Thoracic aortic diseases are generally surgical problems and require surgical treatment (Table 17-1). Acute aortic dissections, rupturing aortic aneurysms, and traumatic aortic injuries are surgical emergencies. Subacute aortic dissection and expanding aortic aneurysms require urgent surgical intervention. Stable thoracic or thoracoabdominal aortic aneurysms (TAAAs), aortic coarctation, or atheromatous disease causing embolization may be considered for elective surgical repair. Increased public awareness of thoracic aortic disease, early recognition of acute aortic syndromes by emergency medical personnel, improved diagnostic imaging technology for the diagnosis of thoracic aortic disease, and an aging population all contribute to the increased number of patients requiring aortic surgery. Furthermore, improvements in the surgical treatment of thoracic aortic diseases combined with increased treatment options such as endovascular stent repair have led to an increased number of patient referrals to centers specializing in the management of patients with thoracic aortic diseases. Improved treatment and survival after aortic surgical procedures often provide a cure for the original disease but have created new and unique problems. An increasing number of patients who have had prior aortic surgical procedures require reoperation for long-term complications of aortic surgery such as bioprosthetic valve or graft failure, aortic pseudoaneurysm at old vascular graft anastomosis, endocarditis, or progression of the original disease process into native segments of the thoracic aorta.

Table 17-1 Diseases of the Thoracic Aorta That Are Amenableto Surgical Treatment

Adapted from Kouchoukos NT, Dougenis D: Surgery of the aorta. N Engl J Med 336:1876, 1997.

The anesthetic management of surgical patients requiring aortic surgery presents some distinctive medical problems in addition to the usual considerations associated with major thoracic or thoracoabdominal operations. The process of repairing or replacing a portion of the thoracic aorta usually requires the temporary interruption of blood flow, creating the potential for ischemia or infarction of almost any major organ system in the body. Strategies to provide organ perfusion, to protect organs from the consequences of hypoperfusion, and to monitor and treat end-organ ischemia during aortic operations are critical aspects of the anesthetic management for thoracic aortic diseases and contribute importantly to the overall success of operations. Some of the procedures performed and managed by surgeons and anesthesiologists for organ protection during thoracic aortic operations, such as partial left-sided heart bypass for distal aortic perfusion, deep hypothermic circulatory arrest (DHCA), selective antegrade or retrograde cerebral perfusion (ACP or RCP), and lumbar cerebrospinal fluid (CSF) drainage, are practiced routinely in no other area of medicine.

GENERAL CONSIDERATIONS FOR THE PERIOPERATIVE CARE OF AORTIC SURGICAL PATIENTS

Patients undergoing thoracic aortic operations of any type share common considerations for the safe conduct of anesthesia and perioperative care (Table 17-2).

Table 17-2 General Considerations for the Anesthetic Care of Thoracic Aortic Surgical Patients

Preanesthetic Assessment

Urgency of the operation (emergent, urgent, or elective)

Pathology and anatomic extent of the disease

Median sternotomy vs. thoracotomy vs. endovascular approach

Mediastinal mass effect

Airway compression or deviation

Preexisting or Associated Medical Conditions

Aortic valve disease

Cardiac tamponade

Coronary artery stenosis

Cardiomyopathy

Cerebrovascular disease

Pulmonary disease

Renal insufficiency

Esophageal disease (contraindications to TEE)

Coagulopathy

Prior aortic operations

Preoperative Medications

Warfarin (Coumadin)

Antiplatelet therapy

Antihypertensive therapy

Anesthetic Management

Hemodynamic monitoring

Proximal aortic pressure

Distal aortic pressure

Central venous pressure

Pulmonary artery pressure and cardiac output

Transesophageal echocardiography

Neurophysiologic monitoring

Electroencephalography (EEG)

Somatosensory evoked potentials (SSEPs)

Motor evoked potentials (MEPs)

