Anesthesia for Vascular Surgery

Chapter 25


Anesthesia for Vascular Surgery




Peripheral Vascular Disease


Atherosclerosis is the most common cause of occlusive disease. This degenerative process involves the formation of atheromatous plaques that may obstruct the vessel lumen and thereby cause a reduction in distal blood flow. The pathophysiologic processes that affect the arteries include plaque formation, which obstructs the lumen (stenosis); thrombosis, which can result in acute ischemia; embolism from microthrombi or atheromatous debris, which decreases distal blood flow; and weakening of the arterial wall with aneurysm formation. The most common risk factors associated with atherosclerosis are shown in Box 25-1. Cigarette smoking and diabetes mellitus are major risk factors in the pathogenesis of atherosclerosis in the peripheral vascular system. Typical symptoms of peripheral occlusive disease include claudication, skin ulcerations, gangrene, and impotence.1 The extent of disability is primarily influenced by the development of collateral blood flow. Initially, collateral blood flow sufficiently meets tissue oxygen demands. As the disease process progresses, supply is unable to meet demand, and limb ischemia becomes symptomatic, requiring therapeutic intervention. The mortality rate associated with patients with vascular disease is two- to sixfold higher than within the general population.2 There is a relationship between inflammation and the development of atherosclerosis. Platelet interaction with leukocytes and other cells that modulate the immune response play a major role in the development of atherosclerosis.3,4 Researchers have discovered heritable genetic factors that predispose patients to developing vascular disease.5



BOX 25-1   Risk Factors Related to the Development of Atherosclerotic Lesions




Treatment for peripheral occlusive disease may range from pharmacologic therapy to surgery. Surgical therapy includes transluminal angioplasty, endarterectomy, thrombectomies, endovascular stenting, and arterial bypass procedures. Some common surgical maneuvers used for bypassing occlusive lesions are aortofemoral, axillofemoral, femorofemoral, and femoropopliteal bypass techniques. Bypass techniques may be classified as inflow or outflow procedures, depending on the level of the obstruction, with the dividing axis being at the level of the groin. Temporary occlusion of the operative artery is mandatory when bypass procedures are used. The response to aortic cross-clamping in patients with aortoiliac occlusive disease produces less hemodynamic variability as compared to patients with aneurysmal disease. The development of collateral circulation provides alternative vascular blood flow in patients with occlusive disease.6,7



Preoperative Evaluation


The atherosclerotic process in occlusive disease is not limited to the peripheral arterial beds and should be expected to be present in the coronary, cerebral, and renal arteries. More than half the mortality associated with peripheral vascular disease results from adverse cardiac events.8 It has been estimated that 42% of patients presenting for abdominal aortic aneurysm (AAA) repair have significant coronary artery disease (CAD).9 The identification and management of cardiac pathology, which often occurs in this patient population, must be managed aggressively to optimize cardiac functioning and decrease morbidity and mortality from cardiac causes. For a complete discussion of a preoperative cardiac evaluation, refer to Chapter 19.


The advantages of β-blockade as relates to factors that affect myocardial oxygen supply and demand have been extensively studied in this patient population, and the judicious use of β-blockers is recommended in patients at high risk for myocardial ischemia and infarction.10 For patients having AAA repairs, there is a 10-fold decrease in cardiac morbidity.11 β-blockade therapy should be instituted days to weeks before surgery and titrated to a target heart rate between 50 and 60 beats per minute (bpm).12 Vascular surgery patients with limited heart rate variability after receiving β-blocking medication exhibit less cardiac ischemia and troponin values postoperatively and have a decreased mortality from all causes 2 years postoperatively.13 It has been suggested that because of their antiinflammatory effects, a statin drug should be instituted 30 days prior to the surgical procedure.14


The presence of concurrent pulmonary, renal, neurologic, and endocrine dysfunction should be identified, and measures should be taken to improve organ function before surgery. Preoperatively, the greater number of comorbidities that exist, the greater the risk of morbidity and mortality during the perioperative period.



