To investigate whether the efficiency of cerebral blood flow autoregulation depends on local brain maturity, we examined the effect of development on the local autoregulatory response. We measured local brain blood flow using laser-Doppler flowmetry and quantitative autoradiography during hemorrhagic hypotension and hypertension (intravenous norepinephrine) in newborn 1- to 2-day-old (P1), 7- to 8-day-old (P8), 16- to 18-day-old (P17), and adult rabbits. Autoregulation within the cortex was less efficient than within the medulla in young (P1 or P8) compared with older rabbits (P17 or adult). Blood pressure increased during development, and the autoregulatory range extended up to 10, 15, 35, and over 100 mmHg about the normal pressure in P1, P8, P17, and adult animals, respectively. Acute severe hypertension readily produced focal areas of hyperemia within the cortex, thalamus, hippocampus, and/or cerebellum in young (P1 and P8) but not P17 animals. Severe hypotension produced profound reductions in blood flow within the cortex and subcortical white matter but not within deep forebrain and brain stem structures. Thus the efficiency of the autoregulatory response in general improved with increasing age and maturity of the brain region indicating that immature brain is susceptible to both ischemia during hypotension and hyperemia during hypertension.
Assessing haemorrhage-critical values of cerebral blood flow by modelling biomechanical stresses on capillaries in the immature brain
