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Anaesthesia for abdominal vascular surgery
VASCULAR ANAESTHESIA
Anaesthesia for abdominal vascular surgery
Learning objectives After reading this article, you should be able to: C understand the indications for surgery of the abdominal aorta C outline the rationale for full and comprehensive preassessment C describe anaesthetic techniques for open or endovascular abdominal aneurysm repair.
Richard J Telford
Abstract Surgery on the abdominal aorta is a major undertaking with a 30-day operative mortality of around 7%. Surgery is indicated to improve claudication in occlusive disease and to protect against aortic rupture in aneurysmal disease. Due to the high incidence of comorbidity in these patients successful outcome requires careful preoperative assessment and meticulous perioperative care. In this article I review the current indications and anaesthetic practices for open and endovascular aneurysm repair.
Preoperative risk assessment Abdominal vascular surgery is a major physiological insult that provokes an increase in oxygen consumption of 40%. Patients presenting for abdominal vascular surgery are frequently unfit with a high incidence of comorbidities, which may mean that they lack the physiological reserve to meet this increased demand. Major morbidity or even mortality may ensue. There is little point in performing a prophylactic operation designed to increase life expectancy if the patient succumbs to the complications of the procedure. Meticulous pre-assessment is essential to assess the risk benefit ratio of the proposed intervention and to enable informed consent to be obtained. Comorbidities include: coronary artery disease, often with impaired ventricular function hypertension pulmonary disease (often related to smoking) renal impairment diabetes mellitus. Careful pre-assessment is required to identify high-risk patients and to optimize the medical management of their comorbidities. Ideally, this should be performed by a consultant vascular anaesthetist as soon as possible after the patient is listed for surgery. The ability to exercise is an excellent indicator of cardiovascular and respiratory fitness. Patients who cannot climb a flight of stairs or walk on level ground at 6 km/hour, frequently have adverse outcomes. Other major cardiac risk factors include: recent myocardial infarction (<1 month) unstable or severe angina decompensated heart failure significant arrhythmias (high-grade atrial ventricular block, symptomatic arrhythmias or supraventricular arrhythmias with uncontrolled ventricular rate) severe valvular heart disease (aortic, mitral stenosis) recent percutaneous coronary intervention (balloon angioplasty <4 weeks, bare metal stent insertion <6 weeks or drug eluting stent insertion <1 year). Pharmacological stress tests (dipyridamoleethallium scintography and dobutamine stress echocardiography) may be used. They have low positive predictive values (20e30%), but a reassuringly high negative predictive value (95e100%) for perioperative cardiovascular complications. Patients with positive stress tests should have coronary angiography. Heart murmurs should be investigated with transthoracic echocardiography.
Keywords Abdominal aortic aneurysms; endovascular aortic aneurysm
Most abdominal vascular surgery involves surgery on the aorta for aneurysmal or symptomatic occlusive disease. Abdominal aortic aneurysms account for more than 5000 deaths per year in England and Wales alone. Most deaths due to ruptured abdominal aortic aneurysms are potentially preventable, because elective repair can be performed in the UK with an operative mortality of less than 7%. By contrast, overall mortality from ruptured abdominal aortic aneurysms is w80%, with an operative survival of 50%. A national screening programme has just been introduced, which is expected to prevent about half the deaths from ruptured aortic aneurysm. The abdominal aorta is aneurysmal when its diameter is greater than 3.0 cm. The prevalence of abdominal aortic aneurysms is rising, and is w10% and 3% in men and women aged more than 65 years. The main risk factors for developing aneurysms are advancing age, family history, smoking and hypertension.
When to operate? Small aneurysms of less than 5 cm rarely rupture. There is no survival benefit from early surgical intervention in these cases; however, patients with small aneurysms should undergo regular ultrasound scanning to monitor the aneurysm size. The estimated annual rupture rate of aneurysms larger than 6 cm is 9%, rising to more than 25% for aneurysms larger than 8 cm. The 5-year survival of patients with aneurysms greater than 5 cm that are not operated on is w20%. Current guidelines are to offer operative intervention when the aneurysm exceeds 5.5 cm. Open repair remains the gold standard treatment. Endovascular aneurysm repair (EVAR) is emerging as a minimally invasive treatment for some abdominal aortic aneurysms that are anatomically suitable.
Richard J Telford BSc(Hons) FRCA is Consultant Anaesthetist at the Royal Devon and Exeter Hospital and former Chairman of the Vascular Anaesthesia Society of Great Britain and Ireland. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
174
Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Cardiopulmonary exercise testing can help identify high-risk patients. It is performed on a cycle ergometer with respiratory gas analysis and simultaneous electrocardiography (ECG) recording. An anaerobic threshold of less than 11 ml/kg/minute, particularly if associated with ECG evidence of ischaemia, is associated with high perioperative mortality.