Monitoring


The extent of perioperative monitoring should be based on the presence of coexisting disease and the type of surgery. Clearly the detection of myocardial ischemia should be a primary objective in patients with vascular disease. Methods for assessing cardiac function include electrocardiographic, pulmonary artery pressure, and transesophageal echocardiography (TEE) monitoring. The effectiveness of pulmonary artery catheters (PACs) in improving patient outcomes has been controversial for years. Many randomized controlled trials have been performed to assess whether they offer any benefit. It was determined that PAC monitoring had no effect on mortality or length of hospital stay. Additionally, there were higher rates of pulmonary embolism, pulmonary infarction, and hemorrhage in the PAC group.15–17 Practice guidelines provided by the American Society of Anesthesiologists Task Force on Pulmonary Artery Catheterization states that the routine use of PACs is not warranted.18


Because of the global nature of atherosclerotic disease, some degree of systemic cardiovascular disease in patients with peripheral vascular disease should be assumed.9 Patients with hypertension and/or angiopathology rely on increased mean arterial pressures to perfuse their vital organs. Thus cerebral and coronary autoregulation occurs at higher than normal pressures. Direct intraarterial blood pressure monitoring allows for near–real-time determination of blood pressure values and is warranted because of dramatic fluctuations that can occur during anesthesia.



Anesthetic Selection


The anesthetic technique chosen for patients having vascular surgery depends on the type of surgical procedure to be performed and the presence of coexisting disease. In certain instances, infiltration of local anesthetic and intravenous sedation may be sufficient, whereas other situations may require the use of general anesthesia. Regional anesthesia for surgery on the lower extremities may decrease the overall morbidity and mortality associated with this patient population. Numerous studies have failed to yield demonstrative advantages for any single anesthetic technique. A comprehensive meta-analysis combining data from 141 studies involving 9559 patients suggested a 30% reduction in mortality for those patients who received a combined general anesthetic and epidural combination. A reduction in the rate of myocardial infarction stroke and respiratory failure was found when epidural anesthesia was used in patients undergoing aortic surgery.19 Several major studies have been conducted evaluating various end points associated with major vascular surgery.20 None of the studies have definitively concluded that superior outcomes depend on the anesthetic technique used.21 The major advantages to using a epidural technique are most noted during the postoperative period. Specific physiologic benefits of an epidural used for major abdominal vascular surgery are summarized in Box 25-2. Some considerations include the findings that inhalation anesthetic agents induce cardioprotection in patients having noncardiac surgery.22 In addition, many vascular patients are receiving anticoagulant therapy; therefore, neuraxial anesthetic techniques must be used with caution to avoid epidural hematoma formation.23



BOX 25-2   Benefits of the Epidural Technique in Vascular Surgery









Postoperative Considerations


Postoperative pain management is an important issue related to peripheral vascular surgery. Most clinicians agree that postoperative administration of narcotics not only provides patient comfort but also contributes to cardiac stability. The use of epidural opioid and local anesthetics in patients recovering from vascular surgery is an important component of postoperative care because pain can greatly enhance sympathetic nervous system stimulation. Despite a decrease in discomfort during the postoperative course, these patients must be monitored in an appropriate surgical unit that is capable of detecting possible adverse events, such as myocardial infarction or respiratory depression, which could be attributed to the administration of epidural opioids and local anesthetics. Presently data are insufficient to confirm that adequate analgesic techniques decrease morbidity and mortality from postoperative complications.24



Abdominal Aortic Aneurysms



Incidence


The incidence of abdominal aortic aneurysm (AAA) is estimated to range between 3% and 10% for patients older than 50 years of age who reside in the western world.25 Improved detection of AAAs is the result of increased screening of asymptomatic aneurysms by noninvasive diagnostic modalities, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonography. The occurrence of AAAs has increased because of the increased age of the general population and the vascular changes that occur as a result of aging.26




