Various mechanisms can lead to the occurrence of CTEPH, the symptoms are, however, the same.

The exact cause of CTEPH is still unclear today. It usually begins with a deep vein thrombosis that can occur through an imbalance in the formation and dissolution of blood clots. The risk factors for the occurrence of a thrombosis are, besides increasing age and familial predisposition, also nicotine consumption, excess weight, varicose veins and hereditary increased coagulability of the blood. Even sitting for longer periods with bent knees (e.g. during long flights), leading to knicks in the veins, stimulates its occurrence. If a thrombus or a part of one is carried via the bloodstream into another organ, blocking a blood vessel there, this is called an embolism.

In the pulmonary embolism, an embolism spreads via the heart into the lung, where it becomes lodged in the branching pulmonary arteries that are reducing in diameter. Up to 30% of those affected with femoral and pelvic vein thrombosis develop this complication, which is sometimes harmless and even without symptoms, while sometimes it can lead to death, depending on the size and localization of the embolism.

In up to 5% of patients these blood clots will not wholly or partially disperse. This can lead to a rise in pressure within the pulmonary vessels and the disease moves on to the chronic stage of chronic thromboembolic pulmonary hypertension.