Endovascular or Open Surgical Therapy for Critical Limb Ischemia

Introduction

Critical limb ischemia (CLI) is a severe form of peripheral arterial disease (PAD) characterized by clinical findings of 1) lower extremity ischemic pain at rest, and/or 2) ischemic tissue loss marked by ulcerations and gangrene, in the setting of specific, objective hemodynamic parameters.1 CLI incidence is estimated to be approximately 500 to 1000 per million per year in the West, with rates anticipated to increase as risk factors such as diabetes, tobacco smoking, and metabolic syndrome increase worldwide.2-5 The natural history of CLI is such that without revascularization 20% to 40% will require amputation, and 6-month mortality exceeds 20%.2,3,5-7 Four-year amputation and mortality rates approach two-thirds.8,9 Recent retrospective review has suggested that overall disease-associated amputation rates may actually be increasing.10 This effect was most pronounced in patients with comorbid diabetes mellitus, who tend to present at younger ages with aggressive disease.10,11 Not surprisingly, providing care for patients with CLI is associated with significant cost to the US healthcare system and is estimated to exceed $3 billion annually.12-17

Revascularization to restore perfusion of the extremity with rest pain and/or tissue loss is the mainstay of therapy.18 No medical therapy has been shown to be effective in salvage of the affected limb itself. However, lifestyle modifications such as smoking cessation as well as lipid-lowering and cardioprotective medications have been shown to reduce the risk of cardiovascular events and mortality in patients with CLI and to increase the patency of both open and endovascular revascularization.11,19-26 Thus these remain an important component of therapy for all patients with CLI and adherence to guidelines established by the American College of Cardiology and the American Heart Association (ACC/AHA) has been demonstrated to reduce major adverse limb events (MALE), major adverse cardiovascular/cerebrovascular events (MACE), and mortality.27

Below we discuss considerations related to open surgical and endovascular therapy, investigations to date comparing the two approaches, and remaining questions and controversies.

Open Surgery for Critical Limb Ischemia

Open surgical techniques utilized in the treatment of CLI consist primarily of endarterectomy and bypass. Determinants of the specific operative approach employed in CLI patients include:

  1. Manifestation of disease (i.e., rest pain vs. tissue loss): Whereas in the former, revascularizing a single level of disease may be sufficient to eliminate debilitating ischemic pain symptoms, in the latter the goal is to re-establish inline, pulsatile flow to the distal extremity to ensure healing.17 This often requires staged procedures to treat multilevel inflow and outflow disease.
  2. Anatomic level(s) of disease: For the treatment of aortoiliac segment disease, bypass is favored over endarterectomy.28 Endarterectomy is a durable option in the treatment of disease in the common femoral, profunda femoris, or proximal superficial femoral arteries; 5-year patency rates as high as 91% have recently been reported.29 More distal stenoses should generally be treated via bypass.30
  3. Patient comorbidities and physiologic reserve: Bypass approaches include those which recapitulate native arterial anatomy directly versus those which utilize an extra-anatomic path. Surgical graft implantation utilizing an intracavitary inflow artery (thoracic/abdominal aorta, iliac artery) exerts more physiologic strain on the patient than extracavitary inflow vessels. Although the former have superior patency rates, they may be avoided in very high risk operative candidates.31-42
  4. Availability of autogenous venous conduit: Whereas prosthetic conduits are acceptable for the aortoiliac reconstructions, autogenous vein outperforms prosthetic in infrainguinal bypasses.43 This is particularly true when the outflow vessel is below the knee.44 Greater saphenous vein is the preferred conduit due to its excellent long-term patency; when this is not available, upper extremity, composite autogenous veins, and cryopreserved allograft saphenous veins are alternatives.43,45-47 Lack of appropriate conduit may alter the risk-benefit balance of bypass to certain distal targets, or preclude it altogether.
  5. Life expectancy: The importance of durability of revascularization for CLI must be emphasized; although overall short-term mortality has historically been high among patients with CLI as discussed above, randomized trials reported in the last decade have demonstrated one-year and two-year survival rates in excess of 85% and 70%, respectively, in patients who receive treatment.48,49 Nevertheless, there does exist a subset of CLI patients with high short-term mortality. The open surgical approach to CLI should take into account these factors and be tailored to an individual patient's trajectory.

While surgical bypass is a durable approach, the major limitations have been related to morbidity and mortality. As alluded to above, rates of these complications vary by the type of surgical approach utilized. Complications which may occur more frequently after surgical bypass include cardiovascular events, wound complications, graft infections, and death.

Endovascular Interventions for Critical Limb Ischemia

Endovascular techniques used in the treatment of CLI include percutaneous angioplasty (using conventional plain, cutting, or drug-coated balloons), atherectomy (mechanical or laser), and stenting (bare metal or drug-eluting).17,20 in the last 20 years the rate of lower limb endovascular interventions has increased more than four-fold.15,50-53

Endovascular modalities for the treatment of CLI enjoyed their greatest initial proliferation in the treatment of aortoiliac disease.17,54-56 However, they have also gained widespread acceptance for femoropopliteal disease.18,57 Continued technological advances have resulted in the ever-expanding application of these interventions to patients with disease characteristics once thought to preclude endovascular interventions, such as Trans-Atlantic Inter-Society Consensus (TASC) D lesions, chronic total occlusions, infrapopliteal disease, and even common femoral and profundal femoral occlusive disease. These newer applications have met some success in CLI, albeit based on limited data.58-64 Determinants of the endovascular technique most apt for individual patient and lesion characteristics remains in evolution.

