Thoracic Aortic Dissection - developinganaesthesia



THORACIC AORTIC DISSECTION

Vanderbilt Mantelpiece, (caryatid detail), Marble, mosaic, oak and cast iron.

Augustus Saint-Gaudens, 1881-3, Metropolitan Museum of Art, New York City.

This beautiful caryatid supports the right edge of the magnificent Vanderbilt Mantelpiece. One would think this to be a significant source of thoracic back pain for the poor caryatid who has born this weight for over 130 years - yet her face carries the most serene expression as if she feels not this burden at all! The Vanderbilt caryatid reminds us that, although we usually expect significant thoracic back pain in cases of aortic dissection, this may not always be the case! Around 4% of cases of aortic dissection surprisingly will have a zero value “ADD risk score”, and occasionally cases are seen even without pain!

THORACIC AORTIC DISSECTION

Introduction

Thoracic Aortic Dissection is a relatively uncommon condition, but a lethal one and so is vital to recognize.

Aortic dissection most commonly involves the thoracic aorta, but dissection can also extend into the abdominal aorta and even into the iliac arteries.

Clinical signs and symptoms can be very non-specific and so a high index of suspicion must be maintained, particularly for high-risk patients. The aortic dissection detection (ADD) risk score can assist in identifying these high-risk patients, although it must be keep in mind that a lack of any high risk factor does not completely exclude the possibility of an aortic dissection.

Treatment must be prompt, as mortality is related directly to delay in management. Untreated thoracic aortic dissection has a mortality rate of about 1% per hour for the first 48 hours, (mortality is 50% at 48 hours).

In general terms Stanford type A dissections will require operation whilst some Stanford type B dissections may be treated medically in an intensive care setting.

Pathophysiology

Classification

The most commonly used is the Stanford classification:

Type A: Proximal dissections, where there is involvement of the ascending aorta, regardless of the site of entry.

Type B: Distal dissections, all dissections not involving the ascending aorta, therefore arch and descending.

Predisposing Factors:

1. Hypertension, including coarctation of the aorta.

2. Familial history. 1

3. Age, peak incidence is 50-70 years.

4. Connective tissue diseases with anomalies of the great vessels:

● Marfans syndrome, (or other connective tissue disorders).

5. Pregnancy

6. Preexisting thoracic aortic aneurysm.

Complications:

1. Transmural rupture of the aorta with hemorrhage.

● This will most commonly be into the left pleural cavity and will usually prove rapidly fatal.

2. Proximal dissections:

● Acute aortic incompetence.

● Cardiac tamponade.

● Coronary artery occlusion leading to myocardial infarction (rare).

3. Aortic branch occlusions, any aortic branch may become occluded due progressive dissection of the intimal flap.

● Carotids leading to stroke.

● Upper limb, clue here may be pulse and blood pressure differentials between arms.

● Paraplegias due to spinal cord ischemia.

● Renal artery involvement.

● Splanchnic ischemia resulting in abdominal pain.

● Lower limb ischemia (if bilateral, this will appear like an aortic saddle embolus however, absence of chest/back pain would help to distinguish this from an aortic dissection).

Prognosis

● Untreated thoracic aortic dissection has a mortality rate of about 1% per hour for the first 48 hours, (mortality is 50% at 48 hours).

● By 3 months mortality reaches 90%.

● Early diagnosis and aggressive management will reduce the mortality to as low as 20%.

Clinical Features

The most consistent symptom will be pain, however the severity and nature of this can be extremely variable, making diagnosis difficult.

Note that many “classic” findings of aortic dissection are frequently absent. A high index for suspicion must therefore be maintained in those patients who are most at risk.

Clinical features include:

1. Pain:

● Acute onset of chest/ back pain is the commonest presenting problem, occurring in up to 95% of cases.

● Typically, this is pleuritic or “tearing” in nature

● In proximal dissections pain may be experienced in the anterior chest, neck, jaw, face. Back/abdominal pain is more suggestive of distal dissections.

