36: Aorto-Occlusive Disease

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CHAPTER 36 Aorto-Occlusive Disease

Luke Osborne, MD

1 Define aortic vascular disease

Aorto-occlusive disease is characterized by atherosclerotic changes within the aorta (usually abdominal) that extend into the iliac and femoral arteries and result in hypoperfusion of vital organs and the lower extremities. Involvement at iliac bifurcation and renal arteries is common; aneurismal changes may be found as well. Seventy percent of aortic aneurysms are infrarenal, and 30% include the renal arteries.

2 What risk factors and coexisting diseases are common in these patients?

Risk factors include smoking, family history, obesity, atherosclerotic disease elsewhere, advanced age and male sex. Common diseases include hypertension, ischemic heart disease, heart failure (HF), chronic obstructive pulmonary disease (COPD), diabetes mellitus, chronic renal disease, and carotid artery disease.

3 What is the natural progression of aortic vascular disease?

The natural history of aneurismal disease is one of continuing enlargement. Operative or endovascular repair should be undertaken when the aneurysm is 6 cm or larger. The incidence of rupture increases with size (9.4% chance of rupture within 1 year for aneurysms 5.5 to 5.9 cm, increasing to 32.5% for aneurysms 7 cm and larger).

National mortality from elective open abdominal aneurysm repair is <5%. For repair of an acutely ruptured aneurysm, perioperative mortality remains high, about 50%. Mortality with open thoracic aneurysm repair ranges from 5% to 14%. Endovascular repair has decreased mortality. The survival rate is 92% at 1 year and 67% at 5 years.

Aorto-occlusive disease results in hypoperfusion distal to the occlusion. Unlike surgery for aneurismal disease, surgery for occlusive disease should be undertaken when the patient becomes symptomatic and fails medical therapy.

4 Describe preoperative preparation of such patients in the presence of concurrent disease

The goal of preoperative evaluation is to optimize any significant organ dysfunction.

image Coronary artery disease: This is the major cause of perioperative mortality and morbidity. Myocardial infarction occurs in 4% to 15% of patients, and HF is noted in 30% of postoperative aneurysm repairs. If the patient has an acute coronary syndrome, decompensated heart failure, severe valvular disease, or significant arrhythmias, he or she should undergo preoperative cardiac testing, including echocardiography, stress testing, and coronary angiography. Similarly coronary artery evaluation may be strongly indicated in the presence of renal insufficiency, history of HF or cerebrovascular disease, or if the patient has diabetes mellitus or cannot achieve a metabolic equivalent (MET) level of four.
image Pulmonary: It has been mentioned that COPD is common. Physical examination, pulmonary function testing, arterial blood gas analysis, and chest radiography may suggest the need for preoperative incentive spirometry and bronchodilator and antibiotic therapy.
image Renal: The incidence of postoperative renal dysfunction in elective infrarenal aorta repair is 3% and up to 30% in thoracic aorta repair. The primary cause of postoperative renal failure is preexisting kidney disease. The degree of renal dysfunction should be characterized, and the patient adequately hydrated. Poorly controlled hypertension should be optimized. If the patient requires a contrast study before surgery, he or she should be well hydrated before dye administration and exposed to a minimum amount of contrast; repeated studies should be avoided.
image Cerebrovascular disease: Signs and symptoms of cerebrovascular insufficiency should be elucidated. Cerebral hypoperfusion during surgery could result in stroke. If carotid stenosis is found, endarterectomy may need to precede aortic repair. Chronically hypertensive patients also have shifts in cerebral autoregulation, and this is a second justification for preoperative management of hypertension.
image Diabetes mellitus: This should be well controlled.

5 List the appropriate intraoperative monitors for aortic surgery

image Standard monitors include pulse oximetry and noninvasive blood pressure and temperature monitoring. Continuous cardiac rhythm monitoring for rhythm (lead II) and ischemia (lead V5) is essential.
image Also essential are the usual anesthesia machine monitors, including capnography and delivered oxygen concentration.
image Intra-arterial monitoring rapidly detects swings in blood pressure and facilitates laboratory analysis.
image Monitoring of urine output with a Foley catheter and urimeter is essential.
image Use of central venous pressures, pulmonary artery catheters, and/or transesophageal echocardiography is appropriate when myocardial dysfunction and valvular disease are present.
image Somatosensory and motor-evoked potential can be useful if spinal cord ischemia is considered a risk.

