Arterial pressure (AP), intracranial pressure (ICP) and cerebral blood flow (CBF) undergo complex spontaneous fluctuations. In spite of the dampening effect of autoregulation, especially in the low-frequency (LF) range, CBF is affected by AP fluctuations. On the other hand, the relationship between CBF and ICP fluctuations in physiological conditions is still incompletely clarified. CBF also depends on changes in cerebral metabolism and chemical cerebrovascular regulation, such as those occurring across wakeÐsleep states (W, wakefulness; QS, quiet sleep; AS, active sleep). The aim of the present work was to evaluate the relationship of LF spontaneous fluctuations in CBF versus those in AP and ICP across the wakeÐsleep cycle in newborn lambs.
Experiments were carried out on male Merino/Border-Leicester cross lambs. Lambs 2Ð5 days old (n = 4) were chronically implanted under general halothane and nitrous oxide anaesthesia with electrodes for electroencephalographic and electromyographic recordings and with catheters in the femoral artery and the subdural space for measuring AP and ICP, respectively. A transit-time ultrasound flow probe (Transonic Systems, Ithaca, NY) was positioned around the superior sagittal sinus for CBF recordings. 72-hour post-operative recovery was allowed. Artifact-free 120 s data sequences sampled at a rate of 100 Hz were selected during stable behavioural states and linearly detrended. Pearson’s correlation coefficient was computed on ICP and CBF data after low-pass filtering at 0.3 Hz. AP and CBF variance was calculated in the LF range (0.016Ð0.3 Hz) by means of spectral analysis and Welch procedure on five overlapped Hanning-windowed 60 s subsets for each original sequence. After completion of the study, lambs were killed under general anaesthesia (150 mg kg-1 sodium pentobarbitone).
The mean correlation coefficient between ICP and CBF was positive and significantly different from 0 (one-sample t test, P < 0.005) in the different states and highest in AS (ANOVA and Tukey’s post-hoc test, P < 0.005). In all states the coefficient of variation of CBF (CVCBF) was significantly higher than that of AP (CVAP) (paired t test, P < 0.005). CVCBF and CVAP were lowest in QS (Mann-Whitney test, P < 0.05).
The positive value of rPC indicates that ICP LF fluctuations are induced by those of CBF. Higher CVCBF vs. CVAP values indicate that CBF fluctuations cannot be explained by AP fluctuations alone. Lowest CVCBF and CVAP values in QS agree with the homeostatic features characterising this state.
All procedures accord with current National guidelines.