The severity of hypotension is proportional to cross-clamp time. Ensuring adequate cardiovascular filling and gradual release of the cross-clamp minimizes declamping hypotension. Vasopressors may be used, but have the potential disadvantage of preferential vasoconstriction of the vasculature above the clamp. Reclamping may be required in resistant hypotension.
Preoperative patient preparation
Renal system The incidence of renal failure after abdominal aortic aneurysm surgery is 5.4%, of which 0.6% require haemodialysis. Mortality exceeds 25% if renal failure occurs. Risk factors for the development of renal failure are shown in Box 1. The renal medulla receives only 20% of the total renal blood flow, making it highly susceptible to hypoxic injury due to its high oxygen-extraction rate. Maintaining an adequate circulating volume reduces the requirement for active reabsorption of salt and water, reducing medullary oxygen requirements and preventing perioperative renal dysfunction. Infrarenal cross-clamping reduces renal blood flow by up to 40% through the alteration of the renineangiotensin system. This can increase renal vascular resistance, causing maldistribution of blood flow away from the medulla and deterioration in glomerular filtration rate. Loop diuretics (for example furosemide) dopamine, mannitol, fenoldapam and N-acetylcysteine have been proposed to protect the kidney. There is no Level 1 evidence to support their use. The mainstay of renal preservation is maintenance of oxygen delivery and the avoidance of nephrotoxins (for example non-steroidal anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, aminoglycosides).
Lifestyle advice should be given to the patient e cessation of smoking and structured exercise programmes may improve cardiorespiratory fitness. All patients presenting for abdominal vascular surgery should receive antiplatelet medication to protect against thromboembolic complications. Statins should be prescribed, because they improve both short- and long-term outcomes following non-cardiac surgery due to plaque stabilization. Routine b blockade for all patients presenting for vascular surgery is associated with increased perioperative mortality. However, some extremely high-risk patients who demonstrate inducible ischaemia on pharmacological stress testing have improved outcomes if they receive carefully titrated b blockade. Modern medical treatment of coronary artery disease with aspirin and statins has negated the benefits of coronary revascularization before non-cardiac surgery. Preoperative coronary artery bypass surgery should be performed only if indicated on prognostic grounds (severe left mainstem disease or severe triple-vessel disease with impaired left-ventricular function). Prophylactic percutaneous coronary intervention has no role in improving fitness for surgery.
Pathophysiology
Spinal cord Spinal cord ischaemia is a rare but devastating complication of aortic surgery. It causes a flaccid paraparesis with dissociated sensory loss. Paraplegia occurs in less than 0.5% of infrarenal abdominal aortic aneurysm repairs; this incidence increases in emergency aortic surgery.
Cardiovascular system Cross-clamping e infrarenal aortic cross-clamping is necessary in open abdominal aortic aneurysm surgery. The increased vascular resistance results in arterial hypertension. Blood pressure typically rises by 7e10%. Cardiac filling pressure also increases because of two factors: (i) blood volume is shifted to the central veins because of reduced venous capacitance in organs distal to the clamp; (ii) a diseased coronary system may be unable to respond to increases in cardiac workload, resulting in cardiac failure. The latter may be exacerbated by the administration of too much fluid before cross-clamping. Studies show reductions of cardiac output of 9e33% after infrarenal cross-clamping. However, some patients with good cardiac performance may increase cardiac output. Vasodilators (for example glyceryl trinitrate) are effective in treating hypertension and cardiac failure, but may exacerbate organ ischaemia by reducing perfusion pressure in the collateral circulation. Patients with severe aorto-occlusive disease often have a well-developed collateral circulation and show minimal response to cross-clamping.
Risk factors for developing renal failure Pre-existing disease C Renal insufficiency C Cardiac failure C Hypertension C Diabetes mellitus C Renal artery artherosclerosis/stenosis Operative factors C Suprarenal clamp C Prolonged clamp time C Hypotension C Massive haemorrhage
Unclamping e may result in a dramatic reduction in blood pressure. The causes for this are as follows: a decrease in systemic resistance due to the removal of the cross-clamp and the release of vasoactive cytokines and metabolites from ischaemic tissues central hypovolaemia due to sequestration of blood in the reperfused organs the release of myocardial depressant factors.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
Others C Increasing age C Contrast nephropathy C Drugs (non-steroidal anti-inflammatory drugs, angiotensinconverting enzyme inhibitors, aminoglycosides)
Box 1
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Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Bowel The incidence of ischaemic colitis after abdominal aortic aneurysm surgery is 0.6%. The inferior mesenteric artery is sacrificed; therefore, bowel supplied by this artery (descending colon, sigmoid and rectum) is dependent on collateral circulation. Risk factors include suprarenal clamps, prolonged clamp times and pre-existing mesenteric artery atherosclerosis. Less severe ischaemic colitis may present with bloody diarrhoea, abdominal pain, unexplained fever or leucocytosis. Persistent postoperative metabolic acidosis with rising lactate concentration should precipitate surgical exploration.
