⚠️ Must-Not-Miss Diagnoses
Tension pneumothorax, massive PE, STEMI with acute heart failure, epiglottitis/upper airway obstruction, anaphylaxis with bronchospasm, cardiac tamponade.
Differential Diagnosis by Key Features
| Clinical Feature | Think Of | Priority |
|---|---|---|
| Acute onset, pleuritic, unilateral, recent immobility | Pulmonary embolism | HIGH |
| Orthopnea, PND, bilateral leg edema, prior heart failure | Acute decompensated heart failure | HIGH |
| Sudden onset, absent breath sounds, trauma or tall thin male | Pneumothorax | HIGH |
| Fever, productive cough, localized crackles | Pneumonia | Moderate |
| Wheezing, known asthma/COPD, triggers | Asthma/COPD exacerbation | Moderate |
| Pallor, fatigue, exertional dyspnea without cardiac/pulmonary findings | Anemia | Moderate |
| Anxiety, perioral tingling, normal SpO2 | Hyperventilation/panic | Low |
Systematic Approach
Immediate
Pulse oximetry + vital signs
SpO2 <90% requires immediate intervention before diagnosis is established. Respiratory rate >30 is a danger sign. Assess work of breathing — accessory muscle use, tripod positioning, inability to speak in full sentences.
History
Onset, quality, associated symptoms
Acute vs chronic dyspnea narrows the differential significantly. Ask about orthopnea (heart failure), pleuritic quality (PE, pleuritis), fever (pneumonia), wheezing (asthma/COPD), and leg swelling (DVT/PE).
Exam
Targeted auscultation and inspection
Unilateral absent breath sounds → pneumothorax. Bilateral crackles + JVD + edema → heart failure. Unilateral consolidation + fever → pneumonia. Wheeze → bronchospasm. Stridor → upper airway obstruction.
Testing
CXR, ECG, BNP, D-dimer based on pretest probability
CXR is the single most useful initial test. ECG evaluates for cardiac etiology. BNP >500 strongly suggests heart failure. D-dimer + Wells score risk-stratifies for PE.
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