Aortic Dissection
December 05, 2009, 19:39 Filed in: Cardiovascular
Aortic Dissection
By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care
Introduction
Aortic dissection is a potentially lethal condition that is the most common acute catastrophe involving the Aorta. It occurs two to three times more frequently than rupture of abdominal aortic aneurysms.
Etiology
The pathogenesis of aortic dissection is not completely understood. It was once thought that a condition known as cystic medial necrosis, a pathological condition of uncertain cause, had to exist along with an intimal tear before a dissection could occur. It is now known that what was once called cystic medial necrosis is a normal part of aging of the Aorta. One thing that is clear is that hypertension is related to aortic dissection. The peak incidence is between 50 and 70 years of age. Dissections are slightly more common in men and in patients who have Marfan's syndrome. Pregnancy is a rare predisposing condition.
Classification
There are two major classification systems for aortic dissections and both are based upon the location of the dissection. The vast majority of aortic dissections occur either at the ascending aorta just above the aortic valve or occur just below the origin of the left subclavian artery. The DeBakey classification calls a Type I dissection one in which the tear originates in the ascending aorta and dissects into the descending aorta. A Type II dissection originates in and is limited to the ascending aorta. A Type III dissection originates in the descending aorta and may progress in either direction. More recently, the Austen and Shumway classification divides dissections into Type A and Type B for ascending and descending dissections respectively.
Pathophysiology
Aortic dissections cause death and morbidity by any or all of several mechanisms. Depending on the location of the dissection, there can be disruption of any of the major branches off of the aorta leading to ischemic injury in the area of distribution of the involved vessels such as the carotids, renals, or coronary arteries. Dissection of the ascending aorta can distort the aortic valve resulting in acute life-threatening aortic insufficiency with resultant congestive heart failure. Thrombi can form and result in distal arterial obstruction in the lower extremities. Exsanguination can occur if the false lumen of the Aorta bleeds into the pleural space or mediastinum and pericardial tamponade can occur if blood dissects into the pericardium. Frequently the systemic blood pressure, particularly proximal to a major aortic obstruction can be very high and very labile and this can result in cerebrovascular accidents.
Presentation
Severe acute chest pain occurs in 95% of patients with aortic dissection. The pain is typically sudden in onset and has a sharp tearing or throbbing quality. The pain is usually centered in the sub sternal region but may be felt in the jaw, precordium, neck, jaw, extremities, epigastrium, or back. Depending on complications there may be neurologic deficits and or evidence of ischemia elsewhere such as blindness or extremity pain. If aortic insufficiency occurs then evidence of acute heart failure may result leading to the appearance of an acute myocardial infarction. Of course, an acute myocardial infarction can also occur because of disruption of a coronary artery at its origin in the proximal aorta. Physical findings vary from shock, to hypertension, to heart failure. Sequential filling of the true and false lumens may produce "double" pulses and there may be unequal blood pressures in the extremities if compromise of the distal artery has ocurred.
Diagnosis
The chest x-ray frequently shows a widened superior mediastinum representing either a mediastinal hematoma or a widened aortic shadow. The initial x-ray may be normal. CT and MRI scanning are fairly specific for aortic dissection and echocardiography, particularly using an esophageal probe can confirm the diagnosis if the area of dissection can be visualized but a negative echocardiogram is not adequate to rule out dissection. Aortography is considered to be the gold standard.
Therapy
Because aortic dissections are life-threatening, specific therapy must be initiated very quickly in many cases. Therapy should not be delayed for aortography if the diagnosis is already apparent. Immediate therapy is aimed at both reducing the blood pressure and at reducing the pulsatile force of left ventricular ejection. Typically, a combination of nitroprusside (Nipride) and propranolol (Inderal) are used to accomplish both goals. Trimethophan camsylate (Arfonad), a ganglionic blocking agent can also be used both to lower the blood pressure and lower the force of left ventricular contraction. Blood pressure should be lowered to the lowest pressure that maintains cerebral and renal perfusion. Definitive surgical repair is indicated for all proximal dissections and for distal dissections with complications (i.e. bleeding, vascular obstruction) while medical therapy is preferred for uncomplicated acute or chronic distal dissections. Repair of proximal dissections may require repair and/or replacement of the aortic valve as well as coronary revascularization.
Outcome
Half of patients with untreated thoracic aortic dissections die within 48 hours and 90% do not survive 6 months. A compilation of the literature in 1983 revealed that for treated type A dissections, medical treatment had a 72% mortality while surgical treatment had a 32% mortality. For type B dissections, medical treatment had a 27% mortality while surgical treatment had a 32% mortality. Ten year survival is around 20% for either type of dissection.
References
Little AG, Anagnostopoulos CE: Aortic Dissections in Thoracic and Cardiovascular Surgery, 4th Edition, Editor W. Glenn, 1983, Appleton-Century-Crofts.
Thompson WL: Hypertensive Urgencies and Emergencies in Textbook of Critical Care, 2nd Edition, Editor Shoemaker et al, 1989, W. B. Saunders Company.
DeBakey ME, Cooley DA, Creech O Jr: Surgical consideration of dissecting aneurysm of the aorta. Ann Surg 142:586, 1955.
