Section VI: Disorders of the Respiratory System and Critical Care Illness
All of the following are typically characterized as an obstructive lung disease EXCEPT:
The answer is A. (Chap. 305) The obstructive lung diseases are characterized by a reduction in the forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) ratio, typically below 0.70. The typical lung diseases that manifest with airways obstruction include chronic obstructive lung disease (which includes emphysema and chronic bronchitis), asthma, bronchiectasis, and bronchiolitis. Asbestosis (Chap. 311) is a lung disease caused by the inhalation of asbestos fibers. It is a fibrotic lung disease and typically manifests with a restrictive ventilator defect and gas transfer defect (reduced diffusion capacity for carbon monoxide [DLCO]) on pulmonary function testing.
A 25-year-old man is brought to the emergency department by ambulance after his family found him unresponsive at home. He has a history of intravenous drug abuse and human immunodeficiency virus (HIV) with medical noncompliance. His last CD4 count was <200/μL. On initial evaluation, his blood pressure is 120/75 mmHg, heart rate is 105 bpm, respiratory rate 8 breaths/min, oxygen saturation (SaO2) of 83%, and temperature of 36.0°C. A blood gas on room air reveals pH of 7.16, partial pressure of carbon dioxide (PCO2) of 70 mmHg, and partial pressure of oxygen (PO2) of 55 mmHg. Which of the following is the most likely diagnosis?
C. Pneumococcal pneumonia
D. Pneumocystis pneumonia
The answer is B. (Chap. 305) The alveolar-arterial oxygen (A-a O2) difference can be helpful in distinguishing hypoventilation (elevated partial pressure of carbon dioxide [PCO2]) as a cause of hypoxemia. The A-a O2 difference on room air should be less than 15 mmHg in a young adult and typically increases slightly with age. The A-a O2 difference cannot easily be interpreted when the fraction of inspired oxygen (FiO2) is greater than 0.21. The A-a O2 difference on room air is elevated in situations of hypoxemia due to ventilation-perfusion (V/Q) mismatch, shunt, or diffusion defect. It will be normal when the hypoxemia is solely due to hypoventilation. In this case, the gradient is 7.5 mmHg, consistent with hypoventilation secondary to a presumed narcotic overdose. Asthma and pulmonary embolism typically cause hypoxemia due to V/Q mismatch. Pneumococcal and pneumocystis pneumonias cause hypoxemia for multifactorial reasons, including shunt.