Short-term regulation of arterial pressure and the calculation of open-loop gain in the intact anesthetized dog.

Open-loop gain of the short-term systemic pressure regulation was determined under closed-loop conditions in the closed chest anesthetized dog (n = 5). For this purpose, cardiac output and mean systemic pressure were varied by ventricular pacing after the production of complete heart block. From the pressure-flow data resistance gain (the ratio of peripheral resistance change to pressure change in the steady state) was obtained by means of a simple model. The value of this gain was automatically estimated by fitting the pressure-flow relation described by the model to the experimental data. The model allows the pressure-flow relation to be straight or curved with or without a zero-flow pressure intercept. The best fit was obtained when the pressure-flow curve was convex to the pressure axis and had no intercept. When the model was linearized about the control values of pressure and flow (operating point), open-loop gain could be calculated from resistance gain. Its averaged value in the control condition, 1.63 +/- 0.45, is in agreement with values found by other investigators in open-loop conditions. During vasoconstriction open-loop gain, at the (new) operating point, increased to 2.51 +/- 0.51; during vasodilation it decreased to 1.17 +/- 0.27. Open-loop gain about an operating point thus can be determined in the intact animal from measurements of mean pressure and mean flow in the steady state.