While uncommon, acute aortic dissection (AAD) is an infrequent but catastrophic disorder.
Aortic dissection is due to the separation of the layers of the aortic wall. A tear in the intimal layer results in the progression of the dissection (either proximal or retrograde) chiefly due to the entry of blood in between the intima and media. An acute aortic dissection is associated with very high mortality; the majority die even before reaching the emergency department. Patients with a chronic aortic dissection (more than 2 weeks) have a slightly better prognosis.
Classically described as a patient complaining of an abrupt onset of severe ‘tearing’ chest pain, presentations can often be more subtle. Physicians correctly suspect the diagnosis in as few as 15% to 43% of cases of verified AAD. If left untreated, mortality approaches 50% in the first 48 hours of onset.
Despite a wealth of literature, a significant number of aortic dissections are missed in the emergency department.
There are two main anatomic classifications used to classify aortic dissection; the DeBakey and Stanford systems. The Stanford system is more frequently employed. It classifies dissections that involve the ascending aorta as type A, regardless of the site of the primary intimal tear, and all other dissections as type B. The DeBakey classification is based upon the site of origin of the dissection. Type 1 originates in the ascending aorta and to at least the aortic arch, while type 2 originates in and is limited to the ascending aorta. Type 3 begins in the descending aorta and extends distally above the diaphragm (type 3a) or below the diaphragm (type 3b). Ascending aortic dissections are almost twice as common as descending dissections.
Predisposing high-risk factors for non-traumatic aortic dissection include:
The incidence of aortic dissection is reported to be 5 to 30 cases per 1 million people per year (compared to the much more common condition of acute myocardial infarction, which affects approximately 4400 cases per 1,000,000 person-years). Regarding emergency department presentations, three AADs are ultimately diagnosed out of every 1000 patients presenting with acute back, chest, or abdominal pain. Age is a risk factor for approximately 75% of aortic dissections occurring in patients who are ages 40 to 70 years, with the majority occurring between the ages of 50 and 65 years. However, there are some significant differences between older adult patients and younger patients with dissections involving the ascending aorta. Older patients are significantly more likely to harbor atherosclerosis, prior aortic aneurysm, iatrogenic dissection, or an intramural hematoma. Younger patients are much less likely to have a history of hypertension, and much more likely to suffer from a connective tissue disorder such as Marfan syndrome. Additionally, AAD is three times more common in men than in women, although women tend to present later and experience worse outcomes.
The aortic wall consists of three layers: the intima, media, and adventitia. Constant exposure to high pulsatile pressure and shear stress leads to a weakening of the aortic wall in susceptible patients resulting in an intimal tear. Following this rent, blood flows into the space between the intima and adventitia, creating a false lumen. Most of these tears take place in the ascending aorta, usually in the right lateral wall where the greatest shear force on the aorta occurs. An AAD can propagate anterograde and/or retrograde and depending on the direction the dissection travels, cause branch obstruction that produces ischemia of affected territory (coronary, cerebral, spinal, or visceral), and for proximal type, A AADs can instigate acute tamponade, aortic regurgitation or aortic rupture.
In an AAD, the true lumen is lined by the intima whereas the false lumen is within the media. In most cases, the true lumen is smaller than the false lumen. Overtimes, the blood flowing through the false lumen leads to development of an aneurysm with the potential for rupture. The three common sites for AAD are as follows:
Clinically, the presentation of AAD is subject to the extent of the dissection, with complaints corresponding to those affected cardiovascular structures. Three fundamental questions that must be addressed when taking a history of a patient with suspected AAD are the quality, radiation, and intensity of pain at the onset. Various studies identified the intensity of onset of pain as the most reliable historical factor. While increasing the probability of AAD when present, classically cited physical findings, such as a discrepancy of blood pressures in the upper extremities, a pulse deficit, or presence of a diastolic murmur, are present in less than 50% of confirmed cases of AAD. Additionally, the presence of chest pain with any neurological finding, the combination of chest and abdominal pain, or chest pain accompanied by limb weakness or paresthesia should alert the clinician to the possibility of AAD.
The pain of AAD is often sudden in onset, reaches maximal severity quickly and can be tearing in nature. In about 10% of patients, the AAD is painless, which is more common in Marfan syndrome.
At the time of presentation, neurological deficits are present in 1/5th of patients. Syncope is also common and may be due to hypovolemia, arrhythmias, MI or an increased vagal tone. If the dissection is antegrade, it may involve the extremity vessels leading to loss of pulses, paresthesias, and pain.
Hoarseness and horner syndrome have also been described in patients with AAD.
If the AAD is leaking or ruptured in the mediastinum, then the patient may have dyspnea and hemoptysis.
Hypertension is very common in AAD; if the patient presents with hypotension then this is a grave sign most likely indicating a rupture. A difference of more than 20mmHg in blood pressure between the arms should raise suspicion of AAD. Other features include:
Routine studies like an ECG and chest x-ray can help differentiate other possible causes for chest pain but can be misleading. The presence of ECG findings consistent with an acute myocardial infarction occurs in eight percent of cases of AAD. Furthermore, while the widening of the aortic silhouette increases the likelihood of AAD, its absence does not reliably exclude the diagnosis. Confirmation of AAD requires cardiovascular imaging to identify the presence of an intimal tear, establish the Stanford classification, and detect valvular or branch involvement. Most society guidelines recommend either CT aortography or transesophageal echocardiogram (TEE) for the diagnosis of AAD. Deciding which modality to employ as a first-line screen should be based on institutional availability and expertise. For most emergency departments (EDs), CT angiography will likely be the first advanced imaging technique on account of its widespread availability.
