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Anaesthesia for abdominal vascular surgery
BLOOD CELLS AND COAGULATION
Anaesthesia for abdominal vascular surgery
Overall survival of patients given early surgery compared with ultrasound surveillance in the management of small aneurysms 1.0
Proportion of patients surviving
Fiona Gamlin
Surgery on the abdominal aorta may be required for aneurysmal dilatation or for symptomatic occlusive atherosclerosis. The risks of non-surgical management of aortic aneurysm depend on aneurysm diameter. Aneurysms greater than 6 cm have an average risk of rupture within 1 year of about 25%. Larger aneurysms expand more rapidly, further increasing the risks, therefore the greater the diameter the greater the risk. In patients with aneurysms smaller than 5 cm, there seems to be no benefit to long-term survival from early surgical intervention (Figure 1). The operative mortality figures for elective aortic surgery are 2–10%, reflecting the higher incidence of associated risk factors, especially in the occlusive atherosclerosis group: • ischaemic heart disease • renal impairment • increasing age • diabetes mellitus • obstructive pulmonary disease • hypertension • smoking. The principal causes of perioperative morbidity and mortality are myocardial ischaemia or infarction, organ failure and respiratory complications. Although improvements in perioperative monitoring and management have reduced morbidity and mortality in elective surgery, the mortality from emergency surgery remains high at about 45% for ruptured aortic aneurysm. The risks of elective or emergency surgery increase dramatically in patients over the age of 80 years.
0.8
Early surgery 0.7
Surveillance 0.6 0
1
2
3
4
5
6
125 187
52 63
Survival time (years) Number at risk Surveillance Early surgery
527 563
497 513
468 489
412 450
229 302
Source: UK Small Aneurysm Trial Participants. Mortality results for randomised controlled trial of early elective surgery or ultrasonographic surveillance for small abdominal aortic aneurysms. Lancet 1998; 352 (9141): 1649–55.
1
General anaesthetic considerations Patients should continue to take regular cardiac medication up to and including the day of surgery. Admission to a high-care area for invasive monitoring and optimization of oxygen delivery in high-risk patients has been shown to improve outcome, but few units have the capacity to achieve this and the benefits probably apply only to those in high-risk groups. The aims of anaesthesia are to avoid exaggerated responses to surgical stimulation, reduce myocardial work and therefore oxygen demand, and maintain cardiac output and oxygen delivery to the tissues. Additional goals include attenuation of the stress response to surgery, preservation of renal function and the provision of postoperative analgesia. Various general anaesthetics can be used, most having some benefits appropriate to aortic surgery (Figure 2), however, many units employ a combination technique of opioid, oxygen, air and low-dose volatile agent to optimize the beneficial effects while minimizing side-effects. The addition of regional anaesthesia, in the form of a thoracic epidural, has several beneficial effects. There is a reduction in haemodynamic instability, the stress response to surgery, and general anaesthetic requirement. There is an increase in endocardial blood flow, and the high-quality analgesia postoperatively reduces the incidence of respiratory and other complications. The main problem related to the use of a thoracic epidural in vascular surgery is the requirement for heparinization for crossclamping, and therefore the potential risk of spinal haematoma. Reports from large numbers of patients suggest that the technique is safe and that the potential benefits outweigh the risks, however, there should be at least 1 hour between catheter insertion and heparin administration. It is also recommended that an epidural
Abdominal aortic surgery Preoperative assessment and optimization Assessment of patients for abdominal vascular surgery is aimed at identifying the risk factors outlined above, and ameliorating them when possible. The most significant factor affecting outcome is perioperative myocardial ischaemia and failure. In addition to baseline ECG and chest radiography, most patients warrant further evaluation of cardiac function (e.g. echocardiography and/or stress testing). The medical management of hypertension, ischaemia or cardiac failure should be optimized before surgery. There is evidence that patients with ischaemic heart disease benefit from perioperative β-blockade, which results in a reduction in episodes of perioperative silent myocardial ischaemia and adverse events.
Fiona Gamlin is Consultant Anaesthetist at St James’s University Hospital, Leeds, UK. She qualified from Newcastle University, and trained in anaesthesia in Newcastle and Leeds. Her interests are perioperative cardiovascular monitoring and the modification of cardiovascular risk factors.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
0.9
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© 2004 The Medicine Publishing Company Ltd
BLOOD CELLS AND COAGULATION
Blood preservation Blood loss during abdominal vascular procedures increases with the complexity of the surgery and with surgery involving a more proximally sited aortic cross-clamp. Transfusion is often recommended if the haematocrit falls below 30% (0.30), and there is some evidence that levels below 28% (0.28) are associated with a higher incidence of myocardial ischaemia in patients with vascular disease. Homologous blood transfusion can be minimized by pre-donation, haemodilution and intraoperative cell salvage. Cell salvage allows for the re-infusion of fresh RBCs with normal concentrations of 2,3-diphosphoglycerate, and without anticoagulant, acid–base or electrolyte disturbance. Wide-bore cannulae, either peripheral or central, are required to allow rapid transfusion of blood and other fluids, preferably warmed. If severe haemorrhage is anticipated, rapid infusion systems capable of delivering warmed fluids should be available. Large-volume blood loss and transfusion result in coagulopathy, requiring fresh frozen plasma and platelet transfusion. Further coagulopathy may occur following gut ischaemia in supra-coeliac clamping, especially if the ischaemic time exceeds 30 minutes. Laboratory coagulation tests, bench-top activated clotting time and thromboelastography can all be used to guide appropriate use of clotting factors. Fibrinolysis may be a problem in complicated or supra-coeliac cases, requiring the use of antifibrinolytic agents (e.g. aprotinin, tranexamic acid).
