Anatomy clinical correlates: Heart

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Introduction0:00–0:17

Have you ever wondered what the secret to someone’s heart is? That's right, a chest x-ray!
All right, so, here at Osmosis we don't actually have the secret to one’s heart, but we do know how to identify the different medical conditions that can affect the heart.Let's start off by identifying the heart borders on a chest x-ray.

Chest X-ray0:17–1:12

The heart silhouette is between the lungs, and the right border, made up by the right atrium, as well as the left border, made up by the left ventricle and part of the left auricle, can be clearly seen.
Above the left auricle, we can identify the pulmonary artery and the aortic arch. And in some clinical circumstances, the silhouette sign can be present, which is when the normal heart silhouette of the heart compared to the lungs is lost.
More appropriately, you might want to think about it as a “loss of the heart silhouette”. The loss of the heart silhouette only occurs when the pathological process is in direct anatomical contact with the heart.
Usually, the middle lobe is seen close to the right border of the heart. So, consolidation in the right middle lobe can also obscure the x-ray silhouette of the right heart border.
All right, now, even though the heart is protected by the sternum and thoracic cage, it’s still susceptible to injury. During penetrating trauma, like, for example, a stab wound, the right ventricle is the most commonly injured structure because of its anterior position in the chest and the fact that it forms the majority of the anterior surface of the heart, followed by the left ventricle which forms the apex of the heart and may be injured as far laterally as the left midclavicular line at the 5th intercostal space.

Penetrating trauma1:12–1:58

The atria are less commonly injured than the ventricles. It’s also worth noting that the lungs overlap most of the anterior surface of the heart, so many penetrating injuries to the heart will also result in concurrent lung injury particularly to the parietal pleura.
Are you ready to listen to your heart? We’re now going to talk about heart auscultation!

Heart auscultation1:58–3:14

The gist of it is to listen to the areas that best project the sound coming from each heart valve. Blood tends to carry the sounds in the direction of its flow so each area is situated superficial to the chamber or vessel into which the blood has passed and in a direct line with the valve orifice.
Let’s start with the aortic valve, which is located posterior to the left of the sternum at the level of the third intercostal space.
To auscultate the aortic valve, you need to move your stethoscope at the second intercostal space, right of the sternal angle.
Moving on to the pulmonary valve, it’s located at level of the left third costal cartilage and is auscultated at the second intercostal space, left to the sternal angle.
The tricuspid valve is posterior to the body of the sternum to the right side at the level of the fourth and fifth intercostal space, and it’s auscultated at the 4th or 5th intercostal area, left to the sternal edge.
The mitral valve is located posterior to the sternum at the level of the fourth costal cartilage to the left and is auscultated at the left 5th intercostal space on the midclavicular lineAnd now let’s talk about conditions that may affect the heart.

Dextrocardia3:14–4:35

First, there’s dextrocardia, which is a rare embryological folding defect where the heart is reversed so the apex is misplaced to the right instead of the left.
Dextrocardia is associated with mirror image positioning of the great vessels and arch of the aorta. Basically, everything that normally is on the left is on the right and vice-versa.
This condition might be part of something called situs inversus, which is a general transposition of the thoracic and abdominal viscera, or it occurs as isolated dextrocardia, where the transposition only affects the heart.
When dextrocardia is associated with situs inversus, the incidence of other cardiac defects is low and the heart usually performs normally.
However, in isolated dextrocardia, the congenital anomaly is complicated by severe cardiac anomalies, such as transposition of the great arteries.
Clinically, dextrocardia can be determined by palpating the apex beat over the right chest. Typically the apex beat, which is the most lateral inferior palpable portion of the heart on the chest wall typically found in the 4th or 5th intercostal space at the mid clavicular line, is on the right side.
An x-ray can then be done to confirm dextrocardia. And while dextrocardia is rare, a myocardial infarction, unfortunately, is not uncommon.

