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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 topics in acute respiratory diseases. This post is designed for you to see what you absolutely need to know going into USMLE Step 2, Step 3 and the ABIM.

Clinical Pearls

  • Minute ventilation (respiratory rate x tidal volume) determines PaCO2
  • Working up a PE is based on pretest probability; look for risk factors and tachycardia
  • Physical exam is key to diagnosing a pneumothorax
  • Keep Light’s criteria simple: fluid LDH or protein 50% or more of serum value -> exudative effusion
  • Treating pneumonia depends on how sick the patient is and where they got sick
  • Aspiration pneumonitis will show immediate infiltrates; aspiration pneumonia takes several days to develop

Acute respiratory failure

Respiratory failure can be hypoxemic (PaO2 <60) or hypercapnic (PaCO2 >50). Diagnosis is made with an ABG to evaluate these two values as well as the A-a gradient. Look to treat the underlying cause as well as the respiratory failure. Proning decreases mortality in ventilated patients!

Hypoxemic failure will be caused by intrinsic lung damage, such as ARDS, pneumonia, pulmonary edema. Provide supplemental O2 to the patient and watch for a rising CO2, which may indicate impending respiratory failure and intubation. Oxygenation is determined by FiO2 and PEEP. Use only as much O2 as needed to maintain their SpO2 90-92% to avoid oxygen toxicity. ARDS can be caused by life-threatening conditions such as sepsis or pancreatitis, and patients should be ventilated with low tidal volumes.

Hypercapnic failure will be caused by worsening of chronic obstructive disease, such as COPD or asthma. PCO2 levels are determined by minute ventilation (respiratory rate x tidal volume), so increasing these variables with BiPAP or mechanical ventilation will improve their status. Excess oxygenation won’t help and may shut off their respiratory drive.

Pulmonary embolism (PE)

Virtually all patients will be tachycardic (or with pulse >90) with a sudden-onset respiratory complaint, such as dyspnea or pleuritic pain. Understanding pretest probability is key, and risk factors are the same for DVT: cancer, prior DVT/PE, prolonged immobility, contraceptive use, or hereditary hypercoagulability. Diagnosis is made with a pulmonary CT angiogram, but caution with AKI and CKD! In renal disease, get a VQ scan to establish the diagnosis. D-dimers are sensitive and are only used to rule out DVT/PE in patients with low pretest probability. Treat with anticoagulation, using oral meds as soon as the patient is stable. A massive/saddle embolism can cause hemodynamic instability and warrants thrombolysis with tPA. 

Pneumothorax (PTX)

These can be traumatic or spontaneous, often in a young healthy male. Physical exam is key: look for respiratory distress, unilateral hyperresonance, distended neck veins, and/or tracheal/mediastinal deviation away from the affected side. Tension PTX causes hemodynamic instability and needle aspiration should be done immediately. If the diagnosis is less clear, you have time to order a CXR. Asymptomatic PTX can be observed.

Pleural effusion

Look for blunting of the diaphragm(s) on CXR. Transudative will be related to hydrodynamics (too much fluid, too little protein), like heart, liver, or renal disease. Exudative will be related to inflammation, like infections or malignancy. To differentiate exudative vs transudative, get pleural fluid analysis with a thoracentesis and apply Light’s criteria. At least one of the three criteria needs to be met to be exudative; otherwise, it is transudative. To simplify this, just look for a pleural fluid value 50% or more of the serum value. This will work 99% of the time in test questions.

Pneumonia

Most pneumonias are viral. In bacterial infections, patients present with a productive cough and signs of infection, and diagnosis is made with CXR. The CURB-65 score (Confusion, Urea, Respirations, low Blood pressure, age >65) can help guide decision-making: <2 can be discharged home, while 2 or more warrants admission. Therapy depends on community vs hospital acquisition. For community acquired, cover strep pneumo with ceftriaxone and azithromycin (covers resistant strains and atypical pathogens). Doxycycline is an alternative to azithromycin to cover atypicals. Unique cases would be patients with cystic fibrosis, who have MRSA infections as children and pseudomonas infections as adults. For hospital acquired, gram negatives are more of a concern. For ventilator-associated pneumonias, cover for MRSA and pseudomonas. For empyemas, treat with aggressive drainage of the pleura via thoracentesis and antibiotic therapy.

Aspiration pneumonitis

Look for patients at high risk for aspiration (intoxication, vomiting, stroke, Parkinson’s, acid reflux, extubation) with respiratory distress and new right lung infiltrates. Aspiration pneumonitis will show infiltrates acutely on CXR, whereas aspiration pneumonia takes a few days to develop. It is common practice to treat with antibiotics either way, but for the test, you’ll monitor for 48 hours after an aspiration event and start antibiotics if symptoms persist or stop antibiotics after 48-72 hours if symptoms resolve. For aspiration pneumonia, use a beta lactam - oral (amoxicillin/clavulanate) if stable or IV (ceftriaxone or ampicillin-sulbactam) if acutely ill.

Further reading:

Lung Story Short: Clearing the Air About Those Pulmonology Step 1 Questions

Radiology for the USMLE: The Chest X-Ray


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