Autoregulation of cerebral blood flow

Blood flow in the cerebral circulation is controlled by a sophisticated regulatory system. Two important components of circulatory regulation are autoregulation and baroreceptor regulation. These fast regulatory systems act on the vessels supplying the brain in order to maintain a constant flow over a wide range of pressures. The control appears to be active within upper and lower limits of pressure. However, these limits shift to higher pressures for hypertensive subjects. The control is most likely mediated by a combination of myogenic and metabolic mechanisms, as well as changes in the activity of the central nervous system. This study focuses on modeling the dynamics and control of blood flow rather than trying to explain the underlying biophysics. The aim of this study is to use a lumped parameter model to reproduce the dynamic changes in pulsatile blood-flow-velocity (BFV) of the middle cerebral artery (MCA) during transient changes in arterial pressure. Measurements are carried out for young, old normatensive, and old hypertensive subjects performing postural change from sitting to standing. BFV is measured in the MCA using transcranial Doppler ultrasound and the pressure is measured using a finapres. The velocity measurements are performed using a head-mount such that the probe does not move when the subject stands up. In order to account for the pressure differences due to gravity, the arm is mounted so that the pressure probe is level with the heart at all times