Preoperative Preparation and Monitoring

Surgical TAA repair requires extensive preoperative planning. The evaluation and management of coexisting cardiac and pulmonary disease are previously discussed in this chapter. Before the day of surgery, the anesthesiologist and the vascular surgeon should discuss the following issues: extent of the aneurysm and technique of surgical repair, plans for distal aortic perfusion, spinal cord ischemia monitoring, renal and spinal cord protection, hemodynamic monitoring, and ventilation strategy.

I routinely have 15 units of packed red blood cells and 15 units of thawed fresh frozen plasma immediately available in the operating room, and additional units are readily obtainable. I use a large cooler to store blood products on ice in the operating room so that they can be returned to the blood bank if not used. Platelets should be available as well. A dedicated critical care technician is helpful in assisting with laboratory testing and retrieving products from the blood bank.

Large-bore intravenous access is obviously important, especially if partial bypass (in contrast to full bypass) is planned, because it is difficult or impossible for the perfusionist to administer fluid or blood products into the closed circuit. I usually insert three 8.5F catheters into the internal jugular and antecubital veins. One of these accepts the pulmonary artery catheter, and the other two are connected to a rapid infuser system (RIS, Haemonetics, Braintree, MA). This system allows the delivery of up to 1500 mL/min of blood products at a temperature of 37 to 38°C. A right radial arterial catheter is used for aneurysms of the descending thoracic aorta because occasionally the cross-clamp is placed proximal to the left subclavian artery, occluding flow to the left upper extremity. When distal aortic perfusion techniques are used, I monitor arterial blood pressure distal to the cross-clamps. This can be accomplished with the placement of a right femoral arterial catheter, or the surgical team can place a catheter directly into the femoral artery or distal aorta. This catheter monitors perfusion pressure to the kidneys, spinal cord, and mesenteric circulation during the time when the cross-clamps are high on the descending aorta, and the lower body is perfused by a shunt or bypass circuit. The radial and femoral arterial pressures should be simultaneously displayed on the anesthesiologist's monitor and a monitor visible to the surgeons and the perfusionists. TEE is used routinely during TAA repair (see Chapter 33 ). When used by a properly trained individual, assessment of left ventricular end-diastolic volume, myocardial ischemia, and valvular function is possible. It is also possible to determine the size and the extent of the aneurysm.

A double-lumen endobronchial tube should be inserted for the purpose of one-lung ventilation (see Chapter 49 ).

2084

Many centers use electrophysiologic monitoring with somatosensory evoked potentials (SSEPs) or motor evoked potentials (MEPs), or both, to monitor for spinal cord ischemia (see Chapter 38 ). These monitoring techniques may be helpful in identifying the important intercostal arteries that perfuse the spinal cord and confirming successful reimplantation into the aortic graft.^[340] If ischemia is identified, cross-clamps can often be moved, upper or lower body blood pressure can be increased to provide perfusion through collaterals, or other measures may be taken to protect the cord (i.e., cerebrospinal fluid [CSF] drainage, induced hypothermia, or intrathecal pharmacologic agents). These techniques are discussed later. There are three general problems with SSEP monitoring when used during TAA repair. First, sensory monitoring is more likely to detect lateral and posterior sensory column ischemia and is a poor monitor for the anterior motor column. As a result, paraplegia can occur despite normal SSEP signals.^[341] Second, inhaled anesthetics ^[342] and hypothermia^[343] can significantly interfere with SSEP signals. Third, ischemia affects peripheral nerves, and ischemia to the lower extremities delays conduction from the usual stimulation sites (e.g., posterior tibial nerve).^[344] To eliminate the peripheral nerves as a confounding factor, spinal stimulation through a lumbar epidural electrode can be used, which may be more specific for ischemic injury than is peripheral monitoring.^[344] Lower extremity and peripheral nerve ischemia can be avoided with the use of distal aortic perfusion techniques. To avoid lower extremity ischemia due to occlusion of the left femoral artery at the insertion site of the retrograde perfusion cannula, my colleagues and I suture a small-caliber graft onto the femoral artery (end-to-side) for cannula insertion, which allows antegrade and retrograde perfusion. These limitations of SSEP monitoring outlined above likely account for the lack of improvement in neurologic outcome in a large prospective series of TAA repairs.^[345] In this same series, the incidence of false-negative responses was 13% and false-positive responses 67%, making identification of critical spinal arteries impossible.

The transcranial MEP technique has been used successfully to monitor the anterior columns of the spinal cord.^{[340] [346] [347]} The technique is relatively simple and can be viewed as a "train-of-four" for the brain and spinal cord. Electrical stimulation over the motor cortex activates α-motor neurons, and evoked electromyographic responses are obtained in lower extremity muscle. Only electromyogenic responses are specific for the status of the motor neurons in the anterior horn gray matter. We place bilateral recording needles in the popliteal fossae (i.e., popliteal nerve) and bilateral surface electrodes over the gastrocnemii and tibialis anterior muscles. We place bilateral stimulating needles in the popliteal fossae to monitor direct muscle responses and level of neuromuscular blockade. During aortic cross-clamping, MEPs are monitored every minute. A reduction of MEP amplitude to less than 25% of baseline is considered an indication of spinal cord ischemia and requires corrective measures. Because signal averaging is not required and the anterior horn cells react with an almost immediate functional loss after the onset of ischemia, the technique can be used to rapidly identify intercostal arteries supplying the spinal cord. The technique can be used to evaluate adequacy of distal aortic perfusion and patency of reimplanted critical intercostal arteries. Careful titration of a short-acting neuromuscular blocker is required to maintain a stable level of neuromuscular blockade. Complete neuromuscular blockade makes MEP monitoring impossible. I use a continuous infusion technique to maintain electrical muscle amplitude of approximately 30% of baseline. Isoflurane,^[348] desflurane,^[349] sevoflurane,^[350] and nitrous oxide^[351] depress synaptic conduction and significantly decrease the amplitude of myogenic MEPs. Although modifications of the stimulating technique have improved monitoring with inhaled anesthetics somewhat, a total intravenous anesthetic technique may be optimal. Fentanyl^[352] and ketamine^[353] have little effect on myogenic MEPs and have been successfully used as a combined anesthetic in a large series of patients undergoing TAA repair with MEP monitoring. ^[333] This series of 210 consecutive patients had the lowest rate of neurologic deficit (2.4%) and permanent paraplegia (1.4%) reported. ^[333]

Body temperature should be monitored at two sites (core and peripheral) to assess cooling and warming when bypass techniques are used. There is an important difference, however, between full and partial bypass with regard to temperature monitoring. ^[354] With full bypass, perfusion is usually into the ascending aorta, and typically the upper body core temperature (i.e., nasopharynx or esophagus) cools and warms fastest, whereas the lower body temperature changes more slowly. With partial bypass, the opposite is true. The blood from bypass is returned into the femoral artery, and the lower body (i.e., rectum or bladder) changes before the upper body changes. This difference is important to recognize to achieve complete cooling and warming because the lagging temperature should be the end point for cooling and warming.