The ABCs of Heart Disease is a system for diagnosing cardiac disease in adults by asking certain questions in a set fashion, the answers to which are certain fundamental observations made from the frontal chest x-ray alone.
The observations start with one that is widely used: The cardiothoracic ratio.
Heart size
The Cardiothoracic ratio is the maximum transverse diameter of the heart divided by the greatest internal diameter of the thoracic cage (from inside of rib to inside of rib).
In normal people, the cardiothoracic ratio is usually less than 50%. Therefore, the cardiothoracic ratio is a handy way of separating most normal hearts from most abnormal hearts.
But it is not infallible.
A heart may be greater than 50% of the cardiothoracic ratio and still be a normal heart.
This can occur if there is an extracardiac cause of cardiac enlargement. Extracardiac causes of cardiac enlargement include:
Inability to take a deep breath because of
· Obesity
· Pregnancy, or
· Ascites
Or abnormalities of the chest that compress the heart such as
· Pectus excavatum deformity, or
· Straight Back Syndrome
Sometimes the heart can be smaller than 50% of the cardiothoracic ratio but still be an abnormal heart. This occurs when there is something obstructing the flow of blood from the ventricles since the ventricles respond at first by undergoing hypertrophy, which does not produce cardiomegaly.
Since not all abnormal hearts are enlarged, definition of cardiac disease in those individuals depends on an assessment of the contours of the heart on the frontal film.
There are 7 “bumps” in all on the frontal film, only 5 of which will have major importance in this system.
Cardiac Contours
1. On the right side of the heart, the first contour is that of the ascending aorta.
This is a low density, almost-straight edge visible just to the right of the trachea anatomically representing the superior vena cava and the brachiocephalic vein but in practice reflecting the size of the ascending aorta. So we shall call this contour the ascending aorta.
The ascending aorta can be small as in ASD or it can be convex outward as in aortic stenosis, aortic regurgitation, and hypertensive cardiovascular disease. The ascending aorta should never project farther to the right than the right heart border in a normal person.
2. Just below the ascending aorta is an indentation where the so-called “double density of left atrial enlargement” will appear when the left atrium enlarges toward the right side of the heart.
Normally, the left atrium forms no border of the heart in the frontal projection. When the left atrium enlarges, it may produce two abnormal contours of the heart. One is on the left side of the heart which we’ll learn about in a moment. The other is the double density of left atrial enlargement and is the less commonly seen of the two.
Where the ascending aorta meets the right atrium, there is usually an indentation. In patients with an enlarged left atrium which projects to the right, there will be two overlapping densities seen where this indentation normally is. One of the densities is the normal right atrium. The other overlapping density is the enlarged left atrium.
The double density may occasionally be seen in normal individuals; always check left heart border for straightening.
3. The last contour on the right side of the heart is the right atrium. In an adult, every disease that causes enlargement of the right atrium also produces enlargement of the right ventricle. So we can consider the right atrium and ventricle together as a single functional unit in adults and we will estimate right-sided cardiac enlargement by observing another cardiac contour, but not the right atrium.
Therefore, for this system the right atrium is not an important contour.
Now we move to the left heart border and start at the top.
4. On the left side of the heart, the first contour is the aortic knob. The aortic knob is a radiographic structure that is formed by the foreshortened aortic arch and a portion of the descending aorta.
In normal people the aortic knob measures less than 35mm when measured from the lateral border of the trachea to the lateral border of the aortic knob.
The knob will be greater than 35 mm due to increased pressure, flow, or changes in the elasticity of the wall e.g. cystic medial necrosis, ASCVD, dissection of the aorta.
5. Just below the aortic knob is the main or undivided segment of the pulmonary artery. The main pulmonary is very important in this system and forms the cornerstone for two of the main categories of disease to follow.
First, you must be able to find the main pulmonary artery segment. Then, you can measure it.
You can find the main pulmonary artery by either locating the first contour below the aortic knob or by finding the adjacent squiggly vessels that are the left pulmonary artery. The main and left pulmonary arteries are always adjacent to each other.
We can measure the main pulmonary artery by drawing a tangent line from the apex of the left ventricle to the aortic knob and then measuring along a perpendicular to that tangent line, the distance between the tangent and the main pulmonary artery.
In normal people, the distance between the tangent and the main pulmonary artery lies within a range of values between 0mm (the main pulmonary is touching the tangent line) to as far away from the tangent line (medially) as 15mm.
Therefore, this sets up two major categories of abnormality.
First, the main pulmonary artery may project beyond the tangent line (greater than 0 mm).
This can occur if there is increased pressure or increased flow in the pulmonary circuit.
Second, the main pulmonary artery may project more than 15 mm away from the tangent line.
This can occur either because there is something intrinsically wrong with the pulmonary artery such as absence or hypoplasia of the pulmonary artery, e.g. Tetralogy of Fallot, truncus. But, these diseases are uncommon in adults.
