How embolism is tested on Step 1
Vignettes give a setting — a long flight, a femur fracture, a laboring patient, a scuba diver, a recent cardiac catheterization — and ask what the embolus is made of, where it came from, or what kind of infarct it caused.
Trace the route: venous emboli → right heart → lungs; left-heart and arterial emboli → systemic organs. A venous clot reaches the brain only through a right-to-left shunt.
Pulmonary thromboembolism
Thrombi from the deep veins of the thigh and pelvis travel through the right heart into the pulmonary arteries. The effect depends on clot size and the patient's cardiopulmonary reserve.
| Embolus size | Consequence |
|---|---|
| Small, peripheral | Often silent; may cause pleuritic chest pain and hemoptysis from infarction |
| Medium | Dyspnea, tachypnea, tachycardia, hypoxemia; infarction more likely with poor bronchial flow (heart failure) |
| Large or saddle (bifurcation of pulmonary trunk) | Acute right heart failure (cor pulmonale), shock, sudden death |
| Many small, recurrent | Chronic thromboembolic pulmonary hypertension |
Pulmonary infarcts are wedge-shaped and hemorrhagic, with the base on the pleural surface.
Systemic and paradoxical emboli
Systemic emboli travel in arterial blood. About 80% start as mural thrombi in the left heart; the rest come from aortic aneurysms, ulcerated plaques, or valve vegetations.
A young patient with a stroke and a DVT should prompt a search for a patent foramen ovale. Transiently raised right-heart pressure (coughing, straining) can push a venous clot across into the left atrium.
Non-thrombotic emboli
A minority of emboli are not blood clots. Each has a distinctive setting.
| Embolus | Setting | Features |
|---|---|---|
| Fat | Fractures of long bones (femur) or pelvis; liposuction | 1–3 days later: hypoxemia, confusion, petechial rash (axilla, conjunctivae), thrombocytopenia |
| Gas (decompression sickness) | Rapid ascent while diving; caisson workers | Nitrogen bubbles: joint pain ("the bends"), dyspnea ("the chokes"); chronic avascular necrosis of femoral head |
| Air | Venous catheters, neck surgery, trauma | >100 mL can obstruct flow; gas lock in the right ventricle |
| Amniotic fluid | Labor, delivery, or immediately after | Sudden dyspnea, hypotension, seizures, DIC; squamous cells and lanugo in maternal pulmonary vessels |
| Cholesterol (atheroembolism) | After vascular procedures or anticoagulation | Livedo reticularis, blue toes, renal failure, eosinophilia |
| Septic | Infective endocarditis, IV drug use | Infected infarcts; tricuspid vegetations → septic pulmonary emboli |
| Tumor | Renal cell or hepatocellular carcinoma invading veins | Tumor fragments in the circulation |
Fat embolism syndrome causes symptoms in only a small fraction of patients with fractures, but microscopic fat emboli are very common after them.
Infarction: what the embolus leaves behind
An infarct is an area of ischemic necrosis. Its color depends on whether blood can still reach the dead tissue.
A 24-year-old man is admitted after a motorcycle crash with a closed femoral shaft fracture. Two days later he becomes confused and short of breath, with oxygen saturation of 84%. There are petechiae across his chest, axillae, and conjunctivae. What is the cause?
- A long-bone fracture releases marrow fat into torn venous sinusoids.
- Fat globules obstruct pulmonary and systemic microvessels, and free fatty acids injure endothelium.
- The 24–72-hour delay and the triad of hypoxemia, neurologic change, and petechial rash are classic.
Answer. Fat embolism syndrome. Management is supportive (oxygen, ventilation if needed); early fixation of the fracture reduces the risk.