Mortality


Elective AAA repair is one of the most frequent vascular surgical procedures, with approximately 40,000 operations performed in the United States annually.27,28 Mortality rates for elective abdominal aortic aneurysmectomies have decreased since the 1970s. The present mortality rate ranges from 1% to 11%, although it is most commonly estimated at 5%. This is compared with mortality rates of 18% to 30% in the 1950s.25,29–35 Advanced detection capabilities, earlier surgical intervention, extensive preoperative preparation, refined surgical techniques, better hemodynamic monitoring, improved anesthetic techniques, and aggressive postoperative management have all contributed to this improvement in surgical outcomes. Data suggest that risk of rupture is very low for AAAs less than 4 cm in diameter, but the risk dramatically increases for AAAs with a 5-cm or greater diameter. Surgical intervention is recommended for AAAs 5.5 cm or greater in diameter.36 Unfortunately, mortality rates for those with undetected or untreated ruptured aortic aneurysms have not followed the trend of those who have surgical intervention. Estimates of mortality resulting from ruptured AAAs vary from 35% to 94%.28,37–40 Combining prehospital with operative mortality, the overall mortality for AAA rupture is 80% to 90%. The 5-year mortality rate for individuals with untreated AAAs is 81%, and the 10-year mortality rate is 100%.33 Other criteria for surgical intervention for AAA include ruptured AAA, 4- to 5-cm AAA with greater than 0.5 cm enlargement in less than 6 months, patients who are symptomatic for AAA, and 5.0-cm AAA or greater for elective repair for patients with a reasonable life expectancy. Early detection and elective surgical intervention can be lifesaving because elective surgical mortality is less than 5% in most series.41




Abdominal Aortic Reconstruction



Patient Selection


As a result of recent advances in surgical and anesthetic techniques, the mortality associated with elective repair of AAAs is fairly low compared with nonsurgical management. Most patients with abdominal aneurysms, including the elderly, are considered surgical candidates. Although advancing age contributes to an increased incidence of morbidity and mortality, age alone is not a contraindication to elective aneurysmectomy.43 However, physiologic age is more indicative of increased surgical risk than chronologic age. Contraindications to elective repair include intractable angina pectoris, recent myocardial infarction, severe pulmonary dysfunction, and chronic renal insufficiency.6 Patients with stable CAD and coronary artery stenosis of greater than 70% who require nonemergent AAA repair do not benefit from revascularization if β-blockade has been established.44 Table 25-1 lists characteristics that define high-risk patients; however, in most cases the presence of an AAA warrants surgical intervention.33



The dimensions of an aneurysm can change over time. Abdominal aortic aneurysms grow approximately 4 mm/yr.45 Aneurysmal vessel dimensions correspond to the law of Laplace:


image


where T = wall tension, P = transmural pressure, and r = vessel radius.


As the radius of a vessel increases, the wall tension increases. Therefore the larger the aneurysm, the more likely the risk of spontaneous rupture. As stated, aneurysms measuring more than 4 to 5 cm in diameter generally require surgical intervention,29 but aneurysms measuring less than 4 to 5 cm should not be considered benign. An aneurysm has the potential to rupture regardless of its size. As the diameter of the aneurysm increases in size, the risk of rupture increases, as shown in Table 25-2.




Patient Preparation


Perioperative myocardial infarction is the most common reason for poor outcomes in noncardiac surgery. Optimization of myocardial oxygen supply and demand and modification of cardiac risk factors is the major goal of preoperative risk reduction. β-blockers, statins, and aspirin are the hallmark pharmacologic treatments for medical management. Prophylactic coronary revascularization is recommended only as per the same indications as the nonoperative setting. Preoperative cardiac testing is recommended only if interpretation of the results will change anesthetic management.46–48


Preoperative fluid loading and restoration of intravascular volume are perhaps the most important techniques used to enhance cardiac function during abdominal aortic aneurysmectomies. Reliable venous access must be secured if volume replacement is to be accomplished. Large-bore intravenous lines and central lines can be used to infuse fluids or blood. Massive hemorrhage is an ever-present threat; therefore, the availability of blood and blood products should be ensured. Provisions for rapid transfusion and intraoperative blood salvage should be confirmed.




