an intramural hematoma (caused by bleeding within the media). since no direct connection exists between the true lumen and the false lumen in these cases, diagnosing an aortic dissection by aortography is difficult if the cause is an intramural hematoma. an aortic dissection secondary to an intramural hematoma should be treated the same as one caused by an intimal tear. diagnosis because of the varying symptoms of aortic dissection, the diagnosis is sometimes difficult to make. concern should be increased in those with low blood pressure, neurological problems, and an unequal pulses. while taking a good history from the individual may be strongly suggestive of an aortic dissection, the diagnosis cannot always be made by history and physical signs alone. often, the diagnosis is made by visualization of the intimal flap on a diagnostic imaging test. common tests used to diagnose an aortic dissection include a ct scan of the chest with iodinated contrast material and a transesophageal echocardiogram. the proximity of the aorta to the esophagus allows the use of higher-frequency ultrasound for better anatomical images. other tests that may be used include an aortogram or magnetic resonance angiogram of the aorta. each of these tests has pros and cons, and they do not have equal sensitivities and specificities in the diagnosis of aortic dissection. in general, the imaging technique chosen is based on the pretest likelihood of the diagnosis, availability of the testing modality, patient stability, and the sensitivity and specificity of the test. d-dimer a measurement of blood d-dimer level may be useful in diagnostic evaluation. a level less than 500 ng/ml may be considered evidence against a diagnosis of aortic dissection, although this guideline is only applicable in cases deemed "low risk" and within 24 hours of symptom onset. the american heart association does not advise using this test in making the diagnosis, as evidence is still tentative. chest x-ray aortic dissection on cxr: note is made of a wide aortic knob. chest radiography may although demonstrate a change in the morphology of the thoracic aorta which can be seen in aortic dissection. classically, new widening of the mediastinum on radiograph is of moderate sensitivity for detecting an ascending aortic dissection; however, this finding is of low specificity, as many other conditions can cause apparent widening of the mediastinum. there are several other associated radiographic findings: the "calcium sign" describes an apparent separation of the intimal calcification from the outer aortic margin by greater than 10 mm. pleural effusions, more commonly in descending aortic dissections, and typically left-sided. other: the obliteration of the aortic knob, depression of the left mainstem bronchus, loss of the paratracheal stripe, and tracheal deviation. importantly, about 12 to 20% of aortic dissections are not detectable by chest radiograph; therefore, a "normal" chest radiograph does not rule out aortic dissection. if there is high clinical suspicion, a more sensitive imaging test (ct angiogram, mr angiography, or transesophageal echo) may be warranted. computed tomography computed tomography angiography is a fast, non-invasive test that gives an accurate three-dimensional view of the aorta. these images are produced by taking rapid, thin-cut slices of the chest and abdomen, and combining them in the computer to create cross-sectional slices. to delineate the aorta to the accuracy necessary to make the proper diagnosis, an iodinated contrast material is injected into a peripheral vein. contrast is injected and the scan performed using a bolus tracking method. this type of scan is timed to injection to capture the contrast as it enters the aorta. the scan then follows the contrast as it flows through the vessel. it has a sensitivity of 96 to 100% and a specificity of 96 to 100%. disadvantages include the need for iodinated contrast material and the inability to diagnose the site of the intimal tear. ct with contrast demonstrating aneurysmal dilation and a dissection of the ascending aorta (type a stanford) chest ct with descending (type b stanford) aortic dissection (red circle) type a dissection with pericardial effusion as a result. mri mri of an aortic dissection aorta descendens with dissectionaorta isthmus magnetic resonance imaging (mri) is also used for the detection and assessment of aortic dissection, with a sensitivity of 98% and a specificity of 98%. an mri examination of the aorta produces a three-dimensional reconstruction of the aorta, allowing the physician to determine the location of the intimal tear and the involvement of branch vessels, and to locate any secondary tears. it is a noninvasive test, does not require the use of iodinated contrast material, and can detect and quantitate the degree of aortic insufficiency. the disadvantage of the mri scan in the face of aortic dissection is that it may be available only in larger hospitals, and the scan is relatively time-consuming, which could be dangerous in people who are already very unwell. due to the high-intensity magnetic fields used during mri, it cannot be used on individuals with metallic implants. in addition, some individuals experience claustrophobia while surrounded by the mri magnet. ultrasound an echocardiogram displaying the true lumen and false lumen of an aortic dissection: in the image to the left, the intimal flap can be seen separating the two lumens. in the image to the right, color flow during ventricular systole suggests that the upper lumen is the true lumen. the transesophageal echocardiogram (tee) is a good test in the diagnosis of aortic dissection, with a sensitivity up to 98% and a specificity up to 97%. it has become the preferred imaging modality for suspected aortic dissection. it is a relatively noninvasive test, requiring the individual to swallow the echocardiography probe. it is especially good in the evaluation of ai in the setting of ascending aortic dissection and to determine whether the ostia (origins) of the coronary arteries are involved. while many institutions give sedation during transesophageal echocardiography for added patient comfort, it can be performed in cooperative individuals without the use of sedation. disadvantages of tee include the inability to visualize the distal ascending aorta (the beginning of the aortic arch), and the descending abdominal aorta that lies below the stomach. a tee may be technically difficult to perform in individuals with esophageal strictures or varices. aortogram main