Abstract
Thoracic aortic disease is often difficult to detect and may remain asymptomatic until presenting acutely, which in turn is associated with high rates of complications, morbidity and mortality. This chapter provides an overview of anaesthesia for both elective and emergency thoracic aortic surgery.
Thoracic aortic disease is often difficult to detect and may remain asymptomatic until presenting acutely, which in turn is associated with high rates of complications, morbidity and mortality. This chapter provides an overview of anaesthesia for both elective and emergency thoracic aortic surgery.
Pathology
Surgical disease of the thoracic aorta includes aneurysm and dissection. These may occur separately or together and may be congenital or acquired. Acquired disease is usually the result of arterial hypertension and atherosclerosis although, historically, syphilis was an important cause. Congenital causes include connective tissue diseases, such as Marfan, Ehlers–Danlos, Turner and Loeys–Dietz syndromes, and polycystic kidney disease.
Aneurysm
A true aneurysm of the aorta is a permanent dilatation that is at least 50% greater than its original diameter, involving all layers of the aorta. A pseudoaneurysm is a rupture through the layers of the aorta, held together by blood, thrombus and surrounding tissues. A dissection is a disruption of the aortic intima with bleeding into the media.
Untreated aneurysms of the descending and thoracoabdominal aorta that are greater than 6 cm in diameter have a 14.1% annual risk of rupture, dissection or death and the 5-year survival in conservatively managed patients is only 10–20%. Indications for surgery are based on individual patient assessment when the predicted operative risk is less than the risk of optimal medical management and include:
A rupture or an acute dissection
Symptomatic enlargement of the aorta – pain or compression of adjacent structures
Aneurysm enlargement of >1 cm per year or a rapid increase in size
An absolute diameter of >6.5 cm, or >6.0 cm in patients with connective tissue disease
Aortic Dissection
Dissection of the aorta is often associated with acute arterial hypertension following physical activity or stress. An intimal tear occurs, usually in the presence of a weakened aortic wall and predominantly involving the middle and outer layers of the media. In this area of weakening, the aortic wall is more susceptible to shear forces produced by pulsatile blood flow in the aorta. The most frequent locations of intimal tears are the areas subjected to the greatest mechanical shear forces; the ascending and isthmic (just distal to the left subclavian artery) segments of the aorta are relatively fixed and thus subject the aortic wall to the greatest amount of mechanical shear stress (Figure 13.1).
Figure 13.1 A dissection of the aorta
Classification of Dissection
The DeBakey classification (Figure 13.2) comprises three types, depending on where the intimal tear is located and which section of the aorta is involved:
Type I: The intimal tear is located in the ascending portion, but the dissection involves all portions (ascending, arch and descending) of the thoracic aorta.
Type II: The intimal tear is in the ascending aorta, but the dissection involves the ascending aorta only, stopping before the origin of the innominate artery.
Type III: The intimal tear is located in the descending segment, and the dissection almost always involves the descending portion of the thoracic aorta only, starting just distal to the origin of the left subclavian artery; type III dissections can propagate proximally into the arch, but this is rare.
The Stanford (Daily) classification (Figure 13.2) comprises two types:
Type A: Dissections that involve the ascending aorta, regardless of where the intimal tear is located and regardless of how far the dissection propagates; clinically, type A dissections run a more virulent course.
Type B: Dissections that involve the aorta distal to the origin of the left subclavian artery.
The survival rate of untreated patients with an aortic dissection is poor, with a 2-day mortality of up to 50% and a 6-month mortality approaching 90%. The usual cause of death is the rupture of the false lumen and a fatal haemorrhage. The overall surgical mortality is approximately 30%, but surgical therapy is often the only viable option for most patients.
Preoperative Assessment
A thorough preoperative assessment should take place, as time allows (Figure 13.3). Specific items to bear in mind include:
Assessment of premorbid functional capacity
Examination for evidence of compression of adjacent structures:
Stridor or dyspnoea are associated with encroachment onto the trachea or left main bronchus
Dysphagia may suggest oesophageal compression
Hoarseness may indicate stretching of the recurrent laryngeal nerve
Baseline neurological examination to document any neurological deficit
Enquiry into any history of angina, MI, CVA or renal dysfunction
Baseline investigations are summarized in Box 13.1. They should be performed where possible according to the urgency of surgery and the stability of the patient. Where possible, coronary angiography should be performed to assess the need for concomitant coronary artery bypass surgery. In aneurysmal disease, CT angiography of the aorta allows 3D reconstruction and surgical planning, specifically the presence of thrombus at clamping sites and the patency of key vessels. Imaging may also indicate the presence of trachea or left main bronchus compression that may make insertion of a double-lumen endotracheal tube challenging. Some centres advocate identification of the greater radicular or Adamkiewicz artery, a large segmental artery usually found between vertebral levels T5 and L2, through which the anterior spinal artery may receive a significant proportion of its blood supply.
