Child's Brain 1: 217-227 (1975)
A New Device for Long-Term Intracranial Pressure Measurement1 G l e n n A . M e y e r , W a r r e n C . L y o n and T h o m a s S. B u s t a r d Department o f Neurosurgery, Medical College of Wisconsin, Milwaukee, Wise., and Nuclear Battery Corporation, Columbia, M d.
K ey Words. Intracranial pressure - chronic measurement • Passive resonance radiotransmitter • Pressure sensitive transducer • Implanted transducer • Monkey Abstract. This paper describes the authors’ approach to the problem of long term intracranial pressure measurement. Several prior devices have not proven use ful for clinical studies of more than a few days duration. Study o f 22 o f the au thors' devices in primates has established the validity of both engineering and medi cal design assumptions. Useful pressure data has been collected for up to 2 months. Redesign to decrease tambour permeability should allow a useful life of months or years.
Introduction
1 This work was supported by A E C Research Contract N o . AT (ll-l)-2243- (In tracranial Pressure Sensor Program).
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Previous Approaches to Chronic Intracranial Pressure Measurement [ 10] Passive resonance radio transmitter. The crucial component of this de vice is a variable capacitance in which the two sides of a capacitor are mounted on opposing sides of an air-filled tambour. The distance between the sides of the capacitor is thus inversely proportional to the intracranial pressure, and the implanted variable capacitor alters the resonant fre quency of this passive, variably tuned circuit [7], The resonant frequency is read by imposing a radio wave upon it through the intact scalp by means of a grid-dip circuit. Unfortunately, this most ingenious device is fragile and requires compensation for changes in body temperature and barometric pressure [1]. One of these difficulties was overcome by adding
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The Authors' Approach The pressure sensing device consists of three fluid-bearing chambers interconnected by capillary tubing [5], The first chamber or tambour is the sensing element. It is a ductile plastic sack filled with Ringer’s solu tion and connected to the storage chamber. The tambour is intended for implantation within the cranial cavity. The storage chamber is a fluidfilled double cavity mechanism with Ringer’s solution on the tambour side of an elastic diaphragm and radioactive fluid on the other side. The radioactive side is connected to a third chamber (sensor or read-out chamber), where it is contained by a slack diaphragm. The storage and sensor chambers are installed subcutaneously, but outside the skull. In operation, as pressure increases within the cranial cavity, a force is exerted on the tambour. This force is transmitted through the saline solu tion to the diaphragm in the storage chamber. The diaphragm deflects and displaces the radioactive fluid from the storage to the sensor chamber where the other diaphragm also deflects. The higher the pressure at the tambour location, the greater the quantity of radioactive fluid in the sen sor chamber. If a radiation detector is placed over the sensor chamber, a measure of the pressure within the cranial cavity is obtained (fig. 1). Our major goals in this phase of the study were to evaluate the following: Sensitivity and accuracy. We wished to evaluate the sensitivity and ac curacy of the implanted device. This data was compared with that ob tained in a long-term bench testing establishing what effect, if any, con tact with tissue fluid had on the transducer. Durability and read-out system. We also wished, firstly, to know what effect implantation trauma and animal movement would have during a several month study, and secondly, determine the observer reliability and reproducibility with the present read-out device.
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an actively transmitting temperature telemeter, thereby allowing accurate continuous temperature compensation [9], This device has not proved practical for widespread use. Other than the Hittman-Meyer transducer, it is the only one suitable for long-term recording. Direct implantation of a pressure-sensitive transducer. These devices have had a wide variety of shapes, sizes, and configurations, all of which require penetration of the scalp by a connecting wire [3, 4, 8], A major problem has been calibration shift after implantation. A fully implantable form of this device with telemetering of data through intact scalp has not yet been developed in practical form.
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D iap hragm
solution
S e n sin g cham ber
Storage reservoir
Tam bour
Fig. I. I C P S design schematic.
Tissue compatibility. Although the histocompatibility of silastic com ponents is well known, we wished to evaluate our fabrication methods to ensure that none of the adhesives etc. used were retained within the silas tic and thereby causing undue amounts of fibrosis and tissue reaction. We also wished to check sterilization technique.
