Introduction

Repair of abdominal aortic aneurysms (AAA) using endovascular stents is now the most common form of corrective intervention compared to open surgery. It is usually performed in specialist centres, being undertaken by multidisciplinary teams (MDTs) consisting of vascular surgeons, interventional radiologists and anaesthetists. This article addresses the anaesthetic implications of endovascular aneurysm repair (EVAR).

AAA prevalence and outcomes

An aneurysm is a permanent, localized dilatation of an artery >50% of the normal diameter of the artery in question. The abdominal aorta is usually 2 cm in diameter and is defined as aneurysmal at 3 cm or above, in 90% of cases this occurs infra-renally. Approximately one in 70 men screened aged 65 years in England will be identified with an aortic aneurysm. With increased diameter, comes increased risk of rupture. Ruptured AAA have extremely high mortality, approximately 80%, when deaths in those not reaching hospital are taken into account. Each year around 3000 men in England and Wales, aged 65 years or over, die from a ruptured AAA, accounting for 1.7% of all deaths in this population. Women are much less likely to develop an AAA (male:female = 3:1) but have higher rupture rates. The incidence and prevalence has decreased over the past 20 years, attributed to reduced rates of smoking, better control of cardiovascular risk factors such as blood pressure and widespread use of antiplatelets and statins. However, the benefit of identifying and treating them prior to rupture remains.

The most significant risk factors for developing an AAA are:

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  Male

  
  
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  Increasing age

  
  
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  Smoking

  
  
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  Hypertension

  
  
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  Genetic factors

  
  
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  Hypercholesterolaemia

  
  
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  Inflammatory vasculitis

  
  
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  Trauma.

Management depends on patient factors and the risk of rupture and includes medical, surveillance or surgical methods. In the UK, surgery is usually indicated once AAA diameter is >5.5 cm, or if >4 cm and has grown >1 cm in a year. Table 1 provides an illustration of rupture rates.

Screening and centralization of services

Given the prevalence and potential impact of AAA, the NHS AAA screening programme was established in 2013, and aims to screen all men at the age of 65 years. Ultrasonography is the imaging modality of choice as it is cheap, non-invasive and has high sensitivity and specificity. Those with aneurysms are referred to a regional vascular service, to be assessed within 12 weeks if 3.0–5.4 cm, or 2 weeks if ≥ 5.5 cm.

Following Getting It Right First Time (GIRFT) recommendations in 2018, vascular services became more centralized, with procedures being delivered at regional, high-volume, specialist centres. This approach facilitates the provision of 24/7 vascular multidisciplinary care, better training opportunities for vascular and interventional radiology trainees and avoids replication of expensive equipment at multiple sites. This model does however increase transfers of potentially very sick patients with its associated risks and there is potential to overwhelm single centres supplying large regions but overall, centralization has led to better patient outcomes.

Surgical techniques – EVAR

EVAR entails the use of a synthetic or fabric tube graft which self-expands in the aorta. The grafts vary from their simplest form such as a standard infra-renal tube graft to complex stent-grafts such as fenestrated, branched or chimney grafts. The surgical and anaesthetic considerations vary immensely depending on graft type, with the infra-renal stent-grafts taking approximately 2 hours with minimal physiological disturbance to highly complex grafts requiring a longer surgical time with a greater physiological insult for patients. The complex grafts require greater expertise and experience and are hence performed in fewer centres. This article will focus on simple, infra-renal EVAR. The repair of an infra-renal AAA is carried out by cannulation of both femoral or iliac arteries and insertion of an expandable aortic graft into the aorta under radiological guidance with the graft being deployed when the operators have established correct positioning. The stent occludes the aneurysmal sac allowing blood to flow across the aneurysm with eventual thrombosis of the aneurysmal sac. Usually, two further grafts are then screened into the iliac vessels acting as sleeves to the main graft. Figure 1 depicts a simple, infra-renal graft in situ.

Simple infra-renal graft in-situ. Permission for use granted by Cook Medical, Bloomington, Indiana.

The less invasive nature of endovascular approaches compared to OR has many advantages. These include a lower physiological insult and stress response, reduced blood loss and associated haemodynamic instability, no cross-clamping induced morbidity and quicker ambulation resulting in shorter hospital stays. It has allowed surgical repair in patients previously deemed too high risk for open surgery and over the past 20 years there has been a move away from OR, towards endovascular approaches. While short term benefit has been apparent, there have been questions regarding the mid-to-longer term outcomes with EVAR.

