Anesthetic Management of Abdominal Aortic Aneurysm Case File

Anesthetic Management of Abdominal Aortic Aneurysm Case File

Lydia Conlay, MD, PhD, MBA, Julia Pollock, MD, Mary Ann Vann, MD, Sheela Pai, MD, Eugene C. Toy, MD

Case 18

An 85-year-old gentleman presents to the preoperative testing clinic with the diagnosis of an 8-cm abdominal aortic aneurysm (AAA) requiring repair discovered by his primary care physician during a routine physical examination. The patient’s past medical history includes wellcontrolled hypertension, hypercholesterolemia, benign prostatic hyperplasia, diabetes mellitus, and an 80-pack-year history of smoking. He has no known drug allergies. His medication list includes metoprolol, lisinopril, and lovastatin. He lives on a farm, and often exerts himself physically doing chores or walking long distances.

The patient is 5 ft 10 in tall and weighs 90 kg. His neck is supple, his mouth opening is more than three fingers breadths, and his airway is classed as a Mallampati 1. His lungs are clear with the exception of occasional sonorous rhonchi, and his heart rhythm is slow but without murmurs.

Given the age of the patient and the anatomy of the aneurysm, the surgeon has made the decision to schedule the placement of an endovascular aorto-bifemoral stent. An open repair of the patient’s abdominal aortic aneurysm will be performed if the endovascular repair cannot be performed successfully.

➤ What are the preoperative concerns for this patient?

➤ How can tests be used to assess his perioperative risks?

➤ What are the most important factors to remember intraoperatively?

ANSWERS TO CASE 18:

Anesthetic Management of Abdominal Aortic Aneurysm

Summary: This is an 85-year-old man with diabetes and a long history of smoking who is undergoing repair of an abdominal aortic aneurysm using an intravascular stent.

➤ Preoperative concerns: The preoperative concerns center on an optimization of his coexisting medical problems such as hypertension, diabetes mellitus, and optimization of his cardiac function.

➤ Tests to assess perioperative risks: In addition to the usual tests obtained before a surgical procedure (hemoglobin, hematocrit, platelet count, chemistry panel, and ECG), preoperative tests are important to assess cardiac function, since the optimization of reversible cardiac disease alters postoperative morbidity and mortality. Appropriate testing includes a history of the patient’s functional capacity at the very least, and noninvasive testing such as a stress test or dobutamine stress ECHO may be required if the assessment of functional capacity is complicated by coexisting diseases such as arthritis or a sedentary lifestyle. Similarly, because of his chronic hypertension and diabetes mellitus, an assessment of renal function is also important. The preoperative assessment of hemoglobin and/ or hematocrit aids in preparing for the potential of sudden large blood losses during the procedure.

➤ Important factors for intraoperative care: Important considerations in the intraoperative care of patients undergoing repair of an AAA are the maintenance of a mean arterial pressure sufficient to provide adequate coronary and cerebral perfusion, and the avoidance of hypertensive episodes which could hasten the aneurysm’s rupture.

ANALYSIS

Objectives

1. Become acquainted with preoperative workup required for the patient undergoing AAA repair.

2. Be able to compare and contrast an AAA repair by open versus endovascular repair.

3. Identify issues basic to anesthetic choice and management for the patient undergoing AAA repair.

4. Obtain a better understanding of the options for pain management following major vascular surgery.

Considerations

This patient’s AAA albeit large was asymptomatic and discovered on a routine physical examination. Though the risk of rupture is a real possibility, the advantages of scheduling his case as an elective procedure following an expeditious preoperative evaluation and preparation outweigh the risks of not proceeding directly to surgery. Optimizing the patient’s medical problems minimizes the postoperative complications, and stratifying his perioperative risk provides valuable information which, in turn, affects his intraoperative and postoperative management.