Invasive Monitoring


Maintaining cardiac function is crucial for a successful surgical outcome; cardiac function should be closely monitored during abdominal aortic reconstruction. Invasive blood pressure monitoring permits beat-to-beat analysis of the blood pressure, immediate identification of hemodynamic alterations related to aortic clamping, and access for blood sampling. However, information obtained from pulmonary artery catheters has been shown to have low sensitivity and low specificity in detecting myocardial ischemia when compared with electrocardiographic and transesophageal echocardiography (TEE). As previously discussed, pulmonary artery catheters are not routinely used unless a specific indication is warranted in these procedures.18,49


By detecting changes in ventricular wall motion, two-dimensional TEE provides a sensitive method for assessing regional myocardial perfusion. TEE is a primary method of intraoperative cardiac assessment in patients undergoing surgery on the heart and the aorta.46,50,51 Wall motion abnormalities also occur much sooner than electrocardiographic changes during periods of reduced coronary blood flow.52 Myocardial ischemia poses the greatest risk of mortality after abdominal aortic reconstruction. Intraoperative monitoring may enable earlier detection and intervention during ischemic cardiac events.



Aortic Cross-Clamping


Abdominal aortic reconstruction may be one of the most challenging situations for the anesthetist. Patients with AAAs tend to be elderly and have varying degrees of coexisting disease. In addition to the risks associated with any major surgical procedure, these patients also experience physiologic changes that are specific to abdominal aortic aneurysmectomies. Perhaps the most dramatic physiologic change occurs with the application of an aortic cross-clamp. Temporary aortic occlusion produces various hemodynamic and metabolic alterations.



Hemodynamic Alterations


The hemodynamic effects of aortic cross-clamping depend on the application site along the aorta, the patient’s preoperative cardiac reserve, and the patient’s intravascular volume. The most common site for cross-clamping is infrarenal, because most aneurysms appear below the level of the renal arteries. Less common sites of aneurysm development are the juxtarenal and suprarenal areas.


During aortic cross-clamping, hypertension occurs above the cross-clamp and hypotension occurs below the cross-clamp. Aortic cross-clamping results in a series of complex metabolic and humoral responses involving the sympathetic and renin-angiotensin-aldosterone systems. There is an absence of blood flow distal to the clamp in the pelvis and lower extremities.6 Increases in afterload cause myocardial wall tension to increase. Mean arterial pressure (MAP) and systemic vascular resistance (SVR) also increase. Cardiac output may decrease or remain unchanged. Pulmonary artery occlusion pressure (PAOP) may increase or display no change. Table 25-3 summarizes the physiologic changes associated with aortic cross-clamping.



Patients with adequate cardiac reserve commonly adjust to sudden increases in afterload without the occurrence of adverse cardiac events. However, patients with ischemic heart disease or ventricular dysfunction are unable to fully compensate, as a result of the hemodynamic alterations. The increased wall stress attributed to aortic cross-clamp application may contribute to decreased global ventricular function and myocardial ischemia. Clinically, these patients experience increases in PAOP in response to aortic cross-clamping. Aggressive pharmacologic intervention is required for restoration of cardiac function during this time. An algorithm that depicts the systemic hemodynamic responses to aortic cross-clamping is shown in Figure 25-1.




Metabolic Alterations


After the application of an aortic cross-clamp, the lack of blood flow to distal structures makes these tissues prone to developing hypoxia. In response to hypoxia, metabolites such as lactate accumulate. Both epinephrine and norepinephrine stimulate myocardial β1-receptors that can increase heart rate and myocardial oxygen demand.


The release of arachidonic acid derivatives also may contribute to the cardiac instability observed during aortic cross-clamping. Thromboxane A2 synthesis, which is accelerated by the application of an aortic cross-clamp, may be responsible for the decrease in myocardial contractility and cardiac output that occurs. Numerous studies have attempted to determine whether cyclooxygenase inhibition caused by the administration of aspirin or ibuprofen before elective aneurysmectomies can preserve myocardial function.


Traction on the mesentery is a surgical maneuver used for exposing the aorta. Mesenteric traction syndrome is associated with this procedure. Decreases in blood pressure and SVR, tachycardia, increased cardiac output, and facial flushing are common responses to mesenteric traction. Although the cause of this syndrome is unknown, it has been associated with high concentrations of 6-ketoprostaglandin F1, the stable metabolite of prostacyclin at the time of mesenteric traction.53 The 6-ketoprostaglandin F1 levels and hemodynamic stability return to preclamp values as reperfusion occurs.


