Most pulmonary emboli come from a deep vein thrombosis in the leg. The clot breaks free, travels through the right heart, and lodges in a pulmonary artery.

Occlusion of a pulmonary artery has two consequences:

• Pressure rises in the pulmonary artery, because flow is blocked.
• The lung supplied by that artery becomes ischemic and may infarct.

How far each one goes depends on the size of the embolus, which decides the size of the vessel it can block, and on the cardiopulmonary status of the patient.

Pathogenesis

Effects of large, medium and small emboli

An embolus that obstructs more than 60% of the pulmonary vasculature is called massive.

The size of the embolus decides how large an artery it can block, and that decides the symptoms.

Small Recurrent showers Medium Large

Symptoms and signs

Saddle embolus · Gross

A large dark thrombus lies across the bifurcation of the opened pulmonary trunk, occluding both main branches at once. An embolus this size causes sudden death, and the lung behind it has had no time to show any change at all.

CTPA · Axial

Contrast fills the pulmonary arteries and the clot appears as a dark defect inside the bright vessel. This is why CT pulmonary angiography is the gold standard: it shows the embolus itself.

Dual circulation

Dual supply

A blocked artery would normally mean dead tissue, but the lung has a second supply. The pulmonary artery brings deoxygenated blood for gas exchange, and the bronchial artery brings oxygenated blood from the systemic circulation, so when one is blocked the other can keep the tissue alive. A small embolus therefore causes bleeding into the alveoli and the tissue itself survives. Infarction follows when that second supply is already inadequate, as it is in congestive heart failure or underlying lung disease.

Pulmonary infarct · Gross

The infarct is wedge-shaped and peripheral, with its base on the pleural surface and its apex pointing toward the occluded vessel. It is dark red because blood from the bronchial circulation continues to leak into tissue that has already died. The pleuritic chest pain comes from the inflamed pleura over the base.

Investigations

Prophylaxis and treatment

Prophylaxis is critical:

• Anticoagulation
• Early walking after an operation
• Elastic stockings
• Intermittent pneumatic compression of the calves
• Isometric leg exercises, for patients confined to bed

Classification

Pulmonary hypertension is either idiopathic — no other disease explains it — or secondary to a condition elsewhere. The World Health Organization scheme sorts the secondary causes by where the problem starts.

Idiopathic pulmonary hypertension

BMPR2 pathway

Bone morphogenetic protein receptor 2 normally inhibits the proliferation of fibroblasts and smooth muscle cells. Inactivating germline mutations in the gene are found in 75% of familial cases and 25% of sporadic cases. Without that signal the endothelium becomes dysfunctional and vascular smooth muscle proliferates, so the vessel wall thickens and the lumen narrows. Only 10 to 20% of people carrying a BMPR2 mutation develop the disease, so modifier genes and environmental triggers must contribute as well.

Morphology

Elastic artery · Gross

An atheroma in a large pulmonary artery, a yellow plaque of the kind normally seen in the aorta and its branches. In the pulmonary circuit an atheroma appears only when the pressure has been raised for a long time, and only in the large elastic arteries.

Small pulmonary artery · H&E

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1 2 3

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Plexiform lesion · H&E

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1 2 3

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Chronic thromboembolic pulmonary hypertension

Emboli that are neither lysed nor fully organized leave fibrous webs and bands inside the arteries. Resistance rises, and the vessels that remain open carry more flow than they were built for. That shear stress drives medial hypertrophy and intimal proliferation in those vessels, and in advanced disease plexiform lesions form. Right heart catheterization then finds a raised mean pulmonary artery pressure, and the echocardiogram shows a hypertrophied right ventricle.

Alveolar hemorrhage · H&E

Alveolar spaces are packed with red cells, with the thin alveolar walls still visible between them. All three syndromes end here, and this explains both the hemoptysis and the ground-glass opacities on imaging.

Etiopathogenesis

Goodpasture syndrome

An autoimmune disease of the teens and twenties, more common in males. The circulating antibody initiates inflammatory destruction of the basement membrane in the renal glomeruli and in the pulmonary alveoli at the same time, which produces the pulmonary-renal syndrome: a proliferative and rapidly progressive glomerulonephritis alongside a hemorrhagic interstitial pneumonitis. Immunofluorescence shows linear deposition of IgG along the glomerular basement membranes in the kidney and along the alveolar septa in the lung.

Morphology

H&E

Large numbers of macrophages fill the alveolar spaces, their cytoplasm loaded with golden-brown pigment, on a background of thickened fibrous septa. The pigment is iron recovered from red cells that were broken down after the bleed. These macrophages are also called heart failure cells.

Prussian blue

This is the same field, stained for iron. Prussian blue turns the hemosiderin inside those macrophages a deep blue, which confirms that the pigment is iron and that bleeding has continued long enough for the iron to be stored. In a child with iron deficiency anemia, the iron is held in these macrophages.

Comparison

Resorption atelectasis

Resorption atelectasis begins with an obstructed airway. Most often the obstruction is excessive secretions such as mucus plugs, or exudate within the smaller bronchi. Bronchial asthma, chronic bronchitis, bronchiectasis and the post-operative state all produce those secretions, so resorption atelectasis accompanies each of them. Aspirated foreign bodies and tumors growing inside a bronchus do the same thing. Air trapped beyond the obstruction is gradually absorbed into the blood, and the alveoli close behind it. The affected lung now occupies less space than it did, so the mediastinum is pulled toward it.

Resorption

A plug fills the bronchus and no fresh air reaches the alveoli beyond it. The dashed outline shows the original volume of the lung, and the collapsed lung lies well inside it. The mediastinum follows the loss of volume toward the affected side.

Clinical findings

Compression atelectasis

Here the airways are open and the problem is outside the lung. Fluid or air collecting in the pleural space raises the pressure around the lung and squeezes it, so the alveoli close under compression. Because the pleural space now holds more than it should, the mediastinum is pushed toward the opposite side. Dullness to percussion indicates fluid, and a malignant effusion is a common source in a patient with a known cancer.

Compression · Chest X-ray

Fluid in the left pleural space. The lower zone is dense where the fluid lies, and the lung above it has been compressed into a smaller volume.

Contraction atelectasis

Focal or generalized fibrosis of the lung or of the pleura prevents the lung from expanding fully. Scar tissue does not stretch, so the alveoli never open to their normal volume. Contraction atelectasis cannot be reversed, because the fibrosis is permanent.

Contraction

The airways are patent and the pleural space is empty, and still the lung remains inside its original outline because fibrous tissue in the lung or the pleura will not let it expand.

Comparison

Discriminating features