Aortic dissection

aortic-dissection

In this dramatic condition a breach in the integrity of the aortic wall allows arterial blood to burst into the media of the aorta. Which is then split into two layers, creating a ‘false lumen’ alongside the existing or ‘true lumen?’

Types (anatomical & clinical):

[A] Type-A:

  • Involves the ascending aorta.
  • Occurs in two-thirds of cases.
  • Frequently extend into the descending aorta.
  • Peak incidence: in the 6th & 7th decades of life.
  • H/O hypertension present in the large majority.
  • Men are twice as frequently affected as women.

[B] Type-B: Involves only the descending aorta distal to the left subclavin artery.

Causes:

  1. Aortic dissection is associated with hypertension  (high blood pressure) and many connective tissue  disorders. Vacuities (inflammation of an artery) is rarely associated with aortic dissection. It can also be the result of chest trauma. 72 to 80% of individuals who present with an aortic dissection have a previous history of hypertension.
  2. A bicuspid aortic valve  (a type of congenital heart disease  involving the aortic valve) is found in 7–14% of individuals who have an aortic dissection. These individuals are prone to dissection in the ascending aorta. The risk of dissection in individuals with bicuspid aortic valve is not associated with the degree of stenosis  of the valve.
  3. Marfan syndrome  is noted in 5–9% of individuals who suffer from aortic dissection. In this subset, there is an increased incidence in young individuals. Individuals with Marfan syndrome tend to have aneurysms of the aorta and are more prone to proximal dissections of the aorta.
  4. Turner syndrome also increases the risk of aortic dissection, by aortic root dilatation.
  5. Chest trauma leading to aortic dissection can be divided into two groups based on etiology: blunt chest trauma (commonly seen in car accidents ) and iatrogenic. Iatrogenic causes include trauma during cardiac catheterization or due to an intra-aortic ballon pump.
  6. Aortic dissection may be a late sequela of cardiac surgery. 18% of individuals who present with an acute aortic dissection have a history of open heart surgery. Individuals who have undergone aortic valve replacement for aortic insufficiency  are at particularly high risk. This is because aortic insufficiency causes increased blood flow in the ascending aorta. This can cause dilatation and weakening of the walls of the ascending aorta.
  7. Syphilis  will only potentially cause aortic dissection in its tertiary stage.

Factors that may predispose to aortic dissection:

  • Hypertension (80% of case)
  • Aortic atherosclerosis.
  • Non specific aortic aneurysm.
  • Aortic coarctation.
  • Collagen disorders: e.g. Marfan’s syndrome.
  • Fibromuscular dysplasia.
  • Previous aortic surgery: e.g. CABG.
  • Trauma.

Clinical features:             

Symptoms: Usually acute, with very sudden onset of extremely severe pain in the chest and back, often starting between the shoulder blades, migrating and frequently describe as ‘tearing’

Signs:

  1. O/E there may be hypertension and asymmetry of brachial, carotid or femoral pulses.
  2. Signs of aortic reflux (e.g. early diastolic murmur)-in type-A dissection.

Investigations:

  1. X-ray chest P/A view: shows-
  • Broadening of the upper mediastinum (in 60%)
  • Distortion of the aorta ‘Knucke’ (in 60%).
  • Left sided pleural effusion (common).
  1. ECG: shows-
  • Left ventricular hypertrophy (in hypertension)
  • Acute myocardial infraction (Usually inferior)
  1. Doppler echocardiography: shows-
  • Aortic regurgitation.
  • A dilated aortic root.
  • Occasionally the flap of the dissection.
  1. Transoesophageal echocardiography (helpful because it can only image the first 3-4 cm of the ascending aorta.
  2. CT scanning and MRI are both high specific.

Prevention: Chronic enlargement of the ascending aorta from aneurysm or previously unrecognized and untreated aortic dissections is repaired electively when it reaches 6 cm (2.4 in) in size and surgery may be recommended between for as little as 4.5 cm (1.8 in) in size if the patient has one of several connective tissue disorders or a family history of ruptured aorta.

Management: Initial management comprises pain control and anti-hypertensive treatment with labetalol to maintain the systolic pressure below 120 mm hg.

Type-A: Emergency surgical reapir under cardiopulmonary bypass.

Type-B:

  1. a) Treated medically, with bed rest & careful control of BP.
  2. b) Surgical intervention required if there is evidence of leakage.

Medication

Aortic dissection generally presents as a hypertensive emergency, and the prime consideration of medical management is strict blood pressure control. The target blood pressure should be a mean arterial pressure  (MAP) of 60 to 75  mmHg,  or the lowest blood pressure tolerated by the patient. Another factor is to reduce the shear-force dP/dt (force of ejection of blood from the left ventricle).

Beta blockers are first line treatment for patients with acute and chronic aortic dissection. In acute dissection, rapidly acting, titratable parenteral agents (such as esmolol, propranolol, or labetalol ) are preferred. Vasodilators such as sodium nitroprusside  can be considered for patients with ongoing hypertension, but they should never be used alone, as they generally cause reflex tachycardia.

Calcium channel blockers can be used in the treatment of aortic dissection, particularly if there is a contraindication to the use of beta blockers. The calcium channel blockers typically used are verapamil and diltiazem , because of their combined vasodilator and negative intoropic  effects.

If the individual has refractory hypertension (persistent hypertension on the maximum doses of three different classes of antihypertensive agents), involvement of the renal arteries in the aortic dissection plane should be considered.

Surgical involves:

  • Replacement of affected part with a Dacron graft.
  • Replacement of aortic valve (sometimes necessary).

Prognosis: Of all people with aortic dissection, 40% die almost straight away and do not reach hospital in time. Of the remainder, 1% dies every hour, making prompt diagnosis and treatment a priorty. Even after diagnosis, 5–20% die during surgery or in the immediate postoperative period. In ascending aortic dissection, if there is a decision that surgery is not appropriate, 75% die within 2 weeks. With aggressive treatment 30-day survival for thoracic dissections may be as high as 90%.

Reference:

  1.  Davidson’s Principles and Practice of Medicine, 21st edition.
  2. Wikipedia the free encyclopedia.

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