Chapter 003. Decision-Making in Clinical Medicine (Part 3)

Diagnostic Hypothesis Generation Cognitive scientists studying the thought processes of expert clinicians have observed that clinicians group data into packets, or "chunks," which are stored in their memories and manipulated to generate diagnostic hypotheses. Because short-term memory can typically hold only 7–10 items at a time, the number of packets that can be actively integrated into hypothesis-generating activities is similarly limited. The cognitive shortcuts discussed above play a key role in the generation of diagnostic hypotheses, many of which are discarded as rapidly as they are formed.A diagnostic hypothesis sets a context for diagnostic steps to follow and provides...
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Chapter 003. Decision-Making in Clinical Medicine (Part 3) Chapter 003. Decision-Making in Clinical Medicine (Part 3) Diagnostic Hypothesis Generation Cognitive scientists studying the thought processes of expert clinicianshave observed that clinicians group data into packets, or chunks, which arestored in their memories and manipulated to generate diagnostic hypotheses.Because short-term memory can typically hold only 7–10 items at a time, thenumber of packets that can be actively integrated into hypothesis-generatingactivities is similarly limited. The cognitive shortcuts discussed above play a keyrole in the generation of diagnostic hypotheses, many of which are discarded asrapidly as they are formed. A diagnostic hypothesis sets a context for diagnostic steps to follow andprovides testable predictions. For example, if the enlarged and quite tender liverfelt on physical examination is due to acute hepatitis (the hypothesis), certainspecific liver function tests should be markedly elevated (the prediction). If thetests come back normal, the hypothesis may need to be discarded or substantiallymodified. One of the factors that make teaching diagnostic reasoning difficult is thatexpert clinicians do not follow a fixed pattern in patient examinations. From theoutset, they are generating, refining, and discarding diagnostic hypotheses. Thequestions they ask in the history are driven by the hypotheses they are workingwith at the moment. Even the physical examination is driven by specific questionsrather than a preordained checklist. While the student is palpating the abdomen ofthe alcoholic patient, waiting for a finding to strike him, the expert clinician is on afocused search mission. Is the spleen enlarged? How big is the liver? Is it tender?Are there any palpable masses or nodules? Each question focuses the attention ofthe examiner to the exclusion of all other inputs until answered, allowing theexaminer to move on to the next specific question. Negative findings are often as important as positive ones in establishingand refining diagnostic hypotheses. Chest discomfort that is not provoked orworsened by exertion in an active patient reduces the likelihood that chronicischemic heart disease is the underlying cause. The absence of a restingtachycardia and thyroid gland enlargement reduces the likelihood ofhyperthyroidism in a patient with paroxysmal atrial fibrillation. The acuity of a patients illness can play an important role in overridingconsiderations of prevalence and other issues described above. For example,clinicians are taught to consider aortic dissection routinely as a possible cause ofacute severe chest discomfort along with myocardial infarction, even though thetypical history of dissection is different from myocardial infarction and dissectionis far less prevalent (Chap. 242). This recommendation is based on the recognitionthat a relatively rare but catastrophic diagnosis like aortic dissection is verydifficult to make unless it is explicitly considered. If the clinician fails to elicit anyof the characteristic features of dissection by history and finds equivalent bloodpressures in both arms and no pulse deficits, he or she may feel comfortable indiscarding the aortic dissection hypothesis. If, however, the chest x-ray shows awidened mediastinum, the hypothesis may be reinstated and a diagnostic testordered [e.g., thoracic computed tomography (CT) scan, transesophagealechocardiogram] to evaluate it more fully. In nonacute situations, the prevalenceof potential alternative diagnoses should play a much more prominent role indiagnostic hypothesis generation. Generation of Diagnostic Hypotheses Because the generation and evaluation of appropriate diagnostic hypothesesis a skill that not all clinicians possess to an equal degree, errors in this process canoccur; in the patient with serious acute illness, these may lead to tragicconsequences. Consider the following hypothetical example. A 45-year-old malepatient with a 3-week history of a flulike upper respiratory infection (URI)presented to his physician with symptoms of dyspnea and a productive cough.Based on the presenting complaint, the clinician pulled out a URI AssessmentForm to improve quality and efficiency of care. The physician quickly completedthe examination components outlined on this structured form, noting in particularthe absence of fever and a clear chest examination. He then prescribed anantibiotic for presumed bronchitis, showed the patient how to breathe into a paperbag to relieve his hyperventilation, and sent him home with the reassurance thathis illness was not serious. After a sleepless night with significant dyspneaunrelieved by ...