Summary of clinical trials

Several randomized control trials (RCTs) have compared EVAR versus OR in the elective treatment of AAA. These include the Endovascular Aneurysm Repair 1 (EVAR 1) trial, the Dutch Randomized Endovascular Aneurysm Management (DREAM) trial, the Open vs. Endovascular Repair of Abdominal Aortic Aneurysm (OVER) trial and the French, Anevrysme de l’aorte abdominale, Chirurgie vs. Endoprothese (ACE) trial. These studies were conducted from 1999 to 2008 and although specifics were different, overall, they found EVAR had lower peri-operative mortality (no difference in ACE) but any survival benefit was lost in the following years. Higher re-intervention rates were seen with EVAR in EVAR 1, DREAM and ACE but not in OVER, possibly as this was the only study that included incision complications from OR.

In EVAR 1, 1252 patients were enrolled, from 37 centres in the UK, between 1999 and 2004. Patients had to be ≥60 years old, have an aneurysm ≥5.5 cm and be fit for either EVAR or OR. They were randomly allocated to EVAR (n = 626) or OR (n = 626). A reduction in all-cause and AAA-related mortality was seen in the EVAR group for up to 6 months. At the 4-year point AAA-related mortality still favoured EVAR but there was no difference in all-cause mortality. Beyond 8 years, OR had significantly lower all-cause and AAA-related mortality. The higher re-intervention rates seen in EVAR (16%) were due to endoleaks, thrombosis, graft kinking and device migration. Further, EVAR was more expensive compared to OR.

The original purpose of EVAR techniques was to provide a surgical alternative for those only deemed eligible for conservative management with AAA ≥5.5 cm. The EVAR 2 trial recruited those not fit for OR and compared outcomes between patients randomized to EVAR (n = 166) or a no-intervention group (n = 172) from 31 UK hospitals. At 4 years, perhaps surprisingly, EVAR did not reduce aneurysm-related, or all-cause, mortality compared to no-intervention. This lack of benefit was felt due to higher-than-expected operative mortality with EVAR (9%; 1.8% in EVAR 1) and lower-than-expected rupture rates in the no-intervention group (9 per 100 person-years). In later follow-up, aneurysm-related mortality was lower in the EVAR group but still didn’t relate to any difference in total mortality between groups. Financially, over 4 years of follow up, EVAR was calculated to be £8649 more expensive than no repair, partly due to higher complication rates requiring re-intervention. Given the mortality in the cohort of patients recruited into EVAR 2, the authors suggested the focus for these patients should be to improve their general fitness rather than rush to EVAR.

More recent work from Shahin et al. contradicts this. A total of 350 patients deemed unfit for OR, underwent EVAR or were managed conservatively. Propensity matching produced 122 pairs of patients from the groups. Overall survival was significantly longer in the EVAR group compared to the conservative group (84 months vs 30 months). One-, three-, and five-year mortality in the EVAR group was 7%, 40% and 68% compared with 25%, 68% and 82% in the conservative group, all P < 0.001. EVAR was also deemed to be cost effective.

The introduction of newer stent grafts, improvements in preoperative planning, imaging and perioperative care, along with increasing EVAR procedures done under local, instead of general anaesthesia, may reduce the complications associated with EVAR. Conclusions from the original RCTs mentioned above, may therefore be less applicable to more contemporary practice, although not all agree. More recent, large population-based registry studies have shown increased use of EVAR with decreased morbidity and mortality despite increasingly elderly and co-morbid patients.

Nonetheless, younger, fitter patients with longer life expectancy will usually be offered OR, avoiding the long-term surveillance of grafts with the risks of repeated radiation and contrast exposure and the potential longer-term complications of EVAR. Older, frailer patients are more likely to be offered EVAR. It is pertinent to remember, vascular patients, in general, have significant associated co-morbidities that adversely impact on their life expectancy regardless of successful surgery on their aneurysm.

NICE abdominal aortic aneurysm: diagnosis and management (NG156) 2020

After a prolonged and controversial, 5-year process, the UK National Institute for Health and Care Excellence (NICE) published its guidelines on the diagnosis and management of AAAs. In 2015, an AAA guideline development committee (GDC), made up of multidisciplinary professional members were appointed by NICE. The NICE technical analysis team presented relevant evidence to the GDC and draft recommendations were published in 2018 for comment by NICE-registered stakeholders. Due to worse long-term outcomes and higher re-intervention rates, EVAR was not deemed to be clinically or cost effective by the GDC and so they recommended against its use in most cases. Exceptions were allowed where OR was contraindicated due to abdominal co-pathology but otherwise, under the draft guidelines, the only management options would be OR or conservative.

