Overall characteristics of arterial pressure control system studied by mild hemorrhage

The overall, static, open-loop gain of the negative-feedback arterial pressure control system was determined repeatedly in anesthetized cats under normal and hypovolumic states. The gain, H, was assessed as (deltaAPI/deltaAPS)-1, where deltaAPI is the immediate fall and deltaAPS the steady-state fall in arterial pressure following a stepwise reduction in blood volume. When the step size was increased from 1.25 to 25% of the total blood volume, H decreased from 30 to less than unity. Considering this amplitude dependence of H, a small amount (1.25%) of quick (in less than 0.3 s) hemorrhage was used to determine H throughout the rest of the experiments. Time-dependent variation of H was evident after a large amount (6.25%) of blood loss. During the initial 10 min, H increased to 126% of the control value, decreased to 43% over the subsequent 20--70 min, and showed a trend toward recovery. The small signal analysis permits a quantitative characterization of the time and input size dependent nature of the arterial pressure control system after various degrees of hemorrhage.