While endovascular therapy has decreased periprocedural mortality and morbidity, concerns have been raised about the durability and cost.65 While its generally more benign morbidity profile makes it tempting to adopt a stepwise, endovascular-first approach, evidence from a landmark randomized controlled trial suggests that the amputation-free survival (AFS) following open surgery after initial attempt at endovascular revascularization is significantly lower than following open surgery at the outset.9 Thus, understanding the conditions determining appropriateness of each therapeutic approach may be of critical importance in optimizing outcomes in CLI patients.

Comparison of Endovascular and Open Surgery for Critical Limb Ischemia

Endovascular and open surgical revascularization are delivered to patients by a variety of vascular specialists. Perhaps not surprisingly, treatment biases among different provider specialties impact decision-making vis-à-vis which modality is offered to patients with CLI.17,66-68 This is compounded by the fact that there is limited high quality data guiding vascular specialists on the optimal treatment approach.

Only one large scale, randomized, controlled trial (RCT) to date has published the results of the direct comparison of endovascular versus open surgery for the treatment of infrainguinal CLI—the United Kingdom (UK) National Institute of Health Research Health Technology Assessment program-sponsored Bypass versus Angioplasty in Severe Ischaemia of the Leg (BASIL) trial.49 Over an approximately five year period from 1999 to 2004, investigators at 27 UK centers randomized 452 patients. Criteria for inclusion included the presence of rest pain or tissue loss (as described above) of arterial etiology. However, patients did not necessarily meet objective hemodynamic parameter criteria, and thus did not strictly fulfill the criteria for a diagnosis of CLI (hence the use of the term "severe limb ischaemia", SLI).20,49 Diagnostic imaging (including angiography 95% of the time) was reviewed by a multidisciplinary team of vascular surgeons and interventional radiologists who had to agree that there was equipoise between open surgical bypass and percutaneous transluminal balloon angioplasty for infrainguinal atherosclerotic disease prior to randomization.49,69 The primary outcome was AFS. Secondary endpoints included all-cause mortality, 30-day morbidity and mortality, reinterventions, health-related quality of life (HRQL), and utilization of hospital resources. Comparison of the two study arms was undertaken in an intention-to-treat fashion. The initial analysis found that AFS at 1 and 3 years was 68% and 57%, respectively, for the cohort assigned to bypass surgery, and 71% and 52%, respectively, for those randomized to angioplasty first—a nonsignificant difference. Surgery was associated with a lower re-intervention rate (18% vs. 26%; 95%CI 0.04-15%). There were no differences in overall mortality, but bypass surgery was associated with increased early morbidity than angioplasty. HRQL was similar in both arms. Hospital costs were higher during the first year for the surgery-first strategy.49 Subsequent longitudinal study of the cohorts has been conducted beyond 3 years post-randomization.9,70 These investigations have revealed that for patients who survived beyond the initial 2-year period (70%), the surgical bypass-first approach was associated with higher overall survival, and a trend towards improved AFS.9 Subgroup analysis demonstrated superiority of autogenous conduit to prosthetic in the outcomes of patients who received surgical bypass.9 Differences in cost became less pronounced beyond the first year following treatment and no differences in HRQL emerged over ensuing years of analysis.70 Thus the trial authors concluded that "... patients predicted to live more than 2 years, and with a useable vein, should usually have bypass surgery first [and] patients expected to live less than 2 years, and those without a useable vein, should usually have balloon angioplasty first."9,20

The BASIL trial represented an important step toward evidence-based decision-making in revascularization of SLI/CLI with infrainguinal disease.65 Nonetheless, several concerns about the BASIL trial's limitations have been raised:

  1. Generalizability to the broader population of CLI patients: As mentioned above, the BASIL trial investigators chose to include patients with SLI rather than adhere to the stricter definition of CLI. A component of the BASIL trial's design included a six-month audit period at the top recruiting centers to determine which segment of the total population of patients with severe limb ischemia was actually being randomized into the trial, the investigators found that only 48 of 585 (8%) patients with SLI were randomized. The reasons for exclusion included aortoiliac disease, anatomic unsuitability or response to medical therapy, refusal of trial entry, lack of equipoise, comorbid conditions, inability to provide informed consent for trial enrollment, and other factors.49 Criteria for equipoise were not clearly defined, thus potentially leading to center-to-center variation.65
  2. Limitation of endovascular technique to conventional balloon angioplasty: The utility of other endovascular approaches, e.g. stenting, were not studied.17,20 There is high quality evidence from a RCT to demonstrate the superiority of the latter to conventional balloon angioplasty alone in the treatment of infrainguinal PAD.71 As such, by the time of study completion, many endovascular interventionalists had adopted a strategy of primary or "bailout" stenting for a failed or suboptimal result following balloon-angioplasty.
  3. Failure to stratify based on anatomic pattern of disease: Approximately one third of patients in the BASIL trial's surgical bypass arm underwent bypass with distal anastomosis to one of the crural arteries (anterior tibial, posterior tibial, peroneal arteries or their branches).49 However their analysis did not account for differences in outcomes that may result from these distal target vessels.11,17
  4. Choice of primary endpoint: It is argued that AFS falls short as an outcome measure because it places disproportionate weight to mortality unrelated to limb salvage, and neglects many limb-related outcomes which are important to SLI/CLI patients.17,72 In the years since the BASIL trial, historic outcomes of interest in CLI such as mortality, vessel/bypass patency, and AFS have been expanded to include broader endpoints such as MALE (above ankle amputation or such major reintervention as thrombectomy/thrombolysis, graft revision, or new bypass graft of the index limb) and additional quality of life measures.2,3,65,72-74 The BASIL trial also did not include an objective measure of hemodynamic success.65

Unresolved Questions and Areas of Further Investigation in the Management of Critical Limb Ischemia

Published studies following the BASIL trial have been marked by a number of flaws including nonrandomization, retrospective design, small sample size, poorly defined interventions, failure to distinguish intermittent claudication from CLI, failure to restrict to infrainguinal anatomic disease pattern, and/or inadequate follow-up.11,17,75 Based on the limited existing data, some vascular specialists have proposed decision-making algorithms which take into account surgical risk, life expectancy, severity of ischemia, anatomic pattern, and conduit availability, and the 2016 ACC/AHA Guideline on the Management of Patients with Lower Extremity Peripheral Arterial Disease lays out clinical and imaging findings that may suggest a benefit to one treatment modality over the other.16,18 However, clinical equipoise and resultant marked variability in the therapy that CLI patients receive depending on region and provider preference appear to persist.11,17,65,76

Given these gaps in knowledge coupled with the substantial impact of CLI on patients who suffer from it, as well as the healthcare system broadly, further RCTs investigating optimal therapy for infrainguinal CLI are crucial. Challenges that RCT investigators face are related to the heterogeneous nature of disease (rest pain vs. tissue loss; femoropopliteal vs. tibial disease) and the treatment goals derived thereof (relief of a single level of obstruction for rest pain vs. re-establishment of in-line flow and/or angiosome-directed revascularization in tissue loss), as well as of the therapeutic options that broadly fall within the categories of endovascular versus open surgical therapy.17,65,77,78

The ongoing Best Endovascular Versus Best Surgical Therapy for Patients with Critical Limb Ischemia (BEST-CLI) trial is a prospective, National Institutes of Health National Heart, Lung, and Blood Institute-funded US and Canadian multicenter, multidisciplinary, pragmatic, open-label, superiority RCT which seeks to enroll 2100 patients from approximately 160 clinical sites in order to assess treatment efficacy, functional outcomes, and total cost of best open surgical and best endovascular revascularization in CLI patients who are candidates for both procedures.11 The trial is pragmatic as the decision for which endovascular or open surgical bypass technique is made by the individual investigator; this allows for the use of therapies which reflect current practice as well as new techniques which come into use during the study period. In addition, the multidisciplinary structure that involves input from the multitude of specialists who provide care for patients with CLI. The primary endpoint is MALE-free survival. Secondary endpoints include total cumulative cost, functional and quality of life outcomes, as well as cost-effectiveness.74 Planned minimum follow-up is 2 years. Cohorts studied will include patients with single-segment saphenous vein and those without such conduit. These are further powered to detect differences in two dichotomous strata based on clinical manifestation of CLI (rest pain alone vs. tissue loss) and anatomic pattern (with or without significant infrapopliteal component).11,74 Thus the design addresses many of the limitations of the BASIL trial and subsequent investigations, and may provide further data about issues previously unexplored in randomized studies (e.g., outcomes of angiosome-directed as opposed to conventional in-line revascularization strategies, as well as anatomic predictors of outcome).18,65

Two additional RCTs in the UK promise to complement the findings of BEST-CLI.17 The BASIL-2 trial is a pragmatic, double arm, open-label, superiority trial of primary vein bypass versus primary best endovascular treatment focused on infrageniculate disease.79 The investigators again seek to enroll the SLI patient group, include a pragmatic array of endovascular interventions, and evaluate AFS and cost-effectiveness, in addition to a number of secondary endpoints.79 In contrast, the BASIL-3 trial seeks to answer questions regarding comparative efficacy of endovascular options, with three randomization arms: 1) plain angioplasty, 2) drug-coated balloon angioplasty, and 3) drug-eluting stents.80

Thus the optimal choice of endovascular versus open surgical revascularization for patients with CLI remains under investigation. The role of determinants such as clinical manifestation of disease, anatomic distribution of lesions, availability of bypass conduits, and patient comorbidities continues to be elucidated. The results of ongoing RCTs will provide vascular specialists much-needed guidance in this arena of vascular disease which has traditionally suffered from a lack of high level evidence.

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