Determination of arterial input impedance spectra from non-invasively recorded pulses

The frequency spectra of modulus and phase of the input impedance (Zin) of large human arteries (abdominal aorta, femoral and subclavian arteries) were computed from transcutaneously recorded, uncalibrated pressure and flow pulses picked up as sphygmograms and Doppler flow velocity pulses, respectively. Since these pulses cannot be calibrated, the modulus (Zin) of the input impedance is calculated in relative units; its spectrum, however, is not influenced by this fact. A modification of the computing procedure makes it possible to determine approximately quasi-continuous frequency spectra of Zin from natural pressure and flow pulses which may be regarded as periodic functions. This is the prerequisite for a detailed analysis of the wave transmission properties of the arterial bed which manifest themselves of the input impedance. For this purpose the peripheral reflection site was moved in a proximal direction by bilateral occlusion of limb arteries. This was done by inflating cuffs placed symmetrically on both sides around the upper or lower parts of the respective limbs. When the occluding cuffs were placed around both lower legs or both thighs, the shortening of the arterial wave transmission line resulted in a marked shift of the first maximum of Zin to higher frequencies in the spectrum of Zin of the abdominal aorta and femoral artery. Bilateral occlusion of the arteries of the forearms or upper arms, however, did not have any measurable influence on Zin of the subclavian artery. Theoretical considerations show that this difference in behaviour of the several parts of the arterial system may be attributed to the varying extent of their inhomogeneity.