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The Open Mind: The Open Mind

Threshold Concepts for Anesthesiologists

Barry, Deborah S. PhD; Littlewood, Keith E. MD

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doi: 10.1213/ANE.0000000000002130
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In Brief

Threshold concepts are an educational framework first envisioned during a national teaching and learning improvement project in the United Kingdom. Meyer and Land1 are the pioneers who conceptualized this framework and proposed the moniker of threshold concepts and troublesome knowledge (TC&TK). Threshold concepts can be at least superficially defined as ways of thinking and practicing that are so fundamental to a discipline that they are required for expertise, that their mastery results in a fundamental transformation of self and one’s relationship with their discipline, and that they are associated with a rupture from prior ways of knowing and doing. Originally developed during work in undergraduate economics, threshold concepts have since been applied in a multitude of disciplines and across learner levels.2 Position papers from Europe have included threshold concepts among the 10 most important pedagogical models for curriculum development3 and a fundamental tool for addressing curriculum failure.4 Although health care educators have not been early adopters, interest in this arena has slowly emerged in recent years.5–11 The purpose of the following discussion is to consider the promise of the threshold concepts framework in health care education generally and in anesthesiology specifically. Threshold concepts provide for the identification and mentored navigation of critical junctures during education, training, and practice in health care. Whether considered from the perspectives of curriculum design, mentorship, competency-based education, and/or life-long learning, threshold concepts are poised to play an important role in the evolution of medical education and practice.


The United Kingdom’s Enhancing Teaching-Learning Environments project was funded by the Economic and Research Council as part of its Teaching and Learning Research Programme.12 From 2001 through 2005, teams explored established techniques as well as developed recommendations for new practices. These included teaching-learning environments, constructive alignment, ways of thinking and practicing in a subject, and TC&TK. The fundamental theory that evolved for threshold concepts can be stated with deceptive simplicity. Meyer and Land1 came to postulate that for any discipline there are certain concepts that one must grasp and use genuinely to move toward authentic mastery of the discipline. The differentiation of threshold concepts from key or core concepts is an important distinction that will be addressed throughout this discussion.

There are several important points regarding this early work. First, although termed as concepts, these critical points in development can be thought of as ways of thinking and practicing. This linkage is apparent from the original milieu of the Enhancing Teaching-Learning Environments project and early publications on TC&TK.1,13 Second, threshold concepts can at first seem surprisingly pedestrian. Examples include the limit in mathematics, opportunity cost in economics, and precedent in law. These might initially seem straightforward concepts that are adequately treated at the level of an introductory course and understood by many generally educated people. But they have been determined by experts in the disciplines to have deeper meaning than commonly understood. Indeed, the ability to understand and manage these concepts in their essence is, in turn, required to continue on the path to expertise. Another important point is that threshold concepts are not the same as core concepts, although typically a subset of the same. Genuine proficiency with threshold concepts results in a fundamental change in the learner and his or her worldview. Further, this change is necessary to reach mastery. Meyer and Land1 suggest that we consider a threshold concept as a portal in a great wall. In Figure 1, the archway can be thought of as the portal and the space within it as a transition from older views and thinking to newer ones. The steps can be considered as core competencies. Although part of the journey to mastery, every competency does not represent a fundamental change in view, and there may be viable, albeit temporary, detours. In contrast, the portal is the only way to move forward. The learner passes through the portal from a place that is often comfortable and proven. They move toward a new space that was previously only hinted at by the old and mostly obstructed view. In the mid of this traverse, there is a state termed liminality, drawn in this analogy from the limen of the portal (Latin limin-, limen transverse beam in a door frame, threshold; probably akin to Latin limus transverse). The state of liminality is no idyllic stroll. It is tumultuous and stressful. An unsettling “rupture in knowing” is described in this process by Schwartzman16 from the work of Heidegger.17 It engages prior “certainties” and makes them unstable and unreliable. Liminality incurs a sense of loss for the old, safe space and as an “in-between” state induces coping mechanisms to deal with this challenge to authenticity and expertise. Meyer and Land1 postulated that true experts have passed through a series of such portals on their path to mastery and could not have achieved expertise had they not made this developmental journey. The challenges of the emotional state of liminality and more nuanced perspectives from educational theory will be addressed later in greater detail.

