The effects of ventricular drainage on the intracranial pressure signal and the pressure reactivity index.

In subarachnoid hemorrhage (SAH) patients intracranial pressure (ICP) is usually monitored via an extraventricular drain (EVD), which can produce false readings when the drain is open. It is established that both the ICP cardiac pulse frequency and long term trends over several hours are often seriously corrupted. The aim of this study was to establish whether or not the intermediate frequency bands [respiratory, Mayer wave and very low frequency (VLF)] were also corrupted. The VLF range is of special interest because it is important in cerebral autoregulation studies. Using a pattern recognition algorithm we retrospectively identified 718 cases of EVD opening in 80 SAH patients. An analysis of differences between closed and open-drain periods showed that ICP amplitude decreased significantly in all of the three lower frequency bands when the EVD was open. A similar analysis of systemic arterial pressure signal revealed similar changes in the same frequency bands that were positively correlated with the ICP changes. Therefore we concluded that the changes in the ICP signal represented real, physiological changes and not artifact. Pressure reactivity index (PRx) values were also computed during closed and open-drain periods. We found a small but statistically significant decrease during open-drain periods. Based on analysis of the change in the PRx distribution during open drainage we concluded that this decrease also represented physiological changes rather than artifact. In summary the ICP respiratory, Mayer wave, and VLF frequency bands are not corrupted when the EVD is open, and it safe to use these for autoregulation studies.