Atrial septal defect
Atrial septal defect (ASD) is a heart defect in which blood flows between the atria (upper chambers) of the heart. Some flow is a normal condition both pre-birth and immediately post-birth via the foramen ovale,
A “shunt” is the presence of a net flow of blood through the defect, either from left to right or right to left. The amount of shunting present, if any, determines the hemodynamic significance of the ASD. A “right-to-left-shunt” typically poses the more dangerous scenario.
During development of the baby, the interatrial septum develops to separate the left and right atria. However, a hole in the septum called the foramen ovale allows blood from the right atrium to enter the left atrium during fetal development. This opening allows blood to bypass the nonfunctional fetal lungs while the fetus obtains its oxygen from the placenta. A layer of tissue called the septum
The many types of atrial septal defects are differentiated from each other by whether they involve other structures of the heart and how they are formed during the developmental process during early fetal development.
secundum atrial septal defect
Most individuals with an uncorrected
Patent foramen ovale
A patent foramen ovale (PFO) is a small channel that has some hemodynamic consequence; it is a remnant of the fetal foramen ovale, which normally closes at birth. In medical use, the term “patent” means open or unobstructed. In about 25% of people, the foramen ovale fails to close properly, leaving them with a PFO or at least with what some physicians classify as a “pro-PFO”, which is a PFO that is normally
Clinically, PFO is linked to stroke, sleep apnea, migraine with aura, and decompression sickness. No cause is established for a foramen ovale to remain open instead of closing naturally, but heredity and genetics may play a role. PFO is not treated in the absence of other symptoms.
The mechanism by which a PFO may play a role in stroke is called paradoxical embolism. In the case of PFO, a blood clot from the venous circulatory system is able to pass from the right atrium directly into the left atrium via the PFO, rather than being filtered by the lungs, and thereupon into systemic circulation toward the brain. PFO is common in patients with atrial septal aneurysms (ASA) which are also linked to cryptogenic (i.e. of unknown cause) strokes.
PFO is more prevalent in patients with cryptogenic stroke than in patients with a stroke of known cause. While PFO is present in only 25% in the general population, the probability of someone having a PFO increases to about 40 to 50% in patients who have had a cryptogenic stroke. Statistically speaking, this is particularly true for patients who have a stroke before the age of 55.
After years of controversy between neurologists and cardiologists, percutaneous PFO closure in addition to antiplatelet therapy is now suggested for patients who meet all the following criteria:
- Age ≤ 60 years
- Embolic-appearing cryptogenic ischemic stroke (ie, no evident source of stroke despite a comprehensive evaluation), and
- PFO with a right-to-left interatrial shunt detected by bubble study (echocardiogram)
PFO closure devices may be implanted via catheter-based
Some data suggest that PFOs may be involved in the pathogenesis of some migraine headaches. Several clinical trials are currently underway to investigate the role of PFO in these clinical situations.
primum atrial septal defect
A defect in the ostium
venosus atrial septal defect
Common or single atrium
Common (or single) atrium is a failure of development of the embryologic components that contribute to the atrial septal complex. It is frequently associated with heterotaxy syndrome.
Mixed atrial septal defect
The interatrial septum can be divided into five septal zones. If the defect involves two or more of the septal zones, then the defect is termed a mixed atrial septal defect.
In unaffected individuals, the chambers of the left side of the heart are under higher pressure than the chambers of the right side because the left ventricle has to produce enough pressure to pump blood throughout the entire body, while the right ventricle needs only to produce enough pressure to pump blood to the lungs.
In the case of a large ASD (> 9 mm), which may result in a clinically remarkable left-to-right shunt, blood shunts from the left atrium to the right atrium. This extra blood from the left atrium may cause a volume overload of both the right atrium and the right ventricle. If untreated, this condition can result in enlargement of the right side of the heart and ultimately heart failure.
Any process that increases the pressure in the left ventricle can cause worsening of the left-to-right shunt. This includes hypertension, which increases the pressure that the left ventricle has to generate to open the aortic valve during ventricular systole, and coronary artery disease which increases the stiffness of the left ventricle, thereby increasing the filling pressure of the left ventricle during ventricular diastole. The left-to-right shunt increases the filling pressure of the right heart (preload) and forces the right ventricle to pump out more blood than the left ventricle. This constant overloading of the right side of the heart causes an overload of the entire pulmonary vasculature. Eventually, pulmonary hypertension may develop.
Pulmonary hypertension will cause the right ventricle to face increased afterload. The right ventricle is forced to generate higher pressures to try to overcome pulmonary hypertension. This may lead to right ventricular failure (dilatation and decreased systolic function of the right ventricle).