Jugular venous oxygen saturation

Lumbar cerebrospinal fluid pressure

Body temperature

Single-lung ventilation for thoracotomy

Double-lumen endobronchial tube

Endobronchial blocker

Potential for bleeding

Large-bore intravenous access

Blood product availability

Antifibrinolytic therapy

Antibiotic prophylaxis

Postoperative Care Considerations and Complications

Hypothermia

Hypotension

Hypertension

Bleeding

Spinal cord ischemia

Stroke

Renal insufficiency

Respiratory insufficiency

Phrenic nerve injury

Diaphragmatic dysfunction

Recurrent laryngeal nerve injury

Pain management

Preanesthetic Assessment

It is important to determine the operative diagnosis because both the anesthetic management and surgical approach are dictated by the anatomic extent of the lesion and the physiologic consequences of the disease. Diseases involving the aortic root, ascending aorta, and proximal aortic arch are generally approached through a median sternotomy, whereas diseases of the distal aortic arch or descending thoracic aorta are approached through a left thoracotomy or thoracoabdominal incision. Sometimes, the operative diagnosis can be established in advance. Other times, a presumptive diagnosis has been made based on patient symptoms or available reports and the definitive diagnosis needs to be verified after patient arrival into the operating room by direct review of the diagnostic studies or by intraoperative transesophageal echocardiography. In either case it is important to discuss the anesthetic and operative plan with the surgical team to be properly prepared for all possible contingencies. Direct review of the actual diagnostic imaging studies such as the angiogram, computed tomographic scan, magnetic resonance image, or echocardiogram not only verifies the operative diagnosis but also provides important information that determines the surgical options. Knowing the size and anatomic extent of aortic pathology provides information about the physiologic impact and consequences of the lesion, permitting the anesthesiologist to anticipate potential difficulties associated with anesthetic procedures, problems related to the surgical repair, and postoperative complications.

Anesthetic Management

Considered as a group, any operative procedure involving the aorta from endovascular stent repairs to open repair of TAAAs is associated with the potential for catastrophic bleeding and cardiovascular collapse. For this reason, continuous diagnostic ECG monitoring, intra-arterial blood pressure monitoring, large-bore vascular access for rapid volume expansion, and ensuring the immediate availability of packed red blood cells can be justified in virtually every patient. Central venous access for monitoring the right atrial pressure and the administration of vasoactive drug therapy to control the circulation can also be justified in almost all cases. Pulmonary artery catheterization to measure pulmonary artery pressures, cardiac output, and mixed venous oxygen saturation is useful for operations involving cardiopulmonary bypass (CPB), DHCA, partial left-sided heart bypass, or cross-clamping of the thoracic aorta. Routine availability and use of intraoperative TEE provide both diagnostic information and the ability to assess ventricular function.

Arguments can be made for using either the left or right radial artery for intra-arterial blood pressure monitoring. A right radial arterial catheter can detect partial occlusion or obstruction of flow into the innominate artery caused by inadvertent placement of the aortic cross-clamp too near to the origin of the innominate artery during the course of operations involving the ascending aorta or aortic arch. A right radial arterial catheter also permits monitoring of blood pressure during repair of the proximal thoracic aorta or distal aortic arch if the left subclavian artery has to be clamped. A left radial arterial catheter must be used for selective ACP via the right axillary artery. Sometimes bilateral radial arterial catheters are necessary. A femoral arterial catheter is necessary to monitor distal aortic pressure when partial left-sided heart bypass is used to provide distal aortic perfusion.

Large-bore peripheral intravenous catheters that are 16 gauge or larger provide satisfactory sites for rapid intravascular volume expansion. An intravenous administration set integrated with a fluid warming unit is desirable, particularly for the rapid administration of blood products. Often, patients coming to the operating room from other areas of the hospital already have established intravascular access with small-bore intravenous catheters at the site of large veins. One approach in this scenario is to exchange the small-bore catheter with a commercially available 7.5- to 8.5-Fr large-bore rapid infusion catheter over a sterile guidewire. The only precaution in the use of these catheters is to ensure that the vein is large enough to accept the larger-diameter catheter. Alternatively, a large-bore central venous catheter, usually an 8.5-Fr introducer sheath, 9-Fr introducer/multiple-access catheter, or hemodialysis catheter, placed in the internal jugular, subclavian, or femoral vein can be used for volume expansion. When a pulmonary artery catheter is necessary, a second introducer sheath for volume expansion can be placed also into the right internal jugular vein. For this procedure, both guidewires should be placed with at least 2 cm of separation between them. Central venous cannulation can be achieved by either anatomic landmark guidance or ultrasound guidance. Ultrasound guidance may increase both the speed and safety of venous cannulation, which is particularly advantageous in emergency operations or when the patient is hemodynamically unstable. A urinary catheter with a temperature probe to measure core temperature, together with a nasopharyngeal temperature probe, is necessary to monitor both the absolute temperature and rate of change of body temperature during deliberate hypothermia and subsequent rewarming. The temperature probe of the pulmonary artery catheter can provide core temperature monitoring, and a rectal temperature probe can be used to monitor shell temperature.

