Acute Aortic Dissection- A “Real” Cardiothoracic Emergency
Introduction
King George II suffered a collapse and died from a type A aortic dissection with pericardial tamponade, the first incidence of aortic dissection ever documented in the literature.
Aortic dissection involving the ascending and descending aorta is associated with high morbidity and mortality rate.
The aortic dissection charitably trusts states that aortic dissection claims the lives of approximately 2,000 persons in the UK and Ireland annually; in contrast, car accidents claim the lives of 1,870 people. The incidence of aortic dissection has risen sharply in the older adult population and is reported to be 8.6/100,000 in ages 60 to 80 years and 32/100,000 for people aged over 80 years.1 The statistics also indicate that physicians correctly suspect a dissection in as few as 15%-43% of cases who proceed to have a verified ascending aortic dissection,2 if left untreated the mortality rate is as high as 50% in the first 48 hours of symptom onset. Physicians must be aware of the clinical characteristics of aortic dissection as a timely referral for cardiothoracic intervention could potentially be a matter of life or death.
Pathophysiology
The aorta is the largest artery in the body, which facilitates the transfer of oxygenated blood from the left ventricle to all the organs in the body via the renal arteries, iliac arteries, inferior mesenteric artery, superior mesenteric artery, and celiac artery.3 Three layers make up the aorta wall: the thick, muscular-elastic tunica media, the outer fibrous tunica adventitia, and the thin tunica intima, which faces the bloodstream. Aortic dissection is a potentially fatal disorder that arises from a rupture in the aortic intimal layer, leading to haemorrhage within the aortic wall and eventual layer separation and complete dissection. The types of dissection vary characterised by the level of the dissection within the aorta itself. Aortic dissection is classified into type A or type B. Type A aortic dissection occurs when the tear in the aorta occurs at the ascending part of the aorta at the branches of the heart whereas type B dissection involves the lower aorta.4
Risk Factors
International studies across Europe, North and South America and Asia indicate the age of risk to vary from 40- 75 years with favour towards increases occurrence in males. There is evidence to suggest that China has the youngest age associated with aortic dissection and this is correlated to being approximately 10-15 years younger than other areas;5 however, this is noted to be approximate as often cases are diagnosed during post mortem.
Genetic disorders such as connective tissue diseases that decrease the structural stability of the aorta wall, such as Marfan syndrome and Ehlers-Danlos syndrome predispose the aorta to dissection and account for 20% of cases.6 Genetic testing and screening are available but require investment and improvement.5
Modifiable risk factors for aortic dissection as a result of prolonged stress on the aorta wall include longstanding hypertension, obesity, smoking, dyslipidaemia, substance abuse such as cocaine use, treatment of infections such as syphilis and TB.6
Other associated risks of dissection include trauma from fall or car accident and there are some associated iatrogenic factors such as cross-clamping, side clamping, graft anastomosis and patch aortaplasty6,7 or the presence of a developed aortic aneurysm.
Signs and Symptoms
Unfortunately, presentations are diverse hence the difficulty in achieving a timely diagnosis. The most common symptoms are sudden onset of severe back or chest pain, which radiates from one to the other with no evidence of an acute cardiac event on ECG. On occasions there may be a recent history of strenuous exercise or the use of recreational drugs which is followed by sudden pain, in this instance, dissection should be included in the initial differentials.6 Along with strokelike symptoms such as abrupt speech difficulties, sight loss, and weakness on one side of the body, individuals may also feel dyspnoea, loss of consciousness, and shortness of breath. The symptoms can vary depending on the level of the dissection and the associated branch that is potentially malperfused due to the presence of a false lumen. It should be understood that aortic dissection can sometimes happen without any signs.4
Diagnostics
While there are no biomarkers as such to aid the diagnosis of an acute dissection there is some evidence to support the use of a D-Dimer. Aorta dissection is said to be associated with increased levels of soluble elastin fragments (sELAF), tenascin-C, smooth muscle myosin heavy chain (sm-MHC), and fibrinogen/ fibrin degradation products, which results in a raised D-Dimer level.8, 9, 10 Theoretically, because the false lumen is exposed in thoracic aorta dissection, the D-dimer would be higher, as the endothelialization of the false lumen would prevent a substantial increase in D-dimer following chronic thoracic aortic dissection, however, there are no confirmed reference ranges.
The use of diagnostic imaging is the most beneficial to clinicians in the diagnosis of aortic dissection and it guides the decision re classification, location of dissection and ultimately the level of urgency required. Currently, transthoracic echo (TTE), transoesophageal echo (TOE), magnetic resonance imaging (MRI), and CT are the primary diagnostic imaging modalities used. The primary reasons for using contrast-enhanced CT angiography are its accessibility, speed, and non-invasiveness.6, 11
Management6
The conservative and surgical management of an acute aorta dissection is identified based on the classification and severity of the dissection. Initial management should aim to obtain blood pressure control and limit fluctuations in pressure with the use of intravenous beta blockers. Medications should be adjusted to maintain a systolic pressure of 100-120mm with a heart rate of 60- 80 beats per minute.6 Type A aortic dissections with the presence of a non-thrombosed false lumen should be considered for emergency surgery. The goal of prompt surgical intervention is to cut off the entry into the false lumen and restore the aortic true lumen using a synthetic interposition graft, either with or without coronary artery re-implantation; however, the primary focus of the surgery is to save a life, therefore, the procedure selection requires an experienced judgement of the anatomical requirements.12
Conclusion
Despite advancements in medicine, acute aortic dissection is a complex life-threatening medical emergency with a recently confirmed mortality rate of 51.4%.13 A high clinical suspicion is imperative to ensure timely identification, investigation and treatment. Clinicians must consider the possibility of dissection in patients who present with chest pain and also be aware that in certain instances aortic dissection may be present with no symptoms. It may soon be possible to anticipate and, to some extent, prevent acute aortic dissection due to advances in our understanding of genetic risk and propensity.
Written by Professor Alan Soo, Cardiothoracic Surgeon, University Hospital Galway and Tara Byrne, Advanced Nurse Practitioner, Cardiothoracic department, Adjunct Clinical Lecturer, Department of nursing University of Limerick Faculty of Education and Health Sciences
References available on request
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