If the ASD is left uncorrected, pulmonary hypertension progresses and the pressure
Most individuals with a significant ASD are diagnosed in utero or in early childhood with the use of ultrasonography or auscultation of the heart sounds during a physical examination.
Some individuals with an ASD have surgical correction of their ASD during childhood. The development of signs and symptoms due to an ASD are related to the size of the intracardiac shunt. Individuals with a larger shunt tend to present with symptoms at a younger age.
Adults with an uncorrected ASD present with symptoms of dyspnea on exertion (shortness of breath with minimal exercise), congestive heart failure, or cerebrovascular accident (stroke). They may be noted in routine testing to have an abnormal chest X-ray or an abnormal ECG and may have atrial fibrillation. If the ASD causes a left-to-right shunt, the pulmonary vasculature in both lungs may appear dilated on chest X-ray, due to the increase in pulmonary blood flow.
The physical findings in an adult with an ASD include those related directly to the intracardiac shunt, and those that are secondary to the right heart failure that may be present in these individuals.
Upon auscultation of the heart sounds, a systolic ejection murmur may be heard that is attributed to the pulmonic valve, due to the increased flow of blood through the pulmonic valve rather than any structural abnormality of the valve leaflets.
In unaffected individuals, respiratory variations occur in the splitting of the second heart sound (S2). During respiratory inspiration, the negative intrathoracic pressure causes increased blood return into the right side of the heart. The increased blood volume in the right ventricle causes the pulmonic valve to stay open longer during ventricular systole. This causes a normal delay in the P2 component of S2. During expiration, the positive intrathoracic pressure causes decreased blood return to the right side of the heart. The reduced volume in the right ventricle allows the pulmonic valve to close earlier at the end of ventricular systole, causing P2 to occur earlier.
In individuals with an ASD, a fixed splitting of S2 occurs because the extra blood return during inspiration gets equalized between the left and right atria due to the communication that exists between the atria in individuals with ASD.
The right ventricle can be thought of as continuously overloaded because of the left-to-right shunt, producing a widely split S2. Because the atria are linked via the atrial septal defect, inspiration produces no net pressure change between them and has no effect on the splitting of S2. Thus, S2 is split to the same degree during inspiration as expiration, and is said to be “fixed”.
In transthoracic echocardiography, an atrial septal defect may be seen on color flow imaging as a jet of blood from the left atrium to the right atrium.
If agitated saline is injected into a peripheral vein during echocardiography, small air bubbles can be seen on echocardiographic imaging. Bubbles traveling across an ASD may be seen either at rest or during a cough. (Bubbles only flow from right atrium to left atrium if the right atrial pressure is greater than left atrial). Because better visualization of the atria is achieved with transesophageal echocardiography, this test may be performed in individuals with a suspected ASD which is not visualized on transthoracic imaging. Newer techniques to visualize these defects involve intracardiac imaging with special catheters typically placed in the venous system and advanced to the level of the heart. This type of imaging is becoming more common and involves only mild sedation for the patient
If the individual has adequate echocardiographic windows, use of the echocardiogram to measure the cardiac output of the left ventricle and the right ventricle independently is possible. In this way, the shunt fraction can be estimated using echocardiography.
Transcranial doppler bubble study
A less invasive method for detecting a PFO or other ASDs than transesophageal ultrasound is transcranial Doppler with bubble contrast. This method reveals the cerebral impact of the ASD or PFO.
The ECG findings in atrial septal defect vary with the type of defect the individual has. Individuals with atrial septal defects may have a prolonged PR interval (a first-degree heart block). The prolongation of the PR interval is probably due to the enlargement of the atria common in
In addition to the PR prolongation, individuals with a
A common finding in the ECG is the presence of incomplete right bundle branch block, which is so characteristic that if it is absent, the diagnosis of ASD should be reconsidered.
Once someone is found to have an atrial septal defect, a determination of whether it should be corrected is typically made. If the atrial septal defect is causing the right ventricle to enlarge a
Drug therapy can be used to minimize the risk of thromboembolism and stroke in PFO. Anticoagulants, such as warfarin, are commonly used to reduce blood clotting, whereas antiplatelet agents, such as aspirin, are used to reduce platelet aggregation and thrombosis.
Evaluation prior to correction
Prior to correction of an ASD, an evaluation is made of the severity of the individual’s pulmonary hypertension (if present at all) and whether it is reversible (closure of an ASD may be recommended for prevention purposes, to avoid such a complication in the first place. Pulmonary hypertension is not always present in adults who are diagnosed with an ASD in adulthood).