The hemodynamic condition of the patient should be reassessed immediately before the induction of general anesthesia. The decrease in arterial pressure in response to anesthetic drugs and subsequent increase in response to tracheal intubation should be anticipated. Both vasopressor drugs and vasodilator drugs should be immediately available to provide precise control of the blood pressure. Intravenous vasodilator drugs being infused to treat preoperative hypertension often need to be reduced in dose or discontinued on induction of general anesthesia. Etomidate is a useful induction agent for patients in cardiogenic shock because it does not attenuate sympathetic nervous system responses and has no direct actions on myocardial contractility or vascular tone. In hemodynamically unstable patients, a narcotic such as fentanyl in combination with a benzodiazepine such as midazolam can be subsequently titrated incrementally to maintain general anesthesia after induction with etomidate. In elective cases, general anesthesia can be induced with routine intravenous hypnotic drugs followed by a narcotic to attenuate the hypertensive responses to tracheal intubation and skin incision. Antibiotic prophylaxis administration should optimally be completed at least 30 minutes before skin incision to achieve adequate bactericidal levels in tissue. Antifibrinolytic therapy, if used, should be administered before full anticoagulation for extracorporeal circulation.

The maintenance of general anesthesia can usually be accomplished with a combination of narcotic analgesics, benzodiazepine sedative hypnotics, an inhaled general anesthetic, and a nondepolarizing muscle relaxant. Anesthetics can be reduced in response to moderate hypothermia in the range of 30°C and then discontinued during deep hypothermia at 18°C and resumed on rewarming. When electroencephalographic (EEG) or somatosensory evoked potential (SSEP) monitoring is required during surgery, barbiturates or bolus doses of propofol are avoided and the dose of the inhaled anesthetic is reduced to 0.5 MAC and kept constant to prevent anesthetic-induced changes in the monitored signals. Propofol, narcotics, and neuromuscular blocking drugs can be used during SSEP monitoring. When intraoperative motor evoked potential (MEP) monitoring is required, total intravenous anesthesia with propofol in combination with remifentanil or similar narcotic without neuromuscular blockade is necessary to ensure consistent reproducible recordings and a good-quality signal. In the majority of cases, the duration of general anesthesia is designed to persist for 1 to 2 hours after patient transfer to the intensive care unit (ICU) to permit a gradual and controlled emergence from general anesthesia. If epidural analgesia is used intraoperatively, a dilute solution of local anesthetic and narcotic is preferred to prevent hypotension caused by sympathetic nervous system blockade and to prevent complete motor or sensory blockade to permit neurologic assessment of lower extremity function.¹

The potential for blood loss and bleeding is always a consideration in operations on the thoracic aorta. The presence of intrinsic disease of the vessel wall, construction of numerous vascular anastomoses in large conducting vessels, need for extracorporeal circulation, and application of deliberate hypothermia all combine to create a situation in which blood loss and transfusion therapy are commonplace. Because blood loss can occur rapidly and unpredictably and be difficult to control, it is often prudent to have fresh frozen plasma and platelets available to provide ongoing replacement of coagulation factors during transfusion of packed red blood cells. The time delay required for laboratory testing to verify the depletion of platelets and clotting factors in the setting of ongoing blood loss is often too long to be useful as a guide for transfusion therapy. Strategies to decrease the risk of bleeding and to conserve blood include discontinuation of anticoagulation and antiplatelet therapy before surgery, antifibrinolytic therapy, the routine use of intraoperative cell salvage, biologic glue, and precise control of arterial pressure and prevention of hypertensive episodes in the perioperative period. The antifibrinolytic agents, ε-aminocaproic acid or tranexamic acid, have been safely used in the setting of thoracic aortic surgery with DHCA. The infusion of an antifibrinolytic agent should be discontinued during the period of DHCA and resumed on reperfusion. Recombinant activated factor VIIa is a synthetic hemostatic agent that promotes hemostasis by binding with tissue factor at the site of tissue injury to promote clot formation. Although experience with this agent has been limited, dramatic responses to this drug have been observed in response to coagulopathic bleeding refractory to conventional therapy in the setting of trauma, cardiac, and aortic surgery.² In the surgical setting, recombinant activated factor VIIa has been administered intravenously in doses up to 90 μg/kg and repeated once after 2 hours. Recombinant activated factor VIIa has an estimated plasma half-life of 2.6 hours and causes a rapid decrease in the prothrombin time.