article: aortography an aortogram involves the placement of a catheter in the aorta and injection of contrast material while taking x-rays of the aorta. the procedure is known as aortography. previously thought to be the diagnostic gold standard, it has been supplanted by other, less-invasive imaging modalities. classification classification of aortic dissection percentage 60% 10–15% 25–30% type debakey i debakey ii debakey iii stanford a (proximal) stanford b (distal) several different classification systems have been used to describe aortic dissections. one such classification is based on chronicity and labels aortic dissections as hyperacute (<24 hours duration), acute (2–7 days), subacute (8–30 days), and chronic (>30 days). the systems commonly in use are based on either the anatomy of the dissection or the duration of onset of symptoms before the presentation. the stanford system is used more commonly now, as it is more attuned to the management of the patient. debakey the debakey system, named after cardiothoracic surgeon michael e. debakey, is an anatomical description of the aortic dissection. it categorizes the dissection based on where the original intimal tear is located and the extent of the dissection (localized to either the ascending aorta or descending aorta or involving both the ascending and descending aorta). type i – originates in ascending aorta, and propagates at least to the aortic arch and often beyond it distally. it is most often seen in patients less than 65 years of age and is the most lethal form of the disease. type ii – originates in the ascending aorta and is confined to it. type iii – originates in the descending aorta and rarely extends proximally, but will extend distally. it most often occurs in elderly patients with atherosclerosis and hypertension. stanford the stanford classification is divided into two groups, a and b, depending on whether the ascending aorta is involved. a – involves the ascending aorta and/or aortic arch, and possibly the descending aorta. the tear can originate in the ascending aorta, the aortic arch, or more rarely, in the descending aorta. it includes debakey types i and ii. b – involves the descending aorta or the arch (distal to the left subclavian artery), without the involvement of the ascending aorta. it includes debakey type iii. the stanford classification is useful as it follows clinical practice, as type a ascending aortic dissections generally require primary surgical treatment, whereas type b dissections generally are treated medically as initial treatment with surgery reserved for any complications. the main indication for surgical repair of type a dissections is the prevention of acute hemorrhagic pericardial tamponade due to leakage of blood through the dissected layers of the intrapericardial proximal aorta. a secondary indication is acute aortic valve insufficiency (regurgitation): ascending aortic dissections often involve the aortic valve, which, having lost its suspensory support, telescopes down into the aortic root, resulting in aortic incompetence. the valve must be resuspended to be reseated, as well as to repair or prevent coronary artery injury. also, the area of dissection is removed and replaced with a dacron graft to prevent further dissection from occurring. however, type b dissections are not improved, from a mortality point of view, by the operation, unless leaking, rupture, or compromise to other organs, e.g. kidneys, occurs. prevention among the recognized risk factors for aortic dissection, hypertension, abnormally high levels of lipids (such as cholesterol) in the blood, and smoking tobacco are considered preventable risk factors. repair of an enlargement of the ascending aorta from an aneurysm or previously unrecognized and untreated aortic dissections is recommended when greater than 5.5 cm (2.2 in) in size to decrease the risk of dissection. repair may be recommended when greater than 4.5 cm (1.8 in) in size if the person has one of the several connective-tissue disorders or a family history of a ruptured aorta. management in an acute dissection, treatment choice depends on its location. for stanford type a (ascending aortic) dissection, surgical management is superior to medical management. for uncomplicated stanford type b (distal aortic) dissections (including abdominal aortic dissections), medical management is preferred over surgery. complicated stanford type b aortic dissections require surgical intervention after initiation of medical therapy. the risk of death due to aortic dissection is highest in the first few hours after the dissection begins, and decreases afterward. because of this, the therapeutic strategies differ for the treatment of an acute dissection compared to a chronic dissection. an acute dissection is one in which the individual presents within the first two weeks. if the individual has managed to survive this window period, their prognosis is improved. about 66% of all dissections present in the acute phase. individuals who present two weeks after the onset of the dissection are said to have chronic aortic dissections. these individuals have been self-selected as survivors of the acute episode and can be treated with medical therapy as long as they are stable. medication aortic dissection generally presents as a hypertensive emergency, and the prime consideration of medical management is to decrease the shear stress in the aortic wall (dp/dt (force of ejection of blood from the left ventricle)) by decreasing blood pressure and the heart rate. the target blood pressure should be a mean arterial pressure (map) of 60 to 75 mmhg, or the lowest blood pressure tolerated. initial decreases should be by about 20%. the target heart rate is less than 65 beats per minute. long-term blood pressure control is required for every person who has experienced aortic dissection. beta blockers are the first-line treatment for patients with acute and chronic aortic dissection. in acute dissection, fast-acting agents can be given intravenously and have doses that are easier to adjust (such as esmolol, propranolol, or labetalol) is preferred. vasodilators such as sodium nitroprusside can be considered for people with ongoing high blood pressure, but they should never be used alone, as they often stimulate a reflexive increase in the heart rate. calcium channel blockers can be used in the treatment of aortic dissection, particularly if a contraindication to the use of beta-blockers exists. the calcium channel blockers typically used are verapamil and diltiazem, because of their