Pulse contour analysis cardiac-output monitoring (LiDCOÔ and PiCCOÔ) is not popular in theatre. Muscle relaxants interfere with the calibration of LiDCOÔ, whereas PiCCOÔ necessitates cannulation of the brachial artery. Heparinization The administration of heparin, 100 IU/kg, before cross-clamping has been shown to reduce thrombotic and embolic events. Additional heparin may be required in the presence of prolonged clamp times. Heparin can be reversed by protamine if bleeding is thought to be due to excessive heparinization. However, protamine should be used with caution as it may lead to myocardial depression, anaphylaxis and pulmonary hypertension.
Conduct of anaesthesia Patients should receive their regular medication on the day of surgery. The aim of anaesthesia is to have a haemodynamically stable, normothermic and pain-free patient on completion of surgery. Most vascular anaesthetists in the UK use a balanced general anaesthetic technique (high-dose opioid, oxygen, air, low-dose volatile agent) with a thoracic epidural. An effective epidural helps ameliorate the stress response to surgery, reducing cardiovascular demands. Epidurals provide highquality postoperative analgesia, facilitating early extubation and reducing the incidence of pulmonary complications. There is no evidence that epidurals reduce mortality. Epidurals can safely be inserted in patients taking aspirin. The situation with clopidogrel is less clear. If an epidural is inserted in a patient taking clopidogrel, careful documentation of the risk:benefit ratio is essential. Patients should be monitored closely for the symptoms and signs of epidural haematoma (that is back pain, bladder dysfunction, leg weakness).
Temperature control Perioperative hypothermia is associated with myocardial ischaemia and dysrhythmias. It contributes to a coagulopathy and increases wound infections. Shivering can increase oxygen consumption up to sixfold, placing excessive demands on the cardiovascular system. Forced air warming devices, fluid warmers and increasing ambient theatre temperatures are used to minimize heat loss. The legs should not be actively warmed during cross-clamping. Haemorrhage and blood-product management Blood loss during abdominal aortic aneurysm surgery is highly variable. Significant bleeding can occur when opening the native aorta because of back-bleeding from the lumbar arteries. Blood loss can result from malpositioned clamps or from leaking anastomoses. Homologous blood transfusion can be minimized by acute normovolaemic haemodilution and intraoperative cell salvage. Pre-donation is not a cost-effective option in the UK. Intraoperative cell salvage provides red cells suspended in normal saline with normal concentrations of 2,3-diphosphoglycerate (free from anticoagulant). Because vascular patients have a high incidence of coronary disease, the haematocrit should be maintained at more than 27% (haemoglobin >9 g/dl). Accurate bedside haemoglobin measurements can be made by a HemocueÒ. Massive haemorrhage results in a dilutional coagulopathy, requiring fresh frozen plasma, cryoprecipitate and platelet transfusions. Appropriate administration of clotting factors is best guided by near-patient testing, using a thromboelastograph. Many centres do not have this technology and rely on laboratorybased coagulation tests, which often lag behind the clinical picture. Appropriate goals are an international normalized ratio <1.5, a platelet count >50 109/litre and fibrinogen concentrations >1 g/dl.
Monitoring In addition to standard monitoring, direct measurement of arterial and central venous pressure, temperature and urine output is mandatory. A five-lead ECG will aid detection of ST segment changes. Although most UK anaesthetists do not routinely measure cardiac output during abdominal aortic aneurysm surgery, the following may be considered in high-risk cases. Pulmonary artery flotation catheters (PAFCs) remain the gold standard, and should be considered in patients with impaired ventricular function. Haemodynamic monitoring with a PAFC in critically ill patients confers no survival benefit. By contrast, trials support their use in the high-risk surgical patient. Transoesophageal echocardiography (TOE) will detect new segmental wall motion abnormalities, which are sensitive indicators of cardiac ischaemia. Left-ventricular volume status can be directly assessed to aid optimization of cardiac output. The costs of training and equipment are high. Studies have not convincingly shown a significant reduction in morbidity or mortality when using TOE.
Postoperative care Patients require close monitoring after abdominal vascular surgery. Traditionally, this has been provided in an intensive care unit (ICU). However, careful patient selection coupled with improvements in anaesthetic and surgical techniques allow early extubation and transfer to a high-dependency unit. Early enteral nutrition is encouraged to maintain gut mucosal integrity and reduce bacterial translocation. Nasogastric tubes are not routinely required. Good glycaemic control is important. Appropriate antacid and thromboembolic prophylaxis should be prescribed, and early mobilization is encouraged.
Oesophageal Doppler monitoring can give an indication of myocardial contractility (velocity) and cardiac filling (flow time corrected for heart rate). A Doppler probe is positioned in the oesophagus to insonate the descending thoracic aorta. A derived cardiac output is calculated via the equipment software. Readings obtained are of limited value during the period of aortic cross-clamping.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
176
Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Emergency abdominal aortic aneurysm surgery
Endovascular aortic aneurysm repair has been used to treat patients deemed unfit for open repair. Randomized studies have shown that endovascular aortic aneurysm repair confers no longterm survival benefit compared with a non-operative approach in unfit patients.
