Clinical Importance of the Compliant Conduit

A primary physiological function of the aorta and its major branches is to convert the highly pulsatile output of the left ventricle to a more nearly uniform and steady flow in the arterioles and capillaries, with a minimum loss of energy (McDonald, 1974). This requires that peripheral vascular input impedance (which is a complex function of arterial resistance, fluid inertance, and arterial compliance [capacitance, distensibility]) be matched to the output impedance of the heart. Thus, a complete mechanical description of the heart and its prospective replacements, or fluid mechanical description of blood flow in the major arteries must take into account arterial compliance. Compliance is defined in this chapter as the fractional change in volume per unit change in pressure: C=(ΔV/V)/ΔP. For cylinders of fixed length, a good approximation to the major blood vessels, this simplifies to twice the fractional change in diameter per unit change in pressure: C=2(ΔD/ΔP)/D. However, it has become commonplace with many authors (Baird, 1977; Megerman and Abbott, 1983; Klein et al.,1982) to omit the constant multiplier, “2”, and this practice will be followed here as well [i.e. C=(ΔD/D)/ΔP; units: % change in diameter/100 mmHg].