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Measurement of intracranial pressure in the unanesthetized rabbit
Brain Research Bulletin,
Vol. 3, pp. 635-638. Printed in the U.S.A.
Measurement of Intracranial Pressure in the Unanesthetized Rabbit T. J. MALKINSON, Division
of Medical
Physiology, Calgary,
W. L. VEALE
Faculty of Medicine, The University Alberta, Canada T2N lN4
(Received MALKINSON, BRAIN
T. J., W. L. VEALE
AND K. E. COOPER
AND K. E. COOPER.
of Calgary
26 May 1978) Measurement
of intrucrunial
pressure
in the unanesthetized
RES. BULL. 3(6) 635-638, 1978-A method is described for the measurement of intracranial pressure in the unanesthetized, minimally restrained rabbit utilizing a modified subarachnoid screw system. The pressures are transmitted from within the cranium via a flexible saline tilled catheter to a fixed external pressure transducer. An index of the relative vertical position of the animal’s skull as compared to the fixed transducer is given by means of a second open-ended pressure measuring catheter, the open end of which is fixed to the subarachnoid screw assembly on the animal’s skull. The system was found to be a reliable method of measurement of intracranial pressures in the minimally restrained rabbit, and could easily be adapted to other animal species. The method is currently being used to assess the effects of fever on intracranial pressures.
rabbit.
Intracranial pressure
Subarachnoid
screw
Cerebrospinal
ACCURATE and continuous measurement of intracranial pressure (ICP) is of importance in neurosurgical practice
both for diagnosis and for interpreting the effectiveness of treatment. Methods which have been developed for the measurement of ICP include: ventriculostomy, subarachnoid cannulation, and implanted subdural or epidural transducers. For several reasons, we chose to measure intracranial pressure by means of the subarachnoid screw system [4]. There is no penetration of the brain. It has been shown [l] that the surgical implantation of a cannula into the lateral cerebral ventricle of the rabbit was in itself cause for an increase in the ventricular fluid pressure due to local trauma and brain edema. It is often difficult to maintain an unobstructed communication between the relatively small lateral cerebral ventricle and the fluid-filled cannula in an unanesthetized animal such as the rabbit over a long period of time. No expensive or delicate pressure transducers are close enough to the animal that could be damaged. When measurements are finished, the catheter can be disconnected easily and the animal housed in normal cages without any specialized animal handling. The system is easily and accurately calibrated before and after the recording. Winn et al. [6] have reported their experience in 147 patients and have monitored a total of 650 patients using the subarachnoid screw system. Of the 147 patients, successful recordings were obtained from 92% and the overall infection rate was 2.1%. They found this system to be a reliable, low-risk method of recording intracranial pressure in patients. They also simultaneously recorded cerebral ventricular pressure in 11 patients and found the pressure recordings to be similar. The 2 main limitations with the method are that the animal must be restrained because of the catheter connection to the pressure transducer, and movement of the animal causes a
Copyright
o 1978 ANKHO
International
fluid
Rabbit
change in the reference level of the transducer. We have attempted to minimize these 2 problems by using a relatively long flexible catheter, minimal restraint of the animal, and by utilizing a second open-ended pressure measuring catheter to give an index of the vertical position of the skull with respect to the fixed pressure transducer. The rabbit was chosen as the experimental animal because considerable research has been done with the rabbit on cerebrospinal fluid (CSF) secretion and absorption [3,5], as well as many other investigators. The rabbit is easily trained to sit quietly for periods of many hours. When placed in only minimal restraint, the body is maintained in 1 position. The head is almost always kept in alignment with the body and the body is maintained in a sitting posture. Animals of uniform age and weight can be easily obtained. The cranial bones are of sufficient thickness to provide a secure anchorage. Intravenous injections are easily made through the marginal ear vein, especially so if a temporary catheter is positioned within the vein. METHOD
A diagramatic representation of the modified subarachnoid screw assembly and its component parts used to measure intracranial pressure is illustrated in Fig. 1. The assembly is machined from stainless steel. The diaphragm is made of silastic. The experimental cap (1-B) is used when ICP is being measured, the protective cap (1-A) is used at all other times. Figure 1-C illustrates the assembly when measuring intracranial pressures. Six adult male New Zealand white rabbits (Oryctolagus cuniculus) were each anesthetized with pentobarbitone sodium (30 mg/kg) via a marginal ear vein. Each animal’s head was shaved and then positioned in a surgical head holder (David Kopf Instruments, Tujunga, CA). A midline incision was then made in
Inc.-0361-9230/78/060635-04$00.90/O
MALKINSON.