Figure 1.
Figure 1.:
The threshold concept is conceptualized as a portal through which one must pass on the road to mastery. On passing through the portal, one’s views and understanding are irreversibly changed. (Photograph by Danial Chitnis14 and licensed through Creative Commons.15)

Meyer and Land1 defined the attributes of threshold concepts that are listed in Table 1 and will be briefly described.

Table 1.
Table 1.:
Features Used to Differentiate Threshold Concepts From Core or Key Concepts

Integrative aspects of threshold concepts refer to not only the unveiling of the interconnectedness of elements but also the resulting enrichment of contextual relationships and deeper understanding. We can even think of an integration of self and discipline.18 This meaningful interconnectedness has emerged as an important part of cognitive function. Within cognitive load theory, integration has to do with the schema that allow consolidated or even automatic management of previously unlinked and overwhelming elements.19 Among simulation educators, a sophisticated method of debriefing involves the exploration of these frames in understanding actions.20

The transformative nature of threshold concepts refers to nothing less than a change in the way one perceives, understands, and is. This has been described in the following way: “Transformative learning involves experiencing a deep, structural shift in the basic premises of thought, feelings, and actions. It is a shift of consciousness that dramatically and irreversibly alters our way of being in the world.”21

This view of transformation incorporates the irreversible nature of threshold concepts. A superficial example is the understanding of “2” as representing 2 things. Once understood in this way, it is difficult to see this symbol as anything else. Subsequently, “II” comes to be understood as a different symbol for 2, part of a related but different system, reflecting the integrative nature of threshold concepts discussed above. At a higher level, the accomplished anesthesiologist can never again look at a clinical situation with his or her novice eyes. Threshold concept mastery results in being unable to see things in previous ways. This is part of what challenges the expert to understand the view of novices or colleagues and patients that do not have the same level of knowledge and expertise in the specialty.

Meyer and Land1 proposed that threshold concepts are often bounded. The new conceptual space will have boundaries with still other new conceptual areas or marking the frontiers of the discipline itself. These may demarcate divisions between disciplines or subdisciplines or “merely” the next obstacle through which a portal must be found and traversed in the journey toward expertise.

Troublesome knowledge is so integral to threshold concepts that early descriptions routinely used both terms (TC&TK). They have since evolved to include the types listed in Table 2.

Table 2.
Table 2.:
Types of Troublesome Knowledge Within the Context of Threshold Concepts and Troublesome Knowledge

The early, relevant description of types of troublesome knowledge that became part of TC&TK is attributed to Perkins.22 His work, in turn, represented an amalgamation of the earlier work of others.23,24 Perkins’22 categorization arose in a discussion defining modern-day constructivism and common challenges for both learners and educators. This seminal relationship of ideas is fitting, remembering that John Dewey is quoted as saying over 80 years ago: “The path of least resistance and least trouble is a mental rut already made. It requires troublesome work to undertake the alteration of old beliefs.”25

Perkins22 considered 4 main categories of troublesome knowledge. These were inert, ritual, conceptually difficult, and foreign knowledge. Inert knowledge, “…sits in the mind’s attic, unpacked only when specifically called for by a quiz or a direct prompt but otherwise gathering dust.” In the study of history, we might list historical facts without connection to an earlier world. In the sciences and math, theories or practice may bounce between the lecture, book, and the test but never move into everyday thinking and doing. This lack of application, reinforcement, interconnectedness, and refinement results in the knowledge remaining unused and “dusty.” Memorizing the date of the Battle of Gettysburg (or the minimum alveolar concentration of isoflurane) in isolation may get points on a test but will not advance one’s ways of thinking and practicing. Such knowledge remains inert unless it is brought into context and used in thoughtful reflection and/or important experiences.