The other reason the main pulmonary artery may be more than 15 mm from the tangent line is the left ventricle and/or aortic knob may enlarge and push the tangent line away from pulmonary artery, e.g. atherosclerosis, HCVD. These are very common diseases.
Young females may normally have prominence of the MPA but rarely does the main pulmonary artery project beyond the tangent.
6. Just below the main pulmonary artery segment (area between the main pulmonary artery and the left ventricle) is a little concavity where the left atrium, when it enlarges on the left side of the heart, will appear.
Filling in at this concavity by an enlarged left atrium produces “straightening” of the left heart border. It may be seen in mitral disease or shunt (VSD, PDA) and rarely left atrial myxoma, papillary muscle dysfunction and chronic CHF.
Sometimes, the left atrial appendage may enlarge as well. A convexity at this contour means the left atrial appendage is enlarged and is seen in mitral valvular disease, usually mitral stenosis.
7. The last cardiac contour is the left ventricle.
Like the right atrium, the left ventricle is not an important contour in this system.
The best way to evaluate which ventricle is enlarged (i.e. right or left) is to look at the corresponding outflow tracts for each ventricle.
If the heart is enlarged and the main pulmonary artery is large (sticks out beyond the tangent line), then the cardiomegaly is made up of at least right ventricular enlargement.
If the heart is enlarged and the aorta is prominent (ascending, knob, descending), then the cardiomegaly is made up of at least left ventricular enlargement
Once one ventricle is determined to be enlarged, then the other ventricle may also be enlarged but there is no way to identify this with a chest x-ray.
Just a few words about the descending aorta.
It is a contour seen through the heart to the left of the thoracic spine. It parallels the spine most of the time and is almost invisible.
It may become tortuous or enlarged as in ASCVD or aortic regurgitation.
So the five important cardiac contours are:
· The ascending aorta
· Indentation where double density of left atrial enlargement will appear
· Aortic knob
· Main pulmonary artery segment
· Indentation where the left atrium, when it enlarges, will appear on the left side of the heart
The right atrium and left ventricle are less important because we evaluate ventricular enlargement by looking at the outflow tracts for each ventricle.
Pulmonary Vasculature
Using just the cardiac contours we can come up with a set of differential diagnoses for cardiac disease, but to make the actual diagnosis we unfortunately have to look at the pulmonary vasculature. It is unfortunate because everyone has difficulty evaluating the pulmonary vasculature. In order to evaluate the pulmonary vasculature, we will examine three parameters.
Before we do that, it is important to know that the pulmonary vasculature will fall into one of the following four categories:
1. Normal
2. Pulmonary venous hypertension
3. Pulmonary arterial hypertension
4. Increased flow
Decreased flow to the lungs is another category but is very difficult to assess accurately, so we’ll not concern ourselves with it.
To evaluate the pulmonary vasculature, we shall look at these three parameters:
1. Measure the right descending pulmonary artery (RDPA).
The RDPA is visible on almost all chest films as a large vessel just to the right of the right heart border. Its diameter can be measured at about the level of the indentation between the ascending aorta and the right atrium.
In normal people, the right descending pulmonary artery is less the 17mm in diameter.
In normal people, in the erect position, the blood flow to the bases is greater than the blood flow to the apices. This is due to the effect of gravity. In normal people, the size of the vessels at the base will therefore be greater than the size of the vessels at the apex.
We can evaluate the size (not the number) of vessels in two imaginary circles about the size of a silver dollar, one drawn at the apex and another at the base of the lung. You have to use the right base because you can’t see the vessels at the left base due to overlap by the heart. So look at the size of the vessels at the right base and compare them to the size of the vessels at either apex. In normal people, the size of the vessels at the base will be greater than the size of the vessels at the apex, so long as the film is done erect.
In people with pulmonary venous hypertension, the blood flow to the apex becomes equal to or greater than the blood flow to the base. Therefore the size of the vessels at the apex becomes equal to or greater than the size of the vessels at the bases – a reversal of the normal distribution pattern. This is known as cephalization.
3. Evaluate the distribution of flow from central to peripheral
Normally, the pulmonary vessels taper gradually from central to peripheral. It doesn't matter whether they are arteries or veins and it doesn’t matter whether it is the lung or any other organ, blood vessels taper gradually from central to peripheral.
In pulmonary arterial hypertension, there is a re-distribution of flow in the lungs from central to peripheral such that the peripheral vessels appear too small for the size of the central vessels from which they come. This discrepancy in the size of the central pulmonary vessels (which are large) compared to the peripheral pulmonary vasculature (which even though it is small is still indistinguishable from normal) is called pruning.
Using the size of the RDPA and the distribution of flow in the lungs—apex to base and central to peripheral, we can define the four states of the pulmonary vasculature.