636
-
TO PRESSURE
VEALE AND COOPER
TRANSDUCER
IICPI
TO PRESSURE
-
TRANSDUCER
DIAPHRAGM
SUBARACHNOIO
A.
B.
FIG. 1. Illustration of the subarachnoid screw system used to monitor Intracranial Pressure in the unanesthetized rabbit. (I-A) The base and protective cap: (1-B) the base and experimental cap; (I-C) the base and experimental cap when measuring intracranial pressures.
the skin, the skin and subcutaneous tissue were retracted to either side and the periostium was scraped from the bone. A twist drill hole was made through the skull in the area where it was desired to monitor the pressure. We selected a site 6 mm posterior to the coronal suture and 4 mm lateral to the sagittal suture. After the hole was made, the depth of the dura from the skull surface was measured. The dura was then opened as was the arachnoid membrane and a small area slightly smaller than the diameter of the lumen of the base of the screw was excised. The base was then screwed into position so that its tip rested on the dura. A secure placement was necessary to ensure that there were no CSF leaks between the threads of the base and surrounding bone. By doing so, a closed compartment was formed between the subarachnoid space and the interior of the base. Two stainless steel screws were inserted into the skull nearby to serve as an anchor for the base and dental cement which was placed around the screws and base assembly. When the dental cement was dry, the lumen of the base was filled with saline. In most cases the pulsations caused by breathing could now be observed as movement of the saline. The protective cap was affixed and a small incision in the skin approximating the position of the base was made. As the midline incision was closed, the base and cap were passed through this lateral incision. Antibiotics were administered post-operatively for 4 days. The fur in the immediate vicinity of the implant was kept closely shaven in order to help keep the preparation clean. Animals were given 7 days to recover from surgery after which conditioning to the experimental setting was accomplished. Experiments were carried out in a quiet, minimally occupied, restricted access room at a temperature of
19.0 * l.O”C. It was important to have a quiet environment in which the animals were comfortable as parameters such as respiration and general awareness, vary considerably with environmental stimuli. Animals were placed gently in an open top box, the top half of the front of which was cut away so that there was no possibility of the animal’s neck or chin resting on a firm surface which might impede blood flow through the vessels in the neck. This box provided only minimal restraint and all animals accepted it readily. The pressure transducer (Statham Instruments Type P23V, Statham Instruments Inc., Oxnard, CA) was calibrated with a saline manometer before and after the experiment. The signal output was displayed on a Beckman Dynograph Recorder (Beckman Instruments Inc.). The calibration range was 0.0 to 10.0 cm saline. Using sterile technique, the protective cap was removed and the lumen of the base was filled with sterile saline. The connecting tubing to the experimental cap was filled with sterile saline and the cap was then rotated into position on the base assembly. It was important to keep the distal end of the connecting tubing open in order to allow pressure and temperature equilibria to be established as the cap was tightened and for a few minutes thereafter. After 2 or 3 minutes, the connecting polyethylene tubing (PE 100: 0.034 i.d., 0.060 o.d.) 45 cm long was connected to the pressure transducer and the pressure waveforms could then be recorded. At the start of the recording period, tests to ensure the patency of the CSF pressure measuring system were performed. These included mild abdominal compression, jugular vein compression, and observation of the resting CSF pressure wave forms. In order to give an index of the vertical position of the
INTRACRANIAL
PRESSURE
637
IN THE RABBIT
animal’s head with reference to the fixed pressure transducer, an open-ended liquid filled catheter was positioned so that the open end was fixed rigidly to the experimental cap; the other end of the catheter was connected via a 45 cm length of tubing to a calibrated pressure transducer (P23V Statham Instruments Inc.) and the signal displayed on the Beckman Dynograph. This method allowed for the remote monitoring of the pressure as the investigator did not have to be in close proximity to the animal in order to note the level of the animal with reference to the fixed pressure transducer. Intracranial pressure data were interpreted according to the following conditions. Each animal quickly adopted a preferred position in which he was comfortable and remained for throughout most of the recording. The ICP and pulsations for any 1 rabbit were used for comparative analysis only when the rabbit was in this position. The data was then point-plotted on graph paper. By using this method, comparative pressures were obtained over the period of recording when the animal was in the same position. By use of the level indicator and close observation, the position of the animal was determined. The reference level of the transducer was chosen as the skull surface at the point of attachment of the base assembly and represented 0.0 mm saline.