Postoperative Care

After completion of the operation, the patient should be transported directly from the operating room into the ICU or postanesthetic care unit for recovery. Transfer of information to the critical care team in advance of receiving the patient is necessary to ensure an uninterrupted transition of care. Immediate application of forced-air warming prevents further temperature drift and restores normothermia even in the moderately hypothermic patient. In the absence of complications and when the medical condition of the patient is stable, the patient can be allowed to emerge from the effects of general anesthesia. Early emergence from general anesthesia is preferable because it permits early postoperative assessment of neurologic function. If the physiologic condition of the patient does not permit safe emergence from general anesthesia, sedation and analgesia can be provided in combination with mechanical ventilatory or circulatory support until the condition of the patient improves.

Common early complications include hypothermia, bleeding, hypertension, hypotension, ischemia, embolism, stroke, agitation and confusion, respiratory failure, and renal failure. Hyperglycemia, anemia, coagulopathy, electrolyte disturbances, and acid-base abnormalities are also common. Frequent hemodynamic assessment is important to control the circulation with short-acting vasoactive drug therapy and to detect cardiac arrhythmias. Arterial blood gas analysis and respiratory assessment are necessary to adjust the level of mechanical ventilatory support and determine the optimal time for safe extubation of the trachea. Laboratory testing to measure electrolyte concentration, hematologic parameters, and coagulation profile is necessary to institute immediate corrective measures. Maintaining glucose concentrations within the normal physiologic range is considered important because hyperglycemia has been associated with increased risk of infection, increased mortality in the ICU, and adverse neurologic outcome. The chest roentgenogram is obtained to verify the proper position of the endotracheal tube and the position of intravascular catheters and to diagnose pneumothorax, atelectasis, pleural effusions, or pulmonary edema. Perioperative antibiotic prophylaxis is typically continued for 48 hours after surgery to decrease the risk of wound and endovascular infections.

THORACIC AORTIC ANEURYSM

An aortic aneurysm is a dilatation of the aorta containing all three layers of the vessel wall that has a diameter of at least 1.5 times that of the expected normal diameter of that given aortic segment. Thoracic aortic aneurysms are common, are detected in 10% of autopsies, have an incidence of 5.9 per 100,000 person-years, and are the most common reason for thoracic aortic surgery. The median age at the time of diagnosis is 65 years, and this lesion occurs two to four times more frequently in males. Common risk factors for thoracic aortic aneurysms include hypertension, hypercholesterolemia, prior tobacco use, collagen vascular disease, and family history of aortic disease. Thoracic aortic aneurysms are classified by their location, size, shape, and etiology. Among thoracic aortic aneurysms, descending thoracic aortic aneurysms are most common, followed by ascending aortic aneurysms, and less often by aortic arch aneurysms.

The anatomic location of the aneurysm and its extent determine its pathophysiologic consequences, operative approaches, and postoperative complications. Aneurysms involving the aortic root and ascending aorta are commonly associated with bicuspid aortic valve or aortic regurgitation (AR). Aneurysms extending into or involving the aortic arch require temporary interruption of cerebral blood flow to accomplish the operative repair. Endovascular stent repair is an option for aneurysms isolated to the descending thoracic aorta ending above the diaphragm. Repair of descending TAAAs requires the sacrifice of some or all of the segmental intercostal arteries branches and is associated with a risk of postoperative paraplegia from spinal cord ischemia or infarction. Aneurysmal disease of the thoracic aorta is often a diffuse process affecting multiple segments of the aorta and producing vessel tortuosity and often coexists in combination with isolated aneurysms of the abdominal aorta.

Most thoracic aortic aneurysms are asymptomatic and discovered incidentally through screening or as a consequence of medical workup for other cardiovascular disease (Box 17-1). The most common initial symptoms of thoracic aortic aneurysm are chest or back pain caused by aneurysmal expansion, rupture, or bony erosion. The mass effect of the aneurysm can cause hoarseness from stretching or compression of the recurrent laryngeal nerve, atelectasis from compression of the left lung, superior vena cava syndrome from compression of the superior vena cava or innominate vein, dysphagia from compression of the esophagus, or dyspnea from compression of the trachea, main stem bronchus, or pulmonary artery. Other symptoms include wheezing, cough, hemoptysis, or hematemesis. Aneurysm of the aortic root causing AR may present as dyspnea on exertion, heart failure, or pulmonary edema. Atherosclerotic aneurysms with mural thrombus may present as embolism, stroke, mesenteric ischemia, renal insufficiency, or limb ischemia.

