2.21 Pneumonia: Lobar, Broncho, and Interstitial

Pulmonary system review for the USMLE Step 1 Exam

• Pneumonia is an infection of the lung parenchyma that deals with gas exchange
• Three classic patterns of pneumonia: lobar, bronchopneumonia, and interstitial pneumonia
• Lobar pneumonia involves a single lobe of the lung and has four stages: congestion, red hepatization, gray hepatization, and resolution
• Most commonly associated with strep pneumoniae, also caused by Legionella and Klebsiella
• Bronchopneumonia is an infection of the bronchi that leads to diffuse infection of adjacent alveoli
• Causative organisms include strep pneumoniae, klebsiella, H influenza, and staph aureus
• Interstitial pneumonia is an ongoing infection in the interstitial areas of alveolar walls
• Causative organisms tend to be different compared to other types of pneumonia and include Mycoplasma pneumoniae, Legionella, chlamydophilia bugs, Coxiella burnetti, and some viruses (influenza and RSV)
• On chest X-ray, lobar pneumonia shows haziness in a single lobe with sharp demarcation, while bronchopneumonia and interstitial pneumonia show more diffuse haziness throughout the lungs
• Interstitial pneumonia tends to be more indolent, and patients may have not so good looking lungs on chest X-ray but are up and walking around (walking pneumonia).