The risks of this procedure emanate from the surgery itself, as well as this patient’s pre-existing medical condition. Even when repaired using an endovascular technique, an AAA repair is still major vascular surgery, and carries the risk of a major blood loss. The anatomic location of the aneurysm with respect to ease of repair and the location of the renal arteries, and the duration of the aorta’s cross clamp time, also influences the operative risk. All in all, his condition is good compared to many patients who undergo this procedure. He has hypertension, peripheral vascular disease (which also carries the risk of coronary artery disease), long-standing diabetes with the risk of endorgan damage, and a long history of smoking, which suggests the possibility of compromised lung function as well. Nevertheless, his functional capacity is good, exceeding 6 to 8 METs. Thus his ASA Physical Status classification is ASA 3. Combining the risks of this operating with the risks emanating from his medical condition, his attendant risk for this procedure is moderate or high.

This patient prefers general anesthesia, although he understands that regional anesthesia such as an epidural catheter for neuraxial blockade is a viable and safe option as well. He will be monitored using the standard intraoperative monitoring paradigm (ECG, blood pressure cuff, pulse oximetry, capnography, and body temperature). Since he is undergoing an AAA repair, an arterial line will be placed to allow continuous monitoring of arterial pressure and frequent blood gas analysis, and a large bore intravenous (16 gauge or larger) will be placed. The use of central venous pressure (CVP) monitors, pulmonary artery catheters, and/or transesophageal echocardiography (TEE) is optional and should be tailored to specific situations. Given his good functional capacity and lack of significant heart disease, the placement of a CVP line is most likely to be adequate for the monitoring of volume status, in addition to providing additional intravenous access. Urine output should be measured via and recorded at frequent time intervals to assist in the assessment of the adequacy of tissue perfusion.

The most important goal of this patient’s intraoperative management is the maintenance of a mean arterial pressure sufficient to provide adequate coronary and cerebral perfusion. This is accomplished by using small doses of anesthetics with minimal side effects in combination, ensuring adequate volume resuscitation, and using vasopressors as necessary. Blood and blood products should be readily available in the event of aneurysm rupture or conversion to open repair.

APPROACH TO

Anesthetic Management of Abdominal Aortic Aneurysm

DEFINITIONS

ABDOMINAL AORTIC ANEURYSM: A focal dilatation of the aorta greater than 50% of its normal arterial diameter. It is often described in relation to the renal arteries as either a suprarenal or an infrarenal abdominal aortic aneurysm.

OPEN REPAIR: Open repair of an AAA involves a midline abdominal incision with excision, the placement of a cross clamp across the width of the aorta above the proximal end of the aneurysm to stop exsanguinations, and replacement by a synthetic graft.

ENDOVASCULAR REPAIR: An endovascular repair is less invasive than an open repair. It is thus associated with less surgical morbidity, enabling it to be considered as a viable therapy for patients considered too ill to be candidates for the traditional open repair. An endovascular repair involves accessing the aorta via bilateral femoral arteries, and then inserting a bifurcated graft across the length of the aneurysm under fluoroscopic guidance.

METABOLIC EQUIVALENT (MET): The metabolic rate consuming 3.5 mL of oxygen per kilogram of body weight per minute or a metabolic rate consuming 1 kcal per kilogram of body weight per hour. (For examples of the energy requirements for common daily activities, please see Table 18–1.)

CLINICAL APPROACH

An AAA is observed at autopsy in 0.5% to 3.2% of the population, and is the primary cause of death in 1.3% of men between 65 and 85 years of age in developed countries. AAAs are more common in men than in women, with a 2:1 preponderance in people under the age of 80, though the frequency of its occurrence approaches 1:1 in older individuals. It is more common in white males than African American males.

Generally, aneurysms that are asymptomatic are followed clinically until they reach 6 cm in diameter, since 6 cm is the maximal size which can be determined by imaging studies alone. Estimating the size or following the progression in size of an aneurysm over 6 cm in diameter requires an invasive intervention.

Abdominal aneurysms requiring treatment are treated surgically by open repair or by endovascular stent placement. Many factors go into the decision making process which determines the type of repair. The size of the aneurysm, its proximity to the renal arteries, the patient’s comorbidities, and operator preference are some of the factors determining the operative approach.