General anaesthesia for use in aortic surgery Advantages Disadvantages Opioids • Cardiovascular system stability • Prolonged postoperative • Analgesia respiratory depression • Reduced stress response Volatile agents • Reduced afterload (vasodilator) • Reduced myocardial • Reduced myocardial oxygen contractility consumption • Reduced diastolic pressure/ • Titratable coronary blood flow • Coronary steal at higher minimum alveolar concentration NItrous oxide • Rapid onset and offset • Myocardial depression • Analgesia Propofol (total intravenous anaesthesia, target-controlled infusion) • Titratable • Reduced myocardial • Reduced afterload (vasodilator) contractility • Postoperative sedation 2
Temperature control Mild hypothermia is associated with vasoconstriction, tachycardia, hypertension and increased myocardial oxygen requirement. In addition, blood viscosity is increased and coagulation adversely affected. Maintenance of normothermia is associated with less biochemical disturbance, improved clinical variables and less myocardial ischaemia. Forced-air warming devices are more effective than a circulating mattress, and all infused fluids should be warmed. The area below the cross-clamp is not actively warmed while the clamp is in place.
should not be placed less than 2 hours before or 12 hours after administration of low molecular weight heparin (LMWH). Monitoring Usual baseline monitoring is required, with the addition of invasive arterial and central venous blood pressure, core temperature and urine output. A five-lead ECG, displaying leads II and V5, is more sensitive than a standard three-lead ECG for the detection of myocardial ischaemia. Pulmonary artery flotation catheters are not beneficial for routine use in abdominal vascular surgery except in patients with poor left ventricular function. In low-risk patients their use is associated with an increase in fluid administration with no improvement in outcome, and it may increase the morbidity associated with excessive fluid. Transoesophageal echocardiography allows real-time monitoring of ventricular filling ejection fraction and cardiac output. Ventricular segmental wall motion abnormalities can be visualized and are more sensitive markers of ischaemia than ECG changes. However, the equipment is expensive and requires a skilled operator. Oesophageal Doppler monitoring is a relatively non-invasive technique for assessing cardiac output. The results are variable if used by an inexperienced operator, and are difficult to interpret when the aortic clamp is in place. In this situation, trends can give valuable information but absolute figures may be inaccurate. Non-invasive cardiac output (NICO) is a technique for assessing cardiac output that utilizes partial rebreathing of carbon dioxide to calculate the cardiac output and some respiratory variables. During aortic surgery, NICO shows an acceptable correlation with thermodilution, although immediately following reperfusion the bias is increased.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
Fluid management In addition to blood loss, there is evaporation from abdominal contents, and also third-space fluid loss, especially into the retroperitoneal space. Replacement with crystalloid or colloid is titrated against central venous or pulmonary artery occlusion pressure, and regular electrolyte analysis is required to detect and correct disturbances of acid–base balance, potassium, calcium and magnesium. The use of Hartmann's solution, rather than normal saline, reduces the development of acidosis. Although there is no evidence of increased incidence of adverse outcomes, the requirement for bicarbonate and blood product transfusion may be reduced. Aortic cross-clamp The placement of an aortic cross-clamp has significant effects on the cardiovascular and neuroendocrine system, the severity increasing the more proximally the clamp is placed. Blood flow to vital structures, including the kidneys, spinal cord and gastrointestinal tract, may be interrupted if supra-renal or supra-coeliac clamping is required. The cardiovascular effects of cross-clamping are less severe in occlusive aortic disease owing to the development of collateral circulation.
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BLOOD CELLS AND COAGULATION
Cardiovascular effects and management: following placement of an aortic cross-clamp, systemic vascular resistance rises causing an increase in mean arterial pressure and left ventricular afterload, an increase in left ventricular end-systolic pressure and wall tension, and a reduction in ejection fraction and cardiac output of up to 40%. The reduction in cardiac output is greater in patients with inadequate intravascular volume, and the cardiovascular changes in general are more severe in supra-coeliac clamping when the splanchnic circulation is interrupted. Patients with normal coronary arteries and myocardial function tolerate these changes and maintain cardiac contractility reasonably well. However, in patients with ischaemic heart disease, the combination of increased myocardial workload and greater intraventricular pressures inhibiting subendocardial blood flow may lead to myocardial ischaemia. This further affects cardiac contractility, central venous and left atrial pressures rise, and left ventricular failure and dysrhythmias may ensue. Before placing the clamp, the patient requires adequate intravascular filling. Afterload reduction is usually achieved by increasing the depth of anaesthesia with a volatile agent. Nitrates, including glyceryl trinitrate and sodium nitroprusside, are generally required only in compromised or emergency patients in whom the additional preload reduction and improved coronary blood flow are beneficial. Inotropic agents may be required if cardiac output is not improved with normalized preload and afterload. During cross-clamping the period of ischaemia causes vasodilatation distal to the clamp, and the accumulation of H+, lactate and other vasoactive metabolites. The release of these agents into the circulation on reperfusion in combination with vasodilatation results in low systemic vascular resistance, reduced venous return and reduced cardiac output. Acidosis and metabolites may directly depress the myocardium and, if hypotension is severe, low coronary blood flow exacerbates the problem. A combination of adequate filling to a central venous pressure above 10 cm H2O or pulmonary artery wedge pressure of 12–15 before de-clamping, and the use of vasoconstrictors and inotropes help to reduce cardiovascular instability. Surgical techniques that minimize the effects are short cross-clamp times and careful, slow release of the clamp.
