Non-invasive measurement of cerebral blood volume in volunteers

Invasive techniques are used for conventional measurements of cerebral blood volume and there is a need for methods which are more readily applicable clinically. We studied 13 healthy volunteers using near infrared spectroscopy and transcranial Doppler sonography. Middle cerebral artery flow velocity (vmca) and changes in cerebral oxygenated haemoglobin (HbO2), deoxygenated Hb (Hb), HbO2 - Hb (Hbdiff) [corrected] and HbO2 + Hb (total haemoglobin; Hbt) were measured at baseline and during graded reduction in arterial oxygen saturation (Spo2) to 85% at hypo-, normo- and hypercapnia. Cerebral blood volume (CBV) values, calculated from regression lines of Hbdiff/SpO2 and HbO2/SpO2 were 5.38 and 4.03 ml 100 g-1 for the two methods, respectively. CBV varied directly with FE'CO2, with mean CBV reactivities of 1.25 and 1.06 ml 100 g-1 kPa-1, respectively. Changes in Hbt were not systematically related to changes in FE'CO2. An increase in vmca and reciprocal decrease in estimated cerebrovascular resistance were observed when oxygen saturation decreased to less than 90-93%. These results suggest that cerebral haemodynamics in individual subjects may be monitored non-invasively providing the methodology is modified to account for hypoxic vasodilatation.