If inert knowledge suffers from a lack of use, ritual knowledge lacks meaningfulness. A common example is the “invert to multiply” fractions rule that may be utilized routinely but is typically associated with neither advancement of understanding nor improved practice. While working with admirable reliability and efficiency, such knowledge will not be pulled into the learner’s view of mathematical relationships unless specifically addressed. It is not a part of understanding but a proven tool. There is nothing wrong with having such tools on our cognitive workbench, unless a fuller understanding and consideration is required for further development. An analogous example in anesthesiology practice might be knowing that resting Vo2 ~ 10 × kg(3/4) without understanding the semilogarithmic nature of the function, without considering relationships to body surface area or metabolism, and relying on linear approximations for daily care.

Conceptually difficult knowledge often presents itself during the study of science in our younger years. Some of the classic examples offered demonstrate that concepts in disagreement with everyday experience remain nebulous and suspect. It is still counterintuitive to many well-educated individuals that a heavier object and lighter object drop at an identically increasing velocity—in a vacuum—or that a ball rolls forever—without friction or other applied force—or that a table is pushing up as much as an object on the table is pushing down. There are few experiences or situations in which thinking in this way is either reinforced or particularly helpful in most of our day-to-day lives. Thus, inert or ritual knowledge may be pressed into service to produce the “right” answer on a test but the underlying concept is poorly applied in reasoning outside the classroom. At some level, conceptually difficult knowledge is not truly believed because it conflicts with intuition and/or prior experience. Within the practice of anesthesiology, learners may struggle with the theoretical basis of issues like uptake and distribution of volatile agents, crisis management, or patient-centeredness as they progress through their careers. Essentially all trainees will demonstrate the ability to adequately answer a question on a multiple choice test, in an oral examination, or among their peers on such concepts. For many, however, this may represent only a superficial level of understanding with shaky support from ritual knowledge.

Foreign knowledge is that which conflicts with our own perspectives. This can be deeply personal in terms of faith, class, race, or nationality. It may also be more superficial. A common example from the study of history is the tendency to apply one’s own moral judgments, knowledge of more recent and related events, and current societal positions to individuals or events of the past. This filtering removes the learner from the historical data and precludes rigorous scholarship.

Meyer and Land1 subsequently built on Perkins’22 work and suggested areas for further consideration. Foreign knowledge is often referred to as alien knowledge. Tacit knowledge, commonly attributed to work in philosophy and described as “we can know more than we can tell”26 addresses the “tradition, inherited practices, implied values, and prejudgments” of communities of practice. Within a surgical suite, we can consider the differences between the cardiac surgery, pediatric surgery, and orthopedic operating rooms. Some of these differences are clear to a casual observer, but many of the underlying shared practices and mental models of team members will be hidden from the novice learner. Additionally, the expert may actually find it difficult to explain the basis of many of their own concepts and practices because those practices are so ingrained, successful, and accepted within this sphere. The novice is thus an outsider to this shared culture, language, and practice.

Other types of troublesome knowledge can be thought of as extensions of the types described above. Student-defended knowledge16 is one such example that, again, reflects the hard and unsettling work of steering off of Dewey’s comfortable, well-worn path. The confusion and emotional turmoil of learning can turn the student toward refuge in current ways of thinking and the comfort of the prior successes of such thinking. This can be thought of as being an opposing force to reflection and eventual acceptance of a new viewpoint and practice.


The state of liminality is crucial to the discussion of threshold concepts. Liminality should be understood first and foremost for the vulnerability required of the learner. Cousins, as quoted by Boys,27 notes the volatility of this point in development:

This space is likened to that which adolescents inhabit, in which they are not yet adults, and not quite children. It is an unstable space in which the learner may oscillate between old and emergent understandings, just as adolescents often move between adult-like and child-like responses to their transitional status. But once a learner enters this liminal space, she is engaged with the project of mastery, unlike the learner who remains in a state of pre-liminality in which understandings are at best vague.