Given the national shift towards EVAR, this resulted in significant controversy and debate across the vascular community. Substantial feedback was accumulated during the subsequent consultation period and stakeholders were critical for several reasons. They felt EVAR outcomes had improved since the RCTs, on which the recommendations were made, were published. Concerns were held regarding skill retention of EVAR (EVAR being the NICE-recommended treatment for ruptured AAAs) and ability to scale up skill levels for OR, given the previous shift towards EVAR. There was further apprehension regarding the capacity for intensive care beds that increased OR procedures would require.

The GDC reviewed the feedback and amendments were agreed. The recommendation against EVAR was removed but it still suggested medical therapy rather than EVAR if not fit for OR. It did suggest consideration of EVAR, only after disclosure to the patient that standard EVAR is neither clinically or cost effective.

Despite this, agreement between NICE and its GDC was not reached and in March 2020, NICE published its final revised guidelines against the advice of its GDC. The final recommendations noted the need for the NHS to move towards OR in a managed and sustainable way but when OR cannot be considered for anaesthetic or morbidity risks, then EVAR can be considered. Interestingly, European guidelines suggest EVAR should be preferred for elective AAA repair in those with ‘reasonable life expectancy’. Only in those with life expectancy >10–15 years do they recommend OR.

The recommendations, listed below, stipulate that surgeons and health systems should:

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  Offer open surgical repair for people with unruptured AAAs meeting the criteria, unless it is contraindicated because of abdominal co-pathology, anaesthetic risk, and/or medical conditions.

  
  
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  Consider EVAR for people with unruptured AAAs who meet the criteria and who have abdominal co-pathology, such as a hostile abdomen, horseshoe kidney, stoma or other considerations, specific to and discussed with the person, that may make EVAR the preferred option.

  
  
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  Consider EVAR or conservative management for people with unruptured AAAs who have risks and/or comorbidities that contraindicate open repair.

The recommendations relevant to anaesthesia and preoperative assessment are as listed below:

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  Consider cardiopulmonary exercise testing (CPET) to assist in shared decision-making.

  
  
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  Preoperative tests should be guided by the NICE guidance for preoperative testing.

  
  
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  Do not use the following risk assessment tools to determine whether or not repair is suitable for a person with an asymptomatic unruptured AAA: British Aneurysm Repair Score, Carlisle Calculator, Comorbidity Severity Score, Glasgow Aneurysm Scale, Medicare risk prediction tool, Modified Leiden Score, Physiological and Operative Severity Score for enUmeration of Mortality (POSSUM), Vascular-POSSUM, Vascular Biochemical and Haematological Outcome Model (VBHOM), Vascular Governance North West (VGNW) risk model.

  
  
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  Epidural should be considered in addition to general anaesthesia for OR, although this is established practice in most centres.

The final recommendation that the GDC advised NICE to publish which NICE rejected can be found elsewhere.

Anaesthetic issues for EVAR surgery

Anaesthetists play a vital role in the perioperative management of vascular patients:

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  Preoperatively:

  
  
  
  
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    preoperative assessment, risk stratification and optimization

    
    
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    MDT discussions regarding risk versus benefit

  
  
  
  
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  Intraoperatively:

  
  
  
  
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    Maintenance of haemodynamic stability, management of significant blood loss and preservation of organ function

    
    
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    Facilitate optimal imaging

  
  
  
  
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  Postoperatively

  
  
  
  
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    Analgesia

    
    
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    Identification of optimal level of postoperative care.

  
  

Preoperative assessment

Vascular patients can be complex due to multiple comorbidities, with EVAR patients typically older and frailer than OR patients. Multiple specialties will be involved in their perioperative care including surgeons, interventional radiologists and anaesthetists and so perioperative planning should adopt a multidisciplinary approach.

Preoperative assessment should ideally be consultant led and include a detailed medical history and clinical examination. Common comorbidities include coronary artery disease, peripheral vascular disease, hypertension, hypercholesterolaemia and chronic obstructive airway disease. A functional assessment, relevant investigations and risk assessment should be undertaken (discussed below). If time permits, lifestyle modifications can be made, comorbidities optimized, and medications rationalized, which may require specialist cardio-respiratory opinion. This process will guide the surgical and anaesthetic plan and assist in shared decision-making and expectation management of patients.

