Cerebral Perfusion Pressure and Nett Cerebral Mean Transit Time in Childhood Hydrocephalus

Abstract

Transit time is defined as the time such that the volume of tracer entering or leaving an organ or tissue is equal to the volume of tracer in that organ or tissue at the equilibrium. This method of measuring cerebral transit time involves introduction of a tracer (sodium pertechnetate) via an antecubital vein. A gamma camera fitted with an ultra high sensitivity collimator is on line to a mini computer. The tracer is flushed rapidly and data are acquired for approximately one minute. The transit time at the aortic arch is calculated from the aortic curve and this is used to correct for variations in central transit time and rate of injection. The results are displayed as a colour coded map (Figure). The nett cerebral mean transit time in 102 normal adult subjects for a whole hemisphere is 5.1 seconds (range 2–8 seconds). The transit time results were compared with the mean intracranial pressure and cerebral perfusion pressures from an overnight ICP study carried out at the same time. CT scans provided an index of ventricular dilatation. The mean transit time value (CBV/CBF) is the reciprocal of circulatory reserve or the degree of reduction in cerebral perfusion pressure. Transit time values correlated significantly (p(0.05) with the actual minus expected cerebral perfusion values.