Intracranial compliance is time-dependent

Compliance is considered a fundamental characteristic of the intracranial system, is measured by the volume-pressure test (VPT), and is considered to be related to the cerebrospinal fluid (CSF) pulse pressure (ΔPcsf). This study was carried out to determine the time-dependent relationship of intracranial compliance and the relationship between intracranial pulse pressure and compliance. In nine dogs divided into two groups, the rate of injection of a test volume and the physical compliance of the intracranial system was altered by opening or closing the skull without altering the base line intracranial pressure. The VPT was carried out by three methods: (a) slow infusion - infusion of 0.3 ml at 1.5 to 0.2 ml/second, (b) fast infusion - infusion of 0.2 ml in 0.5 second, and (c) bolus injection - injection of 0.05 ml as rapidly as possible. With the skull closed, there were linear relationships between diastolic CSF pressure (Pcsf-d) and ΔPcsf; Pcsf-d and slow infusion, fast infusion, or bolus injection; and Pcsf-d and CSF respiratory wave amplitude. With the skull open, the linear relationship was retained only for Pcsf-d vs. ΔPcsf and Pcsf-d vs. bolus injection. Furthermore, ΔPcsf amplitude at any level of Pcsf-d was the same whether the skull was open or closed. It is concluded that intracranial compliance can be divided into two components based on the time constant of the injected volume: physical compliance, which has a short time constant and is related to ΔPcsf, and physiological compliance, which has a longer time constant and is related to the VPT.