Myocardial infarction4:35–7:23

That’s when an artery of the heart is blocked by an embolus, and the myocardium supplied by the occluded vessel no longer receives blood.
If that area can undergo necrosis, resulting in a myocardial infarction. Symptoms of a myocardial infarction include severe crushing chest pain that can often radiate to the back, jaw, left arm, right arm, shoulder, or atypical chest pain that is felt in the abdomen.
Associated symptoms include dyspnea, diaphoresis, which means profuse sweating, as well as nausea and vomiting. The three most common sites of coronary artery occlusion are: the anterior interventricular branch of the left coronary artery approximately 40-50% of the time, the right coronary artery approximately 30-40% of the time, and the circumflex branch of the left coronary artery approximately 15-20% of the time.
Now, dominance of the coronary arterial system also affects what areas of the heart are affected during a myocardial infarction, as dominance determines whether the right or left coronary artery gives off the posterior interventricular branch.
Therefore, during an occlusion to the right or left coronary artery, dominance will determine if the area supplied by the posterior interventricular branch will be affected.
In 67-85% of people, the right coronary artery gives rise to the posterior interventricular branch. In about 8-15% of cases, the left coronary artery is dominant and the posterior interventricular branch comes from the circumflex artery.
In 7-18% of people, there is codominance and both right and left coronary arteries give rise to branches that run in or near the posterior interventricular groove.
So, if the right coronary artery is occluded, then the right atrium, parts of both ventricles and the sino-atrial and atrioventricular nodes are affected along with the area supplied by the posterior interventricular branch which is the inferior adjacent area of ventricles and the posterior third of the interventricular septum.
If the left coronary artery is occluded, then the left atrium, along with parts of both ventricles, the AV bundle, the anterior 2 thirds of the interventricular septum, along with the area supplied by the posterior interventricular artery if it is dominant.
Also remember, the right coronary artery supplies the SA node via the SA nodal branch 60% of the time, and the AV node via the AV nodal branch when it has dominance, so the loss of blood supply to these two nodes also varies during a myocardial infarction.

Conduction system7:23–8:37

Following a myocardial infarction, the conducting system of the heart might be damaged. The left coronary artery gives off the anterior interventricular branch which gives rise to the septal branches that supply the AV bundle in most people.
Additionally, the branches of the right coronary artery mainly supply both the sinoatrial and atrioventricular nodes as we have said before.
The occlusion of one of these arteries can lead to a heart block. In this case, the ventricles will begin to contract independently at their own rate which is approximately 25 to 30 per minute as they do not receive a signal from the SA or AV node, which is slower than their slowest normal rate of 40 to 45 per minute.
If the sinoatrial node has been spared, the atria continue to contract at the normal rate, but the impulse generated by the sinoatrial node doesn’t reach the ventricles.
Damage to either the left or right AV bundle branches leads to a bundle branch block, where excitation passes along the unaffected branch and causes a normal systole of that ventricle only, and the affected ventricle receives conduction via muscle propagation to produce a late asynchronous contraction.
Now, individuals with obstruction of their coronary circulation and severe angina might undergo a coronary bypass graft operation.

Coronary bypass graft operation8:37–9:29

This is when a segment of an artery or vein is connected to the ascending aorta or to the proximal part of a coronary artery and then to the coronary artery distal to the stenosis.
This procedure provides a detour or ‘bypass’ around the stenotic area or blockage re-establishes blood flow to the obstructed cardiac muscle.
It is preferred to use arterial grafts compared to venous grafts as patency is higher in arterial grafts - meaning they are more likely to stay open and allow blood flow than venous grafts.
Preferred graft choices are the internal thoracic artery followed by the radial artery, however in multiple coronary artery disease the great saphenous vein may need to be harvested as it offers lengthy portions.

Percutaneous coronary intervention9:29–10:09

In some individuals with a myocardial infarction, a percutaneous coronary intervention can be done. This is when a catheter with a small inflatable balloon attached to its tip is placed through the obstructed coronary artery.
When the catheter reaches the obstruction, the balloon is inflated and the vessel is stretched to increase the size of the lumen and to improve blood flow.
After dilation of the vessel, an intravascular stent can then be introduced to maintain the vessel open. The catheter can be introduced through the femoral or radial artery to reach the heart and the affected vessel.
However, there is always the risk of arterial hemorrhage when undergoing these procedures. Now, before we move on, can you recall what are the three most common sites of coronary artery occlusion?

Quiz10:09–10:20

Another type of heart catheterization is venous catheterization, where a venous catheter enters the heart to measure intracardiac pressures and study circulation of the functioning heart for things such as congenital heart defects or arrhythmogenic foci.

Cardiac catheterizations10:20–11:12

The cardiac venous catheter is inserted in the femoral vein and travels into the right atrium through the IVC. In order to access the left side of the heart, the catheter must transverse the interatrial septum at the site of the foramen ovale, which is covered in adults by a thin membrane and can be easily punctured, to enter the left atrium.
In addition to cardiac venous catheters, there are also cardiac ablation catheters which we’ll talk about shortly. These deliver radiofrequency ablation to parts of the heart to help correct electrical abnormalities such as supraventricular tachycardias.