The neuroendocrine response to major surgical stress is believed to be mediated by cytokines such as interleukin (IL)-1B, IL-6, and tumor necrosis factor, as well as plasma catecholamines and cortisol.54 These mediators are thought to be responsible for triggering the inflammatory response that results in increased body temperature, leukocytosis, tachycardia, tachypnea, and fluid sequestration. Patients who have an exaggerated plasma stress mediator release had longer operative and cross-clamp times and required a greater number of blood transfusions.



Effects on Regional Circulation


Structures distal to the aortic clamp are underperfused during aortic cross-clamping. Renal insufficiency and renal failure have been reported to occur after abdominal aortic reconstruction. Suprarenal and juxtarenal cross-clamping may be associated with a higher incidence of altered renal dynamics; however, reductions in renal blood flow occur even when aortic cross-clamping occurs below the renal arteries. Infrarenal aortic cross-clamping is associated with a 40% decrease in renal blood flow.55 Renal insufficiency more commonly occurs with suprarenal than infrarenal cross-clamping. Suprarenal clamp time longer than 30 minutes increases the risk of postoperative renal failure. These effects may lead to acute renal failure, which is fatal in 50% to 90% of patients who have undergone aneurysmectomies.56 Neither renal dose dopamine nor mannitol has been definitively proven to preserve or improve renal function postoperatively. Preoperative evaluation of renal function is the best method to assess and anticipate which patients may develop postoperative renal dysfunction. A complete evaluation of renal function is required during the preoperative period.


Spinal cord damage is associated with aortic occlusion. Interruption of blood flow to the greater radicular artery (artery of Adamkiewicz) in the absence of collateral blood flow has been identified as a factor that causes paraplegia in patients having AAA repair. The incidence of neurologic complications increases as the aortic cross-clamp is positioned in a higher or more proximal area. Somatosensory evoked potential (SSEP) monitoring has been advocated as a method of identifying spinal cord ischemia. However, SSEP monitoring reflects dorsal (sensory) spinal cord function and does not provide information regarding the integrity of the anterior (motor) spinal cord.6 Motor evoked potential (MEP) monitoring is capable of determining anterior cord function. This monitoring modality relies on intact neuromuscular functioning for analysis, which limits its use in abdominal aortic aneurysmectomies because neuromuscular blocking drugs are routinely used. Alternative methods for reliable evaluation of spinal cord ischemia are still under investigation.57


Ischemic colon injury is a well-documented complication associated with abdominal aortic resections. Ischemia of the colon is most often attributed to manipulation of the inferior mesenteric artery, which supplies the primary blood supply to the left colon. This vessel is often sacrificed during surgery, and blood flow to the descending and sigmoid colon depends on the presence and adequacy of the collateral vessels. Mucosal ischemia occurs in 10% of patients who undergo AAA repair. In less than 1% of these patients, infarction of the left colon necessitates surgical intervention.56



Aortic Cross-Clamp Release


While the aorta is occluded, metabolites that are liberated as a result of anaerobic metabolism, such as serum lactate, accumulate below the aortic cross-clamp and induce vasodilation and vasomotor paralysis. As the cross-clamp is released, SVR decreases, and blood is sequestered into previously dilated veins, which decreases venous return. Reactive hyperemia causes transient vasodilation secondary to the presence of tissue hypoxia, release of adenine nucleotides,56 and liberation of an unnamed vasodepressor substance, which may act as a myocardial depressant and peripheral vasodilator. This combination of events results in decreased preload and afterload. The hemodynamic instability that may ensue after the release of an aortic cross-clamp is called declamping shock syndrome.58 Evidence demonstrates that venous endothelin (ET)-1 may be partially responsible for the hemodynamic alterations that accompany declamping shock syndrome. Venous ET-1 has a positive inotropic effect on the heart and a vasoconstricting and vasodilating action on blood vessels. Table 25-4 summarizes the most commonly observed hemodynamic responses to aortic declamping and therapeutic interventions.



