If pulmonary hypertension is suspected, a right heart catheterization is usually performed to confirm or rule out the diagnosis. As part of this examination, blood samples are taken from various locations as standard and blood gas analyses are performed. One of the most important measurements during the right heart catheterization is the measurement of the pulmonary arterial occlusion pressure by the so-called wedge maneuver. To measure this value, the catheter with inflated balloon must be advanced into the pulmonary vessels until the wedge position is reached, i.e. the balloon completely occludes a branch of the pulmonary artery. In this study, the investigators want to characterize patients with pulmonary hypertension of different causes in more detail. To do that, two blood samples (totaling approximately 4 mL of blood, one sample directly after occlusion and the other one two minutes later) will be drawn during the right heart catheterization from the above-mentioned wedge position, behind the inflated balloon, and blood gas analyses will be performed on these samples. In addition, various clinical parameters (comorbidities, etc.) will be recorded by means of clinical questionnaires. Follow-up data will be analyzed and correlations with the aforementioned blood gas analyses will be examined. The results of the study will be used to more precisely characterize the still vague concept of secondary pulmonary hypertension. This could help to develop new therapeutic strategies in some subgroups in the future.