Elective aorto-femoral arterial revascularization
This operation is performed to improve the arterial circulation to the lower limbs when significant occlusive disease affects the distal aorta and/or the iliac arteries. It is a major surgical procedure which involves a laparotomy and bilateral groin operations. A prosthetic graft is anastomosed to the infrarenal aorta and femoral arteries to bypass areas of arterial stenosis or occlusion.
Risks of aorto-bifemoral grafting include haemorrhage, coagulopathy, infection, major organ dysfunction (myocardial ischaemia, cerebrovascular accident, renal impairment, ARDS), limb loss, impotence, and ischaemia of the pelvic contents. The risks of an MI or death within 30 days of surgery exceed 5%.
1 Perform a detailed cardiac risk profiling prior to the proposed vascular surgery
2 Optimize medical therapies prior to elective surgery.
2A03; 2A07
A 71-year-old retired taxi driver presents with a history of left thigh claudication that prevents him from walking >75 m. He has already been assessed by a vascular surgeon who arranged for a magnetic resonance angiogram to be performed. This revealed areas of significant stenosis in his left common iliac artery and more minor stenosis in his distal aorta. The vascular surgeon has asked you to assess his fitness for aorto-bifemoral bypass surgery.
He has type II diabetes, controlled hypertension, and a history of angina in the past. He is overweight with a BMI of 38 kg/m2. He continues to smoke 15 cigarettes a day. Five years ago, coronary angiography was performed following a non-ST elevation MI. This demonstrated a tight stenosis in the proximal circumflex artery and 50% stenoses in the left anterior descending and right coronary arteries. The circumflex lesion was treated by the insertion of a bare metal stent. He does not currently suffer from any angina. He has no history of cerebrovascular disease.
His medications include aspirin, ramipril, and insulin. His renal function is impaired (Cr 200 micromoles/L). A 12-lead ECG shows sinus rhythm, with no evidence of ischaemic changes. Resting echocardiography shows no valvular abnormalities, and LV systolic function is within normal limits.
Aorto-bifemoral bypass grafting is a major surgical procedure that is associated with a significant risk of cardiovascular complications or death in the perioperative period. This risk is much higher in patients who have co-existing diseases.
A number of alternative lower-risk management strategies exists for this condition. The anaesthetist can play an important role in estimating the level of risk for individual patients. This information can then be used to inform decisions regarding the most appropriate choice of treatment.
Alternative management strategies that entail lower risks (which may be more appropriate for less fit patients) include conservative treatment; this would include the optimization of medical therapy by adding a statin to the current medications, smoking cessation strategies, and exercise, ideally as part of a structured exercise plan. Endovascular treatment (angioplasty and stent insertion) may be possible for short stenoses within the iliac arteries. Alternative surgical procedures that are less physiologically stressful (and associated with lower rates of perioperative complications/death) involve extra-anatomical grafts and include axillo-femoral (or axillo-bifemoral) and femoro-femoral cross-over grafts.
The revised cardiac risk index (RCRI)* is a simple clinical risk-scoring system that was derived and validated for the prediction of major adverse cardiovascular events (MACE) after surgery. It is currently considered to be one of the best evidence-based clinical risk-scoring systems and is utilized by both European and American preoperative assessment guidelines to help predict cardiac risk in the perioperative period.
One point is allocated for:
◆ History of ischaemic heart disease
◆ History of congestive heart failure
◆ History of cerebrovascular disease
◆ Insulin therapy for diabetes mellitus
◆ Renal impairment (Cr >177 micromoles/L)
◆ High-risk surgery (intraperitoneal, intrathoracic, or suprainguinal vascular).
Rates of major cardiac events in the original validation cohort with 0, 1, 2, or ≥3 risk factors were 0.4%, 0.9%, 7%, and 11%, respectively.
A number of methods can be used to assess the perioperative risk, in addition to clinical risk-scoring systems.
Attempts should be made to quantify exercise tolerance in terms of METs. METs are multiples of the basal metabolic requirement for oxygen (3.5 mL/kg/min). Lists of common activities and their associated METs are available. For example, carrying out light housework requires around 4 METs. This value is important, as being unable to carry out activities that require 4 METs is associated with increased risk, both in the perioperative period and longer term. If possible, it is always better to make an objective assessment of the functional capacity, rather than rely on the patient’s estimates. There are a number of different ways in which this can be done; one of the simplest is to walk up stairs with the patient and observe how they manage this activity, noting any symptoms experienced and how many flights are climbed. The ability to climb two flights of stairs requires between 4 and 5.5 METs, whilst being unable to climb two flights of stairs is associated with increased risks of perioperative morbidity. Another alternative test that can be considered is the six-minute walk test. This measures the distance that can be covered in 6 min over a flat surface. The obvious limitation of any of these functional assessments is that patients with severe peripheral arterial disease will almost certainly be limited by their claudication symptoms.
