Pulsatile hemodynamics of hypertension: systematic review of aortic input impedance

Objective: Input impedance is the frequency-dependent afterload to pulsatile blood flow. Studies of input impedance have been performed as early as the 1960s and have been applied to hypertension (HTN). However, to date, these studies have not been systematically evaluated. This systematic review aims to summarize the literature, interpret existing data from the perspective of impedance theory, and to discuss their potential for generating physiological insights into HTN. Methods: We identified 11 studies wherein computed impedance moduli from both HTN and control (CNT) groups were reported. In addition, we performed bivariate analyses of raw data from three of these studies. Results: Major findings include HTN groups had consistently elevated impedance moduli at 0 Hz (Z(0)) and at heart rate frequency (Z(1)), an increased frequency wherein impedance phase first crosses 0 (f(0)), but no consistent pattern in characteristic impedance (Z(c)), when compared to CNT groups; SBP and DBP are highly correlated with Z(0) and Z(1), moderately correlated with f(0), less correlated with Z(c); the measurement and calculation methods for Z(c) are varied and inconsistent; and a not insignificant proportion of hypertensive individuals have 'normal' Z(0), Z(1) and Z(c) values. These findings are limited by the heterogeneous study populations and small sample sizes. Conclusion: These findings suggest that Z(0), Z(1) and f(0) are significantly associated with HTN, whereas the role of Z(c) is less clear. Additional studies are needed to evaluate these input impedance variables in order to generate substantial implications in clinic settings.