleaveland Clinic, 2024)

Atrioventricular Valve:

1. Tricuspid Valve - Door between your right atrium and right ventricle.

2. Mitral Valve - Door between your left atrium and left ventricle.

Semilunar Valve:

1. Aortic Valve - Opens when blood flows out of your left ventricle to the Aorta (the artery that carries oxygen-rich blood to your body).

2. Pulmonary Valve - Opens when blood flows from your right ventricle to the Pulmonary Artery (the only arteries that carry oxygen-poor blood to your lungs).

2.1.4 Blood Vessels

3 Main Types of Blood Vessels: (Heart | Cleaveland Clinic, 2024)

Arteries - Carry blood away from the heart. Usually, it carries oxygenated blood (except the pulmonary artery)
Veins - Carry blood towards the heart. Usually, it carries deoxygenated blood (except the pulmonary vein)
Capillaries - small blood vessels where the body exchanges the oxygen-rich and oxygen-poor blood.

Figure 12: Artery, Vein and Capillaries (The Editors of Encyclopaedia Britannica, 2024)

Figure 13: Outside of heart (Heart | Cleaveland Clinic, 2024)

Coronary Arteries - The heart receives nutrients through networks of coronary arteries (Heart | Cleaveland Clinic, 2024)

Types of Coronary Arteries:

Left Coronary Artery - Divides into 2 branches:

a. Circumflex Artery: Supplies blood to the left atrium and the side and back of the left ventricle.

b. Left Anterior Descending Artery - Supplies blood to the front and bottom of the left ventricle and the front of the septum.

Right Coronary Artery - Supplies blood to the right atrium, right ventricle, bottom portion of the left ventricle and back of the septum.

2.1.5 The Electrical Conduction System

The Electrical Conduction System of the Heart controls the rhythm and pace of the heartbeat. The list below outlines the different functions of the parts making up the system:

Figure 14: Heart's Electrical System (Anatomy and Function of the Heart’s Electrical System, 2021)

Table 1: Functions of the Different Parts of the Conduction System of the Heart

2.2 Physiology of the Heart

Process of Cardiovascular Cycle:

The Process of the heart pumping the blood results in 2 heartbeats per cycle, producing a lub-dub sound (Blood Flow Through the Heart | Cleaveland Clinic, 2024b)

On the right side ( 1st heartbeat )

Deoxygenated blood from all over your body enters your right atrium through two large veins, your superior vena cava and inferior vena cava. These veins drain blood from your upper body and lower body, respectively, and directly empty it into your right atrium.
The tricuspid valve opens to let blood travel from the right atrium to the right ventricle.
When the right ventricle is full it squeezes, which closes the tricuspid valve and opens the pulmonary valve.
Blood flows through the main pulmonary artery and its branches to the lungs, where it gets oxygen and releases carbon dioxide.

On the left side ( 2nd heartbeat )

Oxygenated blood travels from your lungs to the left atrium through large veins called pulmonary veins. These veins directly empty the blood into the left atrium.
The mitral valve opens to send blood from the left atrium to the left ventricle.
When the left ventricle is full it squeezes, which closes the mitral valve and opens the aortic valve.
The heart sends blood through the aortic valve to rhe aorta, where it flows to the rest of the body.

2.3 Heart rate, Heartbeat and its Complications

Figure 15: parts of a heart (Cawthorne, n.d.)

Normal Heart Beats Per Minute (bpm):

60 - 100 bpms

Each heart beat represents the heart pumping blood to the rest of the body.

Factors affecting heart rate:

Movements ( exercise and relaxation )
Age
Stress
Anxiety
Temperature
Etc

Figure 16: heart wave

Figure 17: ECG of a normal healthy heart activity (J.S.S.G. de Jong, 2011)

Figure 18: ECG of an abnormal heart activity (Leach, 2020)

Heart pumping is driven by its rhythm which is regulated by electrical impulses generated by the Sinus node (SA node), located in the right atrium. Acting as the heart's natural pacemaker, the SA node sends consistent electrical signals that trigger coordinated contractions of the chambers, ensuring efficient blood flow.

These electrical signals can be captured with an Electrocardiogram (ECG), allowing better understanding of the users heart.

The natural heart rhythm can, however, be disrupted by abnormalities within the heart that results in an erratic ECG reading of the heart.

Hence, ICDs are designed just for that ; To detect and correct autonomously by delivering small painless pulses to resist the abnormal heartbeats, Heart Arrhythmias.

2.3.1 Heart Arrhythmias

Heart Arrhythmias refers to the irregularity in heartbeat. There are 2 groups of heartbeat irregularities (Mayo Clinic, 2023):

Tachycardia ( >100 bpm )

Rapid heart rate results in insufficient oxygen-rich blood pumped to the rest of the body due to the right and left ventricles not being properly filled.

- Ventricular Fibrillation (Cardiac Arrest)

Heart quivers instead of contract when electrical signals are not working properly.

Bradycardia ( <60 bpm )

Slowed heart rate results in insufficient oxygen-rich blood pumped to the rest of the body.

