Like with any of the main organ systems, there are a lot of large topics with so many minor details that it can be easy to get lost in the weeds. This review covers the most high yield chronic respiratory diseases. It is designed for you to see what you absolutely need to know going into USMLE Step 2, Step 3 and the ABIM.
- When treating respiratory exacerbations with high dose oral/IV steroids, continue treatment with a taper to avoid adrenal insufficiency
- Prevention is key! LABA/LAMA for COPD, and ICS for asthma
- A rising PCO2 is a bad sign; consider BiPAP in COPD or intubation in asthma
- Obstructive diseases have very low FEV1 and short, flat expiratory loops on spirometry
- Restrictive diseases have low FEV1 and FVC, leading to near-normal FEV/FVC, and have narrowed overall loop on spirometry
- Sleep apnea and obesity hypoventilation are essentially the same obstructive pattern disease with different mechanisms
- History is key to getting to the right diagnosis for pulmonary diseases; pay attention to a patient’s origins, profession, and hobbies
Chronic bronchitis or emphysema often coexist. Look for someone short of breath and coughing chronically, typically a smoker. Decreased FEV1 and decreased FEV1/FVC ratio (< 0.70) is the definitive diagnosis via spirometry. A chest X-ray may show increased AP diameter lung fields. An ABG likely will show elevated PCO2 and low PO2 with normal pH due to elevated serum bicarbonate. An elevated hemoglobin reflects secondary erythrocytosis from chronic hypoxia.
The only ways to decrease mortality are smoking cessation and supplemental O2 (for PaO2 of 88% or less). LABA/LAMA are used individually or together before ICS (inhaled corticosteroids) for maintenance. SABA/SAMA are used as needed only for exacerbations. Look for 3 elements during an exacerbation: increased sputum, cough, and dyspnea. For patients requiring hospitalization, IV or oral steroids can be used with a taper; if increased sputum production, treat with antibiotics. If worsening during an exacerbation, consider noninvasive positive pressure ventilation (NIPPV) with BiPAP to avoid having to intubate!
Asthma can be diagnosed at any age because it is an allergic response (IgE mediated). Look for someone with other atopic conditions (urticaria, nasal polyposis) with dyspnea/cough worse at nighttime, often with wheezing. Spirometry results will mimic that of COPD, but obstruction will be reversible by >12% with SABA or provocable with methacholine. CXRs will likely be unhelpful except to exclude other conditions/complications. During an exacerbation, an ABG should show low CO2 due to hyperventilation; if normal or elevated, consider impending respiratory failure and need for intubation.
ICS are used first for maintenance with SABA as needed. Montelukast can be used in addition to ICS for maintenance. Oral steroids with a taper are used only during severe exacerbations. If exercise-induced, use SABA just prior to exercising.
Chronic inflammation causes permanent dilation of airways and decreased cilia function. Look for a patient with a genetic predisposition to inflammation or recurrent infections (CF is 50% of cases!) who presents with chronic cough, dyspnea, hemoptysis, and recurrent pneumonias. Diagnosis is made with a CT. PFTs will show an obstructive pattern. Treat exacerbations with antibiotics.
Obesity Hypoventilation and Obstructive Sleep Apnea
OHS and OSA can occur at the same time, have different mechanisms, and present the same way (obstructive pattern with elevated PCO2, night-time hypoxia). OSA is caused by neck/upper airway obstruction. OHS is caused by an inability to expand the lungs due to the weight of the chest.
Interstitial Lung Diseases
These generally present the same way; you can differentiate based primarily on the patient’s history and what other organ systems are affected. Idiopathic pulmonary fibrosis is the most common, and don’t forget about environmental exposures (typically the patient’s profession), granulomatous diseases (ie- Sarcoidosis, granulomatosis with polyangiitis), and drug-induced (ie-amiodarone, bleomycin). Physical exam findings will include rales/crackles and digital clubbing. Diagnosis is made with a CT scan or biopsy. PFTs will show a restrictive pattern with decreased lung volumes and DLCO and normal FEV1/FVC (as both are mildly lowered proportionally). Treatments are based on removing offending agents and/or starting steroids. Oxygen can be used for severe cases of hypoxia (SpO2 <88%).
Spirometry Flow-Volume loops
Understanding the physiology will help you more than memorizing how each one is supposed to look. Expiration is above the Flow line, and Inspiration is below the Flow line. Obstructive patterns have a short upslope followed by a flat expiratory phase (see where the FEF arrow is pointing). Restrictive patterns show narrowing of the loop due to decreased lung volumes. Upper airway obstructions affect inspiration, leading to a shallow and flat inspiratory phase.