Risk factors for perioperative renal failure Preoperative renal impairment • Renal artery atherosclerosis • Hypertension • Diabetes mellitus • Contrast toxicity • Heart failure Age Dehydration • Osmotic diuresis from intravenous contrast • Preoperative starvation Operative factors • Suprarenal cross-clamp • Hypotension • Hypovolaemia • Large-volume blood transfusion
3
factors in the prevention of renal impairment are the maintenance of adequate intravascular volume (pulmonary artery wedge pressure 12–15) and cardiac output, and the avoidance of hypotension. If oliguria persists despite these precautions, various agents are used to re-establish or maintain urine output (Figure 4). Although these agents increase urine output, and have theoretical benefits, they have not been shown to improve renal function or outcome when compared with volume expansion alone. Most units employ one or more of these techniques to augment urine output during surgery, especially if a supra-renal clamp is required. Spinal cord ischaemia: normally the spinal cord receives most of its blood supply via the anterior spinal artery. This artery receives blood from the aorta via medullary branches, the most important being the artery of Adamkiewicz, which generally arises between T8 and T12, but may arise anywhere between T1 and L1. Distal supply is via the iliac vessels. Interruption to this blood supply may result in spinal cord ischaemia and a flaccid paralysis, which may be transient or develop into long-term paraplegia. The overall incidence is low (about 0.25%) in aortic aneurysm surgery, but increases in supra-renal or thoraco-abdominal procedures, and in emergency surgery. The most important factors in the preservation of spinal cord function are avoiding prolonged periods of cross-clamp (over 30 minutes), and the identification, preservation and reimplantation of critical vessels. Anaesthetic considerations are principally the maintenance of adequate blood pressure proximal to the clamp to optimize anterior spinal artery perfusion, and the avoidance of hyperglycaemia. Various techniques have been advocated for higher-risk supra-renal surgery including: • somatosensory evoked potentials for the early detection of ischaemia • cord cooling via an epidural catheter • CSF drainage to improve spinal artery perfusion pressure • corticosteroid administration to reduce cord oedema • magnesium administration as a free-radical scavenger. However, there is no clear evidence of improved outcome.
Renal preservation: renal impairment is a significant cause of perioperative morbidity and mortality in abdominal aortic surgery. Mortality increases in line with preoperative creatinine levels. Up to 10% of patients have preoperative renal impairment, and the overall rate of perioperative renal failure in aortic surgery is of the order of 4.5%. Mortality associated with renal failure exceeds 25%. Risk factors for the development of renal failure are shown in Figure 3. Although suprarenal clamping is associated with much higher rates of renal complications (up to 50%), all levels of crossclamp impair renal blood flow. The combination of increased vascular resistance, reduced cardiac output, activation of the renin–angiotensin system and sympatho-adrenal stimulation all serve to impair renal blood flow even with infra-renal cross-clamp. There may also be intra-renal blood flow redistribution away from the medulla, increasing the risk of medullary ischaemia. Oliguria is a poor predictor of renal dysfunction, because many patients demonstrate a reduction in urine output without developing renal impairment. Similarly, maintaining urine output has not been shown to improve outcome. However, it is the only perioperative monitor of renal function available. The most important
ANAESTHESIA AND INTENSIVE CARE MEDICINE
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BLOOD CELLS AND COAGULATION
Beneficial effects of drugs affecting renal function
Theatre preparation for emergency repair of abdominal aortic aneurysm
Mannitol • Plasma expansion • Improved cortical blood flow • Osmotic diuresis • Free-radical scavenging
Equipment • Rapid fluid infusion/warming device • Cell saver • Defibrillator • Forced warming device (upper body)
Loop diuretic (e.g. furosemide) • Diuresis • Sodium excretion
Drug preparation • Induction drugs • Vasopressors Ephedrine, 6 mg/ml Epinephrine, 1:100,000 Phenylephrine, 100 µg/ml • Infusions Dopamine, 4 mg/ml Epinephrine, 100 µg/ml Glyceryl trinitrate, 1 mg/ml • Extras Calcium chloride 10% Atropine, 1 mg in 10 ml Sodium bicarbonate 1.4% Mannitol 10 or 20%
Dopamine • Decreased systemic vascular resistance, increased cardiac output, β-adrenergic effects • Increased renal blood flow, dopaminergic effects • Increased glomerular filtration rate and urine output 4
Emergency abdominal aortic surgery Presentation: emergency abdominal aortic surgery is primarily for symptomatic, leaking or ruptured aneurysm. A symptomatic aneurysm may present with back pain or abdominal pain or tenderness, and requires surgery urgently because there is a high risk of rupture. CT scanning may reveal a contained leak, in which case the management is as for ruptured aneurysm. If the patient shows no evidence of cardiovascular instability and there is little likelihood of a contained leak then he can be managed as for an elective repair. The mortality for urgent repair of symptomatic aneurysm is significantly higher than for elective repair. The classical presentation of a ruptured aneurysm includes back or abdominal pain, shock and a pulsatile abdominal mass.
5
catheter, nasogastric tube and temperature probe. A pulmonary artery catheter may also be considered, however, this is often not practical in a busy situation. Although the homeostatic derangement is much more severe, the general themes of intraoperative management are as for elective repair. The main problems are coagulopathy, requiring early transfusion of clotting products, cardiovascular instability including myocardial ischaemia, and renal injury. In addition, the response to re-perfusion on clamp removal is often exaggerated owing to the additional period of poor tissue perfusion before cross-clamping. Postoperative complications are increased following emergency surgery, and prolonged ICU stays are usual. Predictors for survival to discharge include patient age, the total volume blood product transfusion, duration of surgery, postoperative hypotension and day 1 APACHE scores.