Table 18–1 DAILY ACTIVITIES AND ENERGY REQUIREMENTS
DAILY ACTIVITY	METABOLIC EQUIVALENTS (METS)
Lifting and carrying objects (9-20 kg)	4-5
Lifting and carrying objects > 20 kg	> 6
Walking 1 mile (level) in 20 minutes	3-4
Running/walking on an incline	> 6
Golf	4-5
Gardening (digging)	3-5
Do-it-yourself,wallpapering	4-5
Light housework (ironing, polishing)	2-4
Heavy housework (making beds, scrubbing floors, cleaning windows)	3-6
Competitive sports, swimming, aerobics	> 6
Heavy shoveling, digging ditches	> 6

Abdominal aortic aneurysm repairs, whether open or endovascular, are major vascular procedures with an intermediate risk for perioperative cardiac events. A complete preoperative workup prior to either open or endovascular repair should include a comprehensive history and physical examination. A

detailed assessment of the patient’s exercise tolerance is of paramount importance, and is typically documented in the record in terms of METs. For patients undergoing procedures with an inherent risk at the intermediate level, who are asymptomatic and able to perform at 4 METs or greater, the American College of Cardiology and the American Heart Association (ACC/AHA) recommends further cardiac testing only if the test will change the course of cardiac management (coronary artery stenting or bypass, or myocardial valve replacement, etc.) prior to the operative procedure.

One of the most important goals of the intraoperative management of AAA repair is the maintenance of a mean arterial pressure sufficient to provide adequate coronary and cerebral perfusion. This may be more important in the patient with an AAA. For example, long-standing hypertension, a common finding in patients undergoing AAA repair, is associated with a shift in the autoregulation curve for cerebral perfusion pressure to the right. This means that the blood pressure over which autoregulation preserves cerebral blood flow is higher in hypertensive patients than in the population as a whole

Figure 18–1. Cerebral blood flow is dependent on mean arterial pressure (MAP).

(Figure 18–1). In addition, hypertensive patients may also have left ventricular hypertrophy accompanied by diastolic dysfunction (Figure 18–2). The thickened, hypertrophied ventricle requires more perfusion to meet its oxygen demands. Moreover, coronary perfusion, which happens largely during diastole, may be compromised by diastolic dysfunction.

Monitoring during surgery includes the standard monitors according to guidelines from the American Society of Anesthesiologists (ASA, noninvasive blood pressure monitoring, two-lead electrocardiogram, and end-tidal carbon dioxide concentration, pulse oximetry, and temperature). In addition, for AAA repair, blood pressure is measured by an intra-arterial cannula, and volume status, by central venous pressure monitoring. Urine output is measured periodically to indicate the adequacy of tissue perfusion.

Keeping in mind the hemodynamic goals for this patient, the induction of anesthesia is performed by using agents with minimal cardiovascular effects in combination to minimize the dose of each, in attempt to reduce any undesirable side effects. Prior to induction, an opiate such as fentanyl is administered to reduce the patient’s cardiovascular response to endotracheal intubation. Induction is typically accomplished using a combination of midazolam, more fentanyl, and etomidate. Propofol may be added for blood pressure control if indicated; however, given propofol’s propensity to cause more peripheral vasodilatation and hypotension, etomidate is probably a better choice as the initial induction agent. The maintenance of anesthesia is achieved by a combination of inhalational agent such as isoflurane or desflurane supplemented by boluses of narcotic agents such as fentanyl.

An endovascular AAA repair may be performed under spinal anesthesia if the aneurysm is infrarenal. Indeed, in patients with severe COPD, spinal may be preferable over general anesthesia, though no demonstrable improvement

Figure 18–2. Effects of uncontrolled hypertension—left ventricular hypertrophy.

in outcomes has been observed. The main advantage of a spinal is the avoidance of the need for endotracheal intubation which may predispose to the need for postoperative ventilatory support. However, spinal also has two major disadvantages. First, spinal causes a sympathectomy, which can result in hypotension and complicate the assessment of the patient’s fluid status. Second, if the operation requires an emergent conversion to an open repair, general anesthesia will be required within an emergent time frame, instead of the typical slow induction while carefully titrating the anesthetic agents to the patient’s needs.

