SFT Transducer and its Clinical Application for Continuous Monitoring of Epidural Intracranial Pressure

We devised a semiconductor film strain transducer (SFT) for continuous measurements of epidural or subdural intracranial pressure and blood pressure. The metal diaphragm of this transducer facing the dura mater was made of high quality stainless steel (sus 27, 30μ in thickness). A strip of SFT element is bonded on the counter surface of the diaphragm at its center, and the rim of the diaphragm is fixed at the edge of the stainless steel cylindrical housing base. The inner space is sealed except for an 1 mm diameter polyethylene tube which is used to balance the inner space pressure against the outer. For the detection of SFT resistance change a bridge method was employed. The transducer was calibrated by a comparison method, giving both positive and negative air pressure. We also developed a new telemetric system consisting of transducer bridge circuits with IC amplifier and transmitter circuits. The miniature transmitter measures only 5×1×3 cm in size, but it was still too large to be implanted. The transmitter and its receiving system utilize the pulse transmitting circuits (3 and 80 MHz), and its radius of action is approximately 10 meters. The SFT was used for continuous recording of epidural pressure (EDP) in 41 neurosu rgical patients for a period ranging from one to nine days (3.5 days on an average). No untoward complications such as infection or hemorrhage at the site of implantation were experienced. Induction of general anesthesia caused an acute rise in the EDP in almost all cases. Intraoperative recording of the EDP was performed in the supine, sitting and prone positions. The sitting position proved to be very useful in reducing the EDP for posterior fossa craniectomy. Irrespective of the positioning of the patients, the EDP was restored to a lower and adequate level as soon as an extreme flexion or rotation of the neck was corrected. In one case the development of a postoperative intracerebral hematoma was detected by monitoring the EDP. Telemetric monitoring of the EDP was performed in two patients during their daily life in the neurosurgical ward, such as eating, drinking, sitting, standing or going to stool. The EDP and systemic blood pressure were recorded simultaneously in one of them. When those patients went to stool, the EDP increased sharply at the beginning of straining and the diastolic pressure also increased at the same time, but the systolic pressure decreased. At the end of straining both the systolic and diastolic pressure increased. Of particular interest was the fact that the magnitude of the EDP rise during straining at stool differed significantly by the posture of the patients, such as lying on a bed-pan, seating on a toilet or squatting over a Japanese style toilet.