CORTICAL CEREBRAL BLOOD-FLOW CYCLING - ANESTHESIA AND ARTERIAL BLOOD-PRESSURE

Cycling of various cerebral metabolic substances, arterial vascular di ameter, and flow has been noted by many workers at a frequency near 0. 1 Hz. Suspicion that this phenomenon is dependent on the type of anest hesia led us to investigate the occurrence of cerebral blood flow (CBF ) cycling with different anesthetics. Fifteen Sprague-Dawley rats were anesthetized with either pentobarbital (n = 5, 40-50 mg/kg), alpha-ch loralose (n = 5, 60 mg/kg), or halothane (n = 5, 1-0.5%). Body tempera ture was maintained at 37 degrees C. Femoral arterial and venous cathe ters were placed, and a tracheotomy was performed, permitting artifici al ventilation with 30% O-2-70% N-2. A closed cranial window was forme d over a 3-mm diameter craniotomy. Mean arterial pressure (MABP), arte rial partial pressures of CO2 and O-2 (Pa-CO2 and Pa-O2), and pH were controlled and stabilized at normal values. CBF was determined using l aser Doppler flowmetry. To induce cycling, MABP was transiently and re peatedly lowered by exsanguination. Fast Fourier analysis of selected 64-s flow recordings (n = 38) was performed. CBF cycling was observed, independent of the type of anesthesia, in all animals. In 36 epochs, cycling was induced when MABP was reduced to a mean pressure of 65 +/- 1.5 mmHg. The mean frequency and amplitude were 0.094 +/- 0.003 Hz an d 6.6 +/- 0.5%, respectively. Cycling occurred without blood withdrawa l in two epochs. With the use of the blood-withdrawal epochs (n = 36), all three anesthetics shared a common linear slope between amplitude and blood pressure (P < 0.02) and blood pressure change (P < 0.01). Pe ntobarbital differed from a-chloralose and halothane in the relation b etween cycling frequency and blood pressure. Only pentobarbital exhibi ted correlation between frequency and blood pressure (P < 0.02) and bl ood pressure change (P < 0.001). The occurrence of these oscillations is not related to the type of anesthesia, and they usually occur at MA BP values that are near or just above the lower limit of autoregulatio n. At this pressure level, CBF oscillations would suggest that vasocon strictive and dilatory forces are no longer in balance, but alternativ ely vying for control.