Perioperative management of ruptured aneurysm: the main aim in the management of acute aneurysm rupture is to control blood loss by cross-clamping the aorta as soon as possible. Preoperative resuscitation should be limited to maintaining vital organ function (i.e. cerebral and coronary perfusion). Excessive fluid resuscitation may aggravate the situation by raising the blood pressure and increasing blood loss. Priorities are good venous access with at least two wide-bore cannulae, blood sampling for baseline blood testing, including coagulation, and cross-match of at least 10 units of blood, and rapid transfer to theatre. Two anaesthetists and two operating department practitioners are needed to prepare the patient, theatre equipment and drugs with a minimum of delay (Figure 5). After taking a brief history and attaching non-invasive monitoring, the patient should be prepared and draped before induction. The loss of abdominal muscle tone and sympathetic tone, in conjunction with the effects of anaesthetic drugs and mechanical ventilation may result in a catastrophic fall in blood pressure. Invasive arterial pressure monitoring for induction is useful if access can be achieved without delay. A rapid-sequence induction is used. Many anaesthetists choose a high-dose opioid technique to minimize cardiovascular effects, including hypertension on intubation, which may increase bleeding. Surgery is started as soon as the airway is secure, and blood pressure is maintained at reasonable levels with the rapid infusion of fluids including blood, and the use of pressor agents. Heparin is not required, and once the cross-clamp is in place, a period of relative stability allows for further fluid resuscitation, and the insertion of a central venous
ANAESTHESIA AND INTENSIVE CARE MEDICINE
Endovascular aortic stent procedures The technique of endovascular stent repair of aortic aneurysm via the femoral artery was first described in 1991. If the procedure is technically possible, it is associated with less physiological disturbance, morbidity and mortality than open procedures. There is a considerable reduction in the level of stress hormone release and in nutritional derangement compared with open repair. Early (30 day) mortality in those thought fit for open repair is reported as 2.3% in the Eurostar database, and as 12.5% in those unfit for open repair, however, the relatively high potential for late complications increases late mortality. The rate of conversion to open repair is low (5%), but conversion is associated with high mortality. There is no clear evidence of the long-term effectiveness of stent procedures, and they are generally limited to patients considered unfit for conventional surgery, or to those included in randomized controlled trials.
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BLOOD CELLS AND COAGULATION
Anaesthetic considerations Patients tend to be in the high-risk group for anaesthesia and surgery, therefore careful consideration must be given to preoperative assessment and optimization of medical problems.
Anaesthesia for vascular surgery on extremities
Site: the procedure can be performed in a theatre suite with portable imaging, but about half of the units in the UK prefer to use the higher-quality imaging available in the radiology department. This results in the problems associated with anaesthesia in a distant site not primarily designed for general anaesthesia. The most important consideration is to anticipate everything that could be required, and have it available. The area is often cramped, with large numbers of staff and bulky equipment. The temperature is usually kept low for optimal equipment function, but too low for optimal patient function, and therefore warming with a forced-air warmer is required because warming mattresses under the patient interfere with the imaging. The table on which the patient lies will not tip, therefore anaesthesia must be established before transfer from a theatre trolley or bed. In addition, the mattress is thinner than that on an operating table, and because procedure times can be prolonged, pressure areas need protection.
Andrew B Lumb
Lower limb surgery Most patients presenting with lower limb ischaemia are elderly, often with extensive co-morbidity. Early symptoms include intermittent claudication and leg ulcers, progressing to critical ischaemia (rest pain and tissue loss) or acute ischaemia (white leg). Patients commonly have widespread atherosclerosis involving coronary, cerebral and renal vasculature, and 10-year mortality in these patients is 45%, mainly as a result of coronary and cerebral events. Medical treatment of lower limb ischaemia includes cessation of smoking, exercise programmes, aspirin and control of diabetes mellitus and hypertension. With these simple measures many patients with intermittent claudication improve spontaneously, but 20% of patients progress to critical ischaemia within 6 years and require surgery. Smoking increases the risk of surgical intervention to 80% in 6 years. Revascularization of the leg normally involves bypass of the diseased arterial segment with either synthetic tube grafts or vein. Figure 1 shows arteriograms before and after revascularization. Perioperative mortality for lower limb revascularization is 2–6%, mostly from cardiac causes, and increases with age and more distal bypass surgery. Morbidity is more common and includes myocardial ischaemia and infarction, and less often, heart failure, respiratory and renal complications.
Anaesthesia and analgesia: general and epidural anaesthesia are possible for stent procedures with no clear advantages for either in terms of cardiorespiratory morbidity and mortality. As heparin is administered during the procedure, the timing of the epidural is important. Spinal anaesthesia is also possible, however, owing to the potential for long procedures, conversion to general anaesthesia is relatively high. If regional anaesthesia is to be used without general anaesthesia, sedation may be required to help the patient tolerate prolonged procedures. Overall, the choice of anaesthetic technique depends on the general physical state of the patient. Monitoring: baseline monitoring is principally the same as for open repair. Although there is not the same potential for thirdspace or evaporative fluid losses there is usually a continual, and occasionally brisk, blood loss from the femoral arteriotomy. In addition, though the aorta is occluded for only short periods of time during graft deployment, one femoral artery is occluded for most of the procedure. This may cause significant cardiovascular effects on reperfusion. Urine output also needs careful monitoring, especially if large volumes of potentially nephrotoxic radiological contrast are required. Carbon dioxide may be used as an alternative contrast agent, but the image quality is adversely affected.
Anaesthetic technique The mortality and cardiovascular morbidity of surgery is similar with a regional or general anaesthetic technique. Figure 2 shows the advantages and disadvantages of each technique. Prolonged surgery and potential harvesting of arm veins are important surgical factors in the choice of anaesthetic technique. General anaesthesia: induction of anaesthesia is usually with an intravenous agent followed by a short-acting opioid, and a non-depolarizing muscle relaxant to facilitate intubation and ventilation. Maintenance is achieved with inhaled anaesthetic or intravenous infusions. For both induction and maintenance, care must be taken to administer appropriate doses of anaesthetic agents in elderly patients with poor cardiovascular reserve. Longer-acting opioids (e.g. morphine) may be used for analgesia during maintenance, the effects of which will continue into the postoperative period, but more commonly analgesia is achieved by the simultaneous use of a regional technique.
Recovery: following an uneventful stent procedure, most patients can be woken and transferred to a recovery area with appropriate monitoring. Most patients are in the high-risk group, therefore 24 hours in a high-dependency area are usually indicated. Emergency procedures: there are increasing reports of the use of endovascular repair for ruptured aneurysms with promising results. Various conditions have to be met for the technique to be feasible, such as haemodynamically stable patient, favourable anatomy of the aorta, and rapid availability of facilities and trained staff. If this can be achieved, indications are that emergency stent grafts are a useful technique.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
Andrew B Lumb is Consultant Anaesthetist specializing in vascular surgery at St James’s University Hospital, Leeds, UK. After medical training at St Mary’s Hospital Medical School, London, he trained in anaesthesia at a variety of London hospitals.
