A 71-year-old man with a 6-cm infrarenal aortic aneurysm presented for conventional aortic repair. His past medical history was significant for hypertension, stable angina, hypercholesterolemia, and smoking.
Explain the natural history of aortic aneurysms.
The natural history of all aneurysms is to expand in size. The tendency to rupture primarily depends on wall stress. With increases in aneurysm diameters, the wall stress increases as described by LaPlace law, increasing the risk of rupture:
Wall stress = pressure × radius 2 × wall thickness
The average growth rate of aortic aneurysms is 0.4 cm per year. In a follow-up study of high-risk patients, the overall rupture rate was 3%, with a surgical mortality (elective surgery for aneurysms >6 cm or symptomatic aneurysms) of 4.9%. Approximately 34% of deaths were due to causes unrelated to the aneurysm. The 5-year survival rate for untreated abdominal aortic aneurysms greater than 6 cm was less than 10%, and the 5-year survival rate for untreated abdominal aortic aneurysms less than 6 cm was 50%. Elevated diastolic blood pressure, aneurysm anteroposterior diameter greater than 5 cm, and obstructive pulmonary disease were independent predictors of rupture. Predicted 5-year rupture rates ranged from 2% when these risk factors were absent to 100% when all three risk factors were present.
How is a patient with an aortic aneurysm evaluated preoperatively?
Nearly all patients presenting for aortic surgery have coexisting medical conditions that can significantly affect anesthetic management. Problems include diseases of the cardiovascular, pulmonary, renal, and central nervous systems. The goal of preoperative evaluation is to detect coexisting diseases, assess the risk of adverse outcomes, optimize the patient’s medical status, and devise an anesthetic technique that minimizes complications. It is not always possible to perform a complete preoperative evaluation when surgery is required on an urgent basis, so preoperative optimization of the patient is not always feasible.
It is imperative to evaluate myocardial reserves before aortic surgery. Risk factors for myocardial ischemia include history of previous myocardial infarction, angina, congestive heart failure, male gender, smoking, hypercholesterolemia, diabetes mellitus, and limited exercise tolerance. Patients at low risk for myocardial ischemia may proceed to surgery without further evaluation; this represents a very small subset of patients with aortic aneurysms. Patients with a negative stress test within 2 years of surgery or who have had coronary artery bypass graft surgery without postoperative symptoms probably do not require further work-up for myocardial ischemia. Although stress testing is probably most appropriate for patients with moderate risk, coronary angiography is recommended for patients at high risk for myocardial ischemia.
There are two components to stress testing: “stressing” the myocardium and detection of myocardial ischemia or infarction. “Stressing” is performed by either mechanical (e.g., exercise via treadmill or hand-crank) or pharmacologic means. Pharmacologic stress may involve drugs such as dobutamine that increase myocardial oxygen demand or drugs such as dipyridamole that cause “myocardial steal.” Myocardial ischemia is detected by electrocardiogram (ECG), nuclear studies, or echocardiography. Ischemic myocardium is characterized by changes in ST segment elevations that occur with exercise. On nuclear studies, the absence of nuclear tracer uptake during stress and uptake of nuclear tracers with rest is known as a reversible defect. Fixed defects (i.e., absence of uptake during stress and rest) is consistent with old infarction. Ischemic myocardium is characterized on echocardiography as significant change in wall motion during “stressing.” Myocardial segments that remain akinetic both during rest and during exercise may be assumed to be infarcted. In each of these studies, it is important to determine whether there is myocardium at risk (i.e., myocardium that may become ischemic with stress) as opposed to myocardium that is infarcted. If patients have significant areas of myocardium at risk, further optimization (either pharmacologic or interventional) is warranted before surgery ( Box 34-1 ).