What are the differential diagnoses for a patient presenting with shortness of breath and tachycardia, with a clear chest X-ray (CXR) and normal electrocardiogram (EKG)?

Differential Diagnosis for Shortness of Breath with Tachycardia, Clear Chest X-Ray, and Normal EKG

The most critical diagnoses to rule out immediately are pulmonary embolism, cardiac tamponade, supraventricular tachycardia, and pulmonary arterial hypertension, as these can present with normal chest X-ray and EKG yet cause significant morbidity and mortality if missed. 1, 2

Life-Threatening Diagnoses Requiring Immediate Evaluation

Pulmonary Embolism

• PE is the most dangerous diagnosis that commonly presents with clear lungs on chest X-ray and can have a normal or non-specific EKG 2, 3
• Tachycardia (HR 112) with dyspnea and clear chest X-ray is classic for PE 3
• Calculate the modified Wells' score immediately and check D-dimer if low-to-moderate probability 3
• Obtain CT pulmonary angiography urgently if Wells' score is elevated or D-dimer is positive 2, 3

Cardiac Tamponade

• Presents with elevated systemic venous pressure, tachycardia, dyspnea with clear lungs, and can have normal or non-specific EKG changes 1
• Look specifically for pulsus paradoxus (drop in systolic BP >10 mmHg during inspiration), jugular venous distension, and hypotension 1
• Chest X-ray may show enlarged cardiac silhouette but can be normal early 1
• Bedside echocardiography is essential and diagnostic, showing diastolic collapse of right ventricle/atrium and inferior vena cava dilatation 1, 4
• Electrical alternans on EKG with tachycardia is highly suggestive but not always present 4

Supraventricular Tachycardia (SVT)

• Regular tachycardia at 112 bpm could represent SVT, which causes dyspnea through hemodynamic compromise rather than pulmonary pathology 1
• The drop in blood pressure during SVT is greatest in the first 10-30 seconds and can cause significant dyspnea 1
• Obtain a 12-lead EKG during tachycardia to differentiate AVNRT, AVRT, or atrial tachycardia 1
• Look for pseudo R' wave in V1 or pseudo S waves in inferior leads suggesting AVNRT 1
• Perform vagal maneuvers or give IV adenosine (if hemodynamically stable) while recording a 12-lead EKG to aid diagnosis 1

Acute Coronary Syndrome (Type 2 MI)

• Diabetics and patients with cardiovascular risk factors frequently present with dyspnea as an anginal equivalent without chest pain 2
• Tachycardia with relative hypotension can cause Type 2 MI through supply-demand mismatch 1
• Check high-sensitivity troponin immediately, though elevation alone doesn't distinguish Type 1 from Type 2 MI 1, 2
• Obtain urgent echocardiography to assess for regional wall motion abnormalities 1

Other Important Diagnoses with Clear Chest X-Ray

Pulmonary Arterial Hypertension

• Presents with dyspnea, tachycardia, and clear chest X-ray; physical exam may reveal left parasternal lift (RV hypertrophy) and loud P2 5
• Look for elevated jugular venous pressure with prominent "a" waves and apical S4 gallop (present in 38% of PAH patients) 5
• Absence of orthopnea and paroxysmal nocturnal dyspnea helps distinguish from left heart disease 5
• Transthoracic echocardiography is the essential first diagnostic test to estimate PA systolic pressure and assess RV function 5, 6

Acute Exacerbation of Asthma

• Can present with clear chest X-ray early in exacerbation, especially if no consolidation or hyperinflation yet visible 7
• Assess for wheezing, prolonged expiratory phase, and use of accessory muscles 7
• Perform spirometry if patient stable enough to assess airflow obstruction and bronchodilator response 7
• Check for triggers including medication non-compliance, allergen exposure, or recent respiratory infection 7

Metabolic Acidosis

• Tachypnea and dyspnea can result from respiratory compensation for metabolic acidosis (diabetic ketoacidosis, lactic acidosis, uremia) 2
• Check arterial or venous blood gas for pH, bicarbonate, and anion gap 2
• Obtain point-of-care glucose immediately to rule out severe hyperglycemia or hypoglycemia 2
• Calculate anion gap and assess for lactic acidosis, especially if patient on metformin 2

Anxiety/Panic Attack

• Diagnosis of exclusion after ruling out organic causes 7
• Never attribute symptoms to anxiety without thoroughly excluding organic causes, especially in patients with cardiovascular risk factors 7
• Tachycardia with dyspnea can represent panic disorder, but requires negative workup for life-threatening causes first 7

Tracheobronchomalacia

• Uncommon condition presenting with hypoxemia, tachycardia, and shortness of breath that can mimic PE 3
• Consider in patients with postoperative respiratory distress when PE is ruled out 3
• Diagnosis requires dynamic CT imaging or bronchoscopy 3

Immediate Diagnostic Algorithm

1. Obtain 12-lead EKG immediately during tachycardia to assess rhythm, look for signs of PE (S1Q3T3, right heart strain), ischemia, or SVT patterns 1, 2

2. Perform bedside point-of-care ultrasound/echocardiography to assess for:

   • Pericardial effusion and tamponade physiology 1, 4
   • Right ventricular dilation/dysfunction suggesting PE or PAH 5, 6
   • Left ventricular function and regional wall motion abnormalities 1, 2

3. Check vital signs including pulsus paradoxus and oxygen saturation 1, 2

4. Obtain urgent laboratory tests:

   • High-sensitivity troponin 1, 2
   • D-dimer (if PE suspected and Wells' score calculated) 3
   • Arterial or venous blood gas 2
   • BNP or NT-proBNP 2
   • Complete metabolic panel including glucose and renal function 2

5. If PE remains in differential after initial workup, obtain CT pulmonary angiography urgently 2, 3

Critical Pitfalls to Avoid

• Do not assume clear chest X-ray rules out serious cardiopulmonary disease—PE, tamponade, PAH, and early heart failure can all present with clear lungs 1, 2, 5, 3
• Do not treat wide-complex tachycardia as SVT without confirming diagnosis—giving verapamil or diltiazem for ventricular tachycardia can cause hemodynamic collapse 1
• Do not miss cardiac tamponade by failing to assess for pulsus paradoxus and perform bedside echocardiography 1, 4
• Do not attribute symptoms to anxiety in patients with cardiovascular risk factors without excluding organic causes first 7