In vitro testiiif;. The sensor as described will detect pressure changes between 0 and 400 mm H ,0 . which is the critical range for cranial pressure. Initial testing was performed on the laboratory bench using a water column to achieve a known pres sure on the tambour. The response of the unit was measured with an Eberline Mod el N o. MS-2 countrate meter-channel analyzer and a Model No. PG-2 scintillation detector. The testing was performed to determine the time constant of the device, hysteresis effects, if any, sensitivity to detector, counting chamber separation dis tance, effect o f sterilization, and change in characteristics with time. The characteristics of one of the more sensitive devices are shown in figure 2. The descending pressure point at 25 cm H ,0 was obtained within a few seconds of the time of reaching that pressure, and normally would approach the ascending pressure characteristic value within approximately 30 sec. The line represents a sec ond order least square fit to the data with the exclusion of the one descending pres sure point at 25 cm H ,0 . The standard deviation is 290 cpm. For practical purpos es, the response is linear with pressure, indicating good response of the diaphragms within the elastic range. This device provided somewhat better response than most because o f a very good fit of the diaphragm to the matching body o f the device at zero pressure, and the ratio of the count rate at other pressures to the zero value consequently is high. The normal response is of the order of a factor of four to five over the pressure range of interest, indicating potential for improvement in the fill ing of the devices and the diaphragm fit. A ll devices exhibit linear behavior when filled properly and operated over the design pressure range. When subjected to higher pressures, they do not fail, but change into a decreasing slope response mode
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Methods
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Pressure, cm H 2O
due to contacting o f the storage reservoir diaphragm with the top o f the reservoir and due to depletion o f the tambour fluid. Chronic in vivo resting. We elected to use stump-tail macaque monkeys to en sure accurate evaluation of histocompatibility. We also felt that the skull configura tion and generally upright posture of chair-restrained monkeys would more closely duplicate» implantation in humans. Implantation. The sensing tambour was implanted into the subdural space by making a small cranial and dural opening over the cerebral convexity. The dura ma ter was closed with several sutures. A ll operations were done under sterile condi tions with standard surgical technique including removal o f scalp hair, sterile drap ing, and instrument sterilization. A transverse mid-cerebral scalp incision was used to facilitate exposure.and wound healing. Large skin flaps were developed in both anterior and posterior directions. A ll but two o f the monkeys had been previously chair-trained.
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Fig. 2. Sensor pre-implant characteristics. • = Ascending pressure; o = descending pressure.
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Read-out o j the transducer. The Eberline PG-1 gamma probe and pulse integra tor was used. The rather heavy and bulky photomultiplier tube must be held direct ly over the source (fig. 3). Several sizes o f apertures were evaluated and an aperture just slightly larger than the size of the source was selected to facilitate establishing maximal readings. The Eberline instrument was calibrated each day prior to use with a standard source. In taking readings, two investigators worked as a team. One held the aperture of the photomultiplier tube directly over the implanted source and brought it as close as possible to the skin without actually touching or applying pressure on the skin. The photomultiplier tube was moved slightly in all directions while the other inves tigator observed the rate meter to determine the maximal reading. The roles were then reversed, the investigator, who formerly read the meter, held the probe and a
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Fig. 3. Read-out o f the Hittnian-Meyer transducer.The animal is in a restraining chair during healing o f its scalp wound. A slight bulging o f the scalp over the transducer can be seen beneath the center o f the gamma probe.
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Fig. 4. Low power photomicrograph of a tissue capsule. The pseudomembrane which formed around the sensing tambour is shown. The cellular structure at the bottom is the cerebral cortex which appears perfectly normal. The undulating line just above the brain (in contact at several points) is the normal arachnoidal membrane. The straight line in the middle is the inner wall of the pseudomembrane and the heavy line at the top is the outer wall which was immediately beneath the dura mater.
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second investigator, who formerly held the probe, read the meter and a second inde pendent reading was made. Beginning on the day o f implantation, or the following day, readings were made of all devices. Readings were made with the chair in both the vertical and horizon tal positions. For the most part, daily readings including weekends were made for a period of 2 or 3 weeks or until the device had stabilized. Following this, twice weekly readings were made. In vivo calibration has been carried out with all transducers. This was done by placing a short bevel No. 20 spinal needle into the subarachnoid space either in the lumbar spinal canal or the cisterna magna. The needle was connected to a Statham pressure transducer and Sanborn polygraph for a constant recording o f pulsatile ce rebral spinal fluid pressure. A side arm was connected to a constant pressure infu sion device. This is a bag of intravenous fluids wrapped in a blood pressure cuff so that an accurate infusion pressure can be applied and maintained throughout pres surization runs. The subarachnoid pressure was raised by infusing lactated Ringer’s
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solution to maintain a constant pressure. As soon as stable readings of the radioiso topic pressure transducer were observed, the pressure was raised to another higher level. A descending series of pressures was also done to check for hysteresis. Later studies. One monkey was sacrificed and brain fixation performed in situ following which extensive tissue samples were taken for histologic examination. Samples were taken of both the scalp surrounding the extracranial portions o f the device and the brain and meninges surrounding the pressure sensing tambour (fig. 4). In five other animals, tissue biopsies were taken at the time of transducer replacement or removal. After several weeks o f daily readings, the animals were placed in cages and twice weekly readings were made under mild Sernalyn sedation. Readings were made in the vertical upright, horizontal, and fully inverted positions. The length of the cerebral spinal fluid axis in the study animals was 35-40 cm.