RESULTS
Successful recordings were obtained in all 6 rabbits with the subarachnoid screw system and no infection occurred in any of the animals. In one animal, the lumen of the base had a clot of blood, this clot was removed as a single mass and the base was then irrigated with saline and good recordings were subsequently obtained. The subarachnoid screw system proved to be reliable for monitoring ICP in unanesthetized rabbits for continuous periods of 8 hr or more. The surgical procedure was relatively simple and very few complications occurred. The system remained patent for 3 to 4 weeks. After this time, some closure of the lumen of the base occurred and the waveforms became damped. Responses of the ICP recordings to manipulations such as abdominal compression, jugular vein compression, carotid artery compression were similar to those obtained by other methods of monitoring. Because the CSF pulsations in ICP are partially respiratory in origin, this was a method of determining respiratory rates. Inspiration causes a reduction in ICP; expiration causes an increase in ICP. In most animals, cardiac pulsations could also be seen superimposed upon the respiratory pulsations.
A
-
30 30
set
1
FIG. 2. Illustration of representative intracranial pressure waveforms obtained in 1 rabbit. (2-A) An enlarged portion of the recording; (2-B) the effects of mild abdominal compression on the ICP; (2-C) the effect of bilateral jugular vein compression on the ICP. The duration of compression is indicated by the solid bar.
MALKINSON,
638
It is desirable to have both a protective as well as an experimental cap for the screw in order to provide access to the lumen of the base, to flush it out occasionally and to ensure that no air bubbles are trapped within the system which would tend to damp out or otherwise alter the pressure waveforms. The protective cap, because of its design, is much more suitable for the housing of the animal when experiments are not being carried out. The open-ended catheter provides a reliable index of when the animal’s head is at the same height above the fixed pressure transducer. Because of the behavioral characteristics of the rabbit, the level of the animal’s back did not change significantly during the course of a recording with the level of the head being the main concern. Figure 2-A illustrates a representative portion of the recording obtained in 1 rabbit. Both the respiratory pulsations and the superimposed cardiac pulsations are visible. Figure 2-B illustrates the change in ICP produced before, during, and after mild abdominal compression. Figure 2-C illustrates the changes in ICP produced by bilateral jugular vein compression. The mean intracranial pressure in 6 rabbits was 57.69 + 10.10 mm of saline. The mean pulsation pressure in 6 rabbits was 17.84 * 3.68 mm of saline. The mean pulsation rate in 6 rabbits was 118.00 * 27.95 pulses/min. There appeared to be a periodic fluctuation in the intracranial pressure in some rabbits. This sinusoidal fluctuation has been observed by other investigators [2] who point out the importance of considering these periodic fluctuations before interpreting the results of experiments.
VEALE AND COOPER
setting since little damage can be done to expensive pressure transducers which are some distance away. The surgery is simple and sterility is easily maintained. The brain is not traversed and there is no difficulty in maintaining a free-fluid communication. The recording system is easy to calibrate before and after the experimental protocol and can be very sensitive, depending on the characteristics of the pressure transducer used. The subarachnoid screw can be positioned at any convenient position on the skull and measure the pressure at that point. The implant can easily be removed under anesthesia at the end of the experimental procedure, and the animal could possibly be used for other experiments or the screw could be positioned at another place on the skull. The wound can be allowed to granulate to closure or covered with dental cement. As in any recording system of this type, the fixed location of the pressure transducer relative to the body of the unanesthetized animal which is free to move sometimes makes it difficult to obtain a continuous recording under the same hydrostatic conditions. The system remains patent for approximately 3 to 4 weeks after which time infiltration of the lumen of the screw with bone and scar tissue occurs and the recording is less distinct. Another limitation is that the animal is physically connected to the transducer by the catheter and therefore must have minimal restraint. There is some evidence from experience in humans that the subarachnoid screw preparation may not be suitable for measurement of increases in ICP over relatively long periods of time because of herniation of the brain into the screw opening with subsequent loss of subarachnoid space continuity with the column of fluid.
