Pulmonary Infarction is necrosis, or death of lung tissue due to ischemia (lack of blood flow) to that area of the lung. Most result from a large pulmonary embolism (PE) that blocks or slowwwws the flow of oxygenated blood distal to the blockage. The clot may be caused from a trauma, post operative, or just after birth.
PEs are usually -- but not always --caused by a thrombus (blood clot) that breaks free and lodges in one of the pulmonary arteries. How large the thrombus is will determine how far into the lung it will travel before it lodges.
A small PE will usually lodge in "peripheral pulmonary arteries and are not ordinarily lethal, "according to Emanuel Rubin and Howard M. Reisner in their 2009 book "Essentials of Rubin's Pathology." The reason they are usually benign in nature is because most lung tissue is fed by more than one pulmonary capillary (collateral circulation), and even if a small area becomes necrotized that part of the lung can simply be bipassed.
A large clot, on the other hand, may result in slowed blood flow distal to the PE. The obstruction also mas cause the left ventricle to work extra in an attempt to pump blood past the clot, and this will result in increased pulmonary artery pressure (PAP). This ultimately results in right heart failure. Blood will back up into the lung parychema squeeze the artery distal to the blockage further slowing blood flow.
When blood flow distal to a PE becomes slow enough it's inefficient to meet oxygenation demands of tissue in that area of the lung, tissue necrosis will occur. If a large enough portion of the pulmonary artery is blocked, the blood to the lung may be slowed to the point that the person may die of shock.
Rubin's also notes that a majority of pulmonary infarctions occur in patients with Congested Heart Failure (CHF) and Chronic Obstructive Pulmonary Disease (COPD) because the normal "dual circulation" in these patients may not be functioning due to the disease process. With emphysema this would result from tissue breakdown.
According to urmc.rochester.edu less than 10 percent of PEs will result in pulmonary infarction. Basic pathology notes that three-fourths of PEs occur in the lung bases.
Rubin's also describes pulmonary infarcts as "hemmorhagic, as the bronchial artery pumps blood into the necrotic area. The infarcts are generally pyramidal, with the base of the pyramid on the pleural surface. This pooling of blood will will cause the area to be a raised red-blue region. Once the blood is reabsorbed by surrounding tissue the nectrotic region will become pale and it will ultimately become a fibrous scar."
A study described in Thorax in 1973 (S. Talbot, et. al, "Radiographic signs of embolism and pulmonary infarction," Thorax, 1973, 28, 198) confirmed that 70 percent of pulmonary embolism with pulmonary infarctions could be positively identified with a simple chest x-ray.
According to Leonard V. Crowly in his 2007 book, "An introduction to human disease: pathology and pathophysiology correlations," pulmonary infarction will show up on the chest x-ray as a wedge-shaped area of increased density (incerased whiteness). Other authors have noted that quite often a pulmonary infarct will also present with blunted costrophrenic angles indicating pleural effusions.
Because a PE will not show up on x-ray, PEs that do not result in infarction will generally not show up on a chest x-ray and (absent other disease processes) the x-ray will show up as normal.