出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/05/18 16:12:13」(JST)
| Atrial septal defect | |
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Illustration of an atrial septal defect.
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| Classification and external resources | |
| ICD-10 | Q21.1 |
| ICD-9 | 745.5-745.6 |
| OMIM | 108800 |
| DiseasesDB | 1089 |
| MedlinePlus | 000157 |
| eMedicine | med/3519 article/894813 |
| MeSH | C14.240.400.560.375 |
Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria (upper chambers) of the heart. Normally, the atria are separated by a dividing wall, the interatrial septum. If this septum is defective or absent, then oxygen-rich blood can flow directly from the left side of the heart to mix with the oxygen-poor blood in the right side of the heart, or vice versa.[1] This can lead to lower-than-normal oxygen levels in the arterial blood that supplies the brain, organs, and tissues. However, an ASD may not produce noticeable signs or symptoms, especially if the defect is small.
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 fetus, 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 primum acts as a valve over the foramen ovale during fetal development. After birth, the pressure in the right side of the heart drops as the lungs open and begin working, causing the foramen ovale to close entirely. In approximately 25% of adults,[2] the foramen ovale does not entirely seal.[3] In these cases, any elevation of the pressure in the pulmonary circulatory system (due to pulmonary hypertension, temporarily while coughing, etc.) can cause the foramen ovale to remain open. This is known as a patent foramen ovale (PFO), which is a type of 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 of the heart. This is 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 (>9mm), which may result in a clinically remarkable left-to-right shunt, blood will shunt 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.[4]
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 in order 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 will cause an overload of the entire pulmonary vasculature. Eventually, pulmonary hypertension may develop.
The pulmonary hypertension will cause the right ventricle to face increased afterload. The right ventricle will be forced to generate higher pressures to try to overcome the pulmonary hypertension. This may lead to right ventricular failure (dilatation and decreased systolic function of the right ventricle).
If the ASD is left uncorrected, the pulmonary hypertension progresses and the pressure in the right side of the heart will become greater than the left side of the heart. This reversal of the pressure gradient across the ASD causes the shunt to reverse; a right-to-left shunt will exist. This phenomenon is known as Eisenmenger's syndrome. Once right-to-left shunting occurs, a portion of the oxygen-poor blood will get shunted to the left side of the heart and ejected to the peripheral vascular system. This will cause signs of cyanosis.
Heart of human embryo of about thirty-five days
Atrial septal defect with left-to-right shunt
Illustration depicting atrial septal defect.
As a group, atrial septal defects are detected in 1 child per 1500 live births. PFO are quite common (appearing in 10–20% of adults) but asymptomatic and therefore undiagnosed. ASDs make up 30 to 40% of all congenital heart diseases that are seen in adults.[5]
The ostium secundum atrial septal defect accounts for 7% of all congenital heart lesions. This lesion shows a female preponderance, with a male:female ratio of 1:2.[6]
In May 2013 team of researchers led by Professor Bernard Keavney of British Heart Foundation (BHF) discovered a new gene associated with the disease. According to them a common genetic variation near a gene called MSX1 is strongly associated with the risk of an ASD and this discovery of the particular gene is an important step forward as it will lead to better understanding of why some patients are born with ASD.[7]
There are many types of atrial septal defects. They 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.
The ostium secundum atrial septal defect is the most common type of atrial septal defect, and comprises 6–10% of all congenital heart diseases.
The secundum atrial septal defect usually arises from an enlarged foramen ovale, inadequate growth of the septum secundum, or excessive absorption of the septum primum. Ten to twenty percent of individuals with ostium secundum ASDs also have mitral valve prolapse.[8]
If the ostium secundum ASD is accompanied by an acquired mitral valve stenosis, that is called Lutembacher's syndrome.[9]
Most individuals with an uncorrected secundum ASD do not have significant symptoms through early adulthood. More than 70 percent develop symptoms by about 40 years of age. Symptoms are typically decreased exercise tolerance, easy fatigueability, palpitations, and syncope.
Complications of an uncorrected secundum ASD include pulmonary hypertension, right-sided heart failure, atrial fibrillation or flutter, stroke, and Eisenmenger's syndrome.
While pulmonary hypertension is unusual before 20 years of age, it is seen in 50 percent of individuals above the age of 40. Progression to Eisenmenger's syndrome occurs in 5 to 10 percent of individuals late in the disease process.[9]
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 approximately 25% of people, the foramen ovale fails to close properly, leaving patients with a patent foramen ovale. In medical usage, the term "patent" means open or unobstructed.[10]
Clinically it is linked to decompression sickness, paradoxical embolism and migraine. On echocardiography, there may not be any shunting of blood noted except when the patient coughs.
