Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.
Follow along with First Aid, or with my notes which can be found here: https://www.dropbox.com/sh/fa2307zt7970c19/AADE66sTZnvNjYpMR3ueKDhHa?dl=0
This video is on lung infections, or pneumonia. Pneumonia is an infection of your lung parenchyma, or your respiratory zone. If you remember, that's your respiratory bronchiole, your alveolar sac and your alveoli! Now we can break up pneumonia into several different categories:
1) Lobar pneumonia: one lobe is affected and this can be seen on biopsy and on imaging. There are four stages in lobar pneumonia. The first stage is congestion and is due to edema from inflammation. The second stage is red hepatization due to RBCs and exudate and causes the lung to become firmer. The third stage is grey hepatization when the RBCs get removed. Our last stage is resolution and recovery from the stem cells of our lungs - the type 2 pneumocytes!
2): Bronchopneumonia: affects the bronchioles.
3): Interstitial pneumonia: diffusely affects the space outside your alveolus.
Lobar and bronchopneumonia affects the actual alveoli and is called typical pneumonia. The most common bug is strep pneumoniae. Now interstitial pneumonia affects the outside and thus the symptoms are more mild. We sometimes call this walking pneumonia: the fevers aren't as high, the dyspnea and productive cough isn't as bad, and neither is the chest pain. We call this atypical pneumonia. Not only are the symptoms atypical, but also the causative agents! Agents here include mycoplasma pneumoniae, viruses, and chlamydia.
You should know that the bugs differ in different situations. In the hospital, you'll see more hospital bugs like E. coli, Klebsiella, Pseudomonas and Legionella (especially if they are on a respirator). COPD patients are associated with moraxella, and neonates have a whole host of other agents like group B stre, E.coli etc. Immunocompromised patients may have fungal infections.
Moving on, let's talk about tuberculosis. This bug get's into the lungs and affects the lower lobe. You can get a focal area of caseous necrosis which can calcify (Ghon focus). This can spread via your macrophages to your hilar lymph nodes (Ghon complex). Secondary activation of TB can occur in your upper lobes as TB is an aerobe and needs oxygen and upper lungs happen to be highly ventilated. TB can spread: miliary tuberculosis affects the lungs diffusely, it can spread to your meninges and cause meningitis and can spread to your spine and cause Pott's disease.
Our last topic is abscess and this is often due to aspiration of oropharyngeal content. Common patients include alcoholics, epileptics and hospitalized patients. Anaerobes predominate here. Now what lung does abscess affect? Well in aspiration, it always affects your right lung more, often your lower lobe.