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Mechanistic Case Diagramming

A Tool for Problem-based Learning

Guerrero, Anthony P. S., MD


Without appropriate learning tools to guide the application of knowledge to clinical situations, medical students in a problem-based learning (PBL) curriculum may revert to traditional modes of learning and hence deprive themselves of PBL's educational benefits. The author describes a technique involving the stepwise diagramming of pathophysiologic mechanisms leading from underlying causes to the symptoms and findings presented in a PBL case. This technique can guide clinical reasoning during an initial problem encounter, the identification of pertinent learning issues, and the subsequent integration of knowledge as relevant to the patient's case. The author suggests that training students and tutors to use this technique may allow the former to fully realize the benefits of PBL, including the ability to organize information for use in clinical contexts and the ability to critically reflect upon their own learning.

Dr. Guerrero is assistant professor of psychiatry, director of medical student education in psychiatry, and assistant professor of pediatrics at the University of Hawaiì John A. Burns School of Medicine, Honolulu.

Correspondence and requests for reprints should be addressed to Dr. Guerrero, Department of Psychiatry, Kapi'olani Medical Center for Women and Children, 1319 Punahou Street, Honolulu, HI 96826; telephone: (808) 945-1500; fax: (808) 945-1506; e-mail: 〈〉.

The goal of problem-based learning (PBL) is to endow medical students with the essential skills of clinical reasoning, cooperative learning, and patient-based integration of knowledge. There is evidence that, compared with traditional educational methods, PBL enhances the application of concepts to clinical situations, increases the long-term retention of knowledge, and fosters lifelong interest in learning.1

Such outcomes, however, are not necessarily guaranteed through the mere implementation of a PBL-structured curriculum. More specifically, without the appropriate learning tools to ensure that essential PBL processes are adhered to, students may unwittingly revert to traditional modes of learning such as reciting “cookbook” lists of differential diagnoses for a given symptom or presenting textbook-driven lectures at the expense of group discussion and case reformulation.

In deciding whether or not a learning tool is truly ideal for a PBL curriculum, it is first necessary to identify the specific learning objectives that are optimally fulfilled through PBL. I agree with Barrows2 that the most important goals of PBL include: (1) the structuring of knowledge for use in clinical contexts; (2) the development of an effective clinical reasoning process; (3) the development of effective self-directed learning skills; and (4) increased motivation to learn. Furthermore, I agree with him that the achievement of these goals is best facilitated by what he refers to as “closed-loop” or “reiterative” problem-based learning, in which the initial free-inquiry process and subsequent self-directed learning are followed (in what I refer to as the “reformulative” phase of PBL) by an application of the acquired information back to the case and a critical re-evaluation of the clinical reasoning generated by the case as initially presented. Therefore, the ideal learning tool for PBL should enable students to optimally realize the above four goals and should provide a lasting “thought record” that enables retrospective evaluation of clinical reasoning.

With the hope of optimizing the educational benefits of PBL at the University of Hawaiì John A. Burns School of Medicine, I and my colleagues have encouraged the use of “mechanistic case diagrams” as a learning tool for students. In this article, I present the mechanistic case diagram, which is similar in approach to the “etiologic flowchart” described by Engelberg.3 I then suggest potential uses for this tool in a closed-loop PBL curriculum.

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The mechanistic case diagram is a student-constructed tool whose objective is to trace, in stepwise form, the pathophysiologic mechanisms leading from underlying causes of disease (including genetic, microbiologic, and social) to the clinical signs and symptoms and psychosocial consequences described in a PBL case. I illustrate this basic procedure using a simplified case, whose diagram is shown in Figure 1.