The idea that learners enter into a liminal state in their attempts to grasp certain concepts in their subjects presents a powerful way of remembering that learning is both affective and cognitive and that it involves identity shifts which can entail troublesome, unsafe journeys. Often students construct their own conditions of safety through the practice of mimicry. In our research, we came across teachers who lamented this tendency among students to substitute mimicry for mastery.

Anesthesiologists practice in clinical environments that are often high-intensity, high-risk, and unforgiving of error. In our own individual development as practitioners or as facilitators in the education and training of others, the tension between the inherent risk-taking of new techniques or thinking versus the relative comfort of proven, generally adequate paths is familiar. The model of threshold concepts helps embrace the state of liminality as necessary and expected and allows for educational design to provide for both learner support and patient safety.

Another important aspect of liminality concerns the responses that do not lead to success. Disengagement simply means that the learner backs away from the hypothetical portal. The challenge is too great, the change required too unsettling, and the familiar territory too comfortable, reliable, and functional. The learner now falls behind in terms of sophistication and cannot progress with the same depth and authenticity as their peers who have traversed the portal. An important question arises in the form of how many students and trainees may turn to a different path, simply because of the incomplete and sometimes formidable view afforded from the novice side of the threshold. Mimicry is a superficial copying of expert behaviors, language, and visible strategies. While not robust, mimicry may allow some success in routine situations. This is considered by some to be nothing more than well-disguised disengagement. In truth, it can be a coping mechanism used by anyone during liminality, albeit hopefully short-lived. If we reflect on our own experiences, we may recall times when we felt counterfeit and undeserving of our success, or our position, or the trust, respect, and expectations of our patients and colleagues. Described in just such terms, the imposter phenomenon (IP) has received much attention in education.28 We have all probably felt this phenomenon, at the very least, in our own liminal states. With full recognition of individual variations, it is imperative to consider learner groups that may generally be at high risk of the IP.29,30 If learners feel like outsiders and pretenders in the best of times, the liminal state logically exacerbates these issues and decreases the likelihood of successful transformation. It is unlikely that societal and cultural inequities of gender, class, race, religion, and so on will soon change. This risk of IP must be met first with acknowledgment and then with specific remedies31 for what can become “stuckness”32 in the liminal state. This is appropriate for all learners but especially for high-risk individuals and groups.

Simulation educators are likely to immediately think of a properly run immersive scenario and debriefing as the perfect setting for learners in the liminal state. In fact, much of the language of debriefing similarly considers the transformation required within the learner. In simulation education, most agree that safety and support must be afforded by the learner to preserve the malleability arisen from the emotional forge of a meaningful simulation.33 Simulation experiences may thus provide the best modality available for addressing threshold concepts. Of course, most anesthesiologists in training and certainly in practice spend a small fraction of their professional time in simulation. The lessons of simulation and threshold concepts, however, do logically translate to the clinical environment. Here, too, it will be important to establish an environment that is as supportive and safe as possible to avoid disengagement and/or prolonged mimicry. A further challenge is the paramount issue of patient safety as balanced with these educational goals.


A few examples of defined threshold concepts may be illustrative at this point.1 Within mathematics and applied science, the complex number, which includes a real and an imaginary term, was found early on to be an exemplar. One must “unlearn” a well-honed understanding that the square of any number is positive and work with i, the square root of −1. This is no trivial abstraction, playing roles in everything from theoretical mathematics to signal process engineering. Mathematics also offers the example of limit. A function f(x) = sine(x)/(x), with x approaching zero (x 0) actually approaches 1. This requires a new way of thinking about function behavior and the infinitesimal surroundings of a point. Standard graphs and math skills with prohibition of “dividing by zero” are supplanted. Calculus and its applications rely on a deep understanding of limits. As previously noted, opportunity cost is defined as a threshold concept within economics. This new way of thinking about both included and excluded choices requires that lost opportunity be considered as a choice influence in the same way as the expected rewards of the selected option.

