No abstract

Forty-five measurements of diameters of 12 human cerebral arteries were performed during 10 craniotomies under moderate changes in mean blood pressure and end tidal CO2. The mean change in blood pressure was 30 +/- 16 mm Hg (standard deviation) and that of end tidal CO2 was 14 +/- 6 mm Hg (standard deviation). These changes were induced with nitroprusside, phenylephrine, and adjustment of ventilator rate. Measurements were made through the operating microscope focused at the highest power, with meticulous attention to constant angle and distance from the artery. The mean diameter change in the large cerebral arteries (carotid, middle cerebral artery, vertebral artery) was less than 4%, but the smaller arteries (anterior cerebral artery, M2 segment of middle cerebral artery) showed diameter changes as large as 29% and 21% to end tidal CO2 and blood pressure changes, respectively. These data suggest that at the time of craniotomy, diameters of the large cerebral vessels do not significantly change during moderate variations in blood pressure and CO2, but that larger changes may occur in smaller vessels. This constancy of diameter suggests that the transcranial Doppler velocities obtained during intraoperative monitoring of craniotomies may closely reflect blood flow through the insonated artery.

SUMMARY Cerebral blood flow autoregulation (CBFA) to changes in perfusion pressure has not been previously reported in the rat. A modification of the Kety and Schmidt technique employing I33Xenon was used to measure cerebral blood flow (CBF) in paralyzed adult Sprague Dawley rats passively ventilated with 70% nitrous oxide and 30% oxygen. At a mean arterial blood pressure (MABP) of 121 ± 19 mm Hg, and a mean arterial Pco 2 of 36.2 ± 2.9 mm Hg, mean CBF was 103 ± 22 ml/min/100 gm of brain. CBF responses to hypercarbia were 4.9 ml/min/100 gm per mm Hg change in arterial Fco 2 . CBF was measured during steady state levels of hypo-and hypertension induced by phlebotomy, or by intravenous metaraminol, over the MABP range of 48-205 mm Hg. From a MABP of 80 to 160 mm Hg, CBF remained nearly constant, indicating the presence of CBFA. However, when MABP exceeded 160 mm Hg, CBF became pressure dependent, indicating a "breakthrough" of autoregulation in acute severe hypertension.CEREBRAL BLOOD FLOW autoregulation (CBFA) denotes the adaptive reaction whereby blood flow to the brain is maintained nearly constant despite variations in perfusion pressure.1 In most mammalian species, including man, CBFA has been characterized within the mean arterial blood pressure (MABP) range of approximately 70 to 150 mm Hg.2 ' 5 However, cerebral circulatory responses to blood pressures beyond this range in acute hypertension have not been extensively studied. Some investigators have recently reported that cerebral autoregulatory mechanisms fail at high perfusion pressures, with a so-called "breakthrough" of CBFA taking place. 59Most studies of the cerebral circulation have been carried out in man or in large experimental animals. More recently, CBF has also been studied in the most widely used laboratory animal, the albino rat, by a variety of methods, and values for basal CBF under conditions of normotension have been reported.10 "" However, pressure-flow relationships for the cerebral circulation have not been defined in this species. In the present study we have measured CBF in rats during induced alterations in arterial blood pressure. We have examined a wide range of systemic pressures, paying particular attention to the state of cerebral circulation during acute hypertension. Methods Animal PreparationSprague-Dawley rats of both sexes, weighing 300-600 gm, were employed in this study. Each rat was rapidly tracheotomized under ether anesthesia, paralyzed with tubocurarine (0.5 mg/kg, administered subcutaneously), and passively ventilated with a mixture of 70% nitrous oxide and 30% oxygen by a small animal respirator. Atropine (0.05 mg/kg) was injected subcutaneously to minimize tracheobronchial secretion. Ventilatory rate and tidal volume were adjusted to ensure arterial blood normoxia and normocarbia. The caudal artery was exposed and catheterized with a 30 gauge Teflon catheter. This catheter served to monitor arterial blood pressure with a pressure transducer (Statham, Model P23Dc), and to allow anaerobic withdrawal of arterial blo...