Compliance of isolated porcine coronary small arteries and coronary pressure-flow relations

Compliance of small arteries is important for the interpretation of arterial pressure-flow relations. Six coronary small arteries (mean diameter 189 +/- 46 microns) were cannulated. Pressure-cross-sectional area (CSA) relations were obtained from isolated vessels by slowly varying pressure between 10 and 120 mmHg. Subsequently, small pressure variations were superimposed on constant mean pressures of 10, 30, and 50 mmHg with frequencies of 0.1-10 Hz. Normalized compliance (C0) was calculated as the compliance divided by the CSA at 50 mmHg. In vessels without tone, static C0 was 27.5 +/- 8.9, 6.4 +/- 1.1, and 3.8 +/- 1.0 x 10(-3) mmHg-1 at 10, 30, and 50 mmHg, respectively. At a frequency of 0.1 Hz, C0 decreased to one-third of static C0 at any pressure. Under these conditions, the phase shift between pressure and CSA was rather constant and ranged from -21 to -5 degrees. In four small arteries, smooth muscle tone was induced by the administration of acetylcholine. Activation decreased dynamic C0 by 60%. Two models of coronary input impedance were evaluated: the first model includes viscoelastic properties of the arterial wall and the second takes into account the blood inertia effect. The second model predicts much better the wall mechanics of single small arteries. The viscoelastic model overestimates the frequency dependence of compliance by a factor of 2 and the phase lag by a factor of 4. Moreover this model predicts a strong frequency dependence of induction of tone on compliance and phase lag, whereas this dependence is absent in the experimental results.