This is the gold standard method for objectively assessing exercise capacity. It is now commonly used by anaesthetists as part of the preoperative assessment of patients prior to major surgery. CPET (or CPX) is an integrated test of cardiorespiratory function where a period of graded exercise is performed on either a bicycle ergometer or treadmill. A number of parameters, including peak oxygen consumption (VO2 peak), oxygen consumption at the anaerobic threshold (AT), and ventilatory equivalents for oxygen and carbon dioxide, can be determined accurately during the test. In addition, 12-lead ECG monitoring and ST segment analysis take place to detect any exercise-induced arrhythmias, or myocardial ischaemia. A low VO2 peak, low AT, raised ventilatory equivalents for carbon dioxide, and evidence of myocardial ischaemia, in association with a low AT, have all been associated with increased risks of mortality after major surgery.
This involves performing conventional transthoracic echocardiography at the same time as infusing increasing doses of dobutamine intravenously. The presence and severity of any new ventricular wall motion abnormalities are noted during the period of inotropic and chronotropic stress. The number of segments of the LV that function abnormally can be correlated with the risk of perioperative cardiac complications. The test has a high negative predictive value (90–100%) for perioperative cardiac complications.
This involves IV injection of thallium (a nuclear tracer), which is taken up by the myocardium, and dipyridamole (which induces myocardial stress by causing vasodilatation and tachycardia). Areas of the myocardium with a poor blood supply during stress can be identified by poor thallium uptake. The reversibility of ischaemia can be assessed by repeating the scan after a number of hours when the tachycardia has resolved and the thallium uptake is complete. This test has a similar high negative predictive value for perioperative cardiac complications.
Elevated brain natriuretic peptide (BNP) levels have been shown to be associated with cardiac complications following major surgery. However, at present, there is no consensus regarding optimal cut-off values for use in preoperative risk stratification.
Coronary angiography may be appropriate for selected patients being considered for major vascular surgery—if the patient is suffering from severe angina or has a strongly positive stress test or develops extensive new wall motion abnormalities during stress echocardiography. In these situations, the aim is to identify those patients with very significant coronary artery disease (left main stem or severe triple-vessel disease) whose long-term prognosis may be improved by performing coronary artery bypass grafting (CABG). However, it should be noted that CABG should not be performed routinely in patients with stable coronary artery disease prior to vascular surgery, as this approach does not improve clinical outcomes. Furthermore, it must be emphasized that percutaneous coronary intervention (PCI) with stent insertion is associated with worse outcomes, compared to optimal medical treatment of coronary artery disease, when performed immediately prior to major surgery. This is due to the high incidence of within-stent thrombosis that can occur secondary to the inflammatory and procoagulant effects of major surgery. In addition, there is a requirement for continuous dual antiplatelet therapy after stent insertion (for 3 months after a bare metal stent insertion and 12 months after a drug-eluting stent insertion)—this may have consequences for both haemostasis and regional anaesthesia.
This patient should be on long-term statin therapy for a number of reasons—to reduce the likelihood of future cardiovascular events and death, and to slow the progression of peripheral arterial disease and improve his claudication distance. In addition, when taken around the time of major vascular surgery, statin therapy reduces the risk of death or cardiac complications. These beneficial ‘pleiotropic’ effects of statins may result from an increased atherosclerotic plaque stability caused by reduced levels of vascular inflammation, matrix metalloproteinases, and lipid oxidation. Statins should be continued throughout the perioperative period, as the withdrawal of statin therapy may increase the likelihood of cardiovascular complications by a ‘rebound’ effect.
