LAUSR

It is of increasing importance to understand and predict changes to the systemic and pulmonary circulations in pulmonary hypertension (PH). To do so, it is necessary to describe the circulation in complete quantitative terms. Characteristic impedance (Zc) expresses opposition of the circulation to pulsatile blood flow. Evaluation of systemic and pulmonary Zc relationships according to PH classification has not previously been described. Prospective study of 40 clinically indicated patients referred for CMR and RHC (56 ± 18 years; 70% females, eight mPAP ≤ 25 mmHg, 16 pre‐capillary [Pre‐cPH], eight combined pre‐ and post‐capillary [Cpc‐PH] and eight isolated left‐heart disease [Ipc‐PH]). CMR provided assessment of ascending aortic (Ao) and pulmonary arterial (PA) flow, and RHC, central Ao and PA pressure. Systemic and pulmonary Zc were expressed as the relationship of pressure to flow in the frequency domain. Baseline demographic characteristics were well‐matched across PH subclasses. In those with a mPAP ≤25mHg, systemic Zc and SVR were >2 times higher than pulmonary Zc and PVR. Only Pre‐cPH was associated with inverse pulsatile (systemic Zc 58 [45–69] vs pulmonary Zc 70 [58–85]), but not steady‐state (SVR 1101 [986–1752] vs. PVR 483 [409–557]) relationships. Patients with CpcPH and IpcPH had concordant pulsatile and steady‐state relationships (Graphical Abstract). Measurement of, and the relationship between, systemic and pulmonary Zc in patients according to PH sub‐classification has not previously been described. Systemic Zc was routinely higher than pulmonary Zc, except in patients with newly diagnosed Pre‐cPH, where inverse pulsatile but not steady‐state relationships were observed.