BOX 17-1 Potential Complications of Thoracic Aortic Aneurysms

• Rupture

• Aortic regurgitation

• Trachea or left main stem bronchial compression

• Right pulmonary artery or right ventricular outflow tract obstruction

• Esophageal compression

Leakage or rupture of thoracic aortic aneurysms should be treated as a surgical emergency. Expansion and impending rupture are often heralded by the development of new or worsening pain, often of sudden onset. Rupture is accompanied by the dramatic onset of excruciating pain and hypotension. Rupture of an ascending aortic aneurysm into the pericardial sac causes cardiac tamponade. Rupture of a descending aortic aneurysm may cause hemothorax, aortobronchial fistula, or aortoesophageal fistula. If surrounding tissue does not contain a ruptured aortic aneurysm, the patient will exsanguinate and die.

General Surgical Considerations for Thoracic Aortic Aneurysms

The objective of surgical repair is to replace the aneurysmal segment of aorta with a tube graft to prevent morbidity and mortality as a consequence of aneurysm rupture. Indications for operative repair include the presence of symptoms refractory to medical management, evidence of rupture, an aneurysm diameter of 5.0 to 5.5 cm for an ascending aortic aneurysm, an aneurysm diameter of 6.0 to 7.0 cm for a descending thoracic aneurysm, or an increase in aneurysm diameter greater than or equal to 10 mm/yr. Earlier surgical intervention may be justified in patients with Marfan syndrome, a family history of aortic disease, or dissection. In several series, 1-, 3-, and 5-year survival was as high as 65%, 36%, and 20% for medically treated patients with thoracic aortic aneurysms, respectively. Aneurysm rupture may account for up to 32% to 47% of deaths.³

An important factor that dictates how the surgical repair is performed is the location and extent of the thoracic aortic aneurysm. Thoracic aortic aneurysms of the ascending aorta and aortic arch are approached from a median sternotomy incision. Standard CPB can be used for the repair of aneurysms limited to the aortic root and ascending aorta that do not extend into the aortic arch by cannulating the distal ascending aorta or proximal aortic arch and applying an aortic cross-clamp between the aortic cannula and the aneurysm. Aneurysms that involve the aortic arch require CPB with temporary interruption of cerebral perfusion. Neuroprotection strategies that involve a combination of deliberate hypothermia, selective ACP, and RCP are important to protect the brain from ischemic injury during reconstruction of the aortic arch. Aortic aneurysms that involve the descending thoracic aorta are approached from a lateral thoracotomy or thoracoabdominal incision. Reconstruction of the descending thoracic aorta can be accomplished without extracorporeal circulation by cross-clamping the thoracic aorta or with extracorporeal circulation using partial left-sided heart bypass to provide distal aortic perfusion. Partial left-sided heart bypass is accomplished through cannulation of the left atrium via a left pulmonary vein and cannulation of the distal aorta, internal iliac artery, or femoral artery. If the descending thoracic aortic aneurysm extends into the distal aortic arch, DHCA may be necessary to construct the proximal aortic anastomosis. Operations for descending thoracic aortic aneurysms require consideration of strategies to protect the mesenteric organs, spinal cord, and lower extremities from ischemia as a consequence of temporary interruption of organ blood flow or the sacrifice of collateral vessels to accomplish the repair.

Surgical Repair of Ascending Aortic and Arch Aneurysms

The surgical options for repair of ascending aortic aneurysms depend on the presence of aortic valve disease, aneurysm of the sinuses of Valsalva, and distal extension of the aneurysm into the aortic arch. Intraoperative TEE is useful for evaluating the aortic valve to determine if a valve-sparing surgery is feasible, to determine the aortic valve annular diameter in relation to the diameter of the sinotubular junction to assess aneurysmal dilation of the aortic root, and to detect and quantify the presence of AR after valve repair. The most common aortic valve diseases associated with ascending aortic aneurysm are bicuspid aortic valve or AR caused by dilation of the aortic root (Fig. 17-1). If the aortic valve and aortic root are normal, a simple tube graft can be used to replace the ascending aorta. If the aortic valve is diseased but the sinuses of Valsalva are normal, an aortic valve replacement combined with a tube graft for the ascending aorta without need for re-implantation of the coronary arteries can be performed. If disease involves the aortic valve, aortic root, and ascending aorta, the options include tube graft of the ascending aorta in combination with aortic valve repair, reconstruction of the aortic root with sparing or repair of the aortic valve, bioprosthetic aortic root replacement, composite valve/graft conduit aortic root replacement (Bentall procedure), or replacement of the aortic root with a pulmonary autograft (Ross procedure). Replacement of the aortic root requires re-implantation of the coronary arteries or aortocoronary bypass grafting (Cabrol technique). If there is evidence of significant coronary artery disease, a combined ascending aortic aneurysm repair and coronary artery bypass grafting (CABG) may be necessary.