The next important issue is to consider how threshold concepts have been identified in various areas of education and practice. Several case studies of threshold concepts in a variety of disciplines such as accounting, biology, chemistry, land management, philosophy, and economics have been previously described in the literature.32 Barradell34 reviewed the methodological challenges related to identifying threshold concepts in 3 fields: economics, engineering, and health care. Her review indicates that the majority of research related to identifying threshold concepts has been completed from the instructor perspective, based on their reflections, and discussions of students’ learning.35 Direct observations of student behavior or interviews with students are far less common.36,37 To identify threshold concepts in economics, Davies and Mangan35 utilized a variety of techniques, including a broad survey of introductory economics lecturers from 2 universities, the careful review of student exam responses over several years, the analysis of 120 first-year undergraduate students to 3 open-ended free response questions, and at the end, of course, questionnaire. This approach yielded 2 theoretical tools that can be applied to other disciplines as well as defining characteristics of conceptual change and transformative integrations. The authors also suggest that threshold concepts may exist as a web instead of stand-alone concepts. Thus, transformational shifts in understanding that occur with threshold concepts effect disciplinary understanding.

Park and Light37 utilized a student-centered approach to understanding threshold concepts in chemistry. They conducted in-depth interviews with a subsample of students in an introductory chemistry course to investigate the threshold concept of atomic structure. Their study involved instructors identifying the source of troublesome knowledge, surveying students, and finally selecting a subset of students who held different conceptions of atomic theory before and after the course. Their study was aimed at better understanding student learning difficulties in this particular content arena.

Lucas and Mladevonic38 suggest that emotions may provide evidence of threshold concepts. As applied to the health sciences, students may face difficulties accepting that medicine is not an objective science, and providing diagnoses simply based on lab results is not as certain as they might hope. Uncertainty provides a difficult challenge for many students and may inhibit some students from engaging fully with a discipline. Lucas and Mladecovonic38 discuss the example of accounting, in which students’ preconceived ideas about numbers always providing an objective and clear answer provides a barrier to student learning. This response is also applicable to the health sciences. However, as was previously discussed, barriers to student learning are not the only consideration when identifying threshold concepts. As the threshold concept framework has only been scarcely applied to the health sciences, we have drawn from the methodologies that have been utilized in other fields.

Author K.E.L. has developed an approach for finding possible threshold concepts in clinical reasoning and management. The first step is candid reflection on one’s own career. Clues include concepts that were elusive and/or mastered later than expected and turn out to be fundamental. Discussion with peers can often validate and expand these findings, as has been shown previously within the literature identifying threshold concepts in other fields.38 If these concepts now seem intuitive and very important, and yet are difficult to simply explain to patients, colleagues, or trainees, then transformation from a novice view has occurred. In the second step, consider concepts that elicit faculty laments of “Why don’t they get this? Every year, it is the same thing no matter how we teach it.” Our young colleagues are as smart and competent as were we. The question of why cohort after cohort of overachieving learners continues to lack mastery of certain core competencies despite curricular adaptation is critical. A final clue may be elicited from students and trainees. Threshold concepts can manifest as a wide cross-section of learners stumbling unexpectedly in reaching deep understanding or proficiency. Purposeful discussion with learners who typically fall across the performance spectrum can be very helpful. TC&TK should be considered when such a wide-ranged sample demonstrates very similar misconceptions, “stuckness,” and/or failures of application.


With the basis, origin, and methods of identification of TC&TK now considered, we turn our attention to the potential for application in anesthesiology training and practice. A comprehensive identification of threshold concepts should not arise from one author, group, or institution. Rather, in this section, we shall suggest a few candidates that might be worthy of further consideration and future discussion. Figure 2 illustrates a few of the many possible connections that TC&TK have in the careers of anesthesiologists.

