Background-Current evaluation of pulmonary hypertension (PH) in children invo lves measurement of pulmonary vascular resistance(PVR); however, PVR neglects im portant pulsatile components. Pulmonary artery(PA) input impedance and ventricul ar power(VP) includemean and pulsatile effects and have shown promise as alterna tive measures of vascular function. Here we report the utility of pulsed-wave(P W) Dopplermeasured instantaneous flow and pressure measurements for estimation o f input impedance and VP and use this method to develop a novel parameter: react ivity in complia nce. Methods and Results-An in vitro model of the general pulmonary vasculature was used to obtain impedance and VP, measured by PW Doppler and a reference flo wmeter. Themethodwas then tested in a preliminary clinical study in subjects wit h normal PA hemodynamics(n=4) and patients with PH undergoing reactivity evaluat ion(8 patients; 23 data points). In vitro results showed good agreement between the impedance spectra computed from both flow-measurement methods. Excellent co rrelation was seen in vitro between actual resistance and the zero-frequency(Z0 ) impedance value(r2=0.984). Excellent agreement was also found between Z 0 and PVR in the clinical measurements(j=1.075x+0.73; r=0.993). Furthermore, total VP and VP/cardiac output increased significantly with hypertension(128.73 to 365.9 1 mW and 2.42 to 6.69 mW·mL-1·s-1, respectively). The first-harmonic value of impedance(Z1) was used as a measure of compliance reactivity; older patients exhibited markedly less compliance reactivity than did younger patients. Conclus ions-Input impedance and VP calculated from Doppler measurements and a single- catheter pressure measurement provide comprehensive characterization of PH and r eactivity. Background-Current evaluation of pulmonary hypertension (PH) in children invo lves measurement of pulmonary vascular resistance (PVR); however, PVR neglects im portant pulsatile components. Pulmonary artery (PA) input impedance and ventricul ar power (VP) include mean and pulsatile effects Here we report the utility of pulsed-wave (PW) Dopplermeasured instantaneous flow and pressure measurements for estimation of input impedance and VP and use this method to develop a novel parameter: react ivity in complia nce. Methods and Results-An in vitro model of the general pulmonary vasculature was used to obtain impedance and VP, measured by PW Doppler and a reference flo wmeter. Themethodwas then tested in a preliminary clinical study in subjects wit h normal PA hemodynamics ( n = 4) and patients with PH undergoing reactivity evaluat ion (8 patients; 23 data points). In vitro results showed good agreement between the impedance spec Excellent co rrelation was seen in vitro between actual resistance and the zero-frequency (Z0) impedance value (r2 = 0.984). Excellent agreement was also found between Z 0 and PVR in the clinical measurements ( The total VP and VP / cardiac output increased significantly with hypertension (128.73 to 365.9 1 mW and 2.42 to 6.69 mW · mL-1 · s-1, respectively). The first -harmonic value of impedance (Z1) was used as a measure of compliance reactivity; older patients were marked marked less less compliance reactivity than did younger patients. Conclus ions-Input impedance and VP calculated from Doppler measurements and a single-catheter pressure measurement provide comprehensive characterization of PH and r eactivity.