Correspondence

Clinical Problem-Solving: Diagnosis by Drug Reaction

N Engl J Med 1993; 328:1790-1791June 17, 1993

Article

To the Editor:

The Clinical Problem-Solving case “Weak Reasoning: Diagnosis by Drug Reaction” (Feb. 4 issue)1 underscores how essential a focused history is in the initial evaluation of an acutely ill patient. Given the amount of effort that went into arriving at the ultimate diagnosis of myasthenia gravis, it is ironic that this young woman with chest congestion, shortness of breath, and acute respiratory distress was not asked the simple question, What is your occupation? It would also have been useful to know whether she had come from work, home, or elsewhere. Although there was a brief mention near the end of the case that she had denied exposure to pesticides, her initial presentation could easily have been consistent with metal fume fever, polymer fume fever, or hypersensitivity pneumonitis2. Occupational asthma would be less likely but possible. Such exposures can occur in both large and small industries, affecting both men and women. The value of taking a simple occupational history lies in the fact that all these disorders can be readily reversed by removing the patient from the place of exposure.

James Craner, M.D.
University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School, Piscataway, NJ 08854

2 References

1
Pauker SG, Kopelman RI. Weak reasoning: diagnosis by drug reaction. N Engl J Med 1993;328:336-339
Full Text | Web of Science | Medline

2
Kipen HM, Craner J. Sentinel pathophysiologic conditions: an adjunct to teaching occupational and environmental disease recognition and history taking. Environ Res 1992;59:93-100
CrossRef | Web of Science | Medline

To the Editor:

We are told that the arterial blood gas values measured while the patient was breathing 10 liters of oxygen per minute by face mask were a pH of 7.39, a partial pressure of oxygen of 63 mm Hg, and a partial pressure of carbon dioxide of 38 mm Hg. The low partial pressure of oxygen could have been due to one or more of the following: ventilation-perfusion mismatch, shunting, diffusion abnormalities, hypoventilation, or a low fraction of inspired oxygen. The last two problems are distinguished by a normal alveolar-arterial gradient.

Assume that the fraction of inspired oxygen is roughly 0.40 during ventilation at a rate of 10 liters of oxygen per minute, the barometric pressure is 760 torr, and the partial pressure of water vapor is 47 torr. Solving the alveolar gas equation for this situation yields an alveolar partial pressure of oxygen of approximately 285 mm Hg and an alveolar-arterial gradient of approximately 220 mm Hg.

Given this patient's large alveolar-arterial gradient, it would appear that hypoventilation alone would not explain her hypoxemia. In spite of her normal chest film, it appears that ventilation-perfusion mismatch, shunting, or a diffusion abnormality also contributed to her clinical picture.

R. Bruce Jobe, M.D.
Roseville Kaiser Clinic, Roseville, CA 95678

Author/Editor Response

The authors reply:

To the Editor: Dr. Craner makes the point that an occupational history can uncover important toxic exposures that produce serious and often reversible disease. Because the lungs are commonly affected by industrial toxins, patients in respiratory distress should be asked about their occupation and, for that matter, about their hobbies and pets, and should even be asked where they live. Certain socioeconomic and racial groups living in heavily polluted urban areas are more often exposed to environmental hazards, which can produce pulmonary symptoms. Even when communication is not problematic, we can ask only a limited number of questions. We make hypotheses and follow hunches, but when inconsistencies arise we should gather additional information. In a patient with pulmonary symptoms, an occupational history should be routine. Communication with this young woman was difficult once she was intubated, but in such circumstances a patient's family can be asked.

Dr. Jobe correctly points out that the alveolar-arterial oxygen gradient was large, a finding not typical of hypoventilation alone. Although the diagnosis of myasthenia became apparent as more information was obtained, global hypoventilation was not the leading diagnosis when blood gases were initially measured. If this patient had pure global hypoventilation, one would expect hypercarbia, some degree of respiratory acidosis, and a normal gradient. The clinicians recognized the substantial gradient and pursued it by obtaining a lung scan. The discussant correctly pointed out that the patient's relatively normal partial pressure of carbon dioxide was inappropriate for her degree of hypoxemia, suggesting that hypoventilation and airway disease were contributing causes. With severe hypoventilation and the atelectasis that often accompanies it, shunting occurs and may well contribute to the hypoxemia. Once this patient was intubated, the alveolar-arterial oxygen gradient was less than 50 mm Hg.

Stephen G. Pauker, M.D.
Richard I. Kopelman, M.D.
Tufts University School of Medicine, New England Medical Center, Boston, MA 02111

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