Cerebral near infrared spectroscopy: emitter-detector separation must be increased

We have compared the effect of increasing optode separation (range 0.7-5.5 cm) on the sensitivity of near infrared spectroscopy (NIRS) to discrete red uctions in scalp and cerebral oxygenation in 10 healthy men (mean age 32, r ange 26-39 yr) using multichannel NIRS. During cerebral oligaemia (a mean r eduction in middle cerebral artery flow velocity of 47%) induced by a mean reduction in end-tidal P-CO2 of 2.4 kPa, the decrease in oxyhaemoglobin det ected by NIRS became significantly greater with increasing optode separatio n (P<0.0001). In response to scalp hyperaemia induced by inflation and rele ase of a pneumatic scalp tourniquet, increases in oxyhaemoglobin became sig nificantly smaller with increasing optode separation (P<0.0002). These resu lts are consistent with theoretical models of the behaviour of NIR light in the adult head and support the concept of using multi-detector NIRS to sep arate intra- and extracranial NIR signal changes. However, the emitter-dete ctor separation used by currently available cerebral oximeters is not large enough to provide optimal spatial resolution.