Arterial Input Impedance
Input impedance completely and comprehensively describes an arterial (sub) system. The ratio of the mean arterio-venous pressure drop and mean flow is total vascular resistance or peripheral resistance, Rp. To obtain information about the oscillatory aspects of the arterial system wave shapes of pressure and flow are used; and Fourier analysis applied. The amplitude ratio and the phase difference of the sine waves of pressure and flow give the modulus and phase angle of the impedance (application of Ohm’s law). At zero frequency Rp is found. For intermediate frequencies (1–3 times heart rate) the modulus decreases precipitously and the phase angle is negative. This shows the major contribution of (total) arterial compliance, C. For high frequencies the modulus approaches a constant value and the phase angle is close to zero. This is the contribution of the aortic characteristic impedance. These three elements together give a good description of the input impedance, suggesting the three-element Windkessel as acceptable arterial model.
KeywordsFourier analysis Impedance modulus Impedance phase Peripheral resistance Total arterial compliance Aortic characteristic impedance Impulse response
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