Pulmonary Embolism (PE)

Pulmonary Embolism

V/Q Mismatch & Obstructive Shock

FreeMedSite USMLE STEP 1 REVIEW

Interactive Study:

Definition

Obstruction of pulmonary arteries, leading to a V/Q mismatch where alveoli are ventilated but not perfused.

Presentation

Symptoms

• • Sudden-onset dyspnea
• • Pleuritic chest pain
• • Tachypnea & Tachycardia
• • Respiratory alkalosis (due to hyperventilation)

Massive PE Findings

• • Obstructive shock
• • RV Dilation → ↓ Cardiac Output

Diagnosis

Gold Std

CT Pulmonary Angiography (CTPA)

ECG Pattern

S1Q3T3 (rare) / Sinus Tachycardia (common)

Pathology

Lines of Zahn (fibrin/platelets + red RBCs)

Emboli Types

Mnemonic: F.A.T. B.A.T.

F Fat: Long bone fractures; triad of hypoxemia, neuro, petechial rash.

A Air: Nitrogen bubbles (diving) or central lines. Rx: Hyperbaric O2.

T Thrombus: Most common cause (usually DVT).

B Bacteria: Septic emboli from endocarditis.

A Amniotic: Labor/postpartum; high mortality, can trigger DIC.

T Tumor: Fragments entering circulation (Rare).

High-Yield Pearls

• Lines of Zahn distinguish pre-mortem thrombi from post-mortem clots.

• Fat emboli typically present 24–72 hours after long bone injury.

• PE causes hypoxemia even without lung infiltrates on CXR.

1. What is the most common symptom of a Pulmonary Embolism?

Answer: Sudden-onset dyspnea.

2. Name the triad associated with Fat Embolism Syndrome.

Answer: Hypoxemia, neurological changes, and petechial rash.

3. What histological finding proves a clot was formed before death?

Answer: Lines of Zahn.

4. Which type of embolus is a major complication of labor and can trigger DIC?

Answer: Amniotic fluid embolus.

5. What is the standard treatment for an Air Embolism (decompression sickness)?

Answer: Hyperbaric Oxygen Therapy.

6. What acid-base disturbance is commonly seen in PE due to tachypnea?

Answer: Respiratory alkalosis.

7. In a massive PE, why does cardiac output decrease?

Answer: RV dilation/failure leads to decreased LV filling.

8. What is the most common ECG finding in Pulmonary Embolism?

Answer: Sinus Tachycardia.

9. Where do most thrombotic pulmonary emboli originate?

Answer: Deep veins of the lower extremities (DVT).

10. What management option is used if anticoagulation is contraindicated?

Answer: Inferior Vena Cava (IVC) filter.