Figure 1

Figure 1

I assume for the purposes of this discussion that all research needed to understand this case has been completed. Keeping in mind that I must ultimately explain this patient's presenting symptoms of loss of consciousness and pallor and the key diagnostic findings of poor cardiac contractility and an abnormal electrocardiogram, and keeping in mind the initial hypothesis that this young patient's cardiac disease is most likely attributable to his methamphetamine abuse, I proceed with the first steps of the diagram. I begin by writing “methamphetamine abuse” in a fairly central location on the chalkboard. I will then ask myself: “How exactly did this patient's methamphetamine abuse result in his loss of consciousness associated with pallor?” I use solid arrows to sequentially trace the mechanisms underlying his clinical findings. An example of such a mechanistic sequence would read: “methamphetamine results in (solid arrow) release of epinephrine and norepinephrine, which results in (another solid arrow) increased myocardial demand, which results in functional ischemia, which results in diffuse cardiac necrosis, which results in partial atrioventricular node damage, which results in creation of a slow pathway, which results in abnormal conduction of an atrial impulse, which results in re-entry through the unharmed fast pathway, which results in tachycardia, which results in decreased diastolic filling, which results in decreased cardiac output, which results in poor perfusion, which disrupts normal functioning of the reticulocortical pathways in the brain, which finally results in loss of consciousness.” I enclose “loss of consciousness” in a box to identify it as a presenting symptom and as a relative endpoint on the diagram. Other parts of the diagram are completed in a similar fashion, so that there are mechanisms to explain each of the clinical and laboratory findings.

Therapeutic interventions, such as “digoxin,” are enclosed in ovals and are linked by means of dotted arrows to their exact points of action in the pathophysiologic sequence. In this particular diagram, I see that by increasing contractile force and decreasing atrioventricular conduction, digoxin intervenes at two points in the pathophysiologic sequence.

Finally, I notice that the case describes possible genetic factors (e.g., a family history of substance abuse) and psychosocial factors that possibly contributed to this patient's substance abuse. Hence, I diagram how these factors possibly led to this patient's substance abuse. I then underline and asterisk “genetic factor,” “adolescence,” and “recent emigration” to identify these as the most proximal etiologic factors and relative starting points for the purposes of this diagram.

For clarity's sake, I have tried to organize the diagram so that all solid arrows mean “result in,” and I have tried to make the level of mechanistic detail suit the learning needs of the a PBL tutorial group. So far, the mechanistic diagram I have described is identical in approach to the etiologic map described by Engelberg,3 who used this tool to promote case-based “integrative study” as a supplement to traditional learning in physiology. Next, I describe how using the mechanistic approach “in reverse” can result in a diagram that can organize thought processes specifically elicited during the initial phase of the PBL process.

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The first page of a new PBL case introduces a 19-year-old man with a vague history of substance abuse who presents with loss of consciousness and pallor. Although it may be tempting to refer to a clinical manual and list the differential diagnoses for “loss of consciousness,” clinical reasoning skills would probably be better practiced if I approach this presenting symptom in a mechanistic fashion. Although I may have no prior experience in a clinical setting, by simply asking the question: “What are the mechanisms by which consciousness is interrupted,” I may be able to at least propose that consciousness is related to certain parts of the brain, and that any pathologic process (physiologic and anatomic) affecting normal functioning of the brain could result in loss of consciousness. Relying on the existing database, I could hypothesize: inadequate energy production in the brain (secondary to hypoxemia, poor perfusion, or lack of substrate); physical impingement in the brain (secondary to mass lesions or increased intracranial pressure); and abnormal transmission in the brain (secondary to a seizure or neurotoxins). From these general mechanisms, more specific differential possibilities (e.g., cardiac dysrhythmia, head trauma, etc.) can then be traced backwards, as shown in Figure 2.

Figure 2

Figure 2

By attempting to integrate presenting information into this diagram, I can decide which hypotheses are more supported by the facts than others. For example, the possibility of a cardiac dysrhythmia may be favored, as it can mechanistically explain both the loss of consciousness and the pallor (although I may not know yet exactly how a cardiac dysrhythmia relates to substance abuse). By then reviewing the list of favored possibilities, I may identify additional information needed to rule in or rule out some of the hypotheses. For example, the favored possibilities of a substance-related loss of consciousness and a cardiac dysrhythmia could be investigated by requesting a more detailed history of the substance use and current vital signs. Finally, by recognizing that our understanding of the steps linking “disruption of the brain” to “loss of consciousness” may be unclear, I can identify “the anatomy and physiology of consciousness” as a specific potential learning issue.