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Anaesthesia for abdominal vascular surgery
Overall survival of patients given early surgery compared with ultrasound surveillance in the management of small aneurysms 1.0
Proportion of patients surviving
Fiona Gamlin
Surgery on the abdominal aorta may be required for aneurysmal dilatation or for symptomatic occlusive atherosclerosis. The risks of non-surgical management of aortic aneurysm depend on aneurysm diameter. Aneurysms greater than 6 cm have an average risk of rupture within 1 year of about 25%. Larger aneurysms expand more rapidly, further increasing the risks, therefore the greater the diameter the greater the risk. In patients with aneurysms smaller than 5 cm, there seems to be no benefit to long-term survival from early surgical intervention (Figure 1). The operative mortality figures for elective aortic surgery are 2–10%, reflecting the higher incidence of associated risk factors, especially in the occlusive atherosclerosis group: • ischaemic heart disease • renal impairment • increasing age • diabetes mellitus • obstructive pulmonary disease • hypertension • smoking. The principal causes of perioperative morbidity and mortality are myocardial ischaemia or infarction, organ failure and respiratory complications. Although improvements in perioperative monitoring and management have reduced morbidity and mortality in elective surgery, the mortality from emergency surgery remains high at about 45% for ruptured aortic aneurysm. The risks of elective or emergency surgery increase dramatically in patients over the age of 80 years.
0.8
Early surgery 0.7
Surveillance 0.6 0
1
2
3
4
5
6
125 187
52 63
Survival time (years) Number at risk Surveillance Early surgery
527 563
497 513
468 489
412 450
229 302
Source: UK Small Aneurysm Trial Participants. Mortality results for randomised controlled trial of early elective surgery or ultrasonographic surveillance for small abdominal aortic aneurysms. Lancet 1998; 352 (9141): 1649–55.
1
General anaesthetic considerations Patients should continue to take regular cardiac medication up to and including the day of surgery. Admission to a high-care area for invasive monitoring and optimization of oxygen delivery in high-risk patients has been shown to improve outcome, but few units have the capacity to achieve this and the benefits probably apply only to those in high-risk groups. The aims of anaesthesia are to avoid exaggerated responses to surgical stimulation, reduce myocardial work and therefore oxygen demand, and maintain cardiac output and oxygen delivery to the tissues. Additional goals include attenuation of the stress response to surgery, preservation of renal function and the provision of postoperative analgesia. Various general anaesthetics can be used, most having some benefits appropriate to aortic surgery (Figure 2), however, many units employ a combination technique of opioid, oxygen, air and low-dose volatile agent to optimize the beneficial effects while minimizing side-effects. The addition of regional anaesthesia, in the form of a thoracic epidural, has several beneficial effects. There is a reduction in haemodynamic instability, the stress response to surgery, and general anaesthetic requirement. There is an increase in endocardial blood flow, and the high-quality analgesia postoperatively reduces the incidence of respiratory and other complications. The main problem related to the use of a thoracic epidural in vascular surgery is the requirement for heparinization for crossclamping, and therefore the potential risk of spinal haematoma. Reports from large numbers of patients suggest that the technique is safe and that the potential benefits outweigh the risks, however, there should be at least 1 hour between catheter insertion and heparin administration. It is also recommended that an epidural
Abdominal aortic surgery Preoperative assessment and optimization Assessment of patients for abdominal vascular surgery is aimed at identifying the risk factors outlined above, and ameliorating them when possible. The most significant factor affecting outcome is perioperative myocardial ischaemia and failure. In addition to baseline ECG and chest radiography, most patients warrant further evaluation of cardiac function (e.g. echocardiography and/or stress testing). The medical management of hypertension, ischaemia or cardiac failure should be optimized before surgery. There is evidence that patients with ischaemic heart disease benefit from perioperative β-blockade, which results in a reduction in episodes of perioperative silent myocardial ischaemia and adverse events.
Fiona Gamlin is Consultant Anaesthetist at St James’s University Hospital, Leeds, UK. She qualified from Newcastle University, and trained in anaesthesia in Newcastle and Leeds. Her interests are perioperative cardiovascular monitoring and the modification of cardiovascular risk factors.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
0.9
203
© 2004 The Medicine Publishing Company Ltd
BLOOD CELLS AND COAGULATION
Blood preservation Blood loss during abdominal vascular procedures increases with the complexity of the surgery and with surgery involving a more proximally sited aortic cross-clamp. Transfusion is often recommended if the haematocrit falls below 30% (0.30), and there is some evidence that levels below 28% (0.28) are associated with a higher incidence of myocardial ischaemia in patients with vascular disease. Homologous blood transfusion can be minimized by pre-donation, haemodilution and intraoperative cell salvage. Cell salvage allows for the re-infusion of fresh RBCs with normal concentrations of 2,3-diphosphoglycerate, and without anticoagulant, acid–base or electrolyte disturbance. Wide-bore cannulae, either peripheral or central, are required to allow rapid transfusion of blood and other fluids, preferably warmed. If severe haemorrhage is anticipated, rapid infusion systems capable of delivering warmed fluids should be available. Large-volume blood loss and transfusion result in coagulopathy, requiring fresh frozen plasma and platelet transfusion. Further coagulopathy may occur following gut ischaemia in supra-coeliac clamping, especially if the ischaemic time exceeds 30 minutes. Laboratory coagulation tests, bench-top activated clotting time and thromboelastography can all be used to guide appropriate use of clotting factors. Fibrinolysis may be a problem in complicated or supra-coeliac cases, requiring the use of antifibrinolytic agents (e.g. aprotinin, tranexamic acid).
