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Clinical use of Doppler waveform analysis assumes that vascular resistance is accurately represented by the Doppler indices. This assumption was examined by correlating the pulsatility index (PI) with measures of input impedance including peripheral vascular resistance (Zpr), characteristic impedance (Zo), and reflection coefficient (Rc). Assessment of these parameters from the descending aorta was performed in five chronically instrumented, newborn lambs subjected to administration of norepinephrine and hydralazine. Significant increases in PI, Zpr, Zo, and Rc were seen in response to administration of norepinephrine, and decreases in PI and Zpr occurred with hydralazine use. Significant correlation existed between PI and Zpr throughout the study, but changes in PI did not correlate with changes in Zo and Rc. PI appears to reflect changes in Zpr accurately. However, the lack of ability for PI to assess Zo or Rc requires further investigation.

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