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I believe that students derive the greatest educational benefit from the initial problem-based encounter if it enables them to apply previously learned information to a novel situation; to logically generate and prioritize hypotheses; to systematically identify additional case information and learning issues; and to preliminarily formulate a case. These basic PBL processes would be subverted if students were to rely on clinical manuals, rather than on their own knowledge databases and clinical reasoning skills, to generate differential diagnoses and to propose further workup, or if they were to rely on preconceived (usually exam-motivated) lists of broad topics, rather than on their own knowledge gaps identified through the experience of a patient care problem, to generate learning issues. Mechanistic diagramming, as described in the preceding section, may provide students with a framework for applying their knowledge bases, and for systematically pursuing further clinical investigation while adhering to the basic PBL processes outlined earlier. Furthermore, it may provide them with a patient-based framework that can guide their followup research, which will be integrated during the subsequent, reformulative phase of PBL.

To obtain the most educational benefit from the subsequent, reformulative phase of PBL, students must be given the opportunity to integrate cross-disciplinary knowledge as made relevant to the case, to address psychosocial issues, to engage in cooperative learning, to identify flaws in their logic and further gaps in their knowledge, and to retrospectively evaluate their thought processes during the initial problem encounter. These basic PBL processes might likewise be subverted if students were to present to each other, without interactive feedback, information whose relevance to the case is ambiguous.

As Figure 1 makes clear, the mere task of constructing such a diagram should allow for at least some of the goals of the reformulative phase to be fulfilled. Specifically, instead of leaving the case with only fragmented knowledge of cardiac physiology, students would appreciate how such knowledge enables them to understand this patient's presenting symptoms. Furthermore, in having incorporated certain aspects of this patient's psychosocial history into the diagram, they will not have neglected the holistic perspective that should be integral to any patient-based approach to learning.

Although interactive discussion could occur in its absence, the mechanistic case diagram can also serve as a nidus for group discussion. Individual members may be motivated to share their knowledge as they see gaps in the diagram that can be filled with a mechanistic sequence that they understand. For example, if one student has researched the genetics of drug addiction, she might be able to propose the sequence (shown in Figure 1) leading from “genetic factor” to “methamphetamine abuse.” Also, as a visual representation of the group's thinking, the diagram can clarify areas of confusion among group members and can lead to the identification of further learning issues. For example, if one student believed, in disagreement with the others (and in spite of having prepared a polished lecture on basic cardiovascular physiology), that tachycardia could be manifest only during acute drug use, the presentation of the mechanism leading from “diffuse cardiac necrosis” to “tachycardia” by the other students might enable this student to understand their hypothesis. The group may suggest that the negative urine toxicology and the ECG interpretation of “paroxysmal supraventricular tachycardia” (PSVT) support the latter mechanism. However, they may then discover that, although they incorporated the ECG interpretation of PSVT into the diagram, they were not able to mechanistically explain the individual ECG characteristics (e.g., loss of P waves) that enabled this diagnosis to be made. A further learning issue may therefore be the correlation between cardiac electrophysiology and the electrocardiogram. A revised diagram may then be constructed, with each of the ECG characteristics becoming the boxed endpoints in the diagram.

Finally, if the group had saved the diagram that they had constructed during the initial problem-based encounter, they would be able to critically evaluate their original clinical reasoning. With their newly learned knowledge, they would be able to see that some of their original hypotheses, while mechanistically sound, could have lower priority, given their relatively low incidences (e.g., cerebrovascular atherosclerosis in a 19-year-old). However, they would hopefully appreciate that, even though their understanding of mechanisms might not have been complete, their preliminary proposal that “drug abuse somehow results in a cardiac dysrhythmia, which (via the sequence depicted in Figure 2) ultimately results in loss of consciousness and pallor” was a surprisingly accurate conceptualization that provided the basis for their subsequent, more refined diagram (Figure 1).