Ruptured abdominal aortic aneurysm classically presents with back or abdominal pain, shock and an expanding pulsatile abdominal mass. If the patient is stable, the diagnosis can be confirmed by computed tomography (CT) scanning. ‘Contained’ ruptures that are haemodynamically stable are managed as an elective case. Shocked patients require immediate laparotomy, because aortic cross-clamping is a life-saving manoeuvre (see also article by Stoneham and Howard-Alpe: pp 187e190 of this issue).
Anaesthetic management of endovascular aortic aneurysm repair The anaesthetist should consider: the problems of anaesthesia in the angiography suite the requirement for short periods of apnoea prolonged bilateral femoral occlusion resulting in ischaemic pain the risk (1%) of conversion to an open procedure. General anaesthesia with muscle relaxation and artificial ventilation provides excellent surgical conditions. Epidural, combined spinal and epidural or continuous spinal anaesthesia are appropriate for endovascular aortic aneurysm repair. Sedation is usually required with a benzodiazepine or a target-controlled infusion of propofol. Some units have reported successful management with local infiltration by the surgeon coupled with intravenous sedation. Ischaemic leg pain is best managed with intravenous opioids (for example remifentanil <0.075 mg/kg/ minute). Relative contraindications to local anaesthetic techniques are patient anxiety, previous groin surgery and obesity (body mass index >30). Invasive blood pressure monitoring is necessary. Urinary catheterization is required as the high-contrast load may result in nephropathy. CO2 angiography has been used in some centres. In theory it obviates the need for conventional contrast. In reality the images obtained are not as clear and many use a combination of contrast and CO2 angiography. Renal protection with N-acetylcysteine can be considered, but is controversial. Wide-bore venous access is required, because the rupture of the aorta or of an iliac artery has been reported. The incidence of iliac vessel damage during EVAR is w1%; however, aortic rupture is extremely rare. Anticoagulation is recommended (heparin 5000 IU). The average time of surgery is 2e3 hours.
Endovascular aortic aneurysm repair Endovascular aortic aneurysm repair (EVAR) was developed as a less invasive alternative to open repair. Modular bifurcated stent grafts are placed via open femoral arteriotomies using 16e23 FG deployment devices. EVAR is a combined surgical and radiological procedure, which can be performed in theatre or the angiography suite. Physiological disturbance is lessened as there is no requirement for laparotomy or cross-clamping of the aorta. Patients can be transferred to the ward after a brief period of observation in a high-dependency unit. Hospital stay is reduced. Following endovascular aortic aneurysm repair there is a 65% absolute reduction in early (30-day) mortality compared with open repair. This early survival advantage must be balanced against the risk of endograft-related complications, in particular endoleaks (Figure 1), which necessitate annual CT surveillance. After 4 years, up to 20% of patients require a secondary procedure. If catheter-based therapies cannot salvage the failing endograft, an open repair may be required. This carries a high mortality due to surgical complexity and the requirement of a suprarenal clamp.
Classification of endoleaks
Patient selection for endovascular aortic aneurysm repair Patients with suitable aortic anatomy (up to 70%) can be considered for endovascular aortic aneurysm repair. Criteria for EVAR are an aneurysm neck diameter of 30 mm and a neck length of 10e15 mm. The iliac arteries must be at least 6 mm in diameter to accommodate the large deployment device. Some centres have reported EVAR rates as high as 90%. At present, endovascular aortic aneurysm repair should be reserved for fit elderly patients (aged >80 years) and those patients in whom previous abdominal surgery may make open access to the abdominal aorta difficult (‘the hostile abdomen’). Some patients may choose endovascular repair after the risk:benefit ratio of open versus endovascular repair has been discussed. A Type I (red circled areas): continued direct aortic blood flow past attachment sites (graft migration) Type II (blue circled area): collateral flow from aortic iliac branch vessels Type III (green circled areas): direct flow from loss of device integrity (graft failure)
FURTHER READING Abdominal aortic aneurysm: a service in need of surgery? www.ncepod. org.uk/reports.htm
Figure 1
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Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Hirsch AT, Haskal ZJ, Hertzer NR, et al. ACC/AHA 2005 practice guidelines for the management of patients with peripheral arterial disease. Circulation 2006; 113: e463e654. Kern JW, Shoemaker WC. Meta-analysis of hemodynamic optimization in high-risk patients. Crit Care Med 2002; 30: 1686e92.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
Poldermans D, Hoeks SE, Feringa HH. Pre-operative risk assessment and risk reduction before surgery. J Am Coll Cardiol 2008; 51: 1913e24. POISE Study Group. Effects of extended release metoprolol succinat in patients undergoing non-cardiac surgery (POISE trial): a randomised controlled study. Lancet 2008; 371: 1839e47.