Results
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22 transducers were implanted. O f these devices, 14 were sufficiently sensitive to show a definite and reproducible change in read-out when moving the animal from the vertical to the horizontal position. Therefore a change in pressure of not more than 40 cm HaO was accurately detect ed. It was necessary to remove 3 of the 14 functional devices due to local ly infected wounds. One had separation of the wound edges and exposure of the transducer. Six of the remaining 11 devices became nonfunctional during the first month of study and the other five lost useful function dur ing the second month. The maximum longevity for usefulness in measur ing pressure changes induced by postural manipulation of the monkeys was 2 months. Some of the failed devices responded to larger magnitude changes of subarachnoid pressure induced by infusion of fluid into the subarachnoid space. The authors feel that this will be easily improved (see Discussion). More than 2,000 separate observations of transducer read-out and corresponding data were made. The data were processed and graphs were made of transducer behavior. Failure of the device was gradual in almost all instances becoming complete at 2-60 days after implantation. These data are consistent with a gradual loss of fluid from the sensing tambour (fig. 5). Intensive study of transducer characteristics was made in 22 instances. Several transducers were studied at two intervals after implantation. The subarachnoid pressures of the experimental animals were elevated in steps by infusion of physiological solution from a constant pressure reser-
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Time, days
Time, min
(
voir while measuring the subarachnoid pressure accurately with a Sta tham transducer (see Methods). These studies resulted in the following observations: (1) The functional, less fluid-depleted devices accurately sensed intra cranial pressure (fig. 6). Detailed analysis including curve fitting has had important implications in our final redesign phase [2J. (2) The response time of the Hittman-Meyer transducer is characteristically 7 sec. In sever al instances the response was more delayed, but never longer than 60 sec. (3) Mild hysteresis occurred in four instances with higher readings ob tained on a descending series of pressure steps than on the ascending se ries at the same pressure levels. Pressure steps were usually held for 60 sec. Hysteresis was less with longer duration steps. (4) Holding a high subarachnoid pressure (50 mm Hg) for a prolonged period (8 min) did not result in any decrement of transducer readings. This is evidence against the egress of fluid out of the cranium along the device and within the pseudomembrane which encapsulates it. (5) In five studies the pres sure sensing characteristics of the chronically implanted device were care fully studied before and after removing the scalp and pseudomembrane from the transducer. Encapsulation by animal tissues caused no change in
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Fig. 5. Loss o f sensitivity to postural change o f transducer 021. The read-out o f the rate meter in thousands o f counts per minute is shown at the left. Note the loss o f sensitivity to postural manipulation on the thirteenth d a y .--------- = horizontal;--------- = vertical. Fig. 6. In vivo calibration of transducer 022. The read-out o f the Hittman-Meyer in thousands o f counts per minute is shown on the left ordinate. This is compared to a Statham transducer connected to a needle in thecisterna magna as shown in mm Hgon the right ordinate. Time in minutes is shown on the abscissa.
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transducer characteristics. (6) In many instances a transducer insensitive to postural pressure changes was responsive to infusion-induced pressure changes. In other instances, transducers insensitive to both showed a marked change in read-out induced by finger pressure on the sensing tam bour. Histocompatibility of the transducer in stump-tail macaques has been excellent. Multiple histologic examinations of the tissue surrounding the device show no significant inflammation. The sensing tambour becomes completely enclosed in a very delicate membrane (fig. 4). Complications Three devices became infected and had to be removed, two of these infections were caused by failure to chair-train the animal prior to im plantation, resulting in extreme agitation and attempted auloremoval. In one instance, there was leakage of radioisotope-bearing fluid into the sen sing tambour. In another animal there was a loss of radioisotope into the surrounding scalp and skull. A count rate of 4,000 cpm was observed over this area of the scalp following removal of a transducer which had shown no grossly observable defect (background was 200 cpm). Collec tion of fecal material and urine was made for 2 weeks. Accurate counting of these samples by Eberline Inc. showed no significant increase in count rate above background. The animal has been sacrificed and perfused with preservative. Study of the scalp and skull tissues for radioactivity is under way. Discussion
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In general the results were satisfactory in that the majority of the sen sing devices worked well for at least a brief period of time. Much variabil ity of transducer characteristics was encountered. Part of this is due to the three different study models. There were several complications immedi ately following implantation which would be avoidable with our present experience. We are concerned about the problem of fluid depletion and two alter native ways of correcting this have been considered. First, more osmotic pressure within the sensing tambour may eliminate the loss of fluid from the tambour into the tissues. Alternatively, the silastic tambour might be made less permeable either by sputtering metallic ions onto its surface or by using a less permeable material, perhaps with a bi-layer design.
