R. Aaslid
Hemodynamics AG – Bern, Swaziland
Objective: Earlier attempts at non-invasive determination of ICP used ABP and TCD waveforms to estimate critical closing pressure (CCP). However, the CCP was found to be strongly influenced by vascular tone and a poor indicator of absolute ICP levels.
Material and Methods: An important effect of raised ICP is a decrease in intracranial compliance. This, in turn, causes the pulsations in the ICP waveform to increase. These pulsations are primarily caused by the changes in cerebral blood volume (CBV) throughout the heart cycle. We investigated a relatively simple model of the cerebral circulation where the relative changes in CBV were determined by integrating the difference between the arterial flow velocity (FV) waveform as recorded by TCD, and the venous outflow which we assumed to be non-pulsatile.
Results: In patients with good quality waveform recordings, we found that this simple model gave a good representation of the ICP waveform; in particular it could predict the phase shift between the ABP and the ICP waveforms. We also hypothesized that the shape of the FV waveform was primarily determined by the difference between the ABP and ICP waveforms.
Discussion: The model was used to estimate the cerebrovascular pressure transmission (CPT) in patients with increased ICP. We found a good agreement between the estimated and the measured CPT.
Key words: intracranial pressure dynamics, transcranial Doppler.