Haemophilus influenzae type B impairment of pial vessel autoregulation in rats

To examine the mechanisms of autoregulatory impairment in meningitis, we studied the effects of Haemophilus influenzae type b (Hib) on pial vessels during hemorrhagic hypotension in rats, using a cranial window technique. We prepared cranial windows in barbiturate-anesthetized, mechanically ventilated rats. Artificial cerebrospinal fluid or 10(5) Hib in cerebrospinal fluid (n = 7 each group) was suffused over the pial surface. Pial arteriolar diameter was measured hourly for 4 h. Autoregulation was assessed as the ability of pial arterioles to dilate in response to graded hemorrhagic hypotension at 2 and 4 h. Pial arterioles exposed to Hib dilated progressively to 149 +/- 27% of baseline at 4 h. Vessel diameter in the Hib group was significantly greater than in the control group beginning at 2 h. Autoregulation was progressively impaired in Hib-exposed rats compared with control rats [-5.85 +/- 1.38 versus -8.02 +/- 2.02 and -3.82 +/- 1.57 versus -8.53 +/- 1.72% dilation/kPa fall in mean arterial blood pressure at 2 and 4 h, respectively (p < 0.05)]. These data suggest that autoregulation is impaired in pial arterioles exposed to Hib because involved vessels have a finite dilatory capacity and are close to maximal dilation before hypotensive challenge.