Supraventricular tachycardias11:12–13:36

Supraventricular tachycardias often arise from abnormal electrical activity in the AV node, an accessory electrical pathway, or the atria such as in atrial fibrillation.
Generally, these lead to a ventricular heart rate of over 100 beats per minute, and can lead to things such as shortness of breath, chest pain, and hemodynamic instability.
To fix these electrical abnormalities, we can use cardiac ablation catheterization like we mentioned previously. The cardiac catheter can then be directed to specific structures of the heart such as the right atrium, right ventricle and pulmonary veins where radiofrequency ablation is done to eliminate these accessory electrical pathways to try and restore normal heart function.
One area in the heart where supraventricular tachycardias can occur is the crista terminalis in the right atrium. However, the right phrenic nerve courses along the pericardium over the right atrium, and is at risk of injury during right atrial ablation.
Damage to the right phrenic nerve would cause paresis on the right side of the diaphragm, and can be recognized by elevation of the right hemidiaphragm on x-ray.
Now, a subtype of supraventricular tachycardias is atrial fibrillation, which is actually the most common cardiac arrhythmia.
Atrial fibrillation is when the normal atrial heart contractions are replaced by rapid, irregular, and uncoordinated contractions in different parts of the atrial wall.
These irregular contractions are associated with a major risk of thrombus formation and subsequent systemic thromboembolism such as a stroke.
This occurs because ineffective atrial contraction leads to blood stasis and arterial inflammation. The most common site of thrombus formation is the left atrial appendage, a small sac-like structure of the left atrium.
Speaking of the atria, let’s see what happens when the left atrium of the heart becomes enlarged, which is often the result of pressure or volume overload.
This can result from mitral valve stenosis or regurgitation, as well as left ventricular dysfunction. Now as the most posterior chamber of the heart, the increasing size of the atrium can compress near-by structures, such as the esophagus, leading to difficulty swallowing or dysphagia, or it can compress the left recurrent laryngeal nerve, leading to hoarseness of the voice.

Pericardium13:36–14:17

Ok, now, remember that the heart is contained in a sac called the pericardium? Well, you guessed it, this also has clinical implications.
First, the pericardium has a sweet spot called the transverse pericardial sinus that’s very important for cardiac surgeons.
After the pericardial sac is opened anteriorly, a finger can be passed through the transverse pericardial sinus just posterior to the ascending aorta and pulmonary trunk.
By passing a surgical clamp or a ligature around these large vessels, inserting the tubes of a coronary bypass machine and then tightening the ligature, the surgeon can stop or divert the circulation of blood in these arteries during cardiac surgery.
Alternatively, the pericardium can be affected when fluid accumulates in the pericardial cavity. When there’s too much fluid in the pericardial cavity, it can compress the heart, causing cardiac tamponade.This excess fluid can be removed during a procedure called pericardiocentesis, which is when a wide-bore needle is inserted through the left 5th or 6th intercostal space near the sternum.

Cardiac tamponade14:17–15:11

This approach is possible, because the cardiac notch in the left lung and the shallower notch in the left pleural sac leaves part of the pericardial sac exposed.
The pericardial sac can also be reached through the infrasternal angle, also called the subcostal angle formed by the right and left costal margins, by passing the needle supero-posteriorly.
At this site, the needle avoids the lung and pleura and enters the pericardial cavity. However, there’s a risk of puncturing the internal thoracic artery or its terminal branches.
All right, as a quick recap. On a chest x-ray, the heart silhouette is between the lungs, and the right border, made up by the right atrium, as well as the left border, made up by the left ventricle and part of the left auricle, can be clearly seen.

Review15:11–16:56

Dextrocardia is when the heart is positioned on the right side of the body. It can be isolated or it can be associated with situs inversus, where the organs that are supposed to be on the left are on the right and vice-versa.
The three most common sites of coronary artery occlusion are: the anterior interventricular branch of the left coronary artery, the right coronary artery and the circumflex of the left coronary artery.
A myocardial infarction is when an artery of the heart is blocked by an embolus, and the myocardium supplied by the occluded vessel no longer receives blood.
A coronary bypass graft operation is when preferably a segment of artery is connected to the ascending aorta or to the proximal part of a coronary artery and then to the coronary artery distal to the stenosis.
A percutaneous transluminal coronary angioplasty is when a catheter with a small inflatable balloon at its tip is placed through the obstructed coronary artery to reinstall flow in the respective artery.
Other types of cardiac catheterization include venous catheterization which measures pressures in the heart and arrhythmia foci and ablation catheterization to correct abnormal cardiac pathways.
Atrial fibrillation is a common cardiac arrhythmia which may lead to left atrial appendage thrombus formation. Pericardiocentesis can be done by inserting a needle through the left 5th or 6th intercostal space near the sternum or by inserting the needle through