Hyperventilation-Induced Reduction in Cerebral Blood Flow: Assessment by Positron Emission Tomography

The use of positron emission tomography (PET) has been well documented as a relatively noninvasive method of measuring cerebral blood flow (CBF), both globally and regionally. The utility of readily detecting alterations in CBF is apparent, particularly when applied to the evaluation of therapeutic interventions thought to influence CBF. We report the effects of hypocapnia, an experimental condition of known cerebral vasoconstriction, in ten normal volunteers. Subjects had brain blood flow evaluated utilizing H₂ ¹⁵O as the positron emitter before and after approximately five minutes of hyperventilation. Baseline CBF was measured as a mean ± SD of 61.2 ± 16.3 mL/min/100 g of tissue. Mean baseline arterial blood gas values were PaO₂ 107.4 ± 14 mm Hg, PaCO₂ 37.7 ± 0.89 mm Hg, and pH 7.39 (calculated from mean [H⁺]). Post hyperventilation, global CBF was measured as 31.1 ± 10.8 mL/min/100 g. Mean arterial blood gas values were PaO₂ 141.7 ± 21 mm Hg, PaCO₂ 19.7 ± 5 mm Hg, and pH 7.63 (calculated from mean [H⁺]). CBF decreased by a mean of 49.5 ± 11 percent. Data analysis using the Student's t-est showed a significant change over baseline in PaCO₂ (p<0.001) and CBF (p<0.001), in the hyperventilated state. Correlations were noted between the decrease in CBF and change in PaCO₂ (r = 0.81) as well as between hyperventilation PaCO₂ and the change in CBF (r=0.97). We conclude that, as measured by PET, CBF decreases significantly during a state of artificial hyperventilation to a degree consistent with results seen using other methods. PET appears to be a valuable tool in the assessment of interventions that could influence CBF.