Introduction 

The human heart is remarkable as an organ for pumping oxygenated blood and essential nutrients around the body. The heart beats around 100,000 times a day, pumping approximately 5 litres of blood.  Meanwhile, the heart also removes waste products, such as carbon dioxide, which the body does not need. 

The heart is divided into 4 chambers: 

• The upper chambers - the left atrium and the right atrium 
• The lower chambers - the left ventricle and the right ventricle 

The purpose of the upper chambers (atria) is to receive the blood flowing into the heart, ready to enter the ventricles. Particularly, the left atrium is significant in receiving the oxygenated blood from the lungs and flowing into the left ventricle. The purpose of the lower chambers (ventricles) is to pump blood out of the heart.¹

This article will focus specifically on the normal function of the left atrium versus the effects of an enlarged left atrium on health.  Interestingly, the left atrium enlargement is an indicator of heart disease but also linked to stroke and atrial fibrillation. 

Normal anatomy of the left atrium 

The anatomy of the left atrium is unique from the other chambers. Interestingly, the left atrium has a larger component as well as a smaller component named the left atrial appendage. This atrial appendage is responsible for the control of blood flow.²

When looking at the chest from the front, the left atrium is located at the back of the heart, making it the most posterior of the heart’s chambers. Because of the angled position of the atrial septum (the wall between the atria) and the fact that the openings of the mitral and tricuspid valves are at different levels, this results in the left atrium sitting behind and slightly above the right atrium. Smooth-walled and non-uniform in thickness, the left atrium is considered muscular. Thickness appears to decrease towards the opening of the left and the right pulmonary veins. The walls of the left atrium can be divided into 4, these include the superior, left lateral, anterior and septal.³ In general, the atrial walls are thinner than the ventricular walls as they are more elastic to stretch and receive the incoming blood from the pulmonary veins. 

The mitral valve originates from the left atrioventricular ring, and this importantly divides the left atrium from the left ventricle. The edges of the valve are connected to the left ventricular muscles through their link with the chordae tendinae. Additionally, the tricuspid valve is derived from the right atrioventricular ring, separating the right atrium from the right ventricle.⁴

The pulmonary veins enter the back part of the left atrium, with the veins from the left lung positioned higher than those from the right lung. Just in front of the left atrium lies the transverse pericardial sinus, and ahead of this sinus is the base of the aorta.

Behind the pericardium covering the back wall of the left atrium are several important structures: the point where the trachea splits (tracheal bifurcation), the oesophagus, and the descending part of the thoracic aorta. Furthermore, positioned beyond these structures is the vertebral column.³

Normal function of the left atrium 

The left atrium is significant in the blood flow pathway as the first chamber for oxygenated blood to enter from the pulmonary veins. The left atrium acts as a reservoir during the ventricular systole (ventricular contraction), as an important function. This atrium has a passive conduit role in connecting and transferring the blood to the left ventricle, while ventricular diastole (when the heart muscles relax and enable the blood to fill up the ventricles) occurs while the mitral valve is open. Furthermore, this atrium is a contractile chamber. Atrial contraction occurs during the late stages of diastole. Timing and synchronisation are crucial as the chambers work simultaneously and quickly within the heartbeat.^2,5 The structure of this atrium is adapted for this function and is beneficial for the ideal ventricular filling of oxygenated blood. 

Causes and mechanisms of left atrial enlargement

The mechanisms,  such as increased blood volume and pressure overload, can stretch the walls of the atrium and increase the stress on the walls. Therefore, this mechanism specifically causes the enlargement of the left atrium. This pressure/volume overload can be brought about by the occurrence of mitral stenosis (a mitral valve disease)  or irregularities in left ventricular function. Furthermore, the increased pressure changes in the left atrium cause the insufficient blood filling of the left ventricle.⁶

Other common factors of left atrial enlargement were mentioned by a primary study conducted by Katayama et al. A certain imaging technique was utilised in this study, named Doppler echocardiography. Increased mass index, referring to the thickness and size, of the left ventricle correlated with the increased pressure inside this ventricle. This causes an increase in pressure backwards towards the left atrium, causing the stretching and enlargement observed. Hypertension is also a cause for this increased mass index.⁷ Chronic kidney disease has an effect on atrial enlargement, affecting the atrial rhythmic counts.⁸ Additionally, age-related left atrial enlargement is known to reflect the physiological disturbances alongside the natural process of ageing, rather than a direct effect of ageing.⁹

Effects of left atrial enlargement on function 

As a result of this enlarged left atrium, there is a higher risk of stroke, atrial fibrillation (irregular heart rhythms), and heart failure. This enlarged left atrium also seems to correlate with an increased risk of mortality. A significant overload in volume is seen in the valvular regurgitation. Atrial fibrillation is increased due to proven evidence that the left atrial volume is a better measure than the atrial diameter. There is a loss in atrial contractility. Individuals with symptomatic congenital heart failure experience an elevated filling pressure of the left ventricle. Heart failure is associated with (and may be caused by) the diastolic dysfunction leading to this pressure. Furthermore, the size of the atrium is associated with the prediction of increased risk of death and stroke.

This is independently predictive of the first ischemic stroke.^6,9 Recent findings have shown that left atrium enlargement is associated with a greater risk of severe stroke, particularly with ischemic strokes, in patients who have experienced atrial fibrillation in the past. This enlargement is a predictor of recurrent cardioembolic or cryptogenic strokes.¹⁰ Another study indicated that the left atrial diameter studied signified a higher risk of blood clot formation, known as thromboembolism.¹¹

Examples of treatment options

Single drug interventions can help the reduction of the left atrial size. For example, antihypertensive drugs such as hydrochlorothiazide. This drug demonstrated a greater reduction in size compared to atenolol, captopril, or diltiazem. By influencing the atrial size, this has a knock-on effect on the left ventricular mass.¹² Other medications can be used to focus on atrial fibrillation. Treatments may focus on the underlying issues, such as minimising hypertension and rhythm control. By focusing on hypertension, this can help manage atrial fibrillation. Beta-blockers are also useful in controlling the heart rate, which eventually affects blood pressure.¹³ Furthermore, medical procedures, such as atrial fibrillation ablation, can further help in the left atrial reverse remodelling in patients considered to possess severe enlargement.¹⁴

Summary

The left atrium may appear passive, but it plays an important role in blood flow in the human body. In its normal state, the elastic walls can stretch and receive the incoming blood flow from the pulmonary veins. This means the left atrium acts as a reservoir for oxygenated blood, and also enables blood to be pumped towards the ventricles for systemic circulation. However, when the left atrium is enlarged, there is a higher risk of stroke, atrial fibrillation and heart failure. This is due to the roles of the left atrium not being conducted properly, as the slight change in atrial physiology changes the efficiency of its role. Through imaging, monitoring, and treatment, early intervention is possible. This enlargement may be a marker for potential cardiovascular diseases. The causes and signs of the enlargement could be detected earlier and, therefore, prevent the devastating consequences of this enlarged atrium. Consequences include atrial fibrillation, stroke and even increased risk of death. To analyse and fully understand this chamber, the heart health may be protected and improved long-term.