The open repair of an AAA requires the placement of both proximal and distal clamps across the width of the aorta to allow the graft to be sutured in place in a bloodless field. Prior to placement of the clamp, the patient is anticoagulated with a heparin. With the aorta cross clamped, the major source of perfusion and oxygenation of the patient’s limbs is absent. A local acidemia results, accompanied by the accumulation of inflammatory mediators. The sudden release of these substances when the clamp is removed may cause hypotension, myocardial depression, severe morbidity, and/or death if not recognized and treated promptly. Assuring adequate volume replacement and the ready availability of pressors prior to the opening of the clamp is of paramount importance. Some practitioners also recommend prophylaxis for acidemia with a sodium bicarbonate infusion started at the time of cross clamping. After the clamp has been released, the anticoagulation is reversed with protamine. Protamine can cause profound vasodilatation and hypotension is administered, especially in patients who are hypovolemic. Thus, protamine must be infused slowly and carefully titrated to the patient’s hemodynamics.

The second most important factor to be considered during an open AAA repair is blood loss. In normal patients, large amounts of blood loss (greater than 15% of estimated blood volume) result in a hemodynamic instability manifested by hypotension and tachycardia. In “vasculopaths,” this response is magnified because of the lack of compliance in the patient’s vessels and often, the left ventricle. In this setting, even smaller amounts of sudden blood loss can cause profound drops in blood pressure. Blood loss is treated aggressively with fluids or blood products, depending on the patient’s hemoglobin at the time of infusion. If the hemoglobin is greater than 7 g/dL, most practitioners are comfortable with the infusion of a crystalloid such as normal saline or a colloid such as albumin or hetastarch. If the hemoglobin is less than 7 g/dL, blood transfusions should be utilized to support the bleeding and hemodynamics.

The placement of endovascular stent grafts across the AAA is usually not associated with significant blood loss when compared to open procedures, although there is always the potential for aneurysmal rupture or perforation. Stent grafts also do not require the placement of an aortic cross clamp, hence the degree of incurred acidemia is less. Indeed, acidosis, if present, is perhaps more reflective of an inadequate fluid resuscitation than of limb ischemia.

Stents grafts are usually placed with fluoroscopy support, exposing the patient to contrast dye. If stent placement is difficult, the amount of dye used may be significant enough to cause a contrast-induced nephropathy. Data from the cardiac catheterization laboratory (where all interventions are done under fluoroscopy) suggest that prophylactic use of N-acetyl cysteine and sodium bicarbonate infusions play an adjunctive role in decreasing the incidence of this syndrome.

If the case has an uncomplicated intraoperative course with, for example, no requirement for inotropes or pressors to support vital signs, a minimal need for transfusion, and normal respiratory patterns and effort, the endotracheal tube may be removed at the end of the procedure. Since patients undergoing AAA repair are at moderate to high risk for perioperative myocardial events, testing for a possible myocardial infarction using serial cardiac enzymes and an ECG are routine. Because of the propensity for a major blood loss and dye load, either of which may adversely affect renal function, renal function is also followed postoperatively.

Pain control is an important part of postoperative management, since pain causes hypertension and tachycardia which may be harmful in the presence of coronary disease, to say nothing of the lack of patient comfort. Pain control enables deep breathing, which is vital to prevent postoperative atelectasis. Atelectasis is problematic because it serves as a nidus for pneumonia, which can necessitate a re-intubation of the trachea as well as the need for mechanical ventilation. In the presence of significant incisional pain, patients splint, and minimize movements that worsen the pain—such as deep breathing and ambulation. Ambulation is key in preventing deep vein thrombosis, improving gut motility and optimizing lung mechanics.

