Index of contractility: quantitative analysis of hepatic venous distensibility.

The low-pressure resistance vessels of the splanchnic circulation are passively distensible, and changes in regional blood pressures can lead to large changes in vascular resistance. The relationship between distending blood pressure (Pd) and vascular resistance (R) is described as a constant, the index of contractility (IC) where IC = R x Pd3. IC was derived in an isolated blood-perfused liver and was confirmed in vivo for both pre- and postsinusoidal resistance sites. IC does not change passively in response to wide changes in blood flow or hepatic outflow pressure. IC is dramatically altered in response to active vasoconstriction. In vivo, the presinusoidal IC rose from a control level of 12.2 +/- 4.2 to 92.7 +/- 20.6 IC units (mmHg4.ml-1.min.kg body wt) in response to 1.25 micrograms.kg-1.min-1 norepinephrine intraportal; the postsinusoidal IC rose from 20.4 +/- 2.3 to 59.6 +/- 14.2 IC units. IC reflects resistance changes secondary to active contractile responses independent of the passive consequences of the distensible nature of the resistance sites. We suggest that these concepts can be applied to any vascular bed with distensible resistance vessels.