The literature concerning the role of β-blockers in reducing perioperative risk is somewhat confusing. Early studies of β-blockers in patients with, or at risk of, ischaemic heart disease demonstrated significant reductions in the risk of cardiac death or non-fatal MI after major surgery. These studies prompted the widespread use of β-blockers in patients undergoing vascular surgery. However, a number of studies have since been published that have cast doubt on the appropriateness of this approach. Most recently, the POISE study, a large randomized, placebo-controlled study of the effects of metoprolol in patients undergoing major non-cardiac surgery, has confirmed that perioperative β-blockade does reduce the incidence of MI, but this is at the expense of an increase in all-cause mortality and an excess of cerebrovascular accidents. It is postulated that the increased risk of morbidity and mortality observed in this study relates to hypotension caused by the large doses of metoprolol that were used and the timing of the start of therapy immediately prior to surgery. Notwithstanding the limitations of this study, it has raised concerns that β-blockers may not be appropriate for all patients undergoing vascular surgery. Currently, β-blockers are recommended for patients at the highest risk around the time of vascular surgery (these are patients with evidence of myocardial ischaemia on preoperative stress testing) or for those with multiple (≥3) clinical risk factors. It is also strongly recommended that chronic β-blocker therapy should be continued and not be withdrawn acutely, as this has been shown to lead to rebound increases in myocardial ischaemia and mortality.
This patient should also be on long-term β-blocker therapy as a secondary preventative measure post-MI. In addition, he has multiple clinical risk factors. The optimal time to start therapy remains to be clearly defined, but, by starting some days prior to surgery (ideally up to 30 days), there is ample time to titrate the dose to achieve HRs within the range of 60–70 bpm and avoid hypotension. Currently, β1 selective agents with no intrinsic sympathomimetic activity, such as bisoprolol, are considered the best drugs to use. Finally, it is worth emphasizing that β-blockers are not contraindicated in patients with claudication or chronic obstructive pulmonary disease (COPD).
All patients with peripheral arterial disease should be treated with an antiplatelet drug (most commonly aspirin) to reduce their long-term risk of cardiovascular complications. Historically, antiplatelet therapy was stopped prior to elective surgery to reduce the risk of haemorrhage. However, it is now appreciated that one of the factors contributing to a perioperative MI is the prothrombotic state that develops around the time of surgery. It has been suggested that antiplatelet therapy may reduce the likelihood of perioperative myocardial ischaemia by altering this procoagulant state. Indeed, the withdrawal of aspirin therapy in the perioperative period is associated with an increased incidence of MACE, and, whilst the incidence of bleeding complications may be increased by a factor of up to 1.5 by continuing aspirin, the severity of these complications is not increased. Therefore, it has been suggested that aspirin should only be discontinued if the risks of bleeding outweigh the potential cardiac benefits.
This patient scores four RCRI points and so is in a higher risk category. He is currently complaining of claudication symptoms, which may have a significant impact on his lifestyle but are neither life nor limb-threatening. These important facts must be borne in mind when weighing up the potential benefits and risks of any treatment strategy. Many surgeons, given the non-limb-threatening ischaemia that he is experiencing, would not proceed with aortic surgery in this situation. Alternative options are conservative management that includes exercise and formal smoking cessation strategies or to proceed with a less major form of revascularization treatment such as a right femoral to left femoral artery cross-over graft.
Summary
Reconstructive aortic surgery for aorto-iliac occlusive disease is a very major surgical undertaking, with risks that exceed those associated with an elective aortic aneurysm repair. The anaesthetist should play a significant role in the preoperative assessment of patients being considered for this type of surgery. They should ensure that all co-existing diseases have been optimized and that appropriate secondary preventative medications have been started in advance of any surgical procedure. They should also play an active role in the multidisciplinary team (surgeons, radiologists, and anaesthetists) that discusses treatment options and decides on the optimal treatment plan.
Fleisher LA, Beckman JA, Brown KA, et al. (2007). ACC/AHA 2007 Guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery: a report of the American College of Cardiology/American Heart Association task force on practice guidelines (writing committee to revise the 2002 guidelines on perioperative cardiovascular evaluation for noncardiac surgery). Circulation, 116, e418–99.
Lee TH, Marcantonio ER, Mangione CM, et al. (1999). Derivation and prospective validation of a simple index for prediction of cardiac risk of major noncardiac surgery. Circulation, 100, 1043–9.
POISE study group, Devereaux PJ, Yang H, Yusuf S, et al. (2008). Effects of extended-release metoprolol succinate in patients undergoing non-cardiac surgery (POISE trial): a randomised controlled trial. Lancet, 371, 1839–47.
* Data from Lee, T. H. et al., 1999, 'Derivation and prospective validation of a simple index for prediction of cardiac risk of major noncardiac surgery', Circulation, 100, 10, pp. 1043–1049.