Model-based evaluation of inertial and elastic properties of carotid circulation in pigs and their changes with high-cholesterol diet

Measurements of pulsatile pressure and flow at the input of the left and right carotid arteries, and a lumped parameter model were used to quantify the overall mechanical properties of the carotid circulation and their changes between five control diet pigs (C-pigs) and five pigs (H-pigs) fed a hyperlipidemic diet for 16 weeks. The model (referred to as IVW model) consists of an inertance, L, connected in series with a viscoelastic version of the windkessel. This version differs from the classic Frank-Halesʹs windkessel in that the peripheral resistance, Rp, of the considered portion of arterial circulation is connected in parallel with a complex and frequency dependent compliance, Cc(jω), rather than a constant (purely elastic) compliance. To describe this complex compliance we adopted the Voigt cell. This cell consists of a resistor, Rd, accounting for viscous losses, connected in series with a capacitor, C, representing static compliance. Our IVW model represents the portion of the circulation supplied by either the left or the right carotid artery and is characterised by the four parameters Rp, L, C and Rd. Peripheral resistance, Rp, was calculated as the ratio between mean pressure, , and mean flow, . The other three parameters were estimated by fitting measured to model predicted flows. Paired C-pig and H-pig had similar mean carotid pressure. Average (±SE) C-pig mean pressure was 62.0±4.3 mmHg, and H-pig was 62.7±4.7 mmHg. Estimated compliance, C, averaged 12.8±1.9 10−6g−1cm4s2 in the C-pigs and was reduced by 45% in the hyperlipidemic diet fed pigs (7.1±1.2 10−6g−1cm4s2). A reduction of the overall cross-sectional area of vessels between C-pigs and H-pigs was inferred from a 40% increase in inertance, L. Resistance vessel tone was unchanged as judged from estimates of Rp. No signs of occlusive desease were found in any of the animals. Histomorphological examination of carotid vessels indicated that increased stiffness in H-pigs was associated with a) an abnormal increase in elastic fibres within the tunica media, b) a wave-like, rather than a smooth, appearance of the luminal margin and, c) an apparent thickening of the endothelium