Impedance is the relation between the pressure difference and flow of a linear system, for sinusoidal or oscillatory signals. Impedance completely describes the system and it can be derived from pulsatile pressure difference and pulsatile flow and the application of Fourier analysis. Inversely, when the impedance is known, a given flow allows for the calculation of pressure and vice versa. Systemic arterial and pulmonary arterial input impedance are a comprehensive description of the systemic and pulmonary arterial tree. Input impedances of organ systems may be derived as well. The longitudinal, transverse and characteristic impedance are discussed in Appendix 3.
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