Cardiomegaly in Chest X-ray

Cardiomegaly:

It is a general term used to describe any condition that results in an enlarged heart.

1. Dilative
2. Hypertrophic

1. Dilative: 

The heart can become enlarged due to dilation of the myocardium. Dilated Cardiomyopathy (DCM) is the most common form of non-ischaemic cardiomyopathy. In DCM, the heart becomes weakened and enlarged and cannot pump blood efficiently. The decreased heart function can affect the lungs, liver and other systems (the most common form of non-ischaemic cardiomyopathy). Generalized enlargement of the heart is seen upon normal chest x-ray. Pleural effusion may also be noticed which is due to pulmonary venous hypertension.

2.Hypertrophic:

Just as skeletal muscle, hypertrophy grow in size in response to increase demand, cardiac muscle undergoes hypertrophy when placed under a high workload for a prolong period of time. Some cardiac hypertrophy is normal and reversible. Pathologic hypertrophy is the result of diseases that place increased demand on the heart, such as chronic hypertension, MI and valvular damage.Gross RA enlargement evidence of shift of right border very much into right hemithorax. MPA and RPA shadows prominent. Elevated left bronchial shadow and filling of appendage region of left border indicate left atrial enlargement.

 LVH is the most common type of hypertrophic heart disease. A common cause of LVH is chronic hypertension, which increase the afterload on the left ventricle. This means the left ventricle has to increase contractibility and/or preload to maintain same stroke volume. Overtime the added stress on the left ventricular myocardium results in muscle hypertrophy and remodelling of left ventricle to a less efficient shape and size. This leads to a diminishing ejecting fraction, meaning the heart must work even harder to maintain cardiac output. The larger heart also demands more blood flow and so becomes more susceptible to ischaemic injury.

Causes of Cardiomegaly:

1. Hypertension
2. Valvular heart disease
3. Ischaemic heart disease - previous MI
4. Dilated cardiomyopathy
5. Pericardial effusions e.g. malignancy
6. Right heart dilatation - PHTN
7. ASD
8. Thyroid disorders
9. Infiltrative heart disease - amyloid
10. Congestive heart disease

Extracardiac causes of cardiac enlargement:

1. Portable AP films
2. Obesity
3. Pregnancy
4. Ascites
5. Straight back syndrome
6. Pectus excavatum

 Cardiomegaly is often detected on postero-anterior projection of chest x-ray. The standard method for measuring heart size (Danzer Method) involves measuring the distance from the midline of the spine to the most lateral aspect of the cardiac apex and adding this distance to that found from the same midline to the most lateral aspect of the right atrium. This number is divided by the largest horizontal width of the chest from right to left pleural space (usually found just above the left hemidiaphragmatic surface). This value is known as cardiothoracic ratio i.e. CTR (more than 0.5 indicates cardiomegaly).
CTR = a ratio of
1. Max. horizontal cardiac diameter
2. Max. horizontal thoracic diameter (inner edge of ribs or edge of pleura)

In cardiomegaly chest x-ray, there will be markedly enlarged cardiac silhouette, double contour to the right heart border and splaying of carina. The most lateral contour is of right atrium and the most medial contour is of left atrium.

Right cardiomediastinal contour: (from superior to inferior)

1. Right paratracheal stripe seen in 2/3rd of normal films, made up of right brachiocephalic vein and SVC
2. Arch of azygous vein
3. Ascending aorta in older patients often projects to the right of SVC
4. Superior venacava
5. Right Atrium
6. Inferior venacava

Left cardiomediastinal contour: (from superior to inferior)

1. Left paratracheal stripe made up of LCCA, LSA, LJV
2. Aortic arch +/- aortic nipple
3. Pulmonary artery
4. Auricle of left atrium
5. Left ventricle