If pulmonary hypertension is present, the evaluation may include a right heart catheterization. This involves placing a catheter in the venous system of the heart and measuring pressures and oxygen saturation in the superior vena cava, inferior vena cava, right atrium, right ventricle, and pulmonary artery, and in the wedge position. Individuals with a pulmonary vascular resistance (PVR) less than 7 wood units show regression of symptoms (including NYHA functional class). However, individuals with a PVR greater than 15 wood units have increased mortality associated with the closure of the ASD.
If the pulmonary arterial pressure is more than two-thirds of the systemic systolic pressure, a net left-to-right shunt should occur at least 1.5:1 or evidence of reversibility of the shunt when given pulmonary artery vasodilators prior to surgery. (If Eisenmenger’s physiology has set in, the right-to-left shunt must be shown to be reversible with pulmonary artery vasodilators prior to surgery.)
Surgical mortality due to the closure of an ASD is lowest when the procedure is performed prior to the development of significant pulmonary hypertension. The lowest mortality rates are achieved in individuals with a pulmonary artery systolic pressure less than 40 mmHg. If Eisenmenger’s syndrome has occurred, a significant risk of mortality exists regardless of the method of closure of the ASD. In individuals who have developed Eisenmenger’s syndrome, the pressure in the right ventricle has raised high enough to reverse the shunt in the atria. If the ASD is then closed, the afterload that the right ventricle has to act against has suddenly increased. This may cause immediate right ventricular failure, since it may not be able to pump the blood against pulmonary hypertension.
Surgical closure of an ASD involves opening up at least one atrium and closing the defect with a patch under direct visualization.
Percutaneous device closure involves the passage of a catheter into the heart through the femoral vein guided by fluoroscopy and echocardiography. An example of a percutaneous device is a device which has discs that can expand to a variety of diameters at the end of the catheter. The catheter is placed in the right femoral vein and guided into the right atrium. The catheter is guided through the atrial septal wall and one disc (left atrial) is opened and pulled into place. Once this occurs, the other disc (right atrial) is opened in place and the device is inserted into the septal wall. This type of PFO closure is more effective than drug or other medical therapies for decreasing the risk of future thromboembolism.
Percutaneous closure of an ASD is currently only indicated for the closure of
Percutaneous closure is the method of choice in
Due to the communication between the atria that occurs in ASDs, disease entities or complications from the condition are possible. Patients with an uncorrected atrial septal defect may be at increased risk for developing a cardiac arrhythmia, as well as more frequent respiratory infections.
ASDs, and particularly PFOs, are a predisposing risk factor for decompression sickness in divers because a proportion of venous blood carrying inert gases, such as helium or nitrogen does not pass through the lungs. The only way to release the excess inert gases from the body is to pass the blood carrying the inert gases through the lungs to be exhaled. If some of the inert gas-laden blood passes through the PFO, it avoids the lungs and the inert gas is more likely to form large bubbles in the arterial blood stream causing decompression sickness.
If a net flow of blood exists from the left atrium to the right atrium, called a left-to-right shunt, then an increase in the blood flow through the lungs happens. Initially, this increased blood flow is asymptomatic, but if it persists, the pulmonary blood vessels may stiffen, causing pulmonary hypertension, which increases the pressures in the right side of the heart, leading to the reversal of the shunt into a right-to-left shunt. Reversal of the shunt occurs, and the blood flowing in the opposite direction through the ASD is called Eisenmenger’s syndrome, a rare and late complication of an ASD.
Venous thrombus (clots in the veins) are quite common. Embolizations (dislodgement of thrombi) normally go to the lung and cause pulmonary emboli. In an individual with ASD, these emboli can potentially enter the arterial system, which can cause any phenomenon attributed to acute loss of blood to a portion of the body, including cerebrovascular accident (stroke), infarction of the spleen or intestines, or even a distal extremity (i.e., finger or toe).
This is known as a paradoxical embolus because the clot material paradoxically enters the arterial system instead of going to the lungs.
Some recent research has suggested that a proportion of cases of a migraine may be caused by PFO. While the exact mechanism remains unclear, closure of a PFO can reduce symptoms in certain cases. This remains controversial; 20% of the general population has a PFO, which for the most part, is asymptomatic. About 20% of the female population has migraines, and the placebo effect in migraine typically averages around 40%. The high frequency of these facts finding statistically significant relationships between PFO and migraine difficult (i.e., the relationship may just be chance or coincidence). In a large randomized controlled trial, the higher prevalence of PFO in migraine patients was confirmed, but migraine headache cessation was not more prevalent in the group of migraine patients who underwent closure of their PFOs.
As a group, atrial septal defects are detected in one child per 1500 live births. PFOs are quite common (appearing in 10–20% of adults), but asymptomatic, so undiagnosed. ASDs make up 30 to 40% of all congenital heart diseases that are seen in adults.