Vascular Impedance

Abstract

Impedance is defined as a frequency-dependent term, expressed as modulus and phase graphed against frequency, and quantifying the relationship between pulsatile pressure and pulsatile flow. It embraces (1) longitudinal impedance—the relationship of flow and pressure gradient in an arterial segment, (2) characteristic impedance—the relationship between pulsatile pressure and flow in an artery when there is no reflection from its vascular bed downstream, and (3) input impedance of an artery that supplies (d) a resistive vascular bed whence reflected waves can arise and return to the artery supplying the bed. This is the most common use of the term “impedance". (Input) impedance at zero frequency is also (d), resistance of the bed—principally caused by viscous losses of mean pressure and mean flow. Input impedance studies are readily combined with the study of pressure and flow pulse waveforms in the time domain. Ascending aortic impedance can be used to characterize the load presented to the left ventricle by the whole systemic circulation, and pulmonary vascular impedance, the load presented to the right ventricle by the lungs. Vascular impedance is used to study the effects of aging and disease and of drugs on LV load and LV function. Tuning the LV to vascular load is aided by considering impedance values at frequencies of LV ejection flow harmonics as presented to the LV, and LV function as properties of the LV as it pumps into this frequency-dependent load. Vascular impedance has other uses. The most useful recent application is in studies of cerebral disease, including development of dementia in older persons, in understanding of cerebral vascular function, and in providing a noninvasive method of detecting elevation of intracranial pressure following head injury.