Regional cerebral blood flow during hyperventilation in patients with acute bacterial meningitis

Mechanical hyperventilation is often instituted in patients with acute bacterial meningitis when increased intracranial pressure is suspected. However, the effect on regional cerebral blood flow (CBF) is unknown. In this study, we measured regional CBF (rCBF) in patients with acute bacterial meningitis before and during short-term hyperventilation. In 17 patients with acute bacterial meningitis, absolute rCBF (in ml/100 g min^-1) was measured during baseline ventilation and hyperventilation by single-photon emission computed tomography (SPECT) using intravenous ¹³³Xe bolus injection. Intravenous ^99mTc-HMPAO (hexamethylpropyleneamine oxime) was subsequently given during hyperventilation. In 12 healthy volunteers, rCBF was measured by SPECT and ^99mTc-HMPAO during spontaneous ventilation. Using standard templates to identify regions of interest (ROIs), we calculated rCBF in percentage of cerebellar (^99mTc-HMPAO images) or mean hemispheric (¹³³Xe images) flow for each ROI, the degree of side-to-side asymmetry for each ROI, and the anterior-to-posterior flow ratio. On ¹³³Xe images, absolute rCBF decreased significantly during hyperventilation compared to baseline ventilation in all regions, but the relative rCBF did not change significantly from baseline ventilation (n = 14) to hyperventilation (n = 12), indicating that the perfusion distribution was unchanged. On ^99mTc-HMPAO images (n = 12), relative rCBF and the anterior-to-posterior flow ratio were significantly lower in patients than in controls in the frontal and parietal cortex as well as in the basal ganglia. Focal perfusion abnormalities were present in 10 of 12 patients. Regional cerebral blood flow abnormalities are frequent in patients with acute bacterial meningitis. Short-term hyperventilation does not enhance these abnormalities.