Lifestyle modifications such as smoking cessation, even if surgery is imminent, has benefits, and advice on exercise and nutrition is also recommended. Prehabilitation is the process of enhancing an individual’s functional capacity before surgery to improve postoperative outcomes and has shown promise. A Cochrane review of RCTs comparing the impact of preoperative exercise interventions versus usual care in people having AAA repair was unable to ascertain whether prehabilitation reduced 30-day mortality, pulmonary complications, re-intervention rates or postoperative bleeding. It may however, reduce cardiac and renal complications. High risk of bias and small sample sizes in the four RCTs reviewed led to this uncertainty and the authors called for RCTs of higher quality, larger sample sizes and longer follow-up to identify if it offers benefit.

Cardiac complications cause a large number of perioperative deaths after non-cardiac surgery so identifying cardiac risk is important. Risk factors include a history of ischaemic heart disease, heart failure or cerebrovascular disease, diabetes mellitus, serum creatine >170 micromol/litre, American Society of Anesthesiologists (ASA) ≥3 and reduced functional status. Interventions for secondary prevention of cardiovascular disease should be offered. Pharmacologically, statins and aspirin are recommended for high-risk vascular patients undergoing major vascular surgery and can be continued perioperatively. Other anti-platelets (clopidogrel and ticagrelor) and anti-coagulants (warfarin and direct factor Xa inhibitors) should be stopped. The European Society of Cardiology have guidelines for the cardiovascular assessment and management for patients undergoing non-cardiac surgery.

All patients should have an ECG to exclude arrhythmias or ischaemia and full blood count and urea and electrolytes to identify anaemia or renal impairment, respectively. If raised, baseline troponin and B-type natriuretic peptide (BNP) can indicate increased risk. Valvular heart disease, heart failure, raised BNP or ECG abnormalities should prompt a trans-thoracic echocardiogram. Those with two risk factors and poor functional capacity should have a non-invasive myocardial stress test. Diabetic patients should aim for HbA _1c <69 mmol/litre.

There are no universally accepted risk-stratification models specifically for vascular surgery but, despite their limited sensitivity in predicting perioperative mortality, commonly used scores include Lees revised cardiac risk score or the American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP) scoring system to help guide discussion.

Standard EVAR is classified as an intermediate risk procedure, complex EVAR is higher. Although, in combination with the complexity of patients requiring these procedures, most often undergo an objective assessment of functional performance such as cardiopulmonary exercise testing (CPET). CPET is routinely performed in many units. Results which indicate increased risk in the perioperative period include:

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  Peak oxygen consumption (pV0 ₂ ) of under 15ml/kg/minute

  
  
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  Anaerobic threshold of under 10.2 ml/kg/minute

  
  
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  Any cardiac ischaemia induced during testing.

Suboptimal CPET may lead to referral to relevant teams for further assessment and optimization.

Location

Most hospitals performing EVARs have hybrid theatre suites to facilitate imaging and open surgery if required. These specialist theatres may be in remote sites. Point-of-care testing and blood availability should be easily accessible. Radiation is a hazard for theatre staff, especially the anaesthetist who needs to be in close proximity to the patient. Regular audit of exposure should be undertaken of all staff to allow safe exposure to radiation.

Anaesthesia and intraoperative management

There are three main anaesthetic options for EVAR surgery:

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  general anaesthesia (GA)

  
  
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  central neuroaxial block (spinal, epidural or combined spinal epidural technique)

  
  
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  local anaesthetic.

Studies have attempted to identify superiority of one technique. Dovell et al. compared 30-day mortality, postoperative complications and length of hospital stay between different anaesthetic techniques in patients undergoing standard infrarenal EVAR across 89 UK hospitals. A total of 9783 patients had the procedure, 7069 had GA, 2347 had regional and 367 had local anaesthetic. Thirty-day mortality was significantly lower with regional than GA but not with local and length of stay was shorter with regional and local groups compared to GA; complication rates were the same. In reality, patient and surgical factors may dictate the mode of anaesthetic. These include patient preference, comorbidities (especially significant respiratory disease), ability to lie flat for prolonged periods and ability to breath hold (improves picture quality during surgery). Longer duration of surgery, requirement for vascular access via the upper limbs and potential for major blood loss are the surgical factors that may favour GA. NICE does not recommend a particular mode of anaesthesia in the elective setting and European guidelines only weakly recommend locoregional anaesthesia over GA.

