Epidural Intracranial Pressure Measurement in Rats Using a Fiber-optic Pressure Transducer

Summary

A novel technique to record the pressures within the skull is described. The minimally invasive method uses a fibre-optic pressure sensing system to accurately measure intracranial pressure (ICP) in anaesthetized rats without causing significant brain trauma. The technique may be used in a wide range of experimental models.

Abstract

Elevated intracranial pressure (ICP) is a significant problem in several forms of ischemic brain injury including stroke, traumatic brain injury and cardiac arrest. This elevation may result in further neurological injury, in the form of transtentorial herniation^1,2,3,4, midbrain compression, neurological deficit or increased cerebral infarct^2,4. Current therapies are often inadequate to control elevated ICP in the clinical setting^5,6,7 . Thus there is a need for accurate methods of ICP measurement in animal models to further our understanding of the basic mechanisms and to develop new treatments for elevated ICP.

In both the clinical and experimental setting ICP cannot be estimated without direct measurement. Several methods of ICP catheter insertion currently exist. Of these the intraventricular catheter has become the clinical 'gold standard' of ICP measurement in humans⁸. This method involves the partial removal of skull and the instrumentation of the catheter through brain tissue. Consequently, intraventricular catheters have an infection rate of 6-11%⁹. For this reason, subdural and epidural cannulations have become the preferred methods in animal models of ischemic injury.

Various ICP measurement techniques have been adapted for animal models, and of these, fluid-filled telemetry catheters¹⁰ and solid state catheters are the most frequently used^{11,12,13,14,15}. The fluid-filled systems are prone to developing air bubbles in the line, resulting in false ICP readings. Solid state probes avoid this problem (Figure 1). An additional problem is fitting catheters under the skull or into the ventricles without causing any brain injury that might alter the experimental outcomes. Therefore, we have developed a method that places an ICP catheter contiguous with the epidural space, but avoids the need to insert it between skull and brain.

An optic fibre pressure catheter (420LP, SAMBA Sensors, Sweden) was used to measure ICP at the epidural location because the location of the pressure sensor (at the very tip of the catheter) was found to produce a high fidelity ICP signal in this model. There are other manufacturers of similar optic fibre technologies¹³ that may be used with our methodology. Alternative solid state catheters, which have the pressure sensor located at the side of the catheter tip, would not be appropriate for this model as the signal would be dampened by the presence of the monitoring screw.

Here, we present a relatively simple and accurate method to measure ICP. This method can be used across a wide range of ICP related animal models.

Protocol

1. Skull Penetration

1. Anaesthetize rat with isoflurane (5% induction, 1.5-2% maintenance) in 70% N₂ and 30% O₂. Following induction of anaesthesia, place the rat prone on a warming plate, positioning the rat's nose in an anaesthetic nose cone.
2. Whilst maintaining anaesthesia, secure the head in a stereotaxic frame, inserting the ear-bars until head is stabilised. Ensure breathing is not impaired. (Figure 2-A).
3. Inject scalp subcutaneously with long lasting local anaesthetic, Bupivacaine 0.3 ml 0.5% (Pfizer, Australia) before making a 1.5 cm skin midline head incision. (Sterile instruments and gloves s....

Discussion

The procedure presented here enables a very sensitive and accurate recording of intracranial pressure. This minimally invasive technique avoids significant brain trauma by positioning the pressure sensor in the epidural space and not the brain tissue or ventricles.

The critical steps Include: 1) drilling through the skull – care must be taken not to pierce the dura or damage underlying brain tissue; 2) ensuring a tight seal with the caulking material – if there is any leak, the.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Dental Cement Monomer	Henry Schein	VX- SC500MLL
Dental Cement Polymer	Henry Schein	VX- SC1000GCL4
Dental drill burr- size 12	Gunz Dental	EL104S001012/10
Dental drill burr- size 6	Gunz Dental	EL104S001006/10
Metal Screw	Hardware Store		2 x 4 mm, hexagonal head. (laboratory-modified by 0.7 mm hole drilled through shaft)
SAMBA Control Unit	Harvard Apparatus	50433102
SAMBA Sensor	Harvard Apparatus	50461122	420 LP, 15cm bare fibre, radio-opaque coating
Silagum AV Mono caulking material	Gunz Dental	RG 9152	Vinylpolysiloxanes, hydrogen polysiloxanes, filler, pigments, additives, plantinum catalyst
Terg-A-Zyme	Alconox, Inc.	1304	Enzyme-active powdered detergent