● Pain may be migratory. This feature is very suggestive of aortic dissection.

● Pain may be mild, moderate or severe.

Occasionally aortic dissection may occur, without any apparent pain at all.

2. A small number of dissections occur with relatively little or no pain.

These may present as:

● Sudden death

● Neurological deficits.

● On investigation for other reasons, such as aortic incompetence.

● Recognition of a wide mediastinum on CXR.

3. Cardiovascular:

● The patient is usually hypertensive.

● Hypotension is an ominous sign, usually signifying a proximal dissection with cardiac tamponade or aortic transmural rupture.

● Acute onset of aortic incompetence with anterior chest/neck/jaw/facial pain is very suggestive of a proximal dissection.

● Limb pulse and blood pressure discrepancies may occur and may be transient.

Absence of these signs however cannot rule out the possibility of aortic dissection and there should not be an over reliance on these signs. 4

4. Neurological symptoms:

● Patients may present with neurological symptoms including stroke, para, or mono plegias.

● The combination of chest pain and neurological symptoms is suggestive of aortic dissection.

5. Precipitating event:

● A significant proportion of cases are probably precipitated by acute adrenergic surges as a result of physical or emotional stress. 5

Risk stratification

In general the more high risk findings consistent with an acute aortic dissection that a patient exhibits, the greater will be the likelihood for the diagnosis.

Certain high risk factors have been identified and incorporated into a risk-stratification scoring system known as the Aortic Dissection Detection (ADD) risk score.

This scoring system has been found to be highly sensitive for the detection of aortic dissection, (about 95%) however its specificity is not determined as will require ongoing study.

These high risk factors fall into 3 groupings as follows:

High Risk Predisposing conditions:

● Marfan’s syndrome, (or other connective tissue disease).

● Family history of aortic disease

● Known aortic valve disease

● Recent aortic manipulation

● Known thoracic aortic aneurysm

High Risk Pain characteristics:

Chest, back or abdominal pain that is described as:

● Abrupt in onset and/or Severe in intensity

And

● Tearing or ripping, or sharp, (i.e. pleuritic in nature as opposed to visceral) in quality

High Risk Examination findings:

● Perfusion deficit:

♥ Pulse deficit

♥ Systolic blood pressure deficit

♥ Focal neurological deficit, (in conjunction with pain).

● New onset (or not known to be old) aortic insufficiency murmur (in conjunction with pain).

● Hypotension/ Shock state

The Aortic Dissection Detection (ADD) risk score is determined by the number of categories in which any single risk factor is present.

This will give values of:

0: Low risk, (only approximately 4 % of cases of AD will have this score)

1: Moderate risk, (about 36 % of cases of AD will have this score)

2-3: High risk (about 60 % of cases of AD will have this score)

Note therefore that a score of zero does not completely exclude the possibility of aortic dissection; it merely substantially reduces the possibility.

0: Aortic dissection is less likely, but not definitely excluded.

If the diagnosis is still unclear, aortic imaging should be undertaken if the CXR has a widened mediastinum.

Aortic imaging may be considered if the patient has other significant CVS risk factors or there has been unexplained hypotension.

1: Unless there is strong indication of an alternative diagnosis, (e.g. on CXR, ECG or on clinical assessment supported by testing) then aortic imaging should be obtained

2-3: Proceed to aortic imaging.

Investigations

Blood tests:

1. FBE

● A low Hb is an ominous sign as this suggests leaking or frank rupture of the aorta

2. U&Es/ glucose

3. Clotting profile

● If the patient is taking anticoagulants

5. Troponin

● If significantly elevated, suggests associated myocardial ischemia

6. D-dimers:

● Aortic dissection can result in clot formation in the wall of the aorta in the area of dissection.

Accordingly most patients with aortic dissection will have a positive d-dimer. A negative result however will not exclude the diagnosis. 2, 3

ECG:

● May be normal.

