Widespread reduction of regional cerebral blood flow during hyperventilation-induced EEG slowing ('buildup'). Observation from subtraction of brain imaging with single photon emission computed tomography using technetium-99m hexamethyl-propyleneamine oxime.

To study the pathophysiological mechanisms of hyperventilation-induced EEG showing, i.e., the so-called 'buildup' phenomenon, changes in regional cerebral blood flow (rCBF) were investigated before and during the phenomenon in a 16-year-old woman with headache, thought to be of neurotic origin, by subtraction technique of brain images with single photon emission computed tomography using technetium-99m hexamethyl-propyleneamine oxime (99mTc-HMPAO). The tracer uptake during buildup decreased by 31-42% as compared to baseline values at rest before buildup in all of the measured regions, reflecting a widespread reduction in rCBF. Gas analyses of arterial blood collected during buildup showed a decrease in PaCO2, and increases in PaO2 and pH with a slight decrease in blood pressure and an increase in pulse rate. These results directly demonstrate a close correlation between the hyperventilation-induced EEG and rCBF changes, suggesting that the buildup phenomenon results from cerebral ischemic change, presumably due to cerebral vasoconstriction caused by the PaCO2 decrease.