IN-VIVO CORRELATION OF DOPPLER WAVE-FORM ANALYSIS WITH ARTERIAL INPUTIMPEDANCE PARAMETERS

Previous studies have confirmed that Doppler waveform analysis (DWA) o ffers a valid reflection of changes in peripheral vascular resistance. However, the ability of the pulsatility index (PI), a parameter of DW A, to reflect the dynamic components of the circulation, as assessed b y arterial input parameters, remains uncertain. In addition, the state of the central circulation is considered an important factor influenc ing the accuracy of this technique. This study evaluated the ability o f the aortic PI to reflect alterations of input impedance in a chronic ally instrumented lamb model that was subjected to pharmacologic alter ation of the circulation. Pressure, volumetric flow and continuous-wav e Doppler frequency shift measurements were recorded from the infraren al abdominal aorta. The parameters of input impedance, peripheral vasc ular resistance (Zpr), characteristic impedance (Zo) and reflection co efficient (Rc), were determined and then correlated with changes in th e aortic PI. Initially, perturbations of the circulatory state were cr eated with a vasodilator, hydralazine (HY) and a vasoconstrictor, phen ylephrine (PE). During a second set of experiments, the effect of the reflex heart rate (HR) responses on the PI was evaluated. This was acc omplished by inhibiting reflex HR responses to these vasoactive agents with either trimethophan (TM) or atropine methyl bromide (AMB). In re sponse to HY and HY with TM, significant decreases in the PI and imped ance parameters occurred. Administration of PE and PE with AMB resulte d 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 p ressure (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 t he vasoactive agents, HY with TM and PE with AMB, induced changes in P I 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 pres sure (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 w hen central circulatory factors such as changes in HR are considered.