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Ó 2010 Elsevier Ltd. All rights reserved.
Anaesthesia for abdominal vascular surgery
Learning objectives After reading this article, you should be able to: C understand the indications for surgery of the abdominal aorta C outline the rationale for full and comprehensive preassessment C describe anaesthetic techniques for open or endovascular abdominal aneurysm repair.
Richard J Telford
Abstract Surgery on the abdominal aorta is a major undertaking with a 30-day operative mortality of around 7%. Surgery is indicated to improve claudication in occlusive disease and to protect against aortic rupture in aneurysmal disease. Due to the high incidence of comorbidity in these patients successful outcome requires careful preoperative assessment and meticulous perioperative care. In this article I review the current indications and anaesthetic practices for open and endovascular aneurysm repair.
Preoperative risk assessment Abdominal vascular surgery is a major physiological insult that provokes an increase in oxygen consumption of 40%. Patients presenting for abdominal vascular surgery are frequently unfit with a high incidence of comorbidities, which may mean that they lack the physiological reserve to meet this increased demand. Major morbidity or even mortality may ensue. There is little point in performing a prophylactic operation designed to increase life expectancy if the patient succumbs to the complications of the procedure. Meticulous pre-assessment is essential to assess the risk benefit ratio of the proposed intervention and to enable informed consent to be obtained. Comorbidities include: coronary artery disease, often with impaired ventricular function hypertension pulmonary disease (often related to smoking) renal impairment diabetes mellitus. Careful pre-assessment is required to identify high-risk patients and to optimize the medical management of their comorbidities. Ideally, this should be performed by a consultant vascular anaesthetist as soon as possible after the patient is listed for surgery. The ability to exercise is an excellent indicator of cardiovascular and respiratory fitness. Patients who cannot climb a flight of stairs or walk on level ground at 6 km/hour, frequently have adverse outcomes. Other major cardiac risk factors include: recent myocardial infarction (<1 month) unstable or severe angina decompensated heart failure significant arrhythmias (high-grade atrial ventricular block, symptomatic arrhythmias or supraventricular arrhythmias with uncontrolled ventricular rate) severe valvular heart disease (aortic, mitral stenosis) recent percutaneous coronary intervention (balloon angioplasty <4 weeks, bare metal stent insertion <6 weeks or drug eluting stent insertion <1 year). Pharmacological stress tests (dipyridamoleethallium scintography and dobutamine stress echocardiography) may be used. They have low positive predictive values (20e30%), but a reassuringly high negative predictive value (95e100%) for perioperative cardiovascular complications. Patients with positive stress tests should have coronary angiography. Heart murmurs should be investigated with transthoracic echocardiography.
Keywords Abdominal aortic aneurysms; endovascular aortic aneurysm
Most abdominal vascular surgery involves surgery on the aorta for aneurysmal or symptomatic occlusive disease. Abdominal aortic aneurysms account for more than 5000 deaths per year in England and Wales alone. Most deaths due to ruptured abdominal aortic aneurysms are potentially preventable, because elective repair can be performed in the UK with an operative mortality of less than 7%. By contrast, overall mortality from ruptured abdominal aortic aneurysms is w80%, with an operative survival of 50%. A national screening programme has just been introduced, which is expected to prevent about half the deaths from ruptured aortic aneurysm. The abdominal aorta is aneurysmal when its diameter is greater than 3.0 cm. The prevalence of abdominal aortic aneurysms is rising, and is w10% and 3% in men and women aged more than 65 years. The main risk factors for developing aneurysms are advancing age, family history, smoking and hypertension.
When to operate? Small aneurysms of less than 5 cm rarely rupture. There is no survival benefit from early surgical intervention in these cases; however, patients with small aneurysms should undergo regular ultrasound scanning to monitor the aneurysm size. The estimated annual rupture rate of aneurysms larger than 6 cm is 9%, rising to more than 25% for aneurysms larger than 8 cm. The 5-year survival of patients with aneurysms greater than 5 cm that are not operated on is w20%. Current guidelines are to offer operative intervention when the aneurysm exceeds 5.5 cm. Open repair remains the gold standard treatment. Endovascular aneurysm repair (EVAR) is emerging as a minimally invasive treatment for some abdominal aortic aneurysms that are anatomically suitable.
Richard J Telford BSc(Hons) FRCA is Consultant Anaesthetist at the Royal Devon and Exeter Hospital and former Chairman of the Vascular Anaesthesia Society of Great Britain and Ireland. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
174
Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Cardiopulmonary exercise testing can help identify high-risk patients. It is performed on a cycle ergometer with respiratory gas analysis and simultaneous electrocardiography (ECG) recording. An anaerobic threshold of less than 11 ml/kg/minute, particularly if associated with ECG evidence of ischaemia, is associated with high perioperative mortality.
