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The Left Atrium
The appearance of the left atrium can inform a large number of diagnoses in veterinary echocardiography, and is often exhibits visual changes earlier in the disease course than other cardiac chambers.
A very popular measurement in veterinary echo is the LA:Ao ratio (left atrial to aortic ratio), taken from the right parasternal short axis. This is one of the most difficult views in echocardiography, as it can be easy to under- or over-rotate your image, or cut the base of the heart obliquely so that the aortic valve leaflets look unequal in size.
The aim is to have the aortic valve appear as a circle – if you can see a hint of the outflow track to the left of your image, you have under rotated, and if you see it appearing on the right, you have rotated too far. The above example is typical of the type of image routinely achievable. Textbook-style clarity with all three leaflets perfectly seen may not be possible in every patient in real life.
An LA:Ao ratio >1.5 is considered dilated in dogs, with a little more tolerance given to cats, but even without measuring, it is easy to see from this view that this left atrium is dilated. Also notice how the interatrial septum, seen on the left border (in this view) of the LA, bows into the right side of the heart. This is commonly seen when the left atrium is dilated due to excessive volume or pressure.
3 Common Causes of Left Atrial Dilatation
The left atrium can dilate due to volume overload, pressure overload, or both. The left atrium does not always dilate uniformly in all directions, so it is important to consider other views besides the short axis. In the previous example, left atrial enlargement was obvious even from the right parasternal long axis view. Can you spot the cause from the video below?
It is hopefully obvious that the septal/anterior mitral valve leaflet is very bulky and abnormal. This resulted in severe mitral regurgitation, and significant mitral regurgitation is a very common reason behind a dilated, volume-loaded left atrium in small dogs. Indeed, serial monitoring of left atrial size is recommended to monitor progression of myxomatous mitral valve disease.
Even if your machine was not equipped with Colour or Spectral Doppler, the appearance of the leaflets combined with the dilatation of the left atrium would probably be sufficient information to form the basis of a preliminary diagnosis.
Dilated cardiomyopathy (DCM) is a primary disease of the heart muscle, and is more common in medium-large dogs (such as the Doberman, Boxer, Great Dane, German Shepherd, Setter and Cocker Spaniel). It affects the left side of the heart, with varying degrees of involvement of the right side of the heart. Poor left ventricular systolic function results in high end-systolic and end-diastolic volumes, and the progressive dilatation of the left ventricle may also result in stretching of the mitral valve annulus and the presence of functional mitral regurgitation. This means that the valve leaflets themselves are structurally normal, but blood leaks back into the left atrium during systole because they are no longer meeting properly.
Even if concomitant mitral regurgitation is not present with this disease, as heart failure progresses, diastolic dysfunction – and therefore left atrial dilatation – becomes an inevitability. High end-diastolic pressures in the left ventricle result in high left atrial pressures in order to maintain the diastolic flow of blood into the left ventricle, resulting in this dilatation.
Above: DCM dog with a very dilated left atrium
Other diseases besides DCM are accompanied by diastolic dysfunction. In cats, hypertrophic cardiomyopathy (HCM) is frequently the main culprit for cardiac symptoms, and HCM is a disease of diastole. The hypertrophied heart muscle is stiffened and unable to relax properly – diastolic pressures rise, and this feeds back into the left atrium, which dilates. Even visually, it is possible to appreciate the ‘stiffness’ of the feline heart below – a failure of the heart muscle to relax. Look also at the size of the left atrium in comparison with the left ventricle.
A large number of progressive cardiac diseases lead to heart failure, and serial monitoring is often based around the size of the left atrium. Given the sensitivity of this chamber to both volume and pressure overload, this is a sensible approach. Even when performing a cardiac screen on a new patient, the size of the left atrium in relation to other structures provides immediate supporting information, and can help give weight to a new diagnosis.
Left atrial size is also a powerful prognostic indicator for a number of diseases. It can predict the development of secondary pulmonary hypertension, which can accompany all of the above mentioned diseases in their latter stages, as high left atrial pressures eventually feed back into the pulmonary system and to the right heart.