General anaesthesia for use in aortic surgery Advantages Disadvantages Opioids • Cardiovascular system stability • Prolonged postoperative • Analgesia respiratory depression • Reduced stress response Volatile agents • Reduced afterload (vasodilator) • Reduced myocardial • Reduced myocardial oxygen contractility consumption • Reduced diastolic pressure/ • Titratable coronary blood flow • Coronary steal at higher minimum alveolar concentration NItrous oxide • Rapid onset and offset • Myocardial depression • Analgesia Propofol (total intravenous anaesthesia, target-controlled infusion) • Titratable • Reduced myocardial • Reduced afterload (vasodilator) contractility • Postoperative sedation 2
Temperature control Mild hypothermia is associated with vasoconstriction, tachycardia, hypertension and increased myocardial oxygen requirement. In addition, blood viscosity is increased and coagulation adversely affected. Maintenance of normothermia is associated with less biochemical disturbance, improved clinical variables and less myocardial ischaemia. Forced-air warming devices are more effective than a circulating mattress, and all infused fluids should be warmed. The area below the cross-clamp is not actively warmed while the clamp is in place.
should not be placed less than 2 hours before or 12 hours after administration of low molecular weight heparin (LMWH). Monitoring Usual baseline monitoring is required, with the addition of invasive arterial and central venous blood pressure, core temperature and urine output. A five-lead ECG, displaying leads II and V5, is more sensitive than a standard three-lead ECG for the detection of myocardial ischaemia. Pulmonary artery flotation catheters are not beneficial for routine use in abdominal vascular surgery except in patients with poor left ventricular function. In low-risk patients their use is associated with an increase in fluid administration with no improvement in outcome, and it may increase the morbidity associated with excessive fluid. Transoesophageal echocardiography allows real-time monitoring of ventricular filling ejection fraction and cardiac output. Ventricular segmental wall motion abnormalities can be visualized and are more sensitive markers of ischaemia than ECG changes. However, the equipment is expensive and requires a skilled operator. Oesophageal Doppler monitoring is a relatively non-invasive technique for assessing cardiac output. The results are variable if used by an inexperienced operator, and are difficult to interpret when the aortic clamp is in place. In this situation, trends can give valuable information but absolute figures may be inaccurate. Non-invasive cardiac output (NICO) is a technique for assessing cardiac output that utilizes partial rebreathing of carbon dioxide to calculate the cardiac output and some respiratory variables. During aortic surgery, NICO shows an acceptable correlation with thermodilution, although immediately following reperfusion the bias is increased.
ANAESTHESIA AND INTENSIVE CARE MEDICINE
Fluid management In addition to blood loss, there is evaporation from abdominal contents, and also third-space fluid loss, especially into the retroperitoneal space. Replacement with crystalloid or colloid is titrated against central venous or pulmonary artery occlusion pressure, and regular electrolyte analysis is required to detect and correct disturbances of acid–base balance, potassium, calcium and magnesium. The use of Hartmann's solution, rather than normal saline, reduces the development of acidosis. Although there is no evidence of increased incidence of adverse outcomes, the requirement for bicarbonate and blood product transfusion may be reduced. Aortic cross-clamp The placement of an aortic cross-clamp has significant effects on the cardiovascular and neuroendocrine system, the severity increasing the more proximally the clamp is placed. Blood flow to vital structures, including the kidneys, spinal cord and gastrointestinal tract, may be interrupted if supra-renal or supra-coeliac clamping is required. The cardiovascular effects of cross-clamping are less severe in occlusive aortic disease owing to the development of collateral circulation.
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BLOOD CELLS AND COAGULATION
Cardiovascular effects and management: following placement of an aortic cross-clamp, systemic vascular resistance rises causing an increase in mean arterial pressure and left ventricular afterload, an increase in left ventricular end-systolic pressure and wall tension, and a reduction in ejection fraction and cardiac output of up to 40%. The reduction in cardiac output is greater in patients with inadequate intravascular volume, and the cardiovascular changes in general are more severe in supra-coeliac clamping when the splanchnic circulation is interrupted. Patients with normal coronary arteries and myocardial function tolerate these changes and maintain cardiac contractility reasonably well. However, in patients with ischaemic heart disease, the combination of increased myocardial workload and greater intraventricular pressures inhibiting subendocardial blood flow may lead to myocardial ischaemia. This further affects cardiac contractility, central venous and left atrial pressures rise, and left ventricular failure and dysrhythmias may ensue. Before placing the clamp, the patient requires adequate intravascular filling. Afterload reduction is usually achieved by increasing the depth of anaesthesia with a volatile agent. Nitrates, including glyceryl trinitrate and sodium nitroprusside, are generally required only in compromised or emergency patients in whom the additional preload reduction and improved coronary blood flow are beneficial. Inotropic agents may be required if cardiac output is not improved with normalized preload and afterload. During cross-clamping the period of ischaemia causes vasodilatation distal to the clamp, and the accumulation of H+, lactate and other vasoactive metabolites. The release of these agents into the circulation on reperfusion in combination with vasodilatation results in low systemic vascular resistance, reduced venous return and reduced cardiac output. Acidosis and metabolites may directly depress the myocardium and, if hypotension is severe, low coronary blood flow exacerbates the problem. A combination of adequate filling to a central venous pressure above 10 cm H2O or pulmonary artery wedge pressure of 12–15 before de-clamping, and the use of vasoconstrictors and inotropes help to reduce cardiovascular instability. Surgical techniques that minimize the effects are short cross-clamp times and careful, slow release of the clamp.
