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PATIENT 
Mr. R is a 55-year-old man with chronic obstructive pulmonary disease (COPD) with a chief complaint of dyspnea. He reports that symptoms began 5 days ago with a cough productive of green sputum. The cough worsened, and 4 days ago he had a low-grade fever of 37.2°C. He noticed increasing shortness of breath 3 days ago. He reports that previously he was able to walk about 25 feet before becoming short of breath but now he is short of breath at rest. Last night his fever reached 38.8°C, and today his dyspnea intensified. He is unable to complete a sentence without pausing to take a breath. On physical exam, he appears older than his stated age. He is gaunt, sitting upright, breathing through pursed lips, and in obvious distress. Vital signs are temperature, 38.9°C; RR, 28 breaths per minute; BP, 110/70 mm Hg; pulse, 110 bpm. His pulsus paradox is 20 mm Hg. Lung exam reveals significant use of accessory muscles and markedly decreased breath sounds. Cardiac exam is notable only for diminished heart sounds.
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Your resident is concerned about the adequacy of Mr. R.’s ventilation and suggests checking his pulse oximetry. You remind him that a pulse oximeter will not address the adequacy of the patient’s ventilation nor will it determine whether respiratory failure is present and suggest an ABG.
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An ABG reveals a pH of 7.22, PaCO2 of 70 mm Hg, and PaO2 of 55 mm Hg.
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Always check an ABG when the adequacy of a patient’s ventilation is a concern. Patients with adequate oxygenation may still be in respiratory failure.
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Clearly, Mr. R has several problems that are easily identified, including (1) fever, cough, and history of COPD; (2) respiratory distress; and (3) acidosis. All of these problems are obviously potentially life-threatening. Furthermore, a thorough evaluation of the acidosis may shed light on the status of the other problems.
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At this point, what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis? Given this differential diagnosis, what tests should be ordered?
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RANKING THE DIFFERENTIAL DIAGNOSIS
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Step 1: Determine Whether the Primary Disorder is an Acidosis or Alkalosis by Reviewing the pH
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The low pH confirms the primary disorder is an acidosis.
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Step 2: Determine Whether the Primary Acidosis or Alkalosis is Metabolic or Respiratory by Reviewing the HCO3− and PaCO2
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PaCO2 of 70 mm Hg; Na+, 138 mEq/L; K+, 5.1 mEq/L; HCO3−, 27 mEq/L; Cl−, 102 mEq/L; BUN, 30 mg/dL; creatinine, 1.2 mg/dL.
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The PaCO2 and HCO3− are both elevated. An elevated PaCO2 would lower pH and cause an acidemia ...