The severity of hypotension is proportional to cross-clamp time. Ensuring adequate cardiovascular filling and gradual release of the cross-clamp minimizes declamping hypotension. Vasopressors may be used, but have the potential disadvantage of preferential vasoconstriction of the vasculature above the clamp. Reclamping may be required in resistant hypotension.
Preoperative patient preparation
Renal system The incidence of renal failure after abdominal aortic aneurysm surgery is 5.4%, of which 0.6% require haemodialysis. Mortality exceeds 25% if renal failure occurs. Risk factors for the development of renal failure are shown in Box 1. The renal medulla receives only 20% of the total renal blood flow, making it highly susceptible to hypoxic injury due to its high oxygen-extraction rate. Maintaining an adequate circulating volume reduces the requirement for active reabsorption of salt and water, reducing medullary oxygen requirements and preventing perioperative renal dysfunction. Infrarenal cross-clamping reduces renal blood flow by up to 40% through the alteration of the renineangiotensin system. This can increase renal vascular resistance, causing maldistribution of blood flow away from the medulla and deterioration in glomerular filtration rate. Loop diuretics (for example furosemide) dopamine, mannitol, fenoldapam and N-acetylcysteine have been proposed to protect the kidney. There is no Level 1 evidence to support their use. The mainstay of renal preservation is maintenance of oxygen delivery and the avoidance of nephrotoxins (for example non-steroidal anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, aminoglycosides).
Lifestyle advice should be given to the patient e cessation of smoking and structured exercise programmes may improve cardiorespiratory fitness. All patients presenting for abdominal vascular surgery should receive antiplatelet medication to protect against thromboembolic complications. Statins should be prescribed, because they improve both short- and long-term outcomes following non-cardiac surgery due to plaque stabilization. Routine b blockade for all patients presenting for vascular surgery is associated with increased perioperative mortality. However, some extremely high-risk patients who demonstrate inducible ischaemia on pharmacological stress testing have improved outcomes if they receive carefully titrated b blockade. Modern medical treatment of coronary artery disease with aspirin and statins has negated the benefits of coronary revascularization before non-cardiac surgery. Preoperative coronary artery bypass surgery should be performed only if indicated on prognostic grounds (severe left mainstem disease or severe triple-vessel disease with impaired left-ventricular function). Prophylactic percutaneous coronary intervention has no role in improving fitness for surgery.
Pathophysiology
Spinal cord Spinal cord ischaemia is a rare but devastating complication of aortic surgery. It causes a flaccid paraparesis with dissociated sensory loss. Paraplegia occurs in less than 0.5% of infrarenal abdominal aortic aneurysm repairs; this incidence increases in emergency aortic surgery.
Cardiovascular system Cross-clamping e infrarenal aortic cross-clamping is necessary in open abdominal aortic aneurysm surgery. The increased vascular resistance results in arterial hypertension. Blood pressure typically rises by 7e10%. Cardiac filling pressure also increases because of two factors: (i) blood volume is shifted to the central veins because of reduced venous capacitance in organs distal to the clamp; (ii) a diseased coronary system may be unable to respond to increases in cardiac workload, resulting in cardiac failure. The latter may be exacerbated by the administration of too much fluid before cross-clamping. Studies show reductions of cardiac output of 9e33% after infrarenal cross-clamping. However, some patients with good cardiac performance may increase cardiac output. Vasodilators (for example glyceryl trinitrate) are effective in treating hypertension and cardiac failure, but may exacerbate organ ischaemia by reducing perfusion pressure in the collateral circulation. Patients with severe aorto-occlusive disease often have a well-developed collateral circulation and show minimal response to cross-clamping.
Risk factors for developing renal failure Pre-existing disease C Renal insufficiency C Cardiac failure C Hypertension C Diabetes mellitus C Renal artery artherosclerosis/stenosis Operative factors C Suprarenal clamp C Prolonged clamp time C Hypotension C Massive haemorrhage
Unclamping e may result in a dramatic reduction in blood pressure. The causes for this are as follows: a decrease in systemic resistance due to the removal of the cross-clamp and the release of vasoactive cytokines and metabolites from ischaemic tissues central hypovolaemia due to sequestration of blood in the reperfused organs the release of myocardial depressant factors.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
Others C Increasing age C Contrast nephropathy C Drugs (non-steroidal anti-inflammatory drugs, angiotensinconverting enzyme inhibitors, aminoglycosides)
Box 1
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Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Bowel The incidence of ischaemic colitis after abdominal aortic aneurysm surgery is 0.6%. The inferior mesenteric artery is sacrificed; therefore, bowel supplied by this artery (descending colon, sigmoid and rectum) is dependent on collateral circulation. Risk factors include suprarenal clamps, prolonged clamp times and pre-existing mesenteric artery atherosclerosis. Less severe ischaemic colitis may present with bloody diarrhoea, abdominal pain, unexplained fever or leucocytosis. Persistent postoperative metabolic acidosis with rising lactate concentration should precipitate surgical exploration.