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We have considered the possibility of egress of fluid from within the head following the connecting tubing to the space surrounding the slack diaphragm. If this occurred, it would effectively limit the pressure trans ducing characteristics of the device by placing back pressure on the most distal or slack diaphragm of the device. This possibility was evaluated by doing serial pressure studies after infusion of cerebral spinal fluid. This does not occur within the range of intracranial pressure studied (0-80 mm Hg). This study has established that the connective tissue capsule which in timately surrounds the open slack diaphragm does not interfere with pres sure sensing characteristics. In five studies no difference was seen before and after removing the scalp. Nevertheless, it may be desirable to incor porate some porous material onto the surface of the tubing connecting the sensing tambour. This would eliminate the possibility of trauma to the brain by traction on the sensing tambour. As soon as design characteristics have been finalized, the next step is to incorporate the device into a cerebrospinal fluid shunting device [6], At this step, sensing fluid pressure within the ventricle may be desirable. This can be accomplished by placing a concentric cylindrical tambour in the outer wall of presently used ventricular catheters or by having a dou ble lumen tubing of either the open type or Millipore type. Recent studies have indicated that shunting of hydrocephalic children and maintenance of low pressure within the cranium may be undesirable. First, this may limit or prevent the development of new cerebrospinal fluid absorption pathways. Second, the child's skull may become abnor mally thick and rigid, in some cases limiting further brain growth and contributing to the problem of extreme 'shunt dependency'. An accurate long-term sensor of intracranial pressure would be indispensable to re cently proposed solutions to this problem which allow a child's intracrani al pressure to rise slightly above normal while having the potential for lowering the pressure by a controlled amount.
1 A t kin so n , J . R .; S h u r t l e ff , D. B., and F o l t z , E . L.: Radio telemetry for the measurement of intracranial pressure. J . Neurosurg. 27: 428-432 (1967). 2 Bu stard , T . S.; L y o n , W . C ., and M e yer , G . A .: A nuclear intracranial pressure sensor. Proc. IE E E Nuclear Science Symp., San Francisco 1973.
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References
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G i. enn A . M e y e r . M D . Department of Neurosurgery, The Medical College o f Wisconsin, 8700 West Wisconsin Avenue, M ilwaukee, W153226 (USA)
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3 C o e , J . E .; N e lso n , W . J.; R u d en b er g , F. H ., et al.: Technique for continuous intracranial pressure recording. J . Neurosurg. 27: 370-375 (1967). 4 H u lm e , A . and C ooper , R.: A technique for the investigation o f intracranial pressure in man. J . Neurol. Neurosurg. Psychiat. 29: 154-156 (1966). 5 L y o n , W . C . and M e y er , G . A .: Development o f an indwelling intracranial pres sure sensor-phase II. Nuclear Battery Corporation COO-2243-16, Colum bia, Md. 1973. 6 M e y er , G . A .; L y o n , W . C ., and B u stard , T . S.: Chronic in vivo testing o f a fully implantable intracranial pressure sensor. Proc. IE E E Nuclear Science Symp., San Francisco 1973. 7 O l se n , E . R .; C o ll in s , C . C .; L o u g h b o r o u g h , W. F „ el «/.: Intracranial pres sure measurement with a miniature passive implanted pressure transensor. Amer. J . Surg. 113: 727-729 (1967). 8 R ich a r dson , A .; H id e , T . A ., and E ve r sd en , I. D.: Long term continuous intra cranial pressure monitoring by means o f a modified subdural pressure transdu cer. Lancet ii: 687-690 (1970). 9 S h u lm a n , K . and M arm arou , A .: Analysis o f intracranial pressure in hydroce phalus. Develop. Med. Child. Neurol. 16: 11-16 (1967). 10 T in d a ll , G . T .; M e y e r , G . A ., and Iw ata , K .: Current methods for monitoring patients with head injury. Clin. Neurosurg., Baltimore 19: 98-120 (1972).
