In vivo correlation of Doppler waveform analysis with arterial input impedance parameters

Previous studies have confirmed that Doppler waveform analysis (DWA) offers a valid reflection of changes in peripheral vascular resistance. However, the ability of the pulsatility index (PI), a parameter of DWA, to reflect the dynamic components of the circulation, as assessed by arterial input parameters, remains uncertain. In addition, the state of the central circulation is considered an important factor influencing the accuracy of this technique. This study evaluated the ability of the aortic PI to reflect alterations of input impedance in a chronically instrumented lamb model that was subjected to pharmacologic alteration of the circulation. Pressure, volumetric flow and continuous-wave Doppler frequency shift measurements were recorded from the infrarenal abdominal aorta. The parameters of input impedance, peripheral vascular resistance (Zpr), characteristic impedance (Zo) and reflection coefficient (Rc), were determined and then correlated with changes in the aortic PI. Initially, perturbations of the circulatory state were created with a vasodilator, hydralazine (HY) and a vasoconstrictor, phenylephrine (PE). During a second set of experiments, the effect of the reflex heart rate (HR) responses on the PI was evaluated. This was accomplished by inhibiting reflex HR responses to these vasoactive agents with either trimethophan (TM) or atropine methyl bromide (AMB). In response to HY and HY with TM, significant decreases in the PI and impedance parameters occurred. Administration of PE and PE with AMB resulted in significant increases in PI and each of the impedance parameters. HY and PE induced changes in PI correlated significantly with changes in volumetric flow (r = 0.82, 0.80; p < 0.001), mean arterial blood pressure (r = 0.64, 0.70; p < 0.001) and Zpr (r = 0.77, 0.80; p < 0.001), but not with Zo (r = 0.34, 0.36) and Rc (r = 0.26, 0.31). However, when reflex HR responses were inhibited during the administration of the vasoactive agents, HY with TM and PE with AMB, induced changes in PI correlated significantly with Zo (r = 0.93, 0.89; p < 0.001) and Rc (r = 0.84, 0.83; p < 0.001), and the correlation with mean arterial pressure (r = 0.78, 0.87; p < 0.05) and Zpr (r = 0.92, 0.91; p < 0.05) was significantly greater. These findings indicate that the PI accurately assesses pharmacologically induced changes in the downstream arterial input impedance. The accuracy of this assessment is enhanced further when central circulatory factors such as changes in HR are considered.