Several options are available for postoperative pain control. Prior to the induction of anesthesia, an epidural or intrathecal catheter may be placed in the operating room. Contraindications to these techniques include localized infection at the site of procedure, thrombocytopenia, and medications that alter coagulation such as preoperative use of antiplatelet agents or anticoagulants such as heparin or warfarin. Epidural or intrathecal techniques reduce the postoperative requirements for parenteral narcotics, and therefore improve patient interaction, decrease somnolence, and provide superior pain control. On the other hand, neuraxial techniques, particularly when an opiate is administered, require close monitoring at a level that is not always available within an institution. Neuraxial techniques also carry the same risks of any regional anesthetic with regards to catheter placement, nerve impairment, or an exacerbation of back pain.

Comprehension Questions

18.1. A 54-year-old man presents with a 5.5 cm AAA for repair. He has a past medical history of well-controlled hypertension and diet-controlled diabetes, and is otherwise in excellent physical condition. Which one of the following tests is most appropriate to perform prior to the scheduled AAA repair?

A. Chemical stress test

B. Exercise stress test

C. Coronary angiography

D. An ECG

18.2. In the face of active bleeding and hypotension, fluid resuscitation is performed with which of the following?

A. Colloid such as hetastarch or albumin

B. Crystalloids

C. Blood and blood products

D. As indicated by the hemoglobin and hematocrit

18.3. What actions are indicated prior to the opening of an aortic cross clamp?

A. Be sure that the patient is euvolemic.

B. Obtain additional i.v. access.

C. Ensure an adequate depth of anesthesia.

D. Administer protamine to antagonize the heparin-induced anticoagulation.

ANSWERS

18.1. D. An ECG. This gentleman is in excellent physical condition. On further questioning he states he plays tennis for an hour everyday, is compliant with his medications, doesn’t smoke and has a very healthy diet. In this setting, additional cardiac testing does not give any additional information. While an AAA repair is considered a moderate- to high-risk procedure for cardiac events, testing should only be performed in the setting of positive symptoms and in the event that results from the test would alter management strategy. In this patient with stable and active exercise tolerance, cardiac testing over and above an ECG will be of low yield.

18.2. D. Management of bleeding in the operating room is somewhat of an art. Sudden torrential bleeding which depletes greater than 15% of the patient’s blood volume is usually an emergency and needs to be promptly resuscitated with blood transfusion and cell salvage techniques. If the patient is hemodynamically stable and demonstrates mild hypotension and tachycardia (all signs that point to less than 15% blood volume loss), resuscitation with crystalloids or colloids may be appropriate.

18.3. A. “Be sure that the patient is euvolemic.” Removing the cross clamp in an hypovolemic patient can be life threatening, due to the sudden reduction in systemic vascular resistance and myocardial suppression that accompanies the release of acidotic mediators with the clamp’s removal. Ensuring an adequate blood volume is an important part of the anesthetic plan, especially at this critical point in the procedure. Any required i.v. access should already be in place, and the depth of anesthesia should be stable. Protamine is administered to antagonize the heparin induced anticoagulation when the anticoagulation is no longer needed, that is, after the graft has been placed and the clamp has been removed.

Clinical Pearls

➤ Because of its lower morbidity, an endovascular (as opposed to open) approach to aortic aneurysm repair has expanded the population of patients with AAA who are candidates for surgical treatment.

➤ Patients that present for AAA, whether by open technique or by stent, often have other comorbidities such as hypertension, diabetes, and cardiac disease which significantly impact anesthetic management.

➤ Invasive monitoring such as arterial blood pressure and central venous pressure provide valuable information such as the “beat-to-beat” arterial pressure, volume status, and cardiac functionality both in the operating room and intensive care unit. In the setting of AAA repair, the benefits of invasive monitoring far outweigh the attendant risks.

➤ AAA repair is a moderate- to high-risk procedure that carries the possibilities of complications such as myocardial infarction and stroke in the perioperative period.

References

Baril DT, Kahn RA, et al. Endovascular abdominal aneurysm repair: Emerging developments and anesthetic considerations. J Cardiothorac Vasc Anesth. 2007;21:730-742. 

Norris EJ. Abdominal aortic reconstruction. In: Miller RD, ed. Miller’s Anesthesia. 6th ed. Philadelphia, PA: Churchill Livingstone; 2005.

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