MONITORING INTRACRANIAL DYNAMICS BY TRANSCRANIAL DOPPLER - A NEW DOPPLER INDEX - TRANS SYSTOLIC-TIME

Since the introduction of transcranial Doppler sonography (TCD) severa l investigators have described the relationship between raised intracr anial pressure (ICP) and Doppler waveform. This waveform has been expr essed by several indices, such as the pulsatility index (PI) and the r esistance index (RI). These indices are used to demonstrate the presen ce of raised ICP. In childhood hydrocephalus this information can be u sed to indicate the need for shunt implantation. However, PI and RI do prove to have certain disadvantages as both are strongly influenced b y the heart rate. Moreover, both indices have a broad range of referen ce values, especially in children. Therefore, they are not very reliab le for detecting insidious changes in the ICP. These drawbacks are due to the fact that these indices are composed of blood flow velocity me asurements and do not embody the slope of the TCD waveform itself. An ideal TCD waveform analysis should be performed concerning the time-re lated changes of the velocities. We present a hydrodynamic model, with its electrical analogue, which shows the effects of raised ICP on the intracranial hemodynamic system. Based on these physical findings we define a new Doppler index, the Trans Systolic Time, reflecting specif ic changes in the TCD waveform induced by changes in the mean ICP. The applicability of this index, compared with PI and RI, is illustrated by consecutive simultaneous TCD and AFP measurements in three children with hydrocephalus.