Wednesday, December 15, 2010

Morbidly Obese Man With Respiratory Distress

A morbidly obese (450 lb) individual presents to the emergency department in respiratory distress. Arterial blood gas studies show a PCO2 of 55 mm Hg, a PO2 of 60 mm Hg, and a pH of 7.28. Chest X-ray films are unremarkable, with no evidence of emphysema, tumor, fibrosis, pulmonary infarction, or other disease. Auscultation reveals a rapid but regular heart beat, and the pulse is strong. Which of the following is the most likely explanation of the patient's arterial hypoxemia? 

     A. Decreased capacity for pulmonary diffusion 
     B. Decreased surface area of alveolar capillary membranes 
     C. Hypoventilation of central origin 
     D. Hypoventilation of peripheral origin 
     E. Inequalities of ventilation and perfusion 


         The correct answer is D. It is conceptually worth subclassifying hypoxemia in terms of the groups of the causes listed in the answers. In this case, the patient has no evidence of primary pulmonary or cardiovascular disease, and is known to be morbidly obese. Morbidly obese individuals are vulnerable to the Pickwickian Syndrome (after a character in a Dickens novel), in which pressure from a fatty neck causes intermittent airway obstruction. The many other causes of hypoventilation of peripheral origin include suffocation, submersion, skeletal abnormalities, trauma, phrenic nerve paralysis, polio, and tetanus. 

         Causes for decreased capacity for pulmonary diffusion (choice A) of 02 include processes such as respiratory distress syndrome, emphysema, pulmonary fibrosis, and some granulomatous processes such as sarcoidosis. 

         Causes for decreased surface area of alveolar capillary membranes (choice B) include resection or compression of the lung and emphysema. 

         The usual cause of hypoventilation of central origin (choice C) is respiratory center depression by morphine or barbiturates. 

         Causes for inequalities of ventilation and perfusion (choice E) include chronic bronchitis, asthma, emphysema, bronchiectasis, some granulomatous processes, and tumors.