Overview

Heart enlargement, or cardiomegaly, occurs when an underlying health problem causes the cardiothoracic ratio to exceed 50%. It is a compensatory mechanism, which is temporarily effective because it helps the heart pump blood to meet the body's demands. However, over time, the remodelling of the heart’s structure to perform this function becomes a burden since it increases the workload on the heart.¹ This can lead to inefficient cardiac output and serve as a precursor for heart failure or sudden cardiac death (SCD). Heart enlargement can cause arrhythmia, which is a risk factor for SCD.² This warrants implantable cardioverter defibrillators (ICDs), which can be put under a patient’s skin and aim to restore normal heart function. This article will explore the criteria for recommending ICDs in patients with heart enlargement and how it works as a life-saving option for these patients.

Understanding heart enlargement 

Heart enlargement, or cardiomegaly, is not a disease itself but a symptom of underlying conditions such as hypertension, heart valve disease, coronary artery disease, congenital heart defects, or heart failure. It is commonly diagnosed using chest X-rays, where a cardiothoracic ratio greater than 50% indicates enlargement. These conditions can impair the heart by causing faulty or narrowed valves, reducing blood flow, or affecting the heart muscle and structure. The heart works harder to pump blood to compensate, leading to structural remodelling and enlargement. While initially beneficial, this adaptation becomes unsustainable over time. Sustained heart enlargement can result in stiffening of the heart muscle, reducing its ability to contract effectively and ultimately compromising its function.¹

Patients with heart enlargement may experience:¹

• Shortness of breath
• Chest pain
• Arrhythmia
• Fatigue
• Dizziness
• A reduction in the ability to perform daily activities

Since cardiomegaly is detrimental to the heart’s function, it can lead to SCD. For example, heart enlargement is considered a precursor for heart failure because of the life-threatening arrhythmias caused by the heart becoming too weak to pump blood properly. This enhances the likelihood of SCD because there is a reduced cardiac output.²

What are implantable cardioverter defibrillators?

ICDs emerged nearly 5 decades ago and are now essential in successfully managing fatal arrhythmias. They are usually implanted just below the collarbone or at the side of the chest, and the wires are inserted through a vein that leads to the heart. ICDs are constantly checking for life-threatening arrhythmias, and if they detect one, it elicits a high-energy shock that aims to ‘reset’ the heart. This not only stops the arrhythmia but also restores the normal rhythm of the heart’s natural pacemaker.³

Not only are ICDs life-saving, but they also record the arrhythmias they work against. This means that doctors and clinicians can review this data and make necessary adjustments to the patient’s treatment plan, enhancing personalised medicine.³

The success rate of ICDs is mainly dependent upon the patient’s age. The fact that patients are given this device means it is a supportive therapy rather than treating the underlying cause. The patients are more likely to accumulate comorbidities as they age, which can worsen the heart’s function, rendering ICDs less effective. This is supported by the fact that one study found that the 1-year survival rate was 92%, but the 5-year survival rate for another study was 64%.^4,5 Therefore, ICDs are not solely relied on as a long-term therapy for people with heart enlargement. ACE inhibitors or beta-blockers are likely to be prescribed alongside this management of heart enlargement.

The surgery that inserts the ICD into the patient is generally safe, but a small risk of complications can accompany this:⁶

• ICD bleeding
• Infection
• Wire movement
• Damage to blood vessels, lungs, or heart

What are the criteria for recommending ICDs?

The decision to recommend an ICD for patients with heart enlargement is based on a combination of clinical factors and the health service's guidelines. Therefore, different guidelines have different criteria for recommending ICDs. 

According to the National Institute for Health and Care Excellence (NICE), ICDs are recommended for people who have had serious arrhythmia and may have experienced:⁷

• A cardiac arrest
• Spontaneous sustained ventricular tachycardia
• Sustained ventricular tachycardia without a cardiac arrest, but have a left ventricular ejection fraction less than 35% (which indicates the heart is not pumping blood effectively)

The New York Heart Association classifies heart failure patients into classes based on the severity of their symptoms and the extent to which they interfere with daily life. Therefore, patients with class 2 or 3 heart failure symptoms are considered eligible for an ICD. These patients have limited physical activity, and their less demanding activities cause them to present with fatigue, palpitations, or breathlessness.⁸

If the patient has a family history of heart disease, then they may have a genetic predisposition for a specific disease that causes heart enlargement. Examples of inherited cardiac conditions include hypertrophic cardiomyopathy and dilated cardiomyopathy, which are strongly associated with a high risk of arrhythmias, warranting the use of ICDs.⁹

Summary

Heart enlargement is a product of an underlying health condition that reduces the heart’s ability to pump blood effectively. As a result, the heart is remodelled, temporarily enabling the heart to meet the body’s demand for blood. However, this is detrimental in the long term because the increased stretching and build-up of the heart muscle makes it stiff and impair its contractile ability, leading to arrhythmias and possibly sudden cardiac death. Since the advent of ICDs over 40 years ago, they have demonstrated success in preventing sudden cardiac death through the detection of ventricular arrhythmia. They work by shocking the heart after detecting these arrhythmias, aiming to ‘reset’ the heart to restore the heart’s pacemaker. Some risks of implanting ICDs include infection and wire movement that can interfere with blood vessels. ICDs are recommended for people who have had previous heart-related incidents, such as cardiac arrest or ventricular tachycardia. Many organisations recommend using ICDs if the left ventricular ejection fraction is less than 35% because this suggests poor cardiac output. If patients experience symptoms of heart enlargement or their underlying health conditions during ordinary activity, they are also recommended an ICD.