Figure 19: ECG comparison between Fast and Slow heartbeats

(What Does an Abnormal ECG Mean?, 2023)

2.3.3.1 Common types of Arrhythmias

There are 6 common types of arrhythmia listed as follows (National Heart, Lung, and Blood Institute, 2022):

Figure 20: ECG of patient with Atrial fibrillation (Cadence Heart Centre, 2022)

Atrial Fibrillation (AFib)

An irregular, rapid heart rhythm originating in the atria, often with no distinct pattern, causing uneven atrial contractions and raising the risk of stroke

Figure 21: ECG of patient with Atrial flutter (“Atrial Flutter” Images – Browse 690 Stock Photos, Vectors, and Video, n.d.)

Atrial Flutter (AF)

Fast but regular rhythm also starting in the atria, usually with a characteristic "sawtooth" pattern on ECG, generally more organized than AFib.

Figure 22: ECG of patient with Supraventricular tachycardia (SVT) (“Supraventricular Tachycardia” Images – Browse 115 Stock Photos, Vectors, and Video, n.d.)

Supraventricular Tachycardia (SVT)

Any abnormally fast rhythm arising above the ventricles, often presenting as a sudden increase in heart rate.

Figure 23: ECG of patient with Ventricular tachycardia (VT) (Ventricular Tachycardia | Shutterstock, n.d)

Ventricular Tachycardia (VT)

Rapid rhythm beginning in the ventricles, typically with a wide QRS complex on ECG and a risk of progression to more severe arrhythmias.

Figure 24: ECG of patient with Ventricular Fibrillation (Ventricular Fibrillation | Shutterstock, n.d)

Ventricular Fibrillation (VFib)

A chaotic, disorganized rhythm in the ventricles, leading to ineffective contractions and requiring immediate intervention due to the risk of sudden cardiac arrest.

Figure 25: ECG of patient with Bradycardia (BradyCardia Images – Browse 2,382 Stock Photos, Vectors, and Video, n.d.)

Bradycardia

Slow heart rhythm than the normal heart rhythm of a healthy person.

2.3.2 Factors

Factors that affect the Probability of Arrhythmias (Heart Arrhythmias - Symptoms & Causes | Mount Elizabeth Hospitals, 2022):

Heart scarring from previous heart attack
Too much Alcohol
Too much Caffeine
Diabetes
Stress (High Blood Pressure)
Smoking

2.4 ICD

ICD today as we know, is as small as a matchbox, yet it is able to provide significant advance cardiac care.

2.4.1 Components of ICD

Figure 26: parts of an ICD (Implantable Cardioverter Defibrillator | SingHealth, 2019)

ICD consists of: (Implantable Cardioconverter-Defibrillators(ICD | Stanford Medicine, n.d.)

Pulse Generator - tiny computer that monitors and regulates the heartbeat consistently by generating energy to deliver shocks or pace impulses when arrhythmia occurs.
Lead - wire that deliver electrical signals in between the pulse generator to the heart chamber.
Electrode - located at the end of the leads, help in detecting and delivering signals or energy.
Battery

2.4.2 How does ICD works?

When Heart Arrhythmia occurs,

Electrical signal travel up from the heart to the ICD through the leads, so that ICD can read the heartbeat
When ICD recognize the irregular heartbeat, pulse generator generate energy, delivering shocks or pace impulse to the heart chamber.
Heartbeat wil return to normal state

2.4.3 Functions of ICD

Generally, the Functions of the ICD are (Implantable Cardioverter-Defibrillator (ICD) | Stanford Health Care, n.d.)

Monitor - perpetually monitor's heart electrical activity until battery life is finished.
Detection - detects abnormal heart rhythms through advanced algorithms in the ICD.
Intervention - restoration of normal heart rhythm via

a) Defibrillation (high energy shock for fast paced rhythm detection), and/or

b) Pacing by sending low-energy electrical impulses for slow-paced rhythms.

Data Recording - for health care professionals to review in the RAM and ROM in ICDs.

2.4.4 Types of ICD

There are two types of ICD which has similar capabilities but different function - Transvenous and Subcutaneous.

Figure 27: T-ICD (Lewis & Gold, 2015)

Transvenous ICD (T-ICD)

Leads connect directly into the right ventricular through the veins, with electrodes at the tip to secure contact. T-ICD continuously monitors the heart rhythms for irregularities. Upon detection, it responds with the necessary shock therapies autonomously. It is also able to provide both anti-tachycardia pacing and anti-bradycardia pacing to restore a normal heartbeat (Cawthorne, n.d.).

Figure 28: S-ICD (Lewis & Gold, 2015)

Subcutaneous ICD (S-ICD)

Leads are placed just under the skin. This reduced the unnecessary risks associated with the Transvenous technique. However, S-ICD is not capable of offering any forms of pacing, instead is only able to provide defibrillation shocks to address heart arrythmias (Sahu et al., 2023).

2.4.4.1 Types of Transvenous ICDs

Figure 29: Types of Transvenous ICD (Implantable Cardioverter Defibrillator | SingHealth, 2019)

2.5 Materials in ICD Manufacturing

Since ICD need to be implemented inside the body, the materials used need to be carefully selected to balance our functionality, durability and biocompatibility with the human body. (Biomaterials for pacemakers, defibrillators and neurostimulators | Biomaterials for artificial organs 2016.)

Table 2: Materials used in ICD manufacturing

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