Pulse contour analysis cardiac-output monitoring (LiDCOÔ and PiCCOÔ) is not popular in theatre. Muscle relaxants interfere with the calibration of LiDCOÔ, whereas PiCCOÔ necessitates cannulation of the brachial artery. Heparinization The administration of heparin, 100 IU/kg, before cross-clamping has been shown to reduce thrombotic and embolic events. Additional heparin may be required in the presence of prolonged clamp times. Heparin can be reversed by protamine if bleeding is thought to be due to excessive heparinization. However, protamine should be used with caution as it may lead to myocardial depression, anaphylaxis and pulmonary hypertension.
Conduct of anaesthesia Patients should receive their regular medication on the day of surgery. The aim of anaesthesia is to have a haemodynamically stable, normothermic and pain-free patient on completion of surgery. Most vascular anaesthetists in the UK use a balanced general anaesthetic technique (high-dose opioid, oxygen, air, low-dose volatile agent) with a thoracic epidural. An effective epidural helps ameliorate the stress response to surgery, reducing cardiovascular demands. Epidurals provide highquality postoperative analgesia, facilitating early extubation and reducing the incidence of pulmonary complications. There is no evidence that epidurals reduce mortality. Epidurals can safely be inserted in patients taking aspirin. The situation with clopidogrel is less clear. If an epidural is inserted in a patient taking clopidogrel, careful documentation of the risk:benefit ratio is essential. Patients should be monitored closely for the symptoms and signs of epidural haematoma (that is back pain, bladder dysfunction, leg weakness).
Temperature control Perioperative hypothermia is associated with myocardial ischaemia and dysrhythmias. It contributes to a coagulopathy and increases wound infections. Shivering can increase oxygen consumption up to sixfold, placing excessive demands on the cardiovascular system. Forced air warming devices, fluid warmers and increasing ambient theatre temperatures are used to minimize heat loss. The legs should not be actively warmed during cross-clamping. Haemorrhage and blood-product management Blood loss during abdominal aortic aneurysm surgery is highly variable. Significant bleeding can occur when opening the native aorta because of back-bleeding from the lumbar arteries. Blood loss can result from malpositioned clamps or from leaking anastomoses. Homologous blood transfusion can be minimized by acute normovolaemic haemodilution and intraoperative cell salvage. Pre-donation is not a cost-effective option in the UK. Intraoperative cell salvage provides red cells suspended in normal saline with normal concentrations of 2,3-diphosphoglycerate (free from anticoagulant). Because vascular patients have a high incidence of coronary disease, the haematocrit should be maintained at more than 27% (haemoglobin >9 g/dl). Accurate bedside haemoglobin measurements can be made by a HemocueÒ. Massive haemorrhage results in a dilutional coagulopathy, requiring fresh frozen plasma, cryoprecipitate and platelet transfusions. Appropriate administration of clotting factors is best guided by near-patient testing, using a thromboelastograph. Many centres do not have this technology and rely on laboratorybased coagulation tests, which often lag behind the clinical picture. Appropriate goals are an international normalized ratio <1.5, a platelet count >50 109/litre and fibrinogen concentrations >1 g/dl.
Monitoring In addition to standard monitoring, direct measurement of arterial and central venous pressure, temperature and urine output is mandatory. A five-lead ECG will aid detection of ST segment changes. Although most UK anaesthetists do not routinely measure cardiac output during abdominal aortic aneurysm surgery, the following may be considered in high-risk cases. Pulmonary artery flotation catheters (PAFCs) remain the gold standard, and should be considered in patients with impaired ventricular function. Haemodynamic monitoring with a PAFC in critically ill patients confers no survival benefit. By contrast, trials support their use in the high-risk surgical patient. Transoesophageal echocardiography (TOE) will detect new segmental wall motion abnormalities, which are sensitive indicators of cardiac ischaemia. Left-ventricular volume status can be directly assessed to aid optimization of cardiac output. The costs of training and equipment are high. Studies have not convincingly shown a significant reduction in morbidity or mortality when using TOE.
Postoperative care Patients require close monitoring after abdominal vascular surgery. Traditionally, this has been provided in an intensive care unit (ICU). However, careful patient selection coupled with improvements in anaesthetic and surgical techniques allow early extubation and transfer to a high-dependency unit. Early enteral nutrition is encouraged to maintain gut mucosal integrity and reduce bacterial translocation. Nasogastric tubes are not routinely required. Good glycaemic control is important. Appropriate antacid and thromboembolic prophylaxis should be prescribed, and early mobilization is encouraged.
Oesophageal Doppler monitoring can give an indication of myocardial contractility (velocity) and cardiac filling (flow time corrected for heart rate). A Doppler probe is positioned in the oesophagus to insonate the descending thoracic aorta. A derived cardiac output is calculated via the equipment software. Readings obtained are of limited value during the period of aortic cross-clamping.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:5
176
Ó 2010 Elsevier Ltd. All rights reserved.
