Zero reference level for right heart catheterisation.

Although in the pulmonary circulation small pressure differences may alter the categorisation of patients, there is no consensus on a standard zero reference level (ZRL). In the supine position, ZRL is mostly set at "5 cm below anterior thorax surface", "1/3 thoracic diameter below anterior thorax surface", "mid-thoracic level" or "10 cm above table level". We retrospectively assessed the distance of these four ZRLs from computed tomography-derived right and left atrial centre levels and from one another in patients undergoing right heart catheterisation and calculated the respective differences in pressure readings. We included 196 consecutive patients. The ZRL at "1/3 thoracic diameter" was most often (98.5%) level with the right atrium, and the ZRL at "mid-thoracic level" was level with the left atrium (97.4%), revealing a median (range) pressure difference of -0.3 (-3.0-1.3) and 0.2 (-2.0-1.3) mmHg from the right and left atrial centre level, respectively. The largest differences (8.0 (2.0-15.4) mmHg) were found between the ZRLs "5 cm below anterior thorax surface" and "10 cm above table level". Accordingly, 59% versus 80% of patients would be classified with pulmonary hypertension and 7% versus 38% with elevated left heart pressures. The choice of ZRL strongly influences pulmonary pressure readings and pulmonary hypertension classification. 1/3 thoracic diameter mostly represents the right atrium while the left atrium is best represented by the mid-thoracic level.