A mechanistic case diagram, drawn on the chalkboard, can be used to guide group discussion during the initial phase of the PBL process. This initial diagram (the “hypothesis diagram”), copied on paper by the students, can then guide the research that follows the initial-problem approach. During the subsequent reformulative phase of the PBL process, the group can spend part of their time constructing on the board another mechanistic case diagram (the “summary diagram”) that incorporates relevant new information and the rest of their time evaluating the earlier thinking recorded in their “hypothesis diagram.”

In addition to its use as a PBL learning tool, mechanistic case diagramming has other applications. For example, diagrams constructed by tutorial groups can reveal to the tutor or other curriculum evaluators the extent to which students can apply cross-disciplinary knowledge to patient care problems and the depth of their understanding of basic pathophysiologic mechanisms. Also, the diagrams may reveal learning issues that were and were not prompted by the case as presented, and therefore can potentially be used to guide the revision of PBL cases. For example, if “the pharmacology of calcium-channel blockers” had been an intended learning issue for the case diagrammed in Figures 1 and 2, but the summary diagrams produced by the tutorial groups did not contain any reference to calcium-channel blockers, then one convenient way to ensure that this learning issue is meaningfully covered might be to modify the case so that a calcium-channel blocker is, at some point, part of this patient's management regimen. With this last thought in mind, I might further suggest that the mechanistic case diagram be used (akin to how a different learning tool—the “concept map”—has been used by Edmonson4 at the College of Veterinary Medicine at Cornell University) as a blueprint to guide the creation of cases for a PBL curriculum.

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Mechanistic case diagrams, when applied to problem-based learning, can enhance students' ability to structure their knowledge for use in clinical contexts and to reason effectively when confronted with patient care problems. Such goals are fundamental to problem-based learning, and if optimally fulfilled, can allow students to appreciate the clinical relevance of their learning. Mechanistic case diagrams, therefore, have the potential to enhance students' intrinsic interest in learning. Also, the structure provided by mechanistic case diagrams enables students to evaluate their own learning needs and to subsequently improve the efficiency of their self-directed learning. Finally, the hypothesis diagram and summary diagram encompass both “reverse thinking” (from presenting symptoms to causes) and “forward thinking” (from causes to presenting symptoms) and hence provide closure for Barrows' PBL loop.

All of the above assertions remain to be proven through objective studies. At present, brief training in mechanistic case diagramming is provided to students in their first month at the University of Hawaiì School of Medicine. It may be possible to expand this training and to include a training component for tutors. I and my colleagues are interested to see what effects such training may have on students' abilities to summarize and organize information, to address psychosocial issues, and to conceptualize and reflect during the third year and beyond—all of which, according to Brandon et al.,5 are potential areas for improvement in our program.

In view of the potential lifelong benefits of problem-based training, it would behoove educators to critically evaluate the way their students learn and to investigate new learning tools that effectively advance core PBL processes. As a learning tool that promotes integrative, interactive learning and that enables students to realize the human relevance of their learning, mechanistic case diagramming should be further studied for its value in optimizing the educational benefits of PBL.

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1. Norman GR, Schmidt HG. The psychological basis of problem-based learning: a review of the evidence. Acad Med. 1992;67:557–65.
2. Barrows HS. A taxonomy of problem-based learning methods. Med Educ. 1986;20:481–6.
3. Engelberg J. Complex medical case histories as portals to medical practice and integrative, scientific thought. Am Physiol. 1992;263 (Adv Physiol Educ 8):S45–S54.
4. Edmonson KM. Concept maps and the development of cases for problem-based learning. Acad Med. 1994;69:108–10.
5. Brandon PR, Lindberg MA, Higa TAF. Curriculum Evaluation Bulletin. 1994, October;3:1–3.
© 2001 Association of American Medical Colleges