Overview 

What is sleep apnoea?

Sleep apnoea is a term used to describe a range of breathing-related sleep disorders, generally characterised by apnea and hypopnoea, with apnoea being a lack of breathing for 10 seconds or more while hypopnea is shallow breathing in one's sleep lasting for 10 seconds or more.^1,2 Other symptoms of sleep apnea are: 

• Snoring
• Having a dry mouth upon waking
• A morning headache
• Difficulty sleeping
• Daytime tiredness

Some forms of sleep apnea are:

• Obstructive sleep apnoea:  known as the most common form of sleep apnea, it is caused by an obstruction in the upper airway during sleep, for example, the tongue, and typical symptoms include apnea, hypopnoea and snoring^1,2
• Central sleep apnoea:  a rare form of sleep apnea that occurs when the area of the brain responsible for controlling breathing rate has reduced activity.² This reduction of brain activity is caused by obesity or the use of opioids.² 

Obstructive sleep apnoea is the most common form of sleep apnoea and is linked to cardiovascular issues, which affect the heart and blood vessels; 34% of men and 17% of women are believed to have the condition though it is chronically undiagnosed.² Obstructive sleep apnoea is also a risk factor for many cardiovascular conditions; reduced oxygen intake leads to the heart overexerting itself to account for poor oxygen levels in the blood.²

Risk factors and causes

There are risk factors associated with the onset of sleep apnoea disorders. These include: 

• Obesity: the link between obesity and sleep apnoea is mostly mechanical. Fat deposits in the neck and airway obstruct the airway and reduce the amount of air that can get through²
• The use of opioids: this is specific to central sleep apnoea² 
• Being older
• A narrowed airway is caused by tonsil size
• A history of cardiovascular events such as stroke or heart failure. Central sleep apnea is specifically associated with heart failure²
• Being male²
• A family history of sleep apnea

What is heart enlargement? 

An enlarged heart, also known as cardiomegaly, is when the heart muscles are abnormally thick or stretched, causing the heart to increase in size. Specifically, heart enlargement describes a heart that, at its greatest width, is at least half the size of the chest cavity.³ Heart enlargement is a symptom of an underlying issue and isn’t associated with specific symptoms, however, when symptoms do arise they align with general cardiac insufficiency:³ 

• Dyspnoea, or shortness of breath especially when doing exercise
• Peripheral oedema, or swelling in the lower legs and arms
• Fatigue
• Heart palpitations or the feeling of your heart fluttering³

Heart enlargement is a symptom of an underlying issue. Many cardiovascular issues can cause heart enlargement, some of which are:

• High blood pressure
• Coronary artery disease
• Various forms of cardiomyopathy
• Pulmonary diseases³

Sleep apnoea and how it contributes to heart enlargement 

Sympathetic nervous system overactivation

The sympathetic nervous system is responsible for the fight-or-flight reflex. When oxygen intake decreases, the sympathetic nervous system is triggered causing increased:

• Blood pressure
• Heart rate
• Breathing rate⁴

This is beneficial when scared or in danger but in people with sleep apnoea, the sympathetic nervous system is triggered multiple times a night and the effects continue during the day. Over time, this can lead to heart strain caused by continued high blood pressure and a faster daytime heart rate.⁴ Chronic strain of the heart muscles has been linked to a thickening of the heart muscles, known as left ventricular hypertrophy, causing an enlarged heart.⁵

Low oxygen levels in the blood

Apnoea and hypopnea cause low oxygen levels in the blood. When oxygen levels are low, reactive oxygen species are formed through complex mechanisms creating inflammation in the body, damaging endothelial tissue, and decreasing levels of nitric oxide.⁶ 

Endothelin is a protein made by endothelial cells that makes blood vessels smaller. Nitric oxide is a molecule that reduces and balances the levels of endothelin in the body.⁶ When endothelial cells become dysfunctional due to damage caused by high levels of reactive oxygen species, they begin to release more endothelin than usual.⁶ 

Patients with sleep apnoea have been found to have endothelial dysfunction and low nitric oxide levels, resulting in high levels of endothelin, an increased risk of cardiovascular disorders, and increased blood pressure.^4,6 As cardiovascular disorders and heart strain cause thickening of the heart, this is another way sleep apnoea contributes to heart enlargement.

Complications associated with sleep apnoea that may lead to an enlarged heart 

The decrease in blood oxygen levels leads to the heart working harder to increase the levels of blood in the body. Therefore, many of the complications associated with heart enlargement are related to the cardiovascular system:

• High blood pressure
• Heart failure
• Arrhythmias
• Coronary heart disease 
• Atrial fibrillation^1,2,7

Risk factors 

Multiple risk factors associated with sleep apnea are also risk factors associated with cardiovascular diseases that may contribute to the development of heart enlargement.⁷ Additionally, not everyone that develops sleep apnoea will develop heart enlargement, however, some factors increase the chance of developing an enlarged heart: 

• Age: being older increases the chance of developing sleep apnoea and an enlarged heart
• The male sex: men are twice as likely as women to develop sleep apnoea
• Obesity: obstructive sleep apnoea can be caused by fat deposits
• Race: in America, it was found that African Americans were most likely to develop sleep apnoea
• High blood pressure: can cause strain on the heart causing it to thicken
• Underlying cardiovascular diseases: coronary heart disease, heart failure, arrhythmias⁴

Diagnosis of heart enlargement 

Diagnosis of heart enlargement is done by assessing the heart's size and how well the heart functions:

• A chest X-ray: to determine how large the heart is³ 
• A cardiac MRI scan: a test that uses sound waves to determine the house
• Electrocardiogram or ECG: a test done to measure the electrical activity of the heart
• Echocardiogram: a test that uses sound waves to determine the heart's size and function
• Stress tests: tests done to determine how well the heart responds to activity

Prevention and treatment of heart enlargement 

Reducing the risk of heart enlargement aligns with healthy living practices: 

• Losing weight if obese
• Smoking cessation 
• Alcohol
• Exercise
• Healthy diet^3,7 

Treating heart enlargement means treating the underlying condition. This often means taking medications for hypertension or other cardiovascular disease.³ 

Summary

• Sleep apnoea is a breathing-related sleep disorder causing poor sleep quality and reduced oxygen intake, and is associated with several cardiovascular conditions, in particular heart failure
• Heart enlargement isn’t its own condition but is a symptom of underlying cardiovascular disorders and is found in many patients with heart failure
• Sleep apnoea contributes to an enlarged heart through many mechanisms, but the most prominent is through the overactivation of the sympathetic nervous system, causing an increase in blood pressure, heart rate and breathing rate
• Long-term overactivation of the sympathetic nervous system is associated with higher daytime blood pressure and heart rate, overexerting the heart and causing it to thicken.
• Sleep apnoea is also associated with high endothelin but low nitric acid levels causing increased blood pressure
• Heart enlargement is associated with cardiovascular disorders, being of older age, being male and obesity
• A variety of imaging tests done on the heart are used to diagnose heart enlargement
• Preventive measures of heart enlargement centre around taking on healthy living measures such as exercise, reducing smoking and alcohol intake, healthy eating, and losing weight if obese