Pulmonary circulation begins with rich CO2 (carbon dioxide) and O2 (oxygen) poor blood in the right atrium. Next, this blood flows into the right ventricle. From there, the system pumps the blood into the major pulmonary trunk, which is then divided into the pulmonary arteries of each lung.
The pulmonary capillaries drain into pulmonary venules that flow into two pulmonary veins. These veins deliver CO2-poor and O2-rich blood into the left atrium, eventually reaching the left ventricle and aorta. Usually, around 2% of the blood will follow a different path.
It is shunted (diverted) so that it will circumvent the pulmonary capillaries. This is known as a physiologic shunt. Now, to understand intracardiac and intrapulmonary shunting, it’s essential to take a look at human physiology and anatomy. (1)
Intracardiac Shunting
Intracardiac shunting happens when the cardiac blood circulation in the heart takes a shortcut. These are abnormal pathways for blood flow. Intracardiac shunts are prevalent congenital heart defects. In fact, about 0.8% to 1.2% of live births around the globe have some sort of hereditary heart anomaly. (2)
Intracardiac shunting happens because of a hole in the walls that would otherwise separate high oxygen (arterial) from low oxygen (venous) blood. As a result, both venous and arterial blood end up mixing without getting through the normal pathway.
This could lead to plummeting oxygen levels or a decompression illness. Experts associate these holes in the walls with strokes. Sometimes, although rarely, intracardiac shunting can lead to heart failure, pulmonary hypertension, and heart rhythm problems.
These holes are present in the heart wall that separates the atria. In some cases, the holes can emerge in the lower section of the septum, known as the interventricular septum. Defects in this section include atrial septal defect (ASD) and patent foramen ovale (PFO).
They are referred to as ventricular septal defects (VSD) when affecting the lower heart. Interestingly, everyone has a PFO during the early stages of development. PFOs are a key component in supporting circulation early in life.
But, soon after birth, PFOs spontaneously close in around 70% of people. Whereas approximately 30% of the population still has a residual PFO. Just a fraction of these individuals will need treatment. ASDs, on the other hand, are quite rare. They affect less than 1% of the population. (3)
Diagnosis
An echocardiogram is typically used to diagnose ASDs and PFOs. This tool can help experts visualize the hole and determine the best course of action to mitigate the issue.
Depending on what’s causing the problem, doctors may suggest additional imaging tests, such as a cardiac MRI, cardiac CT, and transesophageal echo (TEE). For a more accurate estimate of the shunt, cardiac catheterization can be used. (4)
Treatment
Intracardiac shunting treatment will vary based on the type of defect the patient is dealing with, including other medical ailments that might affect their body. ASD is almost always in need of treatment since it makes patients susceptible to heart failure and pulmonary hypertension.
Even if ASD can be managed with surgery, a cardiac catheter is often used to solve the issue. A closure device is necessary if PFOs are believed to trigger complications, such as stroke or low oxygen levels. Luckily, most PFOs are benign and won’t need any special attention.
Intrapulmonary Shunting
Intrapulmonary shunts happen in around 20% of patients with non-cirrhotic portal hypertension or cirrhosis. They are often paired with hypoxia and finger clubbing. Intrapulmonary shunting is the primary cause of hypoxemia (lack of blood oxygen) in pulmonary edema. Including other ailments like pneumonia that affect the lungs (5)
So, what causes shunting in the lungs? Shunt happens when venous and arterial blood mix and completely bypass the lungs (extrapulmonary shunt) or bypass the lungs without proper oxygenation (intrapulmonary shunt).
Intrapulmonary shunt develops when blood passes through the lungs but fails to take part in gas exchange. Problems like alveolar filling (with blood, tumor, edema, and pus) can all lead to shunting.
Take pneumonia, for example. Pus starts to fill up the alveoli, which hinders gas exchange. Because there isn’t any air in the alveolus, amplifying the FiO2 level (fraction of inspired oxygen) won’t improve gas exchange. Despite that, it is still worth increasing FiO2.
Cardiac (extrapulmonary) shunting is different. It’s typically recorded in adult patients. Although a lesion prompts communication between the right and left heart, the blood will circulate from left to right.
This causes volume overload and a drop in cardiac output of the right heart. But, not shunt. In time, compensatory changes could occur. These changes can make the blood flow from the right to the left heart. (6)
Diagnosis
Intrapulmonary shunting is mainly recorded by the contrast TTE when bubbles from agitated saline appear in the left atrium within 3 to 6 beats after being spotted in the right side of the heart. But, bubbles won’t usually be present in the absence of vascular dilation. That’s because the lung capillaries will serve as filters. An expert will confirm the diagnosis and administer adequate treatment. (7)
Treatment
How is intrapulmonary shunting treated? Treating an intrapulmonary shunt will vary from person to person. Some patients need oxygen therapy, while others will rely on mechanical ventilation. Long-term oxygen therapy can be a viable opportunity for a more complicated health issue. But, for milder problems, exercise and positive end-expiratory pressure can help. Only a doctor can suggest the best form of treatment.
FAQ
Is intrapulmonary shunting normal?
A healthy lung can be imperfectly perfused and ventilated. Intrapulmonary shunting to a certain, small degree is perfectly normal. But, intrapulmonary shunting can be a problem when there is a serious health complication.
What does it mean when a heart is shunting?
A cardiac shunt happens when the blood follows a pattern that strays or wanders off from systemic circulation. A cardiac (cardiovascular shunt) is an abnormal connection between the systemic and pulmonary circulations. Most of the time, they are caused by congenital heart disease. (8)
Is a shunt the same as a stent?
A stent and a shunt are not completely different concepts. But, they are different in practice. A shunt is a tube (hollow device) meant to connect two body parts. These parts were previously not connected. This is to restore the flow of fluid between these two parts.
A stent is a plastic or metal tube that can be added to a blocked (or narrowed) passageway. It can be permanent or temporary and provides scaffolding. Both devices are manufactured of similar materials. However, they are meant for different tasks. (9)
What are typical examples of conditions associated with intrapulmonary shunting?
Typical examples are extensive pulmonary hemorrhage, aspiration pneumonia, noncardiogenic, and cardiogenic pulmonary edema. These ailments are related to intrapulmonary shunts and cause breathing problems.
