Taking TASC to TASK: Evolving from nomenclature to guidelines

Over the past decade, one priority of the World Health Organization has been to prevent limb amputations. As the number of people with diabetes mellitus is projected to double in the next 15 years, ischemic limb complications are expected to mount with a corresponding increase in resource utilization and related costs, taking an inestimable toll in human terms. Over 90% of diabetes-related amputations are preventable through judicious implementation of available strategies, beginning with primary prevention but including endovascular and surgical revascularization.

The same decade has seen rapid expansion of revascularization technologies, particularly in the field of endovascular interventions. Open surgery has been increasingly focused on limb salvage for patients with critical ischemia at rest, while the relative safety and effectiveness of endovascular strategies has extended applications of catheter-based revascularization to ever broader categories of patients with exertional ischemia – intermittent claudication in its various forms. This trend has accelerated as clinicians are better trained to recognize peripheral artery disease, and patients demand therapeutic strategies that deliver symptom relief more quickly and reliably than conservative care, exercise regimens, and medications can provide.

As the poles divide between bypass surgery for higher risk patients with critical limb ischemia and percutaneous endovascular revascularization for those with exertional symptoms at lower risk, comparative effectiveness studies have become ever more elusive. High-quality evidence is sparse. Few randomized trials have compared surgical to endovascular strategies for the outcomes of limb salvage, cardiovascular and all-cause mortality, functional capacity, quality of life, and cost. Most of the available information is derived from observational studies of varying quality that form an insufficient basis for strong recommendations in practice guidelines. A vanishingly small proportion of patients seem eligible and willing to undergo randomized assignment to surgical versus endovascular revascularization when both are feasible, and unbalanced reimbursement policies create financial disincentives that compound the challenges of conducting this type of clinical research with equanimity.

In the face of this uncertainty, the TransAtlantic Inter-Society Consensus (TASC) Working Group, representing 17 cardiovascular medical and surgical organizations, took a close look at the literature to evolve a standardized system for classification of arterial lesions that reflects their suitability for endovascular, surgical, or hybrid interventions based on anatomical location and morphological complexity. ¹ The new schema extends the previous TASC I and TASC II reports, published in 2000 and 2007, respectively, and extends earlier classifications of lesions in the aorto-iliac and femoropopliteal arterial segments distally to the infrapopliteal vessels. At each anatomical level, obstructive pathology is classified into four categories of complexity. As in the earlier iterations, type A lesions represent the least complex forms such as focal stenosis, types B and C are intermediate, and type D are the most complex situations, including diffuse or occlusive forms of atherothrombotic disease.

Like any schema that attempts to organize disparate information into a logical, practical format, this one has limitations. Lesions at each anatomical level are assessed and categorized separately, without addressing the overall severity of involvement in patients with multilevel disease. The criteria are strictly anatomical, and do not account for the severity of clinical ischemia, which can be influenced by a host of patient-specific factors, including cardiac output, regional vascular resistance, collateral supply, and tissue integrity, to itemize just a few. And while there may be consensus around lesion types classified as A or D, considerable vagary surrounds those in the intermediate categories, which constitute moving targets subject to the evolution of endovascular technology.

Acknowledging these limitations and the paucity of available data, the new document is not a guideline. Treatment decisions, including whether or not to consider revascularization at all and the optimum method of revascularization (endovascular or surgical), have themselves become more complex as the array of available options widens without a corresponding expansion of data from comparative trials. Type A lesions are still most appropriately treated by endovascular methods, and the most complex type D lesions, particularly when they involve distal infrapopliteal arteries in patients with critical limb ischemia, still typically demand surgical bypass, optimally using autologous venous conduits alone or in hybrid combination with adjunctive catheter-based procedures. The management of patients with type B or C lesions straddles the divide, and by delineating these for each arterial segment, the writing group has cast down the gauntlet to prospective investigators to design and execute clinical trials to generate the data necessary to guide practice.

Assuming that the scope of endovascular technology will continue to expand more quickly than effective efforts at disease prevention, greater numbers of patients with lower extremity peripheral artery disease will be presented the option of undergoing catheter-based vascular procedures. For most, one hopes, this will bring the advantage of restoring blood flow and preserving or enhancing mobility as effectively as surgery but with considerably lower procedural morbidity. But we mustn’t lose sight of the distinction between what can be done and what should be done; the latter is invariably more comprehensive. Outcomes will generally be best when these procedures are performed at specialized centers of excellence, but caregivers everywhere can assure that they are deployed based on objective criteria for appropriate use. Linked to this is the need to assure that revascularization takes place at the right point in the course of the disease, neither too early or too late, and only in the context of a holistic plan of management that includes thoughtful discussion of the roles of lifestyle modification, exercise, smoking cessation, and associated risk reduction measures.

The authors of the new TASC report call for consideration of patient features, operator experience, and available resources, but physicians and health systems should respond more to evidence than to circumstance. The American College of Cardiology, American Heart Association, Society for Vascular Medicine, Society for Vascular Surgery, Society for Clinical Vascular Surgery, and TASC, among other societal representatives, are collaborating to develop a clinical practice guideline for management of patients with lower extremity peripheral artery disease. That writing committee, empaneled to assure comprehensive expertise but freedom from relationships with industry or other entities that could confound fair assessment of the science, employs a trustworthy methodology that includes systematic assessment of all available evidence and a robust process for expert review of recommendations. Those crafting the new guideline should consider the TASC schema as they weigh therapeutic alternatives based on pathoanatomy and clinical context.

The TASC group has delineated a classification system that enhances the lexicon, but the work of developing actionable clinical evidence has hardly begun. Those who would generate that evidence must acknowledge how therapeutics inexorably evolve toward less invasive methods. Ultimately, perhaps, every anatomical pattern of disease may become amenable to an endovascular strategy, until earlier identification of patients at risk and more effective preventive strategies reduce or eliminate the need.