Linearity of the vascular pressure-volume relationship of the canine intestine

To test whether the pressure-volume relationship of the canine small intestinal vasculature is linear over the normal range of portal venous pressures (5 to 35 mm Hg), we used two methods to measure volume: (1) the integral of inflow minus outflow (IFD), and (2) tissue activity of 51Cr-labeled erythrocytes (Cr-51). Venous pressures were changed in steps of 5 mm Hg with a servo-controlled system. The tissue was perfused at a constant rate. With venous pressures of 3.6 to 38.6 mm Hg, the vascular compliance was 2.19 +/- 0.42 (SD) ml/kg x mm Hg using IFD, and 1.87 +/- 0.50 ml/kg x mm Hg using Cr-51. Although a quadratic term significantly improved the fit, the effect was small (less than 3 ml/kg with a 30 mm Hg venous pressure change). The control blood volume of the intestinal loop at a venous pressure of 8.6 +/- 1.5 (SD) mm Hg was 86.2 +/- 19.1 ml/kg tissue weight using the mean transit time of a step input of indocyanine green at a perfusion pressure of 106 +/- 29 mm Hg and a flow of 556 +/- 147 ml/min x kg. We conclude that there is no significant change in compliance over the normal venous pressure range of 5-35 mm Hg.