Blood work should include CBC, electrolytes, troponin and renal function. Smooth muscle myosin heavy chain assay is elevated is specific for AAD.
The chest x-ray may show a widened mediastinum or fluid collection in the hemithorax (think rupture). X-ray features that suggest AAD include:
If the patient is stable, CT scan with contrast is recommended. The spiral CT can quickly detect the location of the intimal tear and help the surgeon plan the procedure.
Echocardiography is also a useful modality to detect AAD. TEE is the preferred technique and can be done at the bedside. The key drawback is operator experience. However, if the patient is unstable, TEE is the recommended procedure.
Aortography is not much used today unless there are plans to repair the dissection using stents.
A 12 lead ECG is mandatory to rule out an MI.
Once the diagnosis of AAD is confirmed or highly suspected, urgently consult cardiothoracic or vascular surgery (based on institutional protocols). Acute dissections involving the ascending aorta are considered surgical emergencies. The patient should have an arterial line and central venous catheter for monitoring. A foley should be inserted to assess urine output.
Concurrently commence medical therapy including providing adequate analgesia (morphine is the preferred analgesic, as it decreases sympathetic output as well) and administering a short-acting IV beta-blockers aiming for a heart rate of ~60 bpm (reductions in heart rate and blood pressure reduces aortic wall tension and limit the extent of dissection). If the systolic blood pressure remains elevated, nitroprusside can be added to achieve a systolic blood pressure goal of 100 to 120 mmHg (maintain blood pressure in this range as long as there is no compromise of mentation or urine output). Other agents that can be used to lower blood pressure include esmolol, labetalol or diltiazem.
Surgical therapy for type A AAD involves excision of the intimal tear, obliteration of entry into the false lumen proximally, and reconstitution of the aorta with interposition of a synthetic vascular graft. Surgical intervention for type B AAD tends to be reserved for patients who have a complicated course. Endovascular stent-grafting (TEVAR) has been employed as a less invasive alternative to surgery, primarily for patients with complicated type B dissections.
For AAD involving the ascending aorta, the aorta has to be replaced and the valve has to be assessed. In most cases, the aortic valve may also have to be excised and replaced with a prosthetic valve. Dissections involving the descending aorta are complex and there is a risk of paraplegia. The most difficult dissections are those that involve the aortic arch. Surgical mortality varies from 5-20%. Even those who survive have a long recovery.
Because of the high surgical mortality, today endovascular procedures are being performed for AAD. Endovascular stent placement is associated with significantly lower morbidity compared to surgery but the key is patient selection.
Aortic dissection still carries a very high mortality. At least 30% of patients die after reaching the emergency room and even after surgery, the mortality rates vary from 20-30%. For those who survive surgery, the comorbidity also takes a toll and the quality of life is poor. the highest mortality of an acute aortic dissection is within the first 10 days. Patients with a chronic dissection tend to have a better prognosis, but even their life expectancy is shortened compared to the general population.
Despite the best of circumstances, it is next to impossible to diagnose every case of AAD presenting to the ED. Factors contributing to a high miss rate include perceived mildness of symptoms in some patients with AAD. A second factor leading to misdiagnosis is clinical symptoms and laboratory findings suggesting an alternative diagnosis such as acute coronary syndrome (ACS). A third factor was the lack of expected results such as a pulse deficit or absence of a widened mediastinum on chest x-ray. Approach every chest pain patient as if they could harbor an ADD, establish a risk factor profile, and remain cognizant of more subtle presentations.
Aortic dissection is a medical emergency with very high mortality if it is undiagnosed. The majority of patients present to the emergency department and hence an interprofessional team is the key.
Once the patent with an aortic dissection presents to the emergency room, a standardized system must be in operation to ensure that the diagnosis and management are done without any delay. The triage nurse should be fully aware of the importance of immediate admission of the patient and consult with the emergency physician and cardiac surgeon. The decision on how to make the diagnosis depends on patient stability and availability of imaging tests. The two options include ECHO or a CT scan. An unstable patient should never be sent to the radiology suite. Instead, a cardiologist should be consulted for a bedside echo. The intensivist should be notified as there is an urgent need to lower blood pressure. The operating room should be notified that a patient with a dissection has been admitted. Nurses should be fully aware of the potential complications of aortic dissection and know what medications to use to lower the blood pressure. In addition, nurses should monitor fluid input and output and keep the patient NPO. After surgery, the nurses need to closely monitor the patient for complications. Open communication between the team members is vital to improving outcomes. (Level V)
Several studies show that patient outcomes are improved when managed by an interprofessional team of healthcare professionals that include a cardiologist, intensivist, pulmonologist, nephrologist, cardiac surgeon, interventional radiologist, and anesthesiologist. In addition, the pharmacist must educate the patient about the importance of blood pressure control and compliance with medications. The outcomes of aortic dissection tend to be better in high volume centers compared to small centers which do less than 5 cases a year. (Level V)
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