DISCUSSION
The recording system described can be used on a wide variety of anesthetized or unanesthetized animals. The animal is free to move within the restraints of the experimental
ACKNOWLEDGEMENT
This work Canada.
was supported
by
the Medical Research Council
of
REFERENCES Edvinsson, L., K. C. Nielsen, C. H. Owman and K. A. West. Alterations in intracranial pressure, blood-brain barrier, and brain edema after sub-chronic implantation of a cannula into the brain of conscious animals. Acta Physiol. .scrrncl. 82: 527-531, 1971. Hayden, P. W., D. B. Shurtleff and E. L. Foltz. Ventricular fluid pressure recordings in hydrocephalic patients. Arch. Nrurol. 23: 147-154, 1970. Pollay, M. and F. Curl. Secretion of cerebrospinal fluid by the ventricular ependyma of the rabbit. Am. J. Ph~siol. 213: 10311038, 1967.
Vries, .I. K., D. P. Becker and H. F. Young. A subarachnoid screw for monitoring intracranial pressure. J. Ne~ro.vur,y. 39: 41G419, 1973. 5. Welch, K. Secretion of cerebrospinal fluid by the choroid plexus of the rabbit. Am. J. Physiol. 205: 617-624, 1963. 6. Winn, H. R., R. G. Dacey and J. A. Jane. Intracranial subarachnoid pressure recording: experience with 650 patients. Strr-K. Neurnl. 8: 41-47, 1977. 4.
Vol. 3, pp. 635-638. Printed in the U.S.A.
Measurement of Intracranial Pressure in the Unanesthetized Rabbit T. J. MALKINSON, Division
of Medical
Physiology, Calgary,
W. L. VEALE
Faculty of Medicine, The University Alberta, Canada T2N lN4
(Received MALKINSON, BRAIN
T. J., W. L. VEALE
AND K. E. COOPER
AND K. E. COOPER.
of Calgary
26 May 1978) Measurement
of intrucrunial
pressure
in the unanesthetized
RES. BULL. 3(6) 635-638, 1978-A method is described for the measurement of intracranial pressure in the unanesthetized, minimally restrained rabbit utilizing a modified subarachnoid screw system. The pressures are transmitted from within the cranium via a flexible saline tilled catheter to a fixed external pressure transducer. An index of the relative vertical position of the animal’s skull as compared to the fixed transducer is given by means of a second open-ended pressure measuring catheter, the open end of which is fixed to the subarachnoid screw assembly on the animal’s skull. The system was found to be a reliable method of measurement of intracranial pressures in the minimally restrained rabbit, and could easily be adapted to other animal species. The method is currently being used to assess the effects of fever on intracranial pressures.
rabbit.
Intracranial pressure
Subarachnoid
screw
Cerebrospinal
ACCURATE and continuous measurement of intracranial pressure (ICP) is of importance in neurosurgical practice
both for diagnosis and for interpreting the effectiveness of treatment. Methods which have been developed for the measurement of ICP include: ventriculostomy, subarachnoid cannulation, and implanted subdural or epidural transducers. For several reasons, we chose to measure intracranial pressure by means of the subarachnoid screw system [4]. There is no penetration of the brain. It has been shown [l] that the surgical implantation of a cannula into the lateral cerebral ventricle of the rabbit was in itself cause for an increase in the ventricular fluid pressure due to local trauma and brain edema. It is often difficult to maintain an unobstructed communication between the relatively small lateral cerebral ventricle and the fluid-filled cannula in an unanesthetized animal such as the rabbit over a long period of time. No expensive or delicate pressure transducers are close enough to the animal that could be damaged. When measurements are finished, the catheter can be disconnected easily and the animal housed in normal cages without any specialized animal handling. The system is easily and accurately calibrated before and after the recording. Winn et al. [6] have reported their experience in 147 patients and have monitored a total of 650 patients using the subarachnoid screw system. Of the 147 patients, successful recordings were obtained from 92% and the overall infection rate was 2.1%. They