6 Discuss the physiologic implications of aortic clamping and unclamping

Aortic cross-clamping increases left ventricular afterload. Preparation for this cross-clamping includes increasing depth of anesthesia, vasodilator therapy with nitroglycerin or sodium nitroprusside, and possibly administration of α-blockers or calcium channel blockers. A decrease in renal blood flow occurs in about 90% of patients having thoracic aortic cross-clamping and in about 40% of patients having infrarenal cross-clamping. There is redistribution of renal blood flow to the cortex and a decrease in glomerular filtration. These changes persist to some degree even after cross-clamp removal. The potential for a clinically profound decrease in anterior spinal artery blood flow is more likely in thoracic aortic repair, making spinal cord ischemia a risk; and in certain cases lumbar drainage of spinal fluid has been instituted to mitigate decreases in spinal cord perfusion pressure.

Hypotension and a decrease in systemic vascular resistance are noted after unclamping. Ischemic metabolites, lactic acid, and potassium returning to the central circulation depress contractility and potentiate the hypotension.

7 Review the anesthetic goals for these surgical patients

image Provide anesthesia, analgesia, amnesia, and muscle relaxation.
image Maintain cardiac output and coronary perfusion and limit myocardial workload by controlling heart rate and afterload, especially during aortic cross-clamping.
image Aggressively replace blood loss with crystalloids, colloids, and blood and blood products when necessary. Massive blood loss will likely result in a coagulopathy that will require correction.
image Maintain oxygenation and ventilation, guided by blood gas analysis. Excessive ventilatory pressures may compromise preload.
image The best way to preserve renal function is to maintain normal intravascular volume, cardiac output, and oxygenation.
image Especially in patients with diabetes, blood glucose should be monitored and controlled. Insulin therapy not only controls hyperglycemia but prevents ketoacidosis and may improve wound healing and long-term outcome.
image Hypoperfusion of the anterior spinal artery during aortic cross-clamping or extended hypoperfusion places the patient at risk for paraplegia, especially during thoracic aorta surgery.
image Before unclamping, an intravenous bolus of fluid is wise. It may be necessary to reclamp the aorta or provide some minor degree of occlusion to allow the patient time to equilibrate and stabilize.

8 What can be done intraoperatively to preserve renal function?

The incidence of acute kidney injury after elective repair of infrarenal aneurysms is about 3%. But of these patients the mortality is 40%, and this incidence has not changed in decades.

The biggest predictor of postoperative renal dysfunction is preexisting renal disease. The adequacy of renal perfusion cannot be assumed by the amount of urine output since it does not correlate with postoperative renal function. That fact that urine output can be stimulated with diuretics (furosemide and mannitol) and dopamine does not ensure that renal function will be preserved into the postoperative period.

Major factors that impact postoperative renal function include preoperative renal function, the degree of aortic disease, and the duration of cross-clamping. Therefore optimizing renal function before surgery, maintaining euvolemia and renal perfusion, and minimizing cross-clamp time are paramount. Nephrotoxic medications (e.g., gentamycin) or medications that decrease renal blood flow (nonsteroidal antiinflammatory drugs) should be eliminated from the perioperative period as well.

Dopamine, furosemide, mannitol, and fenoldopam (a selective dopamine type 1 receptor agonist) have all been used to prevent renal failure, but evidence is either lacking or controversial as to the efficacy of these treatments. The evidence that low-dose dopamine (2 mcg/kg/min) has a positive effect is equivocal at best. In addition, dopamine has the harmful property of inducing tachycardia and increased cardiac workload in patients predisposed to myocardial disease. Furosemide may cause renal hypoperfusion in hypovolemic states and induce electrolyte imbalances. Evidence of renal protection with mannitol is inconclusive, but it is still widely used to encourage preclamping diuresis. Many clinicians believe that this osmotic diuretic benefits the kidneys by increasing cortical blood flow and reducing endothelial edema and vascular congestion. Fenoldopam is an antihypertensive agent that dilates renal and splanchnic vessels. This increase in renal blood flow may be advantageous, but more prospective studies are required.

9 What are the potential advantages to postoperative epidural analgesia?

Epidural anesthesia may suppress sympathetic tone and the stress response while providing excellent analgesia. This limits myocardial work by attenuating tachycardia and hypertensive swings in a patients predisposed to cardiac ischemia. It also provides titratable postoperative pain relief without excessive sedation. However, a concern with neuraxial analgesia is hematoma formation in the setting of anticoagulation. It is considered safe practice to use these techniques as long as there is a least 1 hour from epidural placement to heparinization of the patient.

