Right heart catheterization with measurements of pulmonary vascular pressures and cardiac output, and the calculation of pulmonary vascular resistance (PVR) is an essential step in the diagnosis of pulmonary arterial hypertension (PAH). These hemodynamic measurements are of prognostic value, with, however, survival being more closely related to flow (cardiac output) than to pressure (ie, pulmonary artery pressure [Ppa]). The clinical state of PAH patients, as assessed by functional class and exercise capacity, also appears to be more related to cardiac output than to Ppa. Clinical signs of right heart failure are often not clearly related to the progression of pulmonary hypertension, as assessed by Ppa and PVR. These observations may be explained by the fact that heart failure in patients with PAH is the consequence of increased right ventricular (RV) afterload, which is not accurately measured by routine hemodynamic evaluations. fully
Ventricular afterload can be defined either by maximum wall tension or by hydraulic load. The measurement of RV wall tension is problematic because of particular geometry and associated volume measurement uncertainties. However, hydraulic load can be assessed from the morphology of Ppa and flow waves. At a given level of PVR, RV hydraulic load may be increased by a decrease in pulmonary arterial compliance and an increase in wave reflection. These changes affect pressure waves by an increased pulse pressure and late systolic peaking, and they affect flow waves by a shortened acceleration time and by late or midsystolic deceleration. There are data indicating that Ppa and wave qualitative morphology analysis may be of diagnostic and prognostic value in patients with pulmonary hyper-tension.
However, a more accurate quantification of RV hydraulic load can be obtained by a calculation of pulmonary arterial impedance (PVZ) from a spectral analysis of Ppa and flow waves. The results of this analysis are expressed as a PVZ spectrum, consisting of a pressure/flow ratio and a phase angle, both of which are expressed as a function of frequency. A PVZ spectrum includes a measure of total PVR, indexes of wave reflection such as the first minimum of the ratio of pressure and flow moduli or low-frequency phase angle, characteristic impedance (Zc), which corresponds to an inertance to the compliance ratio, and hydraulic load, as evaluated by low-frequency impedance and the amplitude of impedance oscillations.,