By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care
Introduction
Aortic dissection is a potentially lethal condition that is the most common acute catastrophe involving the Aorta. It occurs two to three times more frequently than rupture of abdominal aortic aneurysms.
Etiology
The pathogenesis of aortic dissection is not completely understood. It was once thought that a condition known as cystic medial necrosis, a pathological condition of uncertain cause, had to exist along with an intimal tear before a dissection could occur. It is now known that what was once called cystic medial necrosis is a normal part of aging of the Aorta. One thing that is clear is that hypertension is related to aortic dissection. The peak incidence is between 50 and 70 years of age. Dissections are slightly more common in men and in patients who have Marfan's syndrome. Pregnancy is a rare predisposing condition.
Classification
There are two major classification systems for aortic dissections and both are based upon the location of the dissection. The vast majority of aortic dissections occur either at the ascending aorta just above the aortic valve or occur just below the origin of the left subclavian artery. The DeBakey classification calls a Type I dissection one in which the tear originates in the ascending aorta and dissects into the descending aorta. A Type II dissection originates in and is limited to the ascending aorta. A Type III dissection originates in the descending aorta and may progress in either direction. More recently, the Austen and Shumway classification divides dissections into Type A and Type B for ascending and descending dissections respectively.
Pathophysiology
Aortic dissections cause death and morbidity by any or all of several mechanisms. Depending on the location of the dissection, there can be disruption of any of the major branches off of the aorta leading to ischemic injury in the area of distribution of the involved vessels such as the carotids, renals, or coronary arteries. Dissection of the ascending aorta can distort the aortic valve resulting in acute life-threatening aortic insufficiency with resultant congestive heart failure. Thrombi can form and result in distal arterial obstruction in the lower extremities. Exsanguination can occur if the false lumen of the Aorta bleeds into the pleural space or mediastinum and pericardial tamponade can occur if blood dissects into the pericardium. Frequently the systemic blood pressure, particularly proximal to a major aortic obstruction can be very high and very labile and this can result in cerebrovascular accidents.
Presentation
Severe acute chest pain occurs in 95% of patients with aortic dissection. The pain is typically sudden in onset and has a sharp tearing or throbbing quality. The pain is usually centered in the sub sternal region but may be felt in the jaw, precordium, neck, jaw, extremities, epigastrium, or back. Depending on complications there may be neurologic deficits and or evidence of ischemia elsewhere such as blindness or extremity pain. If aortic insufficiency occurs then evidence of acute heart failure may result leading to the appearance of an acute myocardial infarction. Of course, an acute myocardial infarction can also occur because of disruption of a coronary artery at its origin in the proximal aorta. Physical findings vary from shock, to hypertension, to heart failure. Sequential filling of the true and false lumens may produce "double" pulses and there may be unequal blood pressures in the extremities if compromise of the distal artery has ocurred.
Diagnosis
The chest x-ray frequently shows a widened superior mediastinum representing either a mediastinal hematoma or a widened aortic shadow. The initial x-ray may be normal. CT and MRI scanning are fairly specific for aortic dissection and echocardiography, particularly using an esophageal probe can confirm the diagnosis if the area of dissection can be visualized but a negative echocardiogram is not adequate to rule out dissection. Aortography is considered to be the gold standard.
Therapy
Because aortic dissections are life-threatening, specific therapy must be initiated very quickly in many cases. Therapy should not be delayed for aortography if the diagnosis is already apparent. Immediate therapy is aimed at both reducing the blood pressure and at reducing the pulsatile force of left ventricular ejection. Typically, a combination of nitroprusside (Nipride) and propranolol (Inderal) are used to accomplish both goals. Trimethophan camsylate (Arfonad), a ganglionic blocking agent can also be used both to lower the blood pressure and lower the force of left ventricular contraction. Blood pressure should be lowered to the lowest pressure that maintains cerebral and renal perfusion. Definitive surgical repair is indicated for all proximal dissections and for distal dissections with complications (i.e. bleeding, vascular obstruction) while medical therapy is preferred for uncomplicated acute or chronic distal dissections. Repair of proximal dissections may require repair and/or replacement of the aortic valve as well as coronary revascularization.
Outcome
Half of patients with untreated thoracic aortic dissections die within 48 hours and 90% do not survive 6 months. A compilation of the literature in 1983 revealed that for treated type A dissections, medical treatment had a 72% mortality while surgical treatment had a 32% mortality. For type B dissections, medical treatment had a 27% mortality while surgical treatment had a 32% mortality. Ten year survival is around 20% for either type of dissection.
References
Little AG, Anagnostopoulos CE: Aortic Dissections in Thoracic and Cardiovascular Surgery, 4th Edition, Editor W. Glenn, 1983, Appleton-Century-Crofts.
Thompson WL: Hypertensive Urgencies and Emergencies in Textbook of Critical Care, 2nd Edition, Editor Shoemaker et al, 1989, W. B. Saunders Company.
DeBakey ME, Cooley DA, Creech O Jr: Surgical consideration of dissecting aneurysm of the aorta. Ann Surg 142:586, 1955.