● May show non-specific ST changes.

● Myocardial infarction due to involvement of coronary vessels is described but is rare.

Aortic dissection itself does not typically lead to significant ST elevation.

This is reassuring in cases where PCI is not available and thrombolytics are being considered in a patient with chest pain and ST segment elevation, as thrombolytics will likely be lethal if given to a patient with aortic dissection. 8

CXR:

● May be normal, hence a CXR cannot exclude aortic dissection. Additionally there is wide inter-observer variation in the diagnosis of aortic dissection based on the CXR alone. 6, 7

● It should also be done be to help rule out other causes of chest pain.

Suggestive features of aortic dissection include:

● Widened mediastinum

● Loss of the normal contour of the aortic arch.

● Central displacement of aortic intimal calcification (> 5 mm).

Imaging

The choice of imaging investigation beyond CXR will be between CT with contrast, MRI and TOE and will depend on the expertise available, the stability of the patient and any specific contraindications a patient may have for a particular investigation.

The best first up investigation in most instances will be CT angiogram (i.e. CT scan with contrast).

If the CT scan result is equivocal and index of suspicion remains high, then MRI or TOE will need to be considered.

CT angiogram:

● This in most cases will be the best first up investigation.

● It may be problematic in unstable patients, but in most cases there will be no other option, MRI being even more unsuitable in unstable patients and TOE not being available.

● For patients with contrast allergy, TOE or MRI may be used as alternative options.

● The entire aorta should be imaged, i.e. chest to pelvis.

CT scan with IV contrast. Classical appearances of aortic dissection on axial views. Left: Intimal flap of an aortic dissection seen within the arch of the aorta of a 51 year old male. Right: Intimal flap seen at the level of the upper abdomen, (the “Spalding tennis ball” sign)

MRI:

● This is very sensitive and specific for aortic dissection.

● It will be a good option if the patient has a contrast allergy or renal failure, which makes CT with IV contrast problematic.

● It will be unsuitable for the unstable patient or the patient who is unable to cooperate.

TOE

This is an excellent investigation, (if available).

Advantages include:

● It will give valuable additional information about cardiac function, the state of the aortic valve and whether or not cardiac tamponade is present.

● It is the most sensitive and specific investigation for proximal dissections.

● It can be used on unstable patients within the ED.

Disadvantages include:

● It is very operator dependent.

● The expertise is not readily available.

● It is an invasive procedure and requires patient sedation.

● It cannot readily visualize the distal aorta or branch vessels.

Management

1. Immediate attention to any ABC issues

● Large bore IV access, with pump-set IV line.

2. Establish monitoring:

● Pulse oximeter

● ECG continuous monitoring

● An arterial line is desirable in unstable patients and for those receiving beta blocker/ vasodilator therapy, (as below), but it should not be allowed to delay time to initial definitive investigation or management.

3. Hypotension:

● Most patients with AD have elevated blood pressures. Hypotension is an ominous sign in this condition, suggesting either free rupture with exsanguination or pericardial space bleeding with consequent cardiac tamponade.

● IV fluids may provide some temporizing benefit in the hypotensive patient.

● Inotropes and vasopressors may aggravate the shear forces on the aortic wall.

● Attempts at needle pericardiocentesis, may worsen the situation by loss of tamponading effect on the pericardial bleeding.

● Urgent definitive surgical intervention is the only potentially lifesaving treatment.

4. Analgesia:

● IV opioid analgesia titrated as clinically indicated.

5. Beta blockers:

This is an essential part of management and should be instituted as soon as the diagnosis is made, unless there is an absolute contraindication to its use.

These are used to reduce the shear stress on the aortic wall, to prevent further dissection.

● The best agent to use is an esmolol infusion, because of its short duration of action.

● Alternatively metoprolol 5mg slow I.V boluses can be given up to 15mg titrated to pulse and blood pressure.

6. Vasodilators:

These should be considered for further blood pressure control, if there is not an adequate response to the beta-blocker.

