Unstable state of the arterial pressure control system after a mild hemorrhage.

The stability of the arterial pressure control system after a mild hemorrhage (5 ml/kg body wt) was studied in 10 anesthetized cats by analyzing the responses of arterial pressure and of heart rate and the dead time (L) of heart rate response to a minute, rapid test hemorrhage (1 ml/kg body wt). The overall open-loop gain (H) of the arterial pressure control system was estimated from the ratio of arterial pressure fall immediately after the quick hemorrhage over the steady-state fall. Increases in H and L were correlated with the tendency of the arterial pressure control system to be unstable and oscillate. Oscillation in arterial pressure was always accompanied by a marked increase in heart rate (deltaHR), which represents an enhanced activity of the sympathetic nervous system and serves to restore arterial pressure toward the prehemorrhage level. The present study indicates that the magnitude of deltaHR also serves as a good predictor of arterial pressure oscillation.