The magnitude of the response to unclamping the aorta may be manipulated. Although SVR and MAP decrease, intravascular volume may influence the direction and magnitude of change in cardiac output. Restoration of circulating blood volume is paramount in providing circulatory stability before release of the aortic clamp.7,56,58–60 The site and duration of cross-clamp application, as well as the gradual release of the clamp, influence the magnitude of circulatory instability. For this reason, it is vital that communication between the anesthetist and the surgical team occurs. Partial release of the aortic cross-clamp over time often results in less severe hypotension. An algorithm depicting the systemic hemodynamic response to aortic unclamping is shown in Figure 25-2.




Surgical Approach


The standard approach for elective abdominal aortic reconstruction is the transperitoneal incision. The advantages of this route include exposure of infrarenal and iliac vessels, ability to inspect intraabdominal organs, and rapid closure.61 Unfavorable consequences associated with this approach include increased fluid losses, prolonged ileus, postoperative incisional pain, and pulmonary complications.


The retroperitoneal approach is an alternative to the standard route. Its advantages include excellent exposure (especially for juxtarenal and suprarenal aneurysms and in obese patients), decreased fluid losses, less incisional pain, and fewer postoperative pulmonary and intestinal complications. After implantation with a synthetic graft, the aortic adventitia is closed (Figure 25-3). In addition, the retroperitoneal approach does not elicit mesenteric traction syndrome.61 The reported limitations of this approach are unfamiliarity of surgeons with this technique, poor right distal renal artery exposure, and inability to inspect the integrity of the abdominal contents. Table 25-5 compares the standard and retroperitoneal surgical approaches.





Management of Fluid and Blood Loss


Extreme loss of extracellular fluid and blood should be expected with abdominal aortic aneurysmectomies. The degree of surgical and evaporative losses and third spacing will determine the magnitude of the patient’s fluid volume deficit. Furthermore, the surgical approach, duration of the surgery, and the experience of the surgeon affects the total blood loss. Most blood loss occurs because of back bleeding from the lumbar and inferior mesenteric arteries after the vessels have been clamped and the aneurysm is opened.56,62 The use of heparin also contributes to blood loss. Excessive bleeding, however, can occur at any point during surgery, and blood replacement is commonly administered during abdominal aortic resections.


Owing to the heightened awareness of transfusion-related morbidity, the use of autologous blood via a cell saver system is a standard procedure. Presently, three options are available for administering autologous transfusions: preoperative deposit, intraoperative phlebotomy and hemodilution, and intraoperative blood salvage. Preoperative deposit is becoming more feasible because asymptomatic aneurysms are being detected with greater frequency. Ideally, patients donate their own blood to minimize the intraoperative use of homologous blood products and the subsequent risk of transfusion-related viruses. With anemia and decreased hemoglobin, oxygen transport is decreased, thus making the patient with systemic vascular disease at increased risk for myocardial infarction and stroke. Autotransfusion systems may be used for replacing intraoperative blood loss. In a study at the Mayo Clinic in which intraoperative autologous red-cell salvage was used, 75% less banked blood was transfused. In a prospective study of 100 patients who underwent elective abdominal aortic resections, 80% of the patients received only their own blood.62



Presence of Concurrent Disease



Preoperative Management


The presence of underlying CAD in patients with vascular disease has been well documented. Reports suggest that CAD exists in more than 50% of patients who require abdominal aortic reconstruction and is the single most significant risk factor influencing long-term survivability.7,8,63–65 Myocardial infarctions are responsible for 40% to 70% of all fatalities that occur after aneurysm reconstruction.6,7,32,65 Preoperative cardiac evaluation begins with the identification of risk factors that may contribute to adverse cardiac events and subsequent death. When preoperative CAD exists, an increased incidence of postoperative adverse cardiac complications has been demonstrated.66


The end-point of any method of preoperative cardiac evaluation for aneurysmectomy is identification of functional cardiac limitations. Depending on the degree of cardiac dysfunction, preoperative optimization of cardiac function may range from simple pharmacologic manipulation to surgical intervention. The American College of Cardiology and the American Hospital Association guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery are generally followed when preparing patients for these procedures. Optimizing patient preoperative pathophysiologic states as described in Box 25-4 minimizes the overall rate of morbidity and mortality.