Whichever anaesthetic mode is employed, the overall aims are the same:

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  maintaining patient comfort for 3–4 hours while supine

  
  
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  maintaining temperature and hydration throughout

  
  
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  maintaining stringent blood pressure control especially at time of stent deployment

  
  
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  preparation for major blood loss

  
  
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  monitoring anti-coagulation.

Anaesthetic techniques

Monitoring

Standard Association of Anaesthetists of Great Britain and Ireland (AAGBI) monitoring and invasive arterial blood pressure monitoring, (placed on the opposite side if surgical upper limb vascular access is needed) is required as a minimum. This allows regular blood sampling for blood gas and viscoelastic analysis to guide treatment. The potential for massive blood loss mandates wide-bore intravenous access. Central venous access is case dependent – prolonged operations, complex cases or frail patients with ischaemic heart disease may benefit. A urinary catheter should be placed to allow measurement of hourly urine output and temperature should be monitored with appropriate patient warming devices.

Blood loss

Blood loss can occur from access vessels, during removal of femoral sheath, from damage to large vessels during surgery and aneurysm rupture during stent deployment. Cell salvage should be used for complex fenestrated EVAR surgery and rapid infusion devices should be readily available.

Anticoagulation

All patients receive 5000 units of intravenous heparin prior to cannulation of vessels, to reduce thromboembolic complications such as arterial thrombosis, peripheral emboli, myocardial infarction, colonic ischaemia, deep venous thrombosis and stroke. Further doses may be administered if the procedure is prolonged. Activated clotting time (ACT) guided heparinization may become more common, such as in cardiac surgery, to provide the optimal balance between avoiding thromboembolic complications and increasing bleeding risk. The ongoing ACTION-1 trial is comparing whether ACT guided heparinisation results in more optimal coagulation than 5000 IU as a single dose and although focusing on OR, it should be applicable to EVAR. Protamine is not usually required to reverse heparin unless there is excessive bleeding at the time of closing the groin wounds.

Renal protection

Postoperative renal dysfunction is common in EVAR patients. The main causes are:

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  emboli being dislodged during stent deployment

  
  
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  damage to renal arteries from catheter wires (stenosis or aneurysm)

  
  
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  stent grafts either blocking renal arteries or causing an inflammatory reaction

  
  
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  reperfusion injury from prolonged lower limb ischaemia

  
  
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  intraoperative hypotension or hypovolaemia

  
  
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  use of intravenous contrast agents.

To avoid postoperative renal dysfunction adequate hydration should be maintained, the use of contrast limited as much as possible and nephrotoxic drugs avoided.

Spinal cord monitoring and cerebrospinal fluid (CSF) drainage

There is a risk of spinal cord ischaemia if the aorta is covered by a graft over 20 cm long. However, it is a very rare complication following infra-renal EVAR and more likely with fenestrated or branched EVAR surgery. It is caused by occlusion of the spinal cord feeder vessels by the graft (the largest being the artery of Adamkeiwicz arising from T9-T12), a thromboembolic event, anaemia or perioperative hypotension. Although rarely used, spinal cord monitoring may include observing somatosensory or motor evoked potentials or near infrared spectroscopy. Spinal cord perfusion depends on the difference between mean arterial pressure and mean CSF pressure. CSF drains have a role in the management of patients at high risk of spinal cord ischaemia, by reducing CSF pressure thereby improving spinal cord perfusion, but this invariably is only considered in complex thoraco-abdominal aneurysms which is outside the scope of this manuscript.

Postoperative care

Patient and surgical factors determine whether postoperative care can be delivered at a ward level or on critical care. Complex fenestrated EVARs, prolonged procedures, significant blood loss, significant pre-existing cardiorespiratory disease or cases requiring continued close monitoring, such as spinal cord drains, should be admitted to critical care.

Complications

Immediate surgical complications of EVARs include failed or mal-deployment of stent, arterial rupture and dissection, embolization, and ischaemia of the spinal cord, kidneys or bowel. Longer term compilations include endoleaks, thrombosis and infections. Medical complications include acute coronary events, arrhythmias, cardiac failure, acute renal failure, strokes, venous thromboembolism and post-implantation syndrome.