A New Device for Long-Term Intracranial Pressure Measurement1 G l e n n A . M e y e r , W a r r e n C . L y o n and T h o m a s S. B u s t a r d Department o f Neurosurgery, Medical College of Wisconsin, Milwaukee, Wise., and Nuclear Battery Corporation, Columbia, M d.
K ey Words. Intracranial pressure - chronic measurement • Passive resonance radiotransmitter • Pressure sensitive transducer • Implanted transducer • Monkey Abstract. This paper describes the authors’ approach to the problem of long term intracranial pressure measurement. Several prior devices have not proven use ful for clinical studies of more than a few days duration. Study o f 22 o f the au thors' devices in primates has established the validity of both engineering and medi cal design assumptions. Useful pressure data has been collected for up to 2 months. Redesign to decrease tambour permeability should allow a useful life of months or years.
Introduction
1 This work was supported by A E C Research Contract N o . AT (ll-l)-2243- (In tracranial Pressure Sensor Program).
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Previous Approaches to Chronic Intracranial Pressure Measurement [ 10] Passive resonance radio transmitter. The crucial component of this de vice is a variable capacitance in which the two sides of a capacitor are mounted on opposing sides of an air-filled tambour. The distance between the sides of the capacitor is thus inversely proportional to the intracranial pressure, and the implanted variable capacitor alters the resonant fre quency of this passive, variably tuned circuit [7], The resonant frequency is read by imposing a radio wave upon it through the intact scalp by means of a grid-dip circuit. Unfortunately, this most ingenious device is fragile and requires compensation for changes in body temperature and barometric pressure [1]. One of these difficulties was overcome by adding
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The Authors' Approach The pressure sensing device consists of three fluid-bearing chambers interconnected by capillary tubing [5], The first chamber or tambour is the sensing element. It is a ductile plastic sack filled with Ringer’s solu tion and connected to the storage chamber. The tambour is intended for implantation within the cranial cavity. The storage chamber is a fluidfilled double cavity mechanism with Ringer’s solution on the tambour side of an elastic diaphragm and radioactive fluid on the other side. The radioactive side is connected to a third chamber (sensor or read-out chamber), where it is contained by a slack diaphragm. The storage and sensor chambers are installed subcutaneously, but outside the skull. In operation, as pressure increases within the cranial cavity, a force is exerted on the tambour. This force is transmitted through the saline solu tion to the diaphragm in the storage chamber. The diaphragm deflects and displaces the radioactive fluid from the storage to the sensor chamber where the other diaphragm also deflects. The higher the pressure at the tambour location, the greater the quantity of radioactive fluid in the sen sor chamber. If a radiation detector is placed over the sensor chamber, a measure of the pressure within the cranial cavity is obtained (fig. 1). Our major goals in this phase of the study were to evaluate the following: Sensitivity and accuracy. We wished to evaluate the sensitivity and ac curacy of the implanted device. This data was compared with that ob tained in a long-term bench testing establishing what effect, if any, con tact with tissue fluid had on the transducer. Durability and read-out system. We also wished, firstly, to know what effect implantation trauma and animal movement would have during a several month study, and secondly, determine the observer reliability and reproducibility with the present read-out device.
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an actively transmitting temperature telemeter, thereby allowing accurate continuous temperature compensation [9], This device has not proved practical for widespread use. Other than the Hittman-Meyer transducer, it is the only one suitable for long-term recording. Direct implantation of a pressure-sensitive transducer. These devices have had a wide variety of shapes, sizes, and configurations, all of which require penetration of the scalp by a connecting wire [3, 4, 8], A major problem has been calibration shift after implantation. A fully implantable form of this device with telemetering of data through intact scalp has not yet been developed in practical form.
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D iap hragm
solution
S e n sin g cham ber
Storage reservoir
Tam bour
Fig. I. I C P S design schematic.
Tissue compatibility. Although the histocompatibility of silastic com ponents is well known, we wished to evaluate our fabrication methods to ensure that none of the adhesives etc. used were retained within the silas tic and thereby causing undue amounts of fibrosis and tissue reaction. We also wished to check sterilization technique.
