The final common pathway for death and permanent disability in head injuries and brain diseases is usually increased intracranial pressure. For this reason, measurement and control of intracranial pressure is a major focus of care of these individuals, both acutely and chronically. Since the intracranial contents exist within a rigid container (skull), direct pressure measurements require neurosurgical procedures, with their attendant risks, discomforts, and expense. Existing neurosurgical intracranial monitors can only be used in the hospital (usually ICU) setting, and have limited useful life due to drift and infection. In general, patients in whom the intracranial pressure is of concern are """"""""known"""""""" to the healthcare system - the majority has already had an invasive neurosurgical procedure. These could have in addition (and most other patients would be candidates for) a procedure that left a drift-free, permanent pressure sensor within the cranium. So far in the United States over 72 patents have been issued for devices that purport to facilitate measurement of intracranial pressure in this way. However, none of these devices been developed commercially due to various issues; such as limited lifetime, high cost of parts and assembly, plastic components in contact with extracellular space age rapidly; high cost of manufacturing, poor shielding from stray electromagnetic sources, MRI incompatibility, and so on. To overcome these limitations, we propose an active implantable sensor for intracranial pressure measurement. The sensor core is an oscillator operating at the Industrial-Scientific-Medical (ISM) band of 2.4000- 2.4835 GHz. The sensing component is a capacitor, whose variation with the intracranial pressure changes the oscillation frequency of the oscillator. The oscillator output is coupled to an antenna and can be picked up by an external monitoring unit. The size of the device is less than 1 cm 3. Prototypes of the device and monitoring unit will be developed. Prototype testing will be performed using a dry pressure-testing jig. Each prototype will be tested over its entire pressure range (-25 torr - 200 torr) for frequency response. Tests will be performed at ambient temperatures between 30 degrees C and 42 degrees C. Once the bench-testing phase is complete, an implantation test will be performed using a miniature swine model. The device will be implanted and tested under anesthesia, and then the animal will be allowed to recover. Pressure changes will be induced under anesthesia by induced hypercarbia and tiling the animal. After one week, the animal will be sedated and the device tested again under anesthesia. Then the animal will be killed with an approved technique, and the brain and sensor harvested for analysis. The sensor will be bench-tested according to a standard testing grid. The brain will be analyzed pathologically for changes in the tissue adjacent to the device.
| Neff, Samuel (2005) Measurement of flow of cerebrospinal fluid in shunts by transcutaneous thermal convection. Technical note. J Neurosurg 103:366-73 |
