RHYTHMICAL FLUCTUATIONS OF THE INTRACEREBRAL MICROCIRCULATION STUDIEDIN PIGS

Rhythmic variations in blood flow have been observed in various vascul ar beds, including brain. We have characterized fluctuations of the mi crocirculation in different locations in the brain, and studied the re sponse to changes in arterial carbon dioxide tension, arterial pressur e, and cerebrospinal fluid pressure. Laser Doppler flowmetry was perfo rmed in 20 pentobarbital-anesthetized pigs. Flow probes were positione d on the brain surface and 3, 10, and 20 mm into the cerebral tissue. The protocol included carbon dioxide breathing, hemorrhagic hypotensio n, and infusion into the cisterna magna. Twenty-five periods of low-fr equency oscillations (4.5/min) were found in IO pigs with superimposed spindle-shaped rhythmic variations (0.5/min) of the amplitude in 7. T here were no rhythmic changes in arterial pressure or intracranial pre ssure. Rhythmic activity was most often seen in the probe positioned 2 0 mm into the brain and was often seen in several probes at the same t ime. Animals with rhythmic oscillations before interventions had lower cerebral perfusion pressure and arterial pressure, lower heart rate, and higher laser Doppler signal than the others. Blood loss often init iated oscillations. High intracranial pressure tended to abolish preex isting oscillations. Hypercapnia always abolished preexisting oscillat ions. Oscillations were more frequent if the cerebral perfusion pressu re was in the low range of cerebral autoregulation, occurred more ofte n in the cerebral locations with relatively high local flow, were most likely to be localized, and therefore probably caused by local metabo lic or myogenic variations.