CHANGES IN GLOBAL CEREBRAL BLOOD-FLOW IN HUMANS - EFFECT ON REGIONAL CEREBRAL BLOOD-FLOW DURING A NEURAL ACTIVATION TASK

1. The primary objective of this study was to examine in man, how indu ced changes in global cerebral blood flow (gCBF) affected a regional c erebral blood flow (rCBF) increase resulting from a neural activation task (opening of eyes). A secondary objective was to quantify how such induced changes in gCBF were distributed between representative regio ns of either predominantly grey matter or white matter. 2. Positron em ission tomography with intravenous infusion of (H2O)-O-15 was used to measure gCBF in six normal males. Concomitant measures of rCBF were ob tained in three different regions of interest (ROI): a representative area of predominantly grey matter, a representative area of predominan tly white matter and an area of visual cortex. 3. Cerebral blood flow was altered by establishing steady-state changes in P(CO2) at a near c onstant ventilation of approximately 30 1 min-1. The mean P(ET,CO2) (/- S.D.) levels (mmHg) that resulted were: low, 21.8 +/- 1.8; normal, 39.8 +/- 1.0, and high, 54.8 +/- 1.2. The normal and high levels were obtained by adding appropriate amounts of CO2 to the inspirate. The co rresponding mean gCBF levels across all six subjects with eyes closed were: low, 24.2 +/- 4.6; normal, 37.2 +/- 3.9 and high, 66.8 +/- 7.6 m l min-1 dl-1. 4. Blood flow in grey matter (insular cortex) and white matter (centrum semiovale) at normal levels of P(CO2) averaged 56.8 +/ - 10.1 and 20.3 +/- 3.4 ml min dl-1 respectively. As P(CO2) rose, the increase in rCBF to grey matter was approximately three times greater than that to white matter. 5. An activation state of eyes open in a br ightly lit room was compared to a baseline state of eyes closed in a d arkened room at the three levels of P(CO2) (and hence at three levels of gCBF). Over the whole gCBF range a significant (P = 0.028) effect o f increasing rCBF in the visual cortex ROI was found in response to op ening the eyes; the effect of this activation on rCBF was not signific antly dependent (P = 0.34) on the P(CO2) (and hence gCBF) level. The e ffect of the activation on the rCBF was apparently 'additive' to the r ise of rCBF associated with P(CO2)-related gCBF increase. 6. The resul ts confirm the need to normalize for changes in gCBF during studies of rCBF in response to an activation protocol. They also provide support for the use of an 'additive' model to achieve such normalization prov ided that other cortical areas behave in a similar manner to that of t he visual cortex.