Increases in oxygen consumption without cerebral blood volume change during visual stimulation under hypotension condition

The magnitude of the blood oxygenation level-dependent (BOLD) signal depends on cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate of oxygen (CMRO₂). Thus, it is difficult to separate CMRO₂ changes from CBF and CBV changes. To detect the BOLD signal changes induced only by CMRO₂ responses without significant evoked CBF and CBV changes, BOLD and CBV functional magnetic resonance imaging (fMRI) responses to visual stimulation were measured under normal and hypotension conditions in isoflurane-anesthetized cats at 4.7 T. When the mean arterial blood pressure (MABP) decreased from 89±10 to 50±1 mm Hg (mean±standard deviation, n=5) by infusion of vasodilator sodium nitroprusside, baseline CBV in the visual cortex increased by 28.4%±8.3%. The neural activity-evoked CBV increase in the visual cortex was 10.8%±3.9% at normal MABP, but was negligible at hypotension. Positive BOLD changes of ±1.8%±0.5% (gradient echo time=25 ms) at normal MABP condition became prolonged negative changes of -1.2%±0.3% at hypotension. The negative BOLD response at hypotension starts approximately 1 sec earlier than positive BOLD response, but similar to CBV change at normal MABP condition. Our finding shows that the negative BOLD signals in an absence of CBV changes are indicative of an increase in CMRO₂. The vasodilator-induced hypotension model simplifies the physiological source of the BOLD fMRI signals, providing an insight into spatial and temporal CMRO ₂ changes.