Figure 13.3 Anterior cerebral cannulation utilized in aortic arch surgery. The aorta is opened under DHCA and arterial cannulae placed directly into the ostia of the innominate, left carotid and left subclavian arteries
Bloods: FBC, urea and electrolyte, liver function tests, clotting screen, glucose, cross-match
ECG
Plain posteroanterior and lateral CXR
Pulmonary function tests: basic spirometry and transfer factor
CT aorta
TTE
Coronary angiography
Cardiopulmonary exercise testing to determine maxiumum VO2 and anaerobic threshold
Anaesthesia for Aortic Surgery
Regardless of the extent of the intended surgery, routine cardiac anaesthetic management should be employed – large-bore IV access, invasive arterial pressure monitoring and central venous access. The need for additional monitoring or vascular access is discussed below. Monitoring of Hb concentration, glucose, electrolytes, ACT, ABG and acid–base status must be undertaken at regular intervals throughout the case.
Surgery Involving the Aortic Root and Ascending Aorta
In situations where an AXC can be applied proximal to the innominate artery, management is similar to that of AV surgery. Where root replacement is performed with reimplantation of the coronary arteries, TOE is indicated before CPB to assess the anatomy of the aorta and after CPB to assess valvular and ventricular function.
Surgery Involving the Aortic Arch
This inevitably involves a period of interruption to the cerebral blood supply, necessitating the use of full CPB and deep hypothermic circulatory arrest (DHCA). The venous cannula is commonly placed in either the RA or a femoral vein. The arterial cannula is usually placed in a femoral or right axillary artery, either directly or via a vascular graft to prevent distal limb ischaemia.
The use of DHCA undoubtedly provides cerebral protection, although opinions on the degree of hypothermia required vary. DHCA at 14–20 °C typically provides 20–30 minutes of safe circulatory arrest (without additional cerebral perfusion) whereas moderate hypothermia (20–28 °C) permits only 10–20 minutes of safe arrest. Surrounding the head with ice or using a cold-water jacket may supplement brain cooling and provide an additional margin of safety, although there is no objective evidence of outcome benefit.
In an attempt to reduce the time required for cooling and rewarming on CPB, some centres combine modest hypothermia with either selective antegrade cerebral perfusion or retrograde cerebral perfusion. These perfusion techniques undoubtedly increase the complexity of the surgical procedure and may provide a false sense of security.
Anaesthetic Management
The right radial artery is preferred for uninterrupted BP monitoring, should the AXC be applied proximal to the left subclavian artery. Following induction of anaesthesia, the trachea is intubated using a double-lumen endotracheal tube when access to the descending aorta is required. Additional vascular access includes a femoral arterial line and large-bore femoral venous access (often a haemofiltration catheter) to allow monitoring of the distal arterial pressure and the rapid infusion of fluids.
Monitoring of the brain and core temperature is required during cooling and rewarming, to ensure that excessive thermal gradients are not created. The nasopharynx temperature correlates closely with the cerebral temperature, whereas the rectal or bladder temperature is a reasonable reflection of the core temperature.
Cerebral Oxygenation Monitoring
Near-infrared spectroscopy (NIRS) is increasingly used during aortic surgery, to provide continuous, real-time, non-invasive monitoring of anterior cerebral circulation (see Chapter 31).
A baseline measurement should be obtained as soon as possible, ideally prior to the induction of anaesthesia. The regional cerebral oxygen saturation (rSO2) should be maintained within 25% of the baseline level and a fall below this level should be investigated and addressed promptly. It has been suggested that the use of NIRS is associated with a lower incidence of postoperative neurological dysfunction. Increasing the CPB flow rate, the Hb concentration and the depth of anaesthesia, and decreasing temperature may be effective in increasing the rSO2. An abrupt decrease in rSO2 should prompt exclusion of CPB cannula displacement or cerebrovenous obstruction.
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