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PATIENT 
Mr. C is a 64-year-old man who comes to see you complaining of shortness of breath.
What is the differential diagnosis of dyspnea? How would you frame the differential?
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CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
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The differential diagnosis of dyspnea is extraordinarily broad but following a structured and sequential approach to the patient with dyspnea can allow for the rapid identification of common and serious disorders while also ensuring that other causes are not missed. Heart disease and lung disease constitute the most common serious causes of dyspnea, specifically heart failure (HF), pneumonia, obstructive lung disease (chronic obstructive pulmonary disease [COPD] and asthma), and pulmonary embolism (PE). These four conditions should always be on the differential diagnosis for dyspnea and at the forefront of the clinician’s mind in evaluating the dyspneic patient. A series of other less common disorders represent an immediate risk to life and must also be considered in the very early stages of investigation, as urgent treatment may be necessary. These diagnoses can be remembered using the ACT ASAP mnemonic (Table 15-1).
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Once these common and life-threatening disorders have been considered, a more systematic approach to the many other causes of dyspnea can proceed. The simplest approach to constructing the broad differential diagnosis is to consider the anatomic components of each of these systems, specifically the route of inspired air and oxygen through the cardiopulmonary system and the rest of the body. This allows us to develop a fairly comprehensive differential diagnosis of dyspnea.
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Over the last 2 years, Mr. C has noticed worsening dyspnea on exertion. He now complains of shortness of breath with minimal exertion. He is unable to walk around his house without resting to catch his breath. Several years ago, Mr. C could walk several blocks without any difficulty. He notes that he is unable to sleep lying flat due to shortness of breath (orthopnea), and he has slept on a recliner for the last 6 months. Occasionally, he awakes from sleep acutely short of breath (paroxysmal nocturnal dyspnea). He complains that his feet are swollen.
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Past medical history is notable for an MI 2 years ago. Vital signs are temperature, 37.0°C; RR, 24 breaths per minute; pulse, 110 bpm; BP, 120/78 mm Hg. His pulse is regular with an occasional irregularity. Cardiac exam reveals JVD to the angle of the jaw in ...