Brain eigenfrequency shifting as a sensitive index of cerebral compliance in an experimental model of epidural hematoma in the rabbit: Preliminary study

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Abstract

Objective

To verify brain eigenfrequency shifting after the occurrence of a lesion producing mass effect into the cranial vault.

Design

Experimental animal study.

Setting

Laboratory of experimental surgery affiliated with a university critical care department.

Subjects

Six adult male New Zealand white rabbits.

Interventions

A Camino ICP monitor was placed in the parenchyma, and a 5-Fr balloon-tipped catheter and accelerometer were placed into the epidural space.

Measurements

Before and after the introduction of successive 0.1-mL increments of autologous blood into the balloon, intracranial pressure (ICP) was recorded along with the accelerometer signal obtained during free vibration of the skull triggered by a calibrated hammer. Fast Fourier transformation of the digitized signal provided the eigenfrequency spectrum. The eigenfrequency showing the sharpest decrease after the initial 0.1-mL volume addition was considered as the best frequency, and its variation in response to subsequent 0.1-mL increments represents the brain eigenfrequency shifting.

Main Results

Brain eigenfrequency shifting to lower values occurs for small blood volume increments (up to 0.2 mL). When volume addition becomes >or=to0.3 mL, brain eigenfrequency shifting to higher values is exhibited. The decrease in best frequency after the initial introduction of 0.1 mL is statistically significant (p = .003), in a range of volume in which no significant intracranial pressure difference appears. The respective variation of ICP is explained using a quadratic curve. For volumes of 0 to 0.1 mL, the change in ICP is not statistically significant (p = .08).

Conclusions

Changes of the brain's physical characteristics by mass addition in the cranial vault can be expressed by brain eigenfrequency shifting. The method seems advantageous because it reliably detects mass additions at low levels where no ICP change occurs. Additionally, it provides serial measurements, and it is less invasive than the currently used methods for intracranial compliance. (Crit Care Med 1999; 27:978-984)

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