Risk factors for perioperative renal failure Preoperative renal impairment • Renal artery atherosclerosis • Hypertension • Diabetes mellitus • Contrast toxicity • Heart failure Age Dehydration • Osmotic diuresis from intravenous contrast • Preoperative starvation Operative factors • Suprarenal cross-clamp • Hypotension • Hypovolaemia • Large-volume blood transfusion
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factors in the prevention of renal impairment are the maintenance of adequate intravascular volume (pulmonary artery wedge pressure 12–15) and cardiac output, and the avoidance of hypotension. If oliguria persists despite these precautions, various agents are used to re-establish or maintain urine output (Figure 4). Although these agents increase urine output, and have theoretical benefits, they have not been shown to improve renal function or outcome when compared with volume expansion alone. Most units employ one or more of these techniques to augment urine output during surgery, especially if a supra-renal clamp is required. Spinal cord ischaemia: normally the spinal cord receives most of its blood supply via the anterior spinal artery. This artery receives blood from the aorta via medullary branches, the most important being the artery of Adamkiewicz, which generally arises between T8 and T12, but may arise anywhere between T1 and L1. Distal supply is via the iliac vessels. Interruption to this blood supply may result in spinal cord ischaemia and a flaccid paralysis, which may be transient or develop into long-term paraplegia. The overall incidence is low (about 0.25%) in aortic aneurysm surgery, but increases in supra-renal or thoraco-abdominal procedures, and in emergency surgery. The most important factors in the preservation of spinal cord function are avoiding prolonged periods of cross-clamp (over 30 minutes), and the identification, preservation and reimplantation of critical vessels. Anaesthetic considerations are principally the maintenance of adequate blood pressure proximal to the clamp to optimize anterior spinal artery perfusion, and the avoidance of hyperglycaemia. Various techniques have been advocated for higher-risk supra-renal surgery including: • somatosensory evoked potentials for the early detection of ischaemia • cord cooling via an epidural catheter • CSF drainage to improve spinal artery perfusion pressure • corticosteroid administration to reduce cord oedema • magnesium administration as a free-radical scavenger. However, there is no clear evidence of improved outcome.
Renal preservation: renal impairment is a significant cause of perioperative morbidity and mortality in abdominal aortic surgery. Mortality increases in line with preoperative creatinine levels. Up to 10% of patients have preoperative renal impairment, and the overall rate of perioperative renal failure in aortic surgery is of the order of 4.5%. Mortality associated with renal failure exceeds 25%. Risk factors for the development of renal failure are shown in Figure 3. Although suprarenal clamping is associated with much higher rates of renal complications (up to 50%), all levels of crossclamp impair renal blood flow. The combination of increased vascular resistance, reduced cardiac output, activation of the renin–angiotensin system and sympatho-adrenal stimulation all serve to impair renal blood flow even with infra-renal cross-clamp. There may also be intra-renal blood flow redistribution away from the medulla, increasing the risk of medullary ischaemia. Oliguria is a poor predictor of renal dysfunction, because many patients demonstrate a reduction in urine output without developing renal impairment. Similarly, maintaining urine output has not been shown to improve outcome. However, it is the only perioperative monitor of renal function available. The most important
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Beneficial effects of drugs affecting renal function
Theatre preparation for emergency repair of abdominal aortic aneurysm
Mannitol • Plasma expansion • Improved cortical blood flow • Osmotic diuresis • Free-radical scavenging
Equipment • Rapid fluid infusion/warming device • Cell saver • Defibrillator • Forced warming device (upper body)
Loop diuretic (e.g. furosemide) • Diuresis • Sodium excretion
Drug preparation • Induction drugs • Vasopressors Ephedrine, 6 mg/ml Epinephrine, 1:100,000 Phenylephrine, 100 µg/ml • Infusions Dopamine, 4 mg/ml Epinephrine, 100 µg/ml Glyceryl trinitrate, 1 mg/ml • Extras Calcium chloride 10% Atropine, 1 mg in 10 ml Sodium bicarbonate 1.4% Mannitol 10 or 20%
Dopamine • Decreased systemic vascular resistance, increased cardiac output, β-adrenergic effects • Increased renal blood flow, dopaminergic effects • Increased glomerular filtration rate and urine output 4
Emergency abdominal aortic surgery Presentation: emergency abdominal aortic surgery is primarily for symptomatic, leaking or ruptured aneurysm. A symptomatic aneurysm may present with back pain or abdominal pain or tenderness, and requires surgery urgently because there is a high risk of rupture. CT scanning may reveal a contained leak, in which case the management is as for ruptured aneurysm. If the patient shows no evidence of cardiovascular instability and there is little likelihood of a contained leak then he can be managed as for an elective repair. The mortality for urgent repair of symptomatic aneurysm is significantly higher than for elective repair. The classical presentation of a ruptured aneurysm includes back or abdominal pain, shock and a pulsatile abdominal mass.
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catheter, nasogastric tube and temperature probe. A pulmonary artery catheter may also be considered, however, this is often not practical in a busy situation. Although the homeostatic derangement is much more severe, the general themes of intraoperative management are as for elective repair. The main problems are coagulopathy, requiring early transfusion of clotting products, cardiovascular instability including myocardial ischaemia, and renal injury. In addition, the response to re-perfusion on clamp removal is often exaggerated owing to the additional period of poor tissue perfusion before cross-clamping. Postoperative complications are increased following emergency surgery, and prolonged ICU stays are usual. Predictors for survival to discharge include patient age, the total volume blood product transfusion, duration of surgery, postoperative hypotension and day 1 APACHE scores.