VASCULAR ANAESTHESIA
Emergency abdominal aortic aneurysm surgery
Endovascular aortic aneurysm repair has been used to treat patients deemed unfit for open repair. Randomized studies have shown that endovascular aortic aneurysm repair confers no longterm survival benefit compared with a non-operative approach in unfit patients.
Ruptured abdominal aortic aneurysm classically presents with back or abdominal pain, shock and an expanding pulsatile abdominal mass. If the patient is stable, the diagnosis can be confirmed by computed tomography (CT) scanning. ‘Contained’ ruptures that are haemodynamically stable are managed as an elective case. Shocked patients require immediate laparotomy, because aortic cross-clamping is a life-saving manoeuvre (see also article by Stoneham and Howard-Alpe: pp 187e190 of this issue).
Anaesthetic management of endovascular aortic aneurysm repair The anaesthetist should consider: the problems of anaesthesia in the angiography suite the requirement for short periods of apnoea prolonged bilateral femoral occlusion resulting in ischaemic pain the risk (1%) of conversion to an open procedure. General anaesthesia with muscle relaxation and artificial ventilation provides excellent surgical conditions. Epidural, combined spinal and epidural or continuous spinal anaesthesia are appropriate for endovascular aortic aneurysm repair. Sedation is usually required with a benzodiazepine or a target-controlled infusion of propofol. Some units have reported successful management with local infiltration by the surgeon coupled with intravenous sedation. Ischaemic leg pain is best managed with intravenous opioids (for example remifentanil <0.075 mg/kg/ minute). Relative contraindications to local anaesthetic techniques are patient anxiety, previous groin surgery and obesity (body mass index >30). Invasive blood pressure monitoring is necessary. Urinary catheterization is required as the high-contrast load may result in nephropathy. CO2 angiography has been used in some centres. In theory it obviates the need for conventional contrast. In reality the images obtained are not as clear and many use a combination of contrast and CO2 angiography. Renal protection with N-acetylcysteine can be considered, but is controversial. Wide-bore venous access is required, because the rupture of the aorta or of an iliac artery has been reported. The incidence of iliac vessel damage during EVAR is w1%; however, aortic rupture is extremely rare. Anticoagulation is recommended (heparin 5000 IU). The average time of surgery is 2e3 hours.
Endovascular aortic aneurysm repair Endovascular aortic aneurysm repair (EVAR) was developed as a less invasive alternative to open repair. Modular bifurcated stent grafts are placed via open femoral arteriotomies using 16e23 FG deployment devices. EVAR is a combined surgical and radiological procedure, which can be performed in theatre or the angiography suite. Physiological disturbance is lessened as there is no requirement for laparotomy or cross-clamping of the aorta. Patients can be transferred to the ward after a brief period of observation in a high-dependency unit. Hospital stay is reduced. Following endovascular aortic aneurysm repair there is a 65% absolute reduction in early (30-day) mortality compared with open repair. This early survival advantage must be balanced against the risk of endograft-related complications, in particular endoleaks (Figure 1), which necessitate annual CT surveillance. After 4 years, up to 20% of patients require a secondary procedure. If catheter-based therapies cannot salvage the failing endograft, an open repair may be required. This carries a high mortality due to surgical complexity and the requirement of a suprarenal clamp.
Classification of endoleaks
Patient selection for endovascular aortic aneurysm repair Patients with suitable aortic anatomy (up to 70%) can be considered for endovascular aortic aneurysm repair. Criteria for EVAR are an aneurysm neck diameter of 30 mm and a neck length of 10e15 mm. The iliac arteries must be at least 6 mm in diameter to accommodate the large deployment device. Some centres have reported EVAR rates as high as 90%. At present, endovascular aortic aneurysm repair should be reserved for fit elderly patients (aged >80 years) and those patients in whom previous abdominal surgery may make open access to the abdominal aorta difficult (‘the hostile abdomen’). Some patients may choose endovascular repair after the risk:benefit ratio of open versus endovascular repair has been discussed. A Type I (red circled areas): continued direct aortic blood flow past attachment sites (graft migration) Type II (blue circled area): collateral flow from aortic iliac branch vessels Type III (green circled areas): direct flow from loss of device integrity (graft failure)
FURTHER READING Abdominal aortic aneurysm: a service in need of surgery? www.ncepod. org.uk/reports.htm
Figure 1
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Poldermans D, Hoeks SE, Feringa HH. Pre-operative risk assessment and risk reduction before surgery. J Am Coll Cardiol 2008; 51: 1913e24. POISE Study Group. Effects of extended release metoprolol succinat in patients undergoing non-cardiac surgery (POISE trial): a randomised controlled study. Lancet 2008; 371: 1839e47.
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