What are the perioperative clinical and anesthetic pitfalls that can be encountered during management of patients with thoracic aortic injuries, and how should they be managed?
Coexisting more obvious injuries, lack of obtaining appropriate studies, and misinterpretation of radiographic findings result in diagnostic difficulties and at times missed diagnosis. For example, initially missed aortic injury may be recognized a few days after injury during an emergency sternotomy to relieve a cardiac tamponade and to control bleeding caused by cardiac free wall rupture.
Airway management difficulties may be encountered for several reasons. Bleeding from the aorta may reach the prevertebral space. A hematoma in this area may shift the larynx and trachea anteriorly creating difficulty during laryngoscopy and intubation. A prevertebral hematoma can easily be detected with a lateral neck film. Laryngoscopic visualization of the larynx may be difficult in the presence of a cervical collar placed for documented or suspected cervical spine injury, because neck extension is limited. Combined with a prevertebral hematoma this limitation may make conventional laryngoscopy impossible. However, neck extension should be avoided not only because of the possibility of cervical spine injury but also for preventing sudden hemorrhage from an aortic arch vessel injury; the perivascular hematoma that seals the injury may be distracted by extension of the neck.
Subadventitial and perivascular hematomas or pseudo-aneurysms may compress the left main stem bronchus and cause narrowing of its lumen. Double-lumen tubes have the advantage of preventing spillage of blood into the right bronchus from the left lung during dissection of the aorta. However, forcing a left-sided endobronchial tube blindly into the left main stem bronchus may result in a burst of peribronchial bleeding. Visualization of the left main stem bronchus with a fiberoptic bronchoscope before advancing the endobronchial tube can prevent this complication.
Bleeding into the left, and rarely into the right, hemitho-rax is a frequent complication of thoracic aortic rupture. At times a large quantity of blood may fill the pleural cavity, not only causing collapse of the lung and mediastinal shift but also giving false information when attempting to cannulate the internal jugular or subclavian vein. A free flow of blood may not necessarily indicate vascular puncture; it may be secondary to entry into the blood-filled interpleural space. If this situation is not recognized, rapid infusion of fluids into the pleural space may result in catastrophic complications.
The primary goal of anesthetic management in patients with vascular injuries is to prevent dislodgment of a perivas-cular clot to avoid bleeding. This can be accomplished by deep anesthesia and complete muscle relaxation. This can be achieved easily in hemodynamically stable patients. However, deep anesthesia may not be possible in unstable patients, who may only be able to tolerate oxygen, muscle relaxation, and possibly a minimal dose of intravenous anesthetic. Nevertheless, another important objective is to decrease viscous drag, which is the pulling force against the vascular wall by flowing blood. Viscous drag has the potential to displace the clot sealing the site of injury. It is proportional to the viscosity and blood flow, and inversely proportional to the third power of the radius. Thus, it is important to decrease the contractile force of the myocardium. In a young and healthy patient, the contractile force may increase to maximum levels to compensate for hypovolemia. Blunting this response with anesthetics may produce a catastrophic outcome. These conflicting objectives may be addressed by titration of the anesthetic while monitoring myocardial contractility with the TEE along with measuring the systemic blood pressure. Control of the blood pressure in the preoper-ative period and before clamping of the aorta during surgery is crucial to prevent rupture. Preoperative use of β-blocking or calcium channel blocking agents with or without sodium nitroprusside can usually maintain systolic blood pressure at 80–90 mmHg.
The surgical mortality from traumatic aortic injury averages 20% (5–35%). Mortality appears to correlate with patient age, presence of preoperative hypotension, delay in diagnosis and surgical treatment of the injury, and location of the lesion. Proximal aortic injuries have a greater mortal-ity than isthmic injuries. Also injuries located less than 1 cm from the origin of the left subclavian artery are associated with a greater likelihood of technical difficulty, rupture, and mortality during surgical manipulation than those in other locations.
Spinal cord ischemia resulting in paraplegia or para-paresis is a well-known complication of thoracic aortic injury and its repair. A complete preoperative neurologic evaluation should be documented to avoid confusion regarding neurologic deficits after surgery. The blood sup-ply to the spinal cord is from one anterior and two poste-rior spinal arteries, all of which originate from branches of the vertebral artery at the base of the skull and descend along the cord. The anterior spinal artery supplies the ante-rior two thirds and the posterior spinal arteries the poste-rior third of the cord. Anastomoses between the anterior and posterior blood supply within the cord are rather weak and there are areas with marginal blood supply within the spinal cord. The anterior spinal artery receives a few radic-ular branches that originate from the thoracic intercostals. The largest of those branches is the great radicular artery of Adamkiewicz (arterial radicularis magna).
The two most common mechanisms of spinal cord ischemia in traumatic aortic injury are occlusion of a subclavian artery, which supplies the vertebral and thus the spinal arteries, either by injury or by clamp placement during repair; and surgical interference with the intercostal vessels sup-plying the artery of Adamkiewicz. In 75% of patients, the blood supply to the artery of Adamkiewicz originates from the last four thoracic segments, in 15% from L1 or L2, and in 10% from the fifth to eighth intercostal arteries. The surgical technique is the likeliest cause of spinal cord injury in patients without a preoperative neurologic deficit or long periods of hypotension.
There are three techniques for repair of aortic injuries: clamp and sew; passive shunting between the proximal and distal aorta; and cardiopulmonary bypass. The latter may be achieved in several ways: partial femoral-femoral bypass, left heart bypass (left atrioaortic or atriofemoral), and right atrial to distal aortic bypass. The advantage of left or right atrial to distal aortic bypass techniques is that they can be used with minimal or no anticoagulation.
The clamp and sew technique is associated with the highest rate of spinal cord complications. However, the incidence of paraplegia is lower if the clamp time is less than 30 minutes. The use of passive shunt placement between the proximal and distal portions of the aorta does not signifi-cantly decrease the incidence of neurologic deficit. Partial bypass, e.g., femoral-femoral, decreases the rate of para-plegia, but the need for heparinization is a significant disadvantage of this technique. Left atriofemoral bypass is more frequently used than left atrioaortic or right atrioaortic techniques. If used with a centrifugal pump, these approaches result in the best spinal cord protection. Heparin-bound cannulae are used, which obviates the need for systemic heparinization. Left heart bypass also has two important additional advantages in that unloading the left heart may decrease cardiac complications, and possibly decreases ischemic reperfusion injury. The particular surgical technique will be dictated by surgeon preference and other associated injuries.
The recent development of endovascular stents promises not only to virtually eliminate spinal cord complications but also to decrease the overall morbidity and mortality compared to open repair. Although the routine clinical use of this technique awaits further convincing data, several of the advantages it offers (no need for thoracotomy, single-lung ventilation or aortic bypass, and decreased blood loss) make it an extremely desirable technique.