Perioperative management of ruptured aneurysm: the main aim in the management of acute aneurysm rupture is to control blood loss by cross-clamping the aorta as soon as possible. Preoperative resuscitation should be limited to maintaining vital organ function (i.e. cerebral and coronary perfusion). Excessive fluid resuscitation may aggravate the situation by raising the blood pressure and increasing blood loss. Priorities are good venous access with at least two wide-bore cannulae, blood sampling for baseline blood testing, including coagulation, and cross-match of at least 10 units of blood, and rapid transfer to theatre. Two anaesthetists and two operating department practitioners are needed to prepare the patient, theatre equipment and drugs with a minimum of delay (Figure 5). After taking a brief history and attaching non-invasive monitoring, the patient should be prepared and draped before induction. The loss of abdominal muscle tone and sympathetic tone, in conjunction with the effects of anaesthetic drugs and mechanical ventilation may result in a catastrophic fall in blood pressure. Invasive arterial pressure monitoring for induction is useful if access can be achieved without delay. A rapid-sequence induction is used. Many anaesthetists choose a high-dose opioid technique to minimize cardiovascular effects, including hypertension on intubation, which may increase bleeding. Surgery is started as soon as the airway is secure, and blood pressure is maintained at reasonable levels with the rapid infusion of fluids including blood, and the use of pressor agents. Heparin is not required, and once the cross-clamp is in place, a period of relative stability allows for further fluid resuscitation, and the insertion of a central venous
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Endovascular aortic stent procedures The technique of endovascular stent repair of aortic aneurysm via the femoral artery was first described in 1991. If the procedure is technically possible, it is associated with less physiological disturbance, morbidity and mortality than open procedures. There is a considerable reduction in the level of stress hormone release and in nutritional derangement compared with open repair. Early (30 day) mortality in those thought fit for open repair is reported as 2.3% in the Eurostar database, and as 12.5% in those unfit for open repair, however, the relatively high potential for late complications increases late mortality. The rate of conversion to open repair is low (5%), but conversion is associated with high mortality. There is no clear evidence of the long-term effectiveness of stent procedures, and they are generally limited to patients considered unfit for conventional surgery, or to those included in randomized controlled trials.
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Anaesthetic considerations Patients tend to be in the high-risk group for anaesthesia and surgery, therefore careful consideration must be given to preoperative assessment and optimization of medical problems.
Anaesthesia for vascular surgery on extremities
Site: the procedure can be performed in a theatre suite with portable imaging, but about half of the units in the UK prefer to use the higher-quality imaging available in the radiology department. This results in the problems associated with anaesthesia in a distant site not primarily designed for general anaesthesia. The most important consideration is to anticipate everything that could be required, and have it available. The area is often cramped, with large numbers of staff and bulky equipment. The temperature is usually kept low for optimal equipment function, but too low for optimal patient function, and therefore warming with a forced-air warmer is required because warming mattresses under the patient interfere with the imaging. The table on which the patient lies will not tip, therefore anaesthesia must be established before transfer from a theatre trolley or bed. In addition, the mattress is thinner than that on an operating table, and because procedure times can be prolonged, pressure areas need protection.
Andrew B Lumb
Lower limb surgery Most patients presenting with lower limb ischaemia are elderly, often with extensive co-morbidity. Early symptoms include intermittent claudication and leg ulcers, progressing to critical ischaemia (rest pain and tissue loss) or acute ischaemia (white leg). Patients commonly have widespread atherosclerosis involving coronary, cerebral and renal vasculature, and 10-year mortality in these patients is 45%, mainly as a result of coronary and cerebral events. Medical treatment of lower limb ischaemia includes cessation of smoking, exercise programmes, aspirin and control of diabetes mellitus and hypertension. With these simple measures many patients with intermittent claudication improve spontaneously, but 20% of patients progress to critical ischaemia within 6 years and require surgery. Smoking increases the risk of surgical intervention to 80% in 6 years. Revascularization of the leg normally involves bypass of the diseased arterial segment with either synthetic tube grafts or vein. Figure 1 shows arteriograms before and after revascularization. Perioperative mortality for lower limb revascularization is 2–6%, mostly from cardiac causes, and increases with age and more distal bypass surgery. Morbidity is more common and includes myocardial ischaemia and infarction, and less often, heart failure, respiratory and renal complications.
Anaesthesia and analgesia: general and epidural anaesthesia are possible for stent procedures with no clear advantages for either in terms of cardiorespiratory morbidity and mortality. As heparin is administered during the procedure, the timing of the epidural is important. Spinal anaesthesia is also possible, however, owing to the potential for long procedures, conversion to general anaesthesia is relatively high. If regional anaesthesia is to be used without general anaesthesia, sedation may be required to help the patient tolerate prolonged procedures. Overall, the choice of anaesthetic technique depends on the general physical state of the patient. Monitoring: baseline monitoring is principally the same as for open repair. Although there is not the same potential for thirdspace or evaporative fluid losses there is usually a continual, and occasionally brisk, blood loss from the femoral arteriotomy. In addition, though the aorta is occluded for only short periods of time during graft deployment, one femoral artery is occluded for most of the procedure. This may cause significant cardiovascular effects on reperfusion. Urine output also needs careful monitoring, especially if large volumes of potentially nephrotoxic radiological contrast are required. Carbon dioxide may be used as an alternative contrast agent, but the image quality is adversely affected.
Anaesthetic technique The mortality and cardiovascular morbidity of surgery is similar with a regional or general anaesthetic technique. Figure 2 shows the advantages and disadvantages of each technique. Prolonged surgery and potential harvesting of arm veins are important surgical factors in the choice of anaesthetic technique. General anaesthesia: induction of anaesthesia is usually with an intravenous agent followed by a short-acting opioid, and a non-depolarizing muscle relaxant to facilitate intubation and ventilation. Maintenance is achieved with inhaled anaesthetic or intravenous infusions. For both induction and maintenance, care must be taken to administer appropriate doses of anaesthetic agents in elderly patients with poor cardiovascular reserve. Longer-acting opioids (e.g. morphine) may be used for analgesia during maintenance, the effects of which will continue into the postoperative period, but more commonly analgesia is achieved by the simultaneous use of a regional technique.
Recovery: following an uneventful stent procedure, most patients can be woken and transferred to a recovery area with appropriate monitoring. Most patients are in the high-risk group, therefore 24 hours in a high-dependency area are usually indicated. Emergency procedures: there are increasing reports of the use of endovascular repair for ruptured aneurysms with promising results. Various conditions have to be met for the technique to be feasible, such as haemodynamically stable patient, favourable anatomy of the aorta, and rapid availability of facilities and trained staff. If this can be achieved, indications are that emergency stent grafts are a useful technique.
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Andrew B Lumb is Consultant Anaesthetist specializing in vascular surgery at St James’s University Hospital, Leeds, UK. After medical training at St Mary’s Hospital Medical School, London, he trained in anaesthesia at a variety of London hospitals.
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