An estimated compliance index derived from intracranial pressure recording

The purpose of this study is to test the validity of a proposed compliance index, percent change of compliance per mmHg of intracranial pressure (%CC/mmHg), by comparison of values of the index with corresponding experimentally derived values of compliance. The derivation of %CC/mmHg is based on two assumptions: 1) the pressure-volume characteristic of the craniospinal sac can be linearized for small perturbations about an equilibrium; and 2) during a brief interval in which these perturbations occur the pathophysiologic state of the sac does not significantly change. By rapid infusion of 0.2 ml of mock cerebrospinal fluid (CSF) experimental values of compliance (n = 44) were obtained from 10 piglets during monitoring of intracranial pressure (ICP). A strong correlation (r = 0.89, p < .001, n = 44) was obtained between values of %CC/mmHg and corresponding values of experimentally derived compliance (ml/mmHg). The value of the proposed index of compliance, %CC/mmHg, needs to be evaluated in the clinical setting. However, from the view of validation of ICP instrumentation, high values of both mean ICP and mean %CC/mmHg are contradictory and indicative of the occurrence of instrumentation error due to electronic direct current drift.