Radio Telemetry for the Measurement of Intracranial Pressure

p ERIODIC long-term measurement of intracranial pressure is desirable in m a n y neurosurgical conditions. Perhaps one of the more troublesome diseases from this s tandpoint is hydrocephalus. The status of such a disease requires repeated evaluat ion over a prolonged period, s but the hazards of violating the ventr icular or lumbar subarachnoid cerebrospinal fluid system with repeated needle punctures are formidable2 The applicat ion of radio te lemetry techniques to the general problem of intracranial pressure has been suggested by MacK a y ? 5 The applicat ion of te lemetry to physiologic problems has resulted in the developmen t of m a n y useful systems. Pressure in the human bladder l~ and gastrointestinal t rac t 6 has been studied on a shortterm basis with act ive t ransmi t te rs powered by a chemical ba t te ry . Small act ive t ransmit ters have been constructed tha t will continue to function for a t least a year, bu t this is a relat ively short t ime in te rms of disease processes of neurosurgical concern. Passive t ransmi t te rs t ha t are energized f rom an external radio frequency energy source ra ther than b y chemical batteries have been developed in a var ie ty of forms. Relat ively large units operat ing a t low radio frequencies have been successfully implanted in the body with pulse-rlnglng techniques. ~al Another approach in passive t ransmi t te r development has been to use a t ransistor as a rectifier for radio frequency energy f rom a t r ansmi t t e r outside the body serving as an energy source, storing the