In NORMAL PULMONARY VASCULATURE
The right descending pulmonary artery is less than 17mm
The lower lobe vessels are larger than the upper lobe vessels
There is a gradual tapering of the blood vessels from central to peripheral
In PULMONARY VENOUS HYPERTENSION
The right descending pulmonary artery is usually greater than 17 mm
The upper lobe vessels are equal to or greater than the size of the lower lobe vessels (cephalization)
There is a gradual tapering of the blood vessels from central to peripheral
In PULMONARY ARTERIAL HYPERTENSION
The right descending pulmonary artery is greater than 17mm
The lower lobe vessels are larger than the upper lobe vessels
There is a rapid decrease in the size of the peripheral vessels relative to the central vessels from which they come (pruning)
In INCREASED FLOW
The right descending pulmonary artery is greater than 17 mm
The lower lobe vessels are larger than the upper lobe vessels
There is a gradual tapering of the blood vessels from central to peripheral
All of the blood vessels, everywhere in the lung, are larger than they should be—upper and lower lobe, central and peripheral. But there is more blood flowing through all of the vessels so there appear to be more blood vessel in the lungs than there are in a normal person.
That completes all of the observations you need to answer the set of questions that actually constitute “The ABCs of Heart Disease.” You have just learned the answers. Now, here are the questions.
The ABCs of Heart Disease
The system depends for its success on the questions being asked in this order.
Question A is: Is the left atrium enlarged?
To answer that question, you look at the two places on the frontal chest x-ray where you may see left atrial enlargement - i.e., is there straightening of the left heart border and is there a double-density on the right heart border.
If the answer to question A “Is the left atrium enlarged” is yes, then look at the pulmonary vasculature.
A Heart
Normal Vasculature Remarks
Mitral Regurgitation Enlarged heart
Pulmonary venous hypertension
Mitral stenosis Normal or slightly enlarged heart
Left atrial myxoma
Papillary muscle dysfunction Look for pulmonary edema,
Left heart failure
Increased flow
VSD
PDA Enlarged aorta
Pulmonary arterial hypertension
Mitral stenosis Will also have pulmonary venous hyper.
VSD, PDA Will also have increased flow
If the answer to questions A “Is the left atrium enlarged” is no, then ask question B.
Question B is “Is the main pulmonary artery segment big or bulbous?”
To answer question B, you should draw the tangent line from the apex of the left ventricle to the aortic knob to see if the main pulmonary artery protrudes beyond the tangent line.
If the main pulmonary artery projects beyond the tangent line, the answer to question B “Is the main pulmonary artery enlarged” is yes. If the answer to question B is yes, then look at the pulmonary vasculature.
B Heart
Normal Vasculature
Pulmonic stenosis Left pulmonary artery sometimes big
Idiopathic pulmonary artery dilatation
Hyperdynamic states (e.g. anemia, hyperthyroid)
Pulmonary venous hypertension
None Presupposes left atrial enlargement
Increased flow (Left to right shunts)
ASD
VSD Left atrium not always enlarged
PDA Left atrium not always enlarged
Anomalous pulmonary venous return
Pulmonary arterial hypertension
Primary (idiopathic) Normal lungs
Secondary (e.g. multiple pulmonary emboli, arteritis, COPD, Schistosomiasis-Lungs will be abnormal)
If the answer to question B is no, then ask question C.
Question C is “Is the main pulmonary artery segment concave?” To answer question C, use the same tangent from the apex of the left ventricle to the aortic knob, measure along the same perpendicular to the tangent, but this time look to see if the main pulmonary artery is more than 15 mm away from the tangent line.
If the main pulmonary artery is more than 15 mm away from the tangent line, then the answer to question C “Is the main pulmonary artery segment concave?” is yes.
If the answer to question C is yes, then we’re not going to look at the pulmonary vasculature; we’re going to look at the configuration of the aorta. The reason for this is that almost all “C” hearts have normal pulmonary vasculature so the vasculature won’t help to differentiate one C heart from another.
Instead, we’ll look at the configuration of the aorta.
C Heart
Normal vasculature Remarks
Hypertension Entire aorta prominent
ASCVD Entire aorta prominent
Aortic regurgitation Entire aorta prominent
Aortic stenosis Ascending aorta prominent
Coarctation Notch in descending aorta
Cardiomyopathy Normal aorta
Pulmonary venous hypertension
Can’t be Presupposes left atrial enlargement
Increased flow
None Except in rare truncus, PA should be big
Pulmonary arterial hypertension
Can’t be Presupposes big, not concave, main PA
If the answer to question C is no, then the last question is D, “Is the heart dilated or delta shaped?”
A dilated or a delta-shaped heart is usually one that is greater than 65% of the cardiothoracic ratio (a really large heart) and one with smooth contours such that almost as the same amount of heart projects to both the right and the left of the spine..
The two main entities in the differential diagnosis for a dilated or delta-shaped heart are cardiomyopathy and pericardial effusion.
D Heart
Normal vasculature Remarks
Pericardial effusion Uremia, viral, mets, TB, trauma, post-MI
Cardiomyopathy Alcoholism, beri beri
Multiple valve disease
Coronary artery disease Cardiomyopathy
Ebstein's Big right heart
Corrected transposition
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