ABSTRACT
As its name implies, the term ‘impedance’ is the measure of the opposition to flow presented by a system. By conventional use in physics, and in its most familiar form in the terminology of electric current theory, the term is confined to oscillatory motions or alternating current. Thus, usage is not confined to electric current, but is also used as ‘mechanical impedance’ in vibrating solid systems and as ‘acoustic impedance’ in gas-filled systems. Etymologically, the term ‘resistance’ conveys the same sense as impedance but is, by conventional definition, confined to non-oscillatory or steady motions. Resistance may thus be considered as impedance at zero frequency. All the above terms have a precise physical meaning and can be expressed in mathematical terms. The term ‘impedance’ is frequently used by clinicians in another sense: in a general way, as referring to the impediment or hindrance to blood flow (Wilcken et al., 1964; Miller et al., 1976; Mason, 1978; Finkelstein et al., 1985). Some have described impedance as instantaneous pressure divided by instantaneous flow throughout the cardiac cycle (Urshel et al., 1968a; Urshel et al., 1968b). This concept has no counterpart in the physical sciences and is inappropriate for describing pressure-flow relationships in a system where at any one time pressure depends not only on flow at the same instant, but also on flow at previous instants (this being a consequence of blood inertia, arterial distensibility, and pulse wave travel and reflection). So-called ‘instantaneous impedance’ is a contradiction in terms because it is a frequency-dependent quantity rather than a time-dependent quantity.
