Ramachandran Ramani, MD

Assistant Professor, Department of Anesthesiology

Understanding anesthesia through functional MRI

Functional MR imaging is an objective method of studying the subjective effects of anesthesia. It is a imaging tool being used widely by neuroscientist for studying functional activity in the CNS like speech, movement, visual / auditory activation etc. With fMRI we can measure BOLD (blood oxygen level dependent contrast) a qualitative measure of cerebral metabolism and regional cerebral blood flow (rCBF). BOLD and rCBF are indirect measures of neuronal activity. Our aim is to extend the use of fMRI for improving our understanding of the central effects of anesthesia.

Increase in rCBF in Heshl's cortex: awake, 0.25 MAC Sevo, and 0.5 MAC Sevo

In collaboration with the MRRC (magnetic resonance research center) we have been studying healthy ASA I volunteers under sevoflurane anesthesia. We study the effect of sevoflurane 0.25 and 0.5 MAC as well as the effect of activation under anesthesia. So far, close to 60 subjects have been imaged. In our first protocol the effect of visual, auditory and motor activation was studied under 0.25 MAC sevoflurane. Cerebral metabolism (BOLD) as well as rCBF was measured. Our conclusion from this study was - higher order association regions (like hippocampus, thalamus and cingulate gyrus) since they receive their input from multisynaptic pathways are much more sensitive to 0.25 MAC sevoflurane compared to unimodal association areas. Subsequently we studied the effect 0.25 and 0.5 MAC sevoflurane on auditory activation and memory activation. 0.25 MAC sevoflurane attenuates activation of auditory cortex while 0.5 MAC sevoflurane has profound effects on auditory cortex. This is of clinical relevance because auditory activation closely correlates with midlatency auditory evoked response which in turn correlates with amnesia under anesthesia. When specifically memory was activated (subjects heard a story presented over the headphones) both levels of sevoflurane anesthesia profoundly decreased the memory scores with loss of activation in the prefrontal cortex (primary site in the CNS for short term memory). In future we propose to study titrated doses of propofol as well as narcotic analgesics like Remifentanil.