BOX 25-4   Optimization of Body Systems Prior to Abdominal Aortic Aneurysm Repair











Intraoperative Management



Anesthetic Selection


Several anesthetic techniques are available for abdominal aortic resections. Although each technique has its advantages and disadvantages, a superior technique has not been identified. Anesthetic selection should be based on the following objectives: providing optimum analgesia and amnesia, facilitating relaxation, maintaining hemodynamic stability, preserving renal blood flow, and minimizing morbidity and mortality.



General Anesthesia

Circulatory stability is desirable for patients undergoing AAA reconstruction, especially for those with CAD. All inhalation anesthetics may depress the myocardium and cause hemodynamic instability. Therefore, high concentrations of inhalation agents in patients with moderate to severe decreased ejection fraction should not be used. Because the degree of myocardial depression is dose dependent, it is acceptable to administer inhalation agents at lower inhaled concentrations. Beneficial effects attributed to inhalation agents include the ability to alter autonomic responses, reversibility, rapid emergence, potentially earlier extubation, and neurologic- and cardioprotection.22 Cardiovascular stability provided by opioids has been well documented, and this feature is especially attractive for patients with ischemic heart disease and ventricular dysfunction. Provision of intense analgesia for the initial postoperative period after major abdominal vascular surgery (via the administration of neuraxial opioid) does not alter the combined incidence of major cardiovascular, respiratory, and renal complications.67 Despite the absence of direct myocardial depression, the sympathetic nervous system inhibition that ensues may decrease systemic vascular resistance and heart rate. Therefore, especially in an individual with a moderate to severely decreased ejection fraction, narcotics should be carefully titrated to the patient’s hemodynamic response.





Fluid Management


Maintaining intravascular volume may be an extreme challenge during abdominal aortic resections. Controversy exists regarding whether the administration of crystalloids or colloids affects the overall incidence of morbidity and mortality. Crystalloids may be used for replacing basal and third-space losses at an approximate rate of 10 mL/kg/hr.60 Blood losses initially can be replaced with crystalloids at a ratio of 3:1. The combination of crystalloid and colloid administration is also acceptable. Regardless of the choice of fluid, volume replacement must be dictated by physiologic parameters. Fluid replacement should be sufficient to maintain normal cardiac filling pressures, cardiac output, and urine output of at least 1 mL/kg/hr.7 Patients with limited cardiac reserve can develop congestive heart failure if hypervolemia occurs. As mentioned previously, cell saver blood retrieval is commonly used, and two large-bore intravenous lines in addition to a central venous catheter is warranted.



Hemodynamic Alterations


Hemodynamic changes are likely to occur throughout the procedure. Adequate preoperative sedation should be given before placement of invasive monitoring equipment. Momentary fluctuations in heart rate and blood pressure should be anticipated during induction and intubation. Preoperative replacement of fluid deficits prevents exaggerated responses to vasodilating induction agents. For patients with adequate left ventricular function, hemodynamic stability can be preserved with a “slow” induction using opioids and β-adrenergic blocking agents. Etomidate has minimal myocardial depressant effects and may be most suitable for patients with limited cardiac reserve. The response to mesenteric traction (discussed previously) is also associated with momentary hemodynamic changes.



Renal Preservation


Mortality is four to five times greater in patients who develop acute kidney injury postoperatively. Mechanisms for preserving renal function during aortic cross-clamping include maintaining adequate hydration, avoiding severe and prolonged hypotension, and using renal protection agents such as mannitol, dopamine, furosemide, and N-acetylcysteine. With suprarenal clamp placement, the use of renal cold perfusion at 5° C of hypoosmolar crystalloid solution instilled into the kidney may be an effective renal protection strategy. The theorized mechanism is a decrease in renal metabolic rate resulting in a 7% reduction in oxygen consumption for each degree Celsius drop in temperature.55 However, the best predictor of postoperative renal dysfunction is based on the patient’s preoperative renal function.