10 What specific concerns exist for endovascular repair of the aorta?

Endovascular repair of the aorta is a relatively new technique using stents passed through an artery (usually femoral) and guided to the aorta via fluoroscopy. The patient must be motionless during the procedure; but general, neuraxial, or local anesthesia is appropriate. Local anesthesia has the advantage of requiring less fluid and vasopressor support, although the same degree of pulmonary and cardiac complications occur when compared to general anesthesia. Usually only an arterial line and standard monitors are necessary, but conversion to an open procedure is possible, and aggressive intravenous access is justified. Overall endovascular techniques require less fluid and hemodynamic support, and the 30-day mortality for abdominal aortic aneurysms is less than that for open repairs (3% vs. 4%). The cardiac, pulmonary, renal, and bleeding complications are fewer as well.

11 Describe the primary aspects of management when a patient presents with an acute abdominal aortic rupture

The primary determinants of morbidity and mortality are extent, size, and location. Rupture of the intimal and medial aortic layers is less ominous in the short term as a pseudoaneurysm forms. Although hemorrhage is minimal, proximal increases in afterload and distal ischemia are risks. A larger aortic tear results in rapid and significant blood loss. A retroperitoneal rupture may be temporarily more stable than the almost always fatal intraabdominal rupture.

Resuscitation should be done in the operating room since rapid surgical intervention is necessary to prevent death. Rapid airway control must be obtained to optimize ventilation and oxygenation. The initial goal is to maintain perfusion and oxygenation to the heart and brain. Efforts are directed toward intravascular volume restoration. Multiple large bore intravenous lines are necessary, as is intra-arterial monitoring and probably central venous pressure monitoring as well. Transesophageal echocardiography or pulmonary artery catheterization may be valuable when preexisting myocardial dysfunction is present, but vigorous intravascular restoration and treatment of the inevitable coagulopathy should take precedence. The patient should have 10 units of blood available, universal donor blood if necessary, and the laboratory should be made aware that this case will probably require massive transfusion, with requirements for fresh frozen plasma, platelets, and cryoprecipitate. Disturbances in coagulation are best followed with thromboelastography. Blood pressure may require inotropic or chronotropic support, although volume resuscitation is the mainstay of treatment. A systolic blood pressure of 80 to 100 mm Hg is an ideal goal, although communication about hemodynamic goals should involve the surgeons.

KEY POINTS: Aorto-Occlusive Disease image

1. Coexisting diseases are extremely common and include coronary artery disease, hypertension, COPD, chronic renal disease, and diabetes mellitus. These comorbidities have a very important impact on outcome.
2. Preoperative assessment as guided by the 2007 ACC/AHA Guidelines determines the need for testing and evaluation.
3. The most common cause of perioperative mortality is cardiac disease. Postoperative renal failure also has an important impact on outcome.
4. Goals of anesthesia are to maintain intravascular volume and attenuate the acute and severe hemodynamic changes that occur with vascular cross- and uncross-clamping.

12 Discuss the important elements of postoperative care

image Intensive care unit admission is absolutely essential.
image Cardiac: Patients should be monitored for ischemia and hypotension. The incidence of myocardial ischemia is greatest about the third postoperative day.
image Pulmonary: Many patients remain intubated after surgery if poor baseline pulmonary function exists, if they have left ventricular dysfunction, or if massive fluid shifts have occurred. Epidural analgesia may be of benefit in weaning the patient from mechanical ventilation.
image Renal: Monitoring electrolytes, renal markers, and urine output is important for assessing kidney failure and fluid status.
image Other complications include bleeding, graft dysfunction, bowel ischemia, stroke, embolic events, and extremity hypoperfusion.

SUGGESTED READINGS

1. Greenberg R.K., Lytle B. Endovascular repair of thoracoadominal aneurysms. Circulation. 2008;117:2288-2296.

2. Jones D.R., Lee H.T. Perioperative renal protection: best practice and research. Clin Anaesthesiol. 2008;22:193-215.

3. Kee S.T., Dake M.D. Endovascular stent grafting. In: Jaffe R.A., Samuels S.I., editors. Anesthesiologist’s manual of surgical procedures. ed 3. Philadelphia: Lippincott Williams and Wilkins; 2004:313-316.

4. Oliver W.C.Jr, Nuttall G.A., Cherry K.J., et al. A comparison of fenoldopam with dopamine and sodium nitroprusside in patients undergoing cross-clamping of the abdominal aorta. Anesth Analg. 2006;103:833-840.

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