● Blood pressure should be maintained at a systolic of 100-120mmHg.

● This is best done with a GTN or nitroprusside infusion.

● Note that vasodilators should only be commenced following the administration of a beta blocker.

This is because vasodilators on their own by reducing the afterload may result in an increased velocity of blood ejection from the ventricle which will increase the shear stress on the aortic wall and therefore risk further dissection. Prior administration of beta blocker will help prevent this effect.

7. Surgery:

● All proximal dissections will require urgent operation.

● Some cases may be suitable for less invasive endovascular stenting procedures.

● Some distal dissections may be managed medically in ICU providing there are no major branch occlusion complications.

Disposition

All patients with type A aortic dissection, and who are suitable candidates for surgery, will need transfer to a hospital with facilities for cardiac surgery.

Those with type B who are suitable for medical management, may be managed in ICU under the close care of a Vascular surgeon as well as the Intensivist.

Appendix 1

Radiology of the normal aorta:

Left: Chest x-ray of a normal thoracic aorta. Arrows indicate arch, and arrowheads show ascending and descending aorta. 9

Right: CT reconstruction showing normal anatomy of the thoracoabdominal aorta with standard anatomic landmarks for reporting aortic diameters: 9

Anatomic locations:

1. Aortic sinuses of Valsalva

2. Sinotubular junction

3. Mid ascending aorta (midpoint in length between Nos. 2 and 4).

4. Proximal aortic arch (aorta at the origin of the innominate artery)

5. Mid aortic arch (between left common carotid and subclavian arteries)

6. Proximal descending thoracic aorta (begins at the isthmus, approximately 2 cm distal to left subclavian artery).

7. Mid descending aorta (midpoint in length between Nos. 6 and 8).

8. Aorta at diaphragm (2 cm above the celiac axis origin).

9. Abdominal aorta at the celiac axis origin.

Vanderbilt Mantelpiece, Marble, mosaic, oak and cast iron.

Augustus Saint-Gaudens, 1881-3, Metropolitan Museum of Art, New York City.

References

1. Hasham S. N et al. Non-syndromic genetic predisposition to aortic dissection: a newly recognized, diagnosable and preventable occurrence in families. Ann Emerg Med 43 (1): 79, January 2004.

2. Eggebrecht H et al. Value of plasma fibrin d-dimers for detection of acute aortic dissection. J Am Coll Card 44 (4): 804 August 18, 2004.

3. Weber W eat al. D-dimer in acute aortic dissection. Chest 123 (5); 1375, May 2003.

4. Teece S. Peripheral pulses to exclude thoracic aortic dissection. Emerg Med J. 21; 589, September 2004.

5. Hatzaras I. S. et al. Role of exertion as inciting events for acute aortic dissection. Am J Cardiol 100 (9): 1470, November 1, 2007.

6. von Kodolitsh T et al. Chest radiography for the diagnosis of acute aortic syndrome. Am J Med 116; 73, January 15, 2004.

7. Jagannath A.S et al. Aortic dissection: A statistical analysis of the usefulness of plain chest radiographic findings. Am J Rad 147; 1123, December 1986.

8. Weiss P et al. How many patients with acute dissection of the thoracic aorta would erroneously receive thrombolytic therapy based on the electrocardiographic findings on admission? Am J Cardiol 72: 1329, December 1, 1993.

9. Guidelines for the Diagnosis and Management of Patients With Thoracic Aortic Disease. J Am Coll Cardiol, 2010. 55 (14): p. e27-129; p.71 for the ADD Risk Score

10. Rogers , A.M., et al., Sensitivity of the Aortic Dissection Detection Risk Score, a Novel Guideline-Based Tool for Identification of Acute Aortic Dissection at Initial Presentation / Clinical Perspective. Circulation, 2011. 123(20): p. 2213-2218

Dr. J. Hayes.

Reviewed 20 September 2012.

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