In vitro testiiif;. The sensor as described will detect pressure changes between 0 and 400 mm H ,0 . which is the critical range for cranial pressure. Initial testing was performed on the laboratory bench using a water column to achieve a known pres sure on the tambour. The response of the unit was measured with an Eberline Mod el N o. MS-2 countrate meter-channel analyzer and a Model No. PG-2 scintillation detector. The testing was performed to determine the time constant of the device, hysteresis effects, if any, sensitivity to detector, counting chamber separation dis tance, effect o f sterilization, and change in characteristics with time. The characteristics of one of the more sensitive devices are shown in figure 2. The descending pressure point at 25 cm H ,0 was obtained within a few seconds of the time of reaching that pressure, and normally would approach the ascending pressure characteristic value within approximately 30 sec. The line represents a sec ond order least square fit to the data with the exclusion of the one descending pres sure point at 25 cm H ,0 . The standard deviation is 290 cpm. For practical purpos es, the response is linear with pressure, indicating good response of the diaphragms within the elastic range. This device provided somewhat better response than most because o f a very good fit of the diaphragm to the matching body o f the device at zero pressure, and the ratio of the count rate at other pressures to the zero value consequently is high. The normal response is of the order of a factor of four to five over the pressure range of interest, indicating potential for improvement in the fill ing of the devices and the diaphragm fit. A ll devices exhibit linear behavior when filled properly and operated over the design pressure range. When subjected to higher pressures, they do not fail, but change into a decreasing slope response mode
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Methods
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Pressure, cm H 2O
due to contacting o f the storage reservoir diaphragm with the top o f the reservoir and due to depletion o f the tambour fluid. Chronic in vivo resting. We elected to use stump-tail macaque monkeys to en sure accurate evaluation of histocompatibility. We also felt that the skull configura tion and generally upright posture of chair-restrained monkeys would more closely duplicate» implantation in humans. Implantation. The sensing tambour was implanted into the subdural space by making a small cranial and dural opening over the cerebral convexity. The dura ma ter was closed with several sutures. A ll operations were done under sterile condi tions with standard surgical technique including removal o f scalp hair, sterile drap ing, and instrument sterilization. A transverse mid-cerebral scalp incision was used to facilitate exposure.and wound healing. Large skin flaps were developed in both anterior and posterior directions. A ll but two o f the monkeys had been previously chair-trained.
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Fig. 2. Sensor pre-implant characteristics. • = Ascending pressure; o = descending pressure.
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Read-out o j the transducer. The Eberline PG-1 gamma probe and pulse integra tor was used. The rather heavy and bulky photomultiplier tube must be held direct ly over the source (fig. 3). Several sizes o f apertures were evaluated and an aperture just slightly larger than the size of the source was selected to facilitate establishing maximal readings. The Eberline instrument was calibrated each day prior to use with a standard source. In taking readings, two investigators worked as a team. One held the aperture of the photomultiplier tube directly over the implanted source and brought it as close as possible to the skin without actually touching or applying pressure on the skin. The photomultiplier tube was moved slightly in all directions while the other inves tigator observed the rate meter to determine the maximal reading. The roles were then reversed, the investigator, who formerly read the meter, held the probe and a
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Fig. 3. Read-out o f the Hittnian-Meyer transducer.The animal is in a restraining chair during healing o f its scalp wound. A slight bulging o f the scalp over the transducer can be seen beneath the center o f the gamma probe.
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Fig. 4. Low power photomicrograph of a tissue capsule. The pseudomembrane which formed around the sensing tambour is shown. The cellular structure at the bottom is the cerebral cortex which appears perfectly normal. The undulating line just above the brain (in contact at several points) is the normal arachnoidal membrane. The straight line in the middle is the inner wall of the pseudomembrane and the heavy line at the top is the outer wall which was immediately beneath the dura mater.
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second investigator, who formerly held the probe, read the meter and a second inde pendent reading was made. Beginning on the day o f implantation, or the following day, readings were made of all devices. Readings were made with the chair in both the vertical and horizon tal positions. For the most part, daily readings including weekends were made for a period of 2 or 3 weeks or until the device had stabilized. Following this, twice weekly readings were made. In vivo calibration has been carried out with all transducers. This was done by placing a short bevel No. 20 spinal needle into the subarachnoid space either in the lumbar spinal canal or the cisterna magna. The needle was connected to a Statham pressure transducer and Sanborn polygraph for a constant recording o f pulsatile ce rebral spinal fluid pressure. A side arm was connected to a constant pressure infu sion device. This is a bag of intravenous fluids wrapped in a blood pressure cuff so that an accurate infusion pressure can be applied and maintained throughout pres surization runs. The subarachnoid pressure was raised by infusing lactated Ringer’s
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solution to maintain a constant pressure. As soon as stable readings of the radioiso topic pressure transducer were observed, the pressure was raised to another higher level. A descending series of pressures was also done to check for hysteresis. Later studies. One monkey was sacrificed and brain fixation performed in situ following which extensive tissue samples were taken for histologic examination. Samples were taken of both the scalp surrounding the extracranial portions o f the device and the brain and meninges surrounding the pressure sensing tambour (fig. 4). In five other animals, tissue biopsies were taken at the time of transducer replacement or removal. After several weeks o f daily readings, the animals were placed in cages and twice weekly readings were made under mild Sernalyn sedation. Readings were made in the vertical upright, horizontal, and fully inverted positions. The length of the cerebral spinal fluid axis in the study animals was 35-40 cm.
