Cor pulmonale, also known as pulmonary heart disease, is a type of heart disease where the right side of the heart enlarges and fails over time due to high blood pressure in the lungs or pulmonary hypertension. It is usually caused by lung diseases like chronic obstructive pulmonary disease (COPD) that result in low oxygen levels and resistance within the pulmonary arteries of the lungs. The enlarged right ventricle must work harder to pump blood into the lungs, causing it to thicken and eventually fail if left untreated.