Figure 2.
Figure 2.:
A few of many possible connections between threshold concepts & troublesome knowledge (TC&TK) and various roles and career stages in anesthesiology. EPA indicates entrustable professional activities.

Interprofessional practice (IPP) is an everyday occurrence within anesthesiology. As an obvious example, many anesthesiologists practice in an anesthesia care team model. More importantly, anesthesia care is typically delivered in a setting that involves providers from other medical specialties, nursing, and allied health care. The growing evidence that improved IPP is related to improved patient outcomes39 adds to the urgency of this discussion. Interprofessional education is still new or nonexistent in many curricula and training programs. Most practitioners have thus come to some way of understanding and practicing within this increasingly complex environment even if without reflection or awareness of IPP competencies40 or theory. Current practice often arises from trial and error, emotionally charged experiences and outcomes, and incomplete understanding rather than a defined, mentored experience with structured feedback.

Cognitive load is a concept that is difficult to appreciate and manage. As we gain expertise, we are clearly handling more and more complex challenges with greater aptitude. It is logical to believe that, with enough experience, we can handle huge amounts of discrete data and respond appropriately because this is exactly what appears to happen superficially. This applies to everyday experiences from learning to drive to the clinical challenge of taking care of complex patients. However, it is important to understand that we all have a severely limited number of cognitive elements that we can handle at one time.22 It is the organization of what were once many elements into a single element that creates schema that are tested and refined. These eventually allow experts to handle as single elements that which is cognitively overloading for the novice. Coming to understand (and believe) that the working memory bottleneck is insurmountable promotes meta-cognition of our own development. The cognitive biases of heuristics and intuition41 become connected to everyday work. With this understanding, the use of cognitive aids, perhaps a threshold concept in and of itself in medicine,42 has a scientific basis and is not a crutch for the lazy or weak.

The Illness Script43 is a reorganization of knowledge from the classically taught horizontal structure of system and disease into a vertical structure that helps consider defining features and differences. This network becomes the framework of clinical reasoning, especially in terms of system 1 thinking.41,44 The formulation of illness scripts appears to be necessary in the development of clinical expertise. These concepts are now a part of the curriculum at many medical schools, but for many established practitioners, this was a path traveled alone throughout their career. Introducing illness scripts as a threshold concept could smooth and speed this crucial development in clinical reasoning.

Threshold concepts may be especially helpful in the era of Entrustable Professional Activities45 and Milestones.46 If TC&TK can be identified among these expectations, then curricula and assessment can be proactively designed to prevent learners from simply “checking a box” with superficial performances. Within the threshold concept framework, educators can instead provide the opportunity for deep learning and demonstration of successful mastery. This will, in turn, prepare them for the next developmental challenge. Meaningful metrics should help validate progress and, as previously noted,34 may help engagement during these “unsafe journeys.”

The authors, as members of an education team, have found a notable threshold concept among our second-year medical students. Ventilation-perfusion (V/Q) mismatch met the criteria for threshold concepts. This is an apt example because most students could perform adequately on formative test questions and the concept seems straightforward. However, performance deteriorated when students were, for example, challenged to explain the changes seen in progression from moderate to severe chronic obstructive pulmonary disease. It became clear that few students were facile with changing V/Q relationships throughout the lung. The faculty therefore provided support by articulating the fundamental importance of mastering this concept in pulmonary medicine, conducting sessions with worked examples and clinical presentations, and demonstrating the interconnectedness of V/Q matching throughout pulmonary medicine.


It is important to note that threshold concepts, as a theory, have not yet been universally accepted within the education community. From the time of early adoption, there have been well-argued challenges to the validity, utility, and/or practicality of threshold concepts. Additionally, the threshold concept framework is relatively new and rapidly evolving. This evolution is occurring both within and across a wide variety of disciplines. An ever-increasing number of educators have generated an impressive array of books, presentations, articles, and meetings devoted to threshold concepts.2 In point of fact, adoption has occurred before either rigorous demonstration of validity or development of best practices. This, in turn, has resulted in variations of interpretation and implementation that, for critics, calls into further question whether threshold concepts represent a coherent, definable, and transferable set of ideas and practices.