PFO has been linked to strokes and 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 atria into the left atria via the PFO, and ultimately into systemic circulation.[11] PFO is common in patients with atrial septal aneurysms (ASA) which are also linked to cryptogenic (i.e. of unknown cause) strokes.[12] PFO is more prevalent in patients with cryptogenic stroke than in patients with a stroke of known cause.[13]
Treatments for PFO include surgical closure, percutaneous device closure, anticoagulant therapy, and antiplatelet agents.[12]
There is debate within the neurology and cardiology communities about the role of a PFO in cryptogenic neurologic events such as strokes and transient ischemia attacks (TIAs) without any other potential cause. Some data suggested that PFOs may be involved in the pathogenesis of some migraine headaches.[14] Several clinical trials are currently underway to investigate the role of PFO in these clinical situations.[14]
A defect in the ostium primum is occasionally classified as an atrial septal defect,[15] but it is more commonly classified as an atrioventricular septal defect.[16][17] Ostium primum defects are less common than ostium secundum defects.[18]
A sinus venosus ASD is a type of atrial septum defect in which the defect in the septum involves the venous inflow of either the superior vena cava or the inferior vena cava.
A sinus venosus ASD that involves the superior vena cava makes up 2 to 3% of all interatrial communication. It is located at the junction of the superior vena cava and the right atrium. It is frequently associated with anomalous drainage of the right-sided pulmonary veins into the right atrium (instead of the normal drainage of the pulmonary veins into the left atrium).[19]
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.[20]
The inter atrial septum can be divided into 5 septal zones. If the defect involves 2 or more of the 5 septal zones, then the defect is termed a mixed atrial septal defect.[4]
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 physical examination.
Some individuals with an ASD will have undergone 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 will present with symptoms of dyspnea on exertion (shortness of breath with minimal exercise), congestive heart failure, or cerebrovascular accident (stroke). They may be noted on 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.[21]
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, there may be a systolic ejection murmur that is attributed to the pulmonic valve. This is due to the increased flow of blood through the pulmonic valve rather than any structural abnormality of the valve leaflets.
In unaffected individuals, there are respiratory variations 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, there is a fixed splitting of S2. The reason that there is a fixed splitting of the second heart sound is that the extra blood return during inspiration gets equalized between the left and right atrium 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. It may be possible to see bubbles travel across an ASD either at rest or during a cough. (Bubbles will only flow from right atrium to left atrium if the RA pressure is greater than LA).
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 that are 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 typically.
If the individual has adequate echocardiographic windows, it is possible to use the echocardiogram to measure the cardiac output of the left ventricle and the right ventricle independently. In this way, it is possible to estimate the shunt fraction using echocardiography.
A less invasive method for detecting a PFO or other ASDs than transesophagal ultrasound is Transcranial Doppler with bubble contrast.[22] 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 that is common in ASDs and the increased distance due to the defect itself. Both of these can cause an increased distance of internodal conduction from the SA node to the AV node.[23]
In addition to the PR prolongation, individuals with a primum ASD have a left axis deviation of the QRS complex while those with a secundum ASD have a right axis deviation of the QRS complex. Individuals with a sinus venosus ASD exhibit a left axis deviation of the P wave (not the QRS complex).
A common finding in the ECG is the presence of incomplete right bundle branch block. The presence of a right bundle branch block 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 has to be made.
Surgical mortality due to 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 of less than 40 mmHg.
If Eisenmenger's syndrome has occurred, there is significant risk of mortality 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 the pulmonary hypertension.
Closure of an ASD in individuals under age 25 has been shown to have a low risk of complications, and individuals have a normal lifespan (comparable to a healthy age-matched population). Closure of an ASD in individuals between the ages of 25 and 40 who are asymptomatic but have a clinically significant shunt is controversial. Those that perform the procedure believe that they are preventing long-term deterioration in cardiac function and preventing the progression of pulmonary hypertension.
Methods of closure of an ASD include surgical closure and percutaneous closure.
