Transcranial Doppler Sonography Indicates Critical Brain Perfusion During Hemorrhagic Hypotension in Dogs

This study investigated the effects of hemorrhagic hypotension on cerebral blood flow velocity and brain electrical activity (by electroencephalogram [EEG]).Eleven mongrel dogs were anesthetized with isoflurane (1 minimum alveolar anesthetic concentration [MAC]) and catheters were placed into both femoral arteries and veins for mean arterial blood pressure (MAP) measurement, blood withdrawal, and drug administration. Brain temperature, arterial blood gases, and pH were maintained constant. EEG was recorded from temporoparietal recording sites versus a frontal reference. A pulsed transcranial Doppler (TCD) probe (2 MHz, Transpect Trademark, Medasonics) was placed on the dura via a temporal bone window to measure mean (Vmean, cm/s) and diastolic blood flow velocity (Vdiast, cm/s) in the middle cerebral artery. At the end of the surgical preparation, isoflurane was discontinued and all animals received fentanyl (bolus, 25 micro gram/kg intravenously (IV); infusion, 50 micro gram centered dot kg-1 centered dot h-1 IV) plus 50% N2 O/O2 during 30 min of equilibration. After recordings of baseline data, the dogs were hemorrhaged at a rate of 80-100 mL/min. The observation interval was 14 min. EEG spectral edge frequency (SEF 95%) and Vmean did not change when MAP was decreased from 109 +/- 10 to 63 +/- 7 mm Hg. This indicates preserved neuronal function and intact autoregulation of cerebral blood flow. Below MAP of 49 +/- 9 mm Hg, a shift of the EEG to lower frequencies was associated with decreases in Vmean and Vdiast. EEG burst suppression occurred at a MAP of 31 +/- 7 mm Hg, paralleled by a loss of the diastolic flow velocity pattern. The results of this study are consistent with findings in patients with increased intracranial pressure where the loss of diastolic blood flow velocity occurs at a level when cerebral perfusion decreases below the cerebral ischemic threshold. (Anesth Analg 1995;81:1203-7)