| Summary: | Raised intracranial pressure (ICP > 20 mm Hg) occurs in at last thirty percent of severely head injured patients and is associated with a significant increase in the number of deaths and severely disabled survivors. A study of the ICP waveform may yield information of relevance to the management of raised ICP either as a predictor of raised ICP or of its underlying cause. An observational study in thirty severely head injured patients was carried out, where fifteen hundred ICP and blood pressure (BP) waveform samples were collected under microcomputer control and analysed through the use of spectral methods in a systems analysis aproach to quantifying pressure transmission across the cerebrovascular bed. The amplitude transfer functions, as a measure of cerebrovascular pressure transmission, were calculated for the first 4 cardiac harmonics from the ICP and BP spectra and were found to cluster into four classes: those with an overall flat amplitude transfer function (curve type 1), and those with an elevated low frequency response (curve type 2), those with an elevated high frequency response (curve type 3) and those exhibiting both an elevated low and an elevated high frequency response (curve type 4). Curve types 2 and 4 (elevated low frequency, elevated low frequency in combination with elevated high frequency) were most often associated with raised ICP (ICP > 20 mm Hg) whereas curve types 1 and 3 (flat, elevated high frequency) were most often associated with ICP less than 15 mm Hg. To aid in the interpretation of these clinically observed forms of cerebrovascular pressure transmission, an expermental investigation into the relationship between cerebrovascular resistance, craniospinal compliance and cerebrovascular pressure transmission was performed. Before such an experimental study could be performed, an improved method of measuring craniospinal compliance was needed. Existing measures of lumped craniospinal compliance such as the volume pressure response method (VPR) were considered too variable chiefly as a result of the sequence using an electronic square wave pressure generator to produce a small (0.05 ml), exact and reproducible transient volume increase into the CSF space. The new method was validated against the VPR method in physical and animal models, where it accurately followed compliance changes and demonstrated significantly less variation between measurements. The experimental studies were then performed in animal models of raised ICP, arterial hypercarbia and arterial hypertension, using the improved compliance technique. The data obtained from those studies were an aid to the interpretation of the four forms of cerebrovascular pressure transmission that were observed in the clinical studies. A flat amplitude transfer function (curve type 1) indicates equal transmission of all harmonics of the arterial pressure waveform through to the CSF space and was found in the experimental studies associated with ICP less than 15 mm Hg and with normal craniospinal compliance normal cerebrovascular resistance and a negative fundamental phase shift. This form of cerebrovascular pressure transmission may characterize a functionally normal cerebrovascular bed.
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