Although invasive, surgical closure is particularly beneficial because additional drug therapy is not needed.[24] This is considered to be the gold standard to prevent PFO and paradoxical embolism.[24]
Percutaneous device closure involves the passage of a catheter into the heart through the femoral vein guided by fluoroscopy and echocardiography.[12] An example of a percutaneous device is the Cardia PFO occluder which has discs that can expand to a variety of diameters at the end of the catheter.[25] The catheter is placed in the right femoral vein and guided into the right atria.[25] The catheter is guided through the atrial septal wall and one disc (left atrial) is opened and pulled into place.[25] Once this occurs, the other disc (right atrial) is opened in place and the device is inserted into the septal wall.[25] This type of PFO closure is more effective than drug or other medical therapies for decreasing the risk of future thromboembolism.[12]
Drug therapy can be used to minimize risk of thromboembolism and stroke in PFO. Anticoagulants, such as warfarin, are commonly used to reduce blood clotting, whereas antiplatelet agents, such as aspirin, to reduce platelet aggregation and thrombosis.[13]
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 that 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 saturations in the SVC, IVC, right atrium, right ventricle, pulmonary artery, and in the wedge position. Individuals with a pulmonary vascular resistance (PVR) of less than 7 wood units show regression of symptoms (including NYHA functional class). On the other hand, individuals with a PVR of greater than 15 wood units have increased mortality associated with closure of the ASD.
If the pulmonary arterial pressure is more than 2/3 the systemic systolic pressure, there should be a net left-to-right shunt of 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, it must be proven that the right-to-left shunt is reversible with pulmonary artery vasodilators prior to surgery.)
Until the early 1990s, surgery was the usual method for closing all ASDs. Now, thanks to medical advances, doctors can use catheter procedures to close secundum ASDs, the most common type of ASD. During the procedure, the doctor inserts a catheter (a thin, flexible tube) into a vein in the groin (upper thigh) and threads it to the heart's septum. The catheter has a tiny umbrella-like device folded up inside it. When the catheter reaches the septum, the device is pushed out of the catheter and positioned so that it plugs the hole between the atria. The device is secured in place and the catheter is withdrawn from the body. Within 6 months, normal tissue grows in and over the device. There is no need to replace the closure device throughout the patient's life. Doctors often use echocardiography (echo) or transesophageal echo (TEE) as well as angiography to guide them in threading the catheter to the heart and closing the defect. TEE is a special type of echo that takes pictures of the heart through the esophagus (the passage leading from the mouth to the stomach). Catheter procedures are much easier on patients than surgery because they involve only a needle puncture in the skin where the catheter is inserted. This means that recovery is faster and easier. The outlook for patients having this procedure is excellent. Closures are successful in more than 9 out of 10 patients, with no significant leakage. Rarely, a defect is too large for catheter closure and surgery is needed.
Surgical closure of an ASD involves opening up at least one atrium and closing the defect with a patch under direct visualization.
Percutaneous closure of an ASD is currently only indicated for the closure of secundum ASDs with a sufficient rim of tissue around the septal defect so that the closure device does not impinge upon the SVC, IVC, or the tricuspid or mitral valves. The Amplatzer Septal Occluder (ASO) is commonly used to close ASDs. The ASO consists of two self-expandable round discs connected to each other with a 4 mm waist, made up of 0.004–0.005´´ Nitinol wire mesh filled with Dacron fabric. Implantation of the device is relatively easy. The prevalence of residual defect is low. The disadvantages are a thick profile of the device and concern related to a large amount of nitinol (a nickel-titanium compound) in the device and consequent potential for nickel toxicity.
Percutaneous closure is the method of choice in most centres.[26]
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 defects may be at increased risk for developing a cardiac arrhythmia, as well as more frequent respiratory infections.[18]
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.[27][28] 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 there is an increase in the blood flow through the lungs. Initially, this increased blood flow is asymptomatic, but if it persists, the pulmonary blood vessels may stiffen, causing pulmonary hypertension. The pulmonary hypertension increases the pressures in the right side of the heart, leading to the reversal of the shunt into a right-to-left shunt. Once the reversal of the shunt occurs, and the blood begins flowing in the opposite direction through the ASD, that is called Eisenmenger's syndrome. The syndrome is 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. This can cause any phenomenon that is 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 migraine may be caused by patent foramen ovale. While the exact mechanism remains unclear, closure of a PFO can reduce symptoms in certain cases.[29][30] This remains controversial. 20% of the general population have a PFO, which for the most part, is asymptomatic. 20% of the female population have migraines. And, the placebo effect in migraine typically averages around 40%. The high frequency of these facts makes 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 patent foramen ovale in migraine patients was confirmed, but migraine headache cessation was not more prevalent in the group of migraine patients that underwent closure of their patent foramen ovale.[31]
|coauthors= (help)|accessdate= requires |url= (help)|accessdate= requires |url= (help)|date= (help); |accessdate= requires |url= (help)This article incorporates public domain material from the United States Department of Health and Human Services document "National Heart, Lung, and Blood Institute".
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| リンク元 | 「心房中隔二次口開存症」 |
| 関連記事 | 「defect」「secundum」「atrial」「ostium」「septal」 |