Postoperative Considerations


Cardiac, respiratory, and renal failure are the most common complications observed postoperatively in patients recovering from abdominal aortic reconstruction. Cardiovascular function must be closely monitored in the intensive care unit (ICU) for at least 24 hours after surgery. Maintaining adequate blood pressure, intravascular fluid volume, and myocardial oxygenation is paramount during this period. Myocardial infarction frequently contributes to postoperative morbidity and mortality; serial cardiac enzyme analysis may be justified. Pharmacologic agents used in the treatment of hypertension must also be available.


Most patients require ventilatory assistance during the postoperative period. Vigilant monitoring of respiratory function is mandatory, especially when epidural catheters are used for postoperative analgesia. To address the significant number of serious postoperative complications, which are noted in Box 25-5, intensive and continuous assessment of the patient condition is vital. Patients are admitted to the ICU for high-acuity care.



BOX 25-5   Postoperative Considerations for Patients Having Abdominal Aortic Aneurysm Repair





Juxtarenal and Suprarenal Aortic Aneurysms


Although most AAAs occur below the level of the renal arteries, 2% extend proximally and involve the renal or visceral arteries.60,70 Juxtarenal aneurysms are located at the level of the renal arteries, but they spare the renal artery orifice. More proximal suprarenal aneurysms include at least one of the renal arteries and may involve visceral vessels. The effects of aortic cross-clamping for juxtarenal or suprarenal aneurysms are similar to those for infrarenal aortic occlusions; however, the magnitude of hemodynamic alterations increases as the aorta is clamped more proximally.


Renal failure, although possible during infrarenal aortic cross-clamping, occurs more often because of suprarenal aortic occlusion. Maintaining adequate intravascular volume and administering osmotic and loop diuretics may minimize renal ischemia and dysfunction.


Paraplegia is possible when the blood supply to the spinal cord is interrupted by aortic cross-clamping at or above the level of the diaphragm. Increasing the MAP or decreasing cerebrospinal fluid (CSF) pressure by placing a catheter in the subarachnoid space to drain CSF may be used as a means to increase spinal cord perfusion pressure.7,69,70 Total body hypothermia and multimodal neurological monitoring including somatosensory and motor evoked potentials can be used to decrease the incidence of paraplegia. Neurologic deficits can become evident weeks after surgery. Box 25-6 summarizes the complications that may result from juxtarenal or suprarenal aortic occlusion.



BOX 25-6   Potential Complications of Juxtarenal or Suprarenal Aortic Occlusion





Ruptured Abdominal Aortic Aneurysm


A high mortality rate of 80% to 90% is associated with a ruptured AAA, whereas postoperative mortality is estimated to range from 40% to 50%.71 Endovascular aortic repair is being used to treat ruptured AAAs and may decrease the overall mortality. The most common symptoms of ruptured AAAs include a triad of severe abdominal discomfort or back pain, hypotension, and a pulsatile mass.72 Other common symptoms include syncope, groin or flank pain, hematuria, and groin hernia. Risk factors associated with an increase in mortality in patients with a ruptured AAA are noted in Box 25-7.



BOX 25-7   Risk Factors Associated with an Increased Risk of Mortality in Patients with Abdominal Aortic Aneurysm Rupture




(From Cronenwett JL, Johnston W. Rutherford’s Vascular Surgery. 7th ed. Vol. 2. Philadelphia: Saunders; 2010.)


Hypotension and a history of cardiac disease are two factors associated with the poorest prognosis.37,39 Patients with these symptoms should be immediately transferred to the operating room for surgical exploration. When hypotension is absent, more time is available for a comprehensive preoperative assessment and testing.


Once the patient arrives in the operative suite, a brief preoperative evaluation, establishing venous access, and provisions for fluid and blood product administration can be completed. Hemodynamic stability must be the primary objective, and anesthetic induction and maintenance agents must be selected on a case-by-case basis.

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May 31, 2016 | Posted by in ANESTHESIA | Comments Off on Anesthesia for Vascular Surgery

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