An early and eloquent challenge of threshold concepts is that of Rowbottom,47 written within a few years of Meyer and Land’s1 original description. More recent discussions34,48 echo these concerns, albeit with recommendations for possible remedies of shortcomings. One of the most common criticisms of threshold concepts is that they are inadequately defined to be unique and identifiable. This is a reasonable concern. None of Meyer and Land’s1 criterion are absolute, and different disciplines and authors seem to emphasize particular areas over others as noted above. The questions can be reasonably asked, for example, of why a threshold concept is different than any other key or core concept with such loose definitions, how is transformation measured and how much is enough, or are threshold concepts the same for everyone—ie, is this some externally defined phenomena or actually an individual internal process? Such questions will need to be addressed with serious scholarship in the further development of practices and theory. Whether this work will be done and the results thereof satisfy the critics of threshold concepts remains to be seen. There is precedent, for example, in the case of cognitive load theory, for those with apparently incompatible viewpoints49–52 to actually come together on at least some aspects of educational practice.53 The eventual place of threshold concepts in general educational theory is, at this point, unknown. What is clear is that a large cohort of educators from a wide range of disciplines have found value and substance in threshold concepts and formed an enthusiastic international community reflecting their experience.


In closing, let us consider the use of threshold concepts from an effort–benefit perspective. The introduction of this discussion proposed that the threshold concepts framework provides for the definition and mentored navigation of critical junctures during education, training, and practice in health care. Successful implementation would mean that essential mastery of restrictive points in development could be anticipated, supported, and verified. Superficial performance that wastes the time and benefit of subsequent educational encounters could be minimized in frequency and duration. Alternatively, a learner unable or unwilling to step into and through liminality despite counseling, educational resources, and alternative modalities could be identified as suspended or stuck. Understanding liminality and the IP can help better understand and help this colleague.

Even in the unlikely event that threshold concepts turn out to be a passing fad in health care, there is really nothing lost. The work done would still be well invested, regardless of one’s belief in the theory discussed. Key concepts in the development of an anesthesiologist that are necessary for success will have been identified and addressed in curriculum and practice. Consider additionally that Entrustable Professional Activities and Milestones are defined ways of thinking and practicing that will require evaluation. Some of these will almost certainly emerge as challenging to learners to an extent that surprises and frustrates the expert. TC&TK helps understand these occurrences. Even for those who do not subscribe to TC&TK, the mere identification of such unexpected hurdles should be helpful. Thus, even as their educational underpinnings are being debated, TC&TKs should promote improvements ranging from better educational design to more efficient life-long learning. Whether adopted incrementally or in total, we suggest that the model of threshold concepts is invaluable in understanding the career development of clinicians.

The next chapter in the evolution of threshold concepts will presumably occur on several fronts. The continued debate of the theoretical basis and the refinement of criterion will be necessary for more complete acceptance by critics in the educational community. There is every indication that the adoption of this framework is continuing to grow and the community of advocates increasing along with continuing reports of the worldwide experience in implementing threshold concepts. It is now time to quantitatively investigate the effectiveness of the threshold concepts framework in improving learner outcomes. This, in the end, will define the roles of the threshold concepts framework in everyday educational practice and life-long learning.


Name: Deborah S. Barry, PhD.

Contribution: This author helped with research of threshold concept identification methods in the education literature, summation of these findings and pertinent references for manuscript, clarification of example threshold experience from experiences during University of Virginia preclinical pulmonary system as member of education team, and drafting reviews and collaboration in preparation for submission.

Name: Keith E. Littlewood, MD.

Contribution: This author helped with literature review of threshold concepts; summation of origins, basis, and examples of threshold concepts; discussion of identification in anesthesiology and potential uses; discovery of example from preclinical system; and review of drafts.

This manuscript was handled by: Edward C. Nemergut, MD.


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