Results
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22 transducers were implanted. O f these devices, 14 were sufficiently sensitive to show a definite and reproducible change in read-out when moving the animal from the vertical to the horizontal position. Therefore a change in pressure of not more than 40 cm HaO was accurately detect ed. It was necessary to remove 3 of the 14 functional devices due to local ly infected wounds. One had separation of the wound edges and exposure of the transducer. Six of the remaining 11 devices became nonfunctional during the first month of study and the other five lost useful function dur ing the second month. The maximum longevity for usefulness in measur ing pressure changes induced by postural manipulation of the monkeys was 2 months. Some of the failed devices responded to larger magnitude changes of subarachnoid pressure induced by infusion of fluid into the subarachnoid space. The authors feel that this will be easily improved (see Discussion). More than 2,000 separate observations of transducer read-out and corresponding data were made. The data were processed and graphs were made of transducer behavior. Failure of the device was gradual in almost all instances becoming complete at 2-60 days after implantation. These data are consistent with a gradual loss of fluid from the sensing tambour (fig. 5). Intensive study of transducer characteristics was made in 22 instances. Several transducers were studied at two intervals after implantation. The subarachnoid pressures of the experimental animals were elevated in steps by infusion of physiological solution from a constant pressure reser-
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224
Time, days
Time, min
(
voir while measuring the subarachnoid pressure accurately with a Sta tham transducer (see Methods). These studies resulted in the following observations: (1) The functional, less fluid-depleted devices accurately sensed intra cranial pressure (fig. 6). Detailed analysis including curve fitting has had important implications in our final redesign phase [2J. (2) The response time of the Hittman-Meyer transducer is characteristically 7 sec. In sever al instances the response was more delayed, but never longer than 60 sec. (3) Mild hysteresis occurred in four instances with higher readings ob tained on a descending series of pressure steps than on the ascending se ries at the same pressure levels. Pressure steps were usually held for 60 sec. Hysteresis was less with longer duration steps. (4) Holding a high subarachnoid pressure (50 mm Hg) for a prolonged period (8 min) did not result in any decrement of transducer readings. This is evidence against the egress of fluid out of the cranium along the device and within the pseudomembrane which encapsulates it. (5) In five studies the pres sure sensing characteristics of the chronically implanted device were care fully studied before and after removing the scalp and pseudomembrane from the transducer. Encapsulation by animal tissues caused no change in
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Fig. 5. Loss o f sensitivity to postural change o f transducer 021. The read-out o f the rate meter in thousands o f counts per minute is shown at the left. Note the loss o f sensitivity to postural manipulation on the thirteenth d a y .--------- = horizontal;--------- = vertical. Fig. 6. In vivo calibration of transducer 022. The read-out o f the Hittman-Meyer in thousands o f counts per minute is shown on the left ordinate. This is compared to a Statham transducer connected to a needle in thecisterna magna as shown in mm Hgon the right ordinate. Time in minutes is shown on the abscissa.
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transducer characteristics. (6) In many instances a transducer insensitive to postural pressure changes was responsive to infusion-induced pressure changes. In other instances, transducers insensitive to both showed a marked change in read-out induced by finger pressure on the sensing tam bour. Histocompatibility of the transducer in stump-tail macaques has been excellent. Multiple histologic examinations of the tissue surrounding the device show no significant inflammation. The sensing tambour becomes completely enclosed in a very delicate membrane (fig. 4). Complications Three devices became infected and had to be removed, two of these infections were caused by failure to chair-train the animal prior to im plantation, resulting in extreme agitation and attempted auloremoval. In one instance, there was leakage of radioisotope-bearing fluid into the sen sing tambour. In another animal there was a loss of radioisotope into the surrounding scalp and skull. A count rate of 4,000 cpm was observed over this area of the scalp following removal of a transducer which had shown no grossly observable defect (background was 200 cpm). Collec tion of fecal material and urine was made for 2 weeks. Accurate counting of these samples by Eberline Inc. showed no significant increase in count rate above background. The animal has been sacrificed and perfused with preservative. Study of the scalp and skull tissues for radioactivity is under way. Discussion
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In general the results were satisfactory in that the majority of the sen sing devices worked well for at least a brief period of time. Much variabil ity of transducer characteristics was encountered. Part of this is due to the three different study models. There were several complications immedi ately following implantation which would be avoidable with our present experience. We are concerned about the problem of fluid depletion and two alter native ways of correcting this have been considered. First, more osmotic pressure within the sensing tambour may eliminate the loss of fluid from the tambour into the tissues. Alternatively, the silastic tambour might be made less permeable either by sputtering metallic ions onto its surface or by using a less permeable material, perhaps with a bi-layer design.
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We have considered the possibility of egress of fluid from within the head following the connecting tubing to the space surrounding the slack diaphragm. If this occurred, it would effectively limit the pressure trans ducing characteristics of the device by placing back pressure on the most distal or slack diaphragm of the device. This possibility was evaluated by doing serial pressure studies after infusion of cerebral spinal fluid. This does not occur within the range of intracranial pressure studied (0-80 mm Hg). This study has established that the connective tissue capsule which in timately surrounds the open slack diaphragm does not interfere with pres sure sensing characteristics. In five studies no difference was seen before and after removing the scalp. Nevertheless, it may be desirable to incor porate some porous material onto the surface of the tubing connecting the sensing tambour. This would eliminate the possibility of trauma to the brain by traction on the sensing tambour. As soon as design characteristics have been finalized, the next step is to incorporate the device into a cerebrospinal fluid shunting device [6], At this step, sensing fluid pressure within the ventricle may be desirable. This can be accomplished by placing a concentric cylindrical tambour in the outer wall of presently used ventricular catheters or by having a dou ble lumen tubing of either the open type or Millipore type. Recent studies have indicated that shunting of hydrocephalic children and maintenance of low pressure within the cranium may be undesirable. First, this may limit or prevent the development of new cerebrospinal fluid absorption pathways. Second, the child's skull may become abnor mally thick and rigid, in some cases limiting further brain growth and contributing to the problem of extreme 'shunt dependency'. An accurate long-term sensor of intracranial pressure would be indispensable to re cently proposed solutions to this problem which allow a child's intracrani al pressure to rise slightly above normal while having the potential for lowering the pressure by a controlled amount.
1 A t kin so n , J . R .; S h u r t l e ff , D. B., and F o l t z , E . L.: Radio telemetry for the measurement of intracranial pressure. J . Neurosurg. 27: 428-432 (1967). 2 Bu stard , T . S.; L y o n , W . C ., and M e yer , G . A .: A nuclear intracranial pressure sensor. Proc. IE E E Nuclear Science Symp., San Francisco 1973.
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References
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G i. enn A . M e y e r . M D . Department of Neurosurgery, The Medical College o f Wisconsin, 8700 West Wisconsin Avenue, M ilwaukee, W153226 (USA)
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3 C o e , J . E .; N e lso n , W . J.; R u d en b er g , F. H ., et al.: Technique for continuous intracranial pressure recording. J . Neurosurg. 27: 370-375 (1967). 4 H u lm e , A . and C ooper , R.: A technique for the investigation o f intracranial pressure in man. J . Neurol. Neurosurg. Psychiat. 29: 154-156 (1966). 5 L y o n , W . C . and M e y er , G . A .: Development o f an indwelling intracranial pres sure sensor-phase II. Nuclear Battery Corporation COO-2243-16, Colum bia, Md. 1973. 6 M e y er , G . A .; L y o n , W . C ., and B u stard , T . S.: Chronic in vivo testing o f a fully implantable intracranial pressure sensor. Proc. IE E E Nuclear Science Symp., San Francisco 1973. 7 O l se n , E . R .; C o ll in s , C . C .; L o u g h b o r o u g h , W. F „ el «/.: Intracranial pres sure measurement with a miniature passive implanted pressure transensor. Amer. J . Surg. 113: 727-729 (1967). 8 R ich a r dson , A .; H id e , T . A ., and E ve r sd en , I. D.: Long term continuous intra cranial pressure monitoring by means o f a modified subdural pressure transdu cer. Lancet ii: 687-690 (1970). 9 S h u lm a n , K . and M arm arou , A .: Analysis o f intracranial pressure in hydroce phalus. Develop. Med. Child. Neurol. 16: 11-16 (1967). 10 T in d a ll , G . T .; M e y e r , G . A ., and Iw ata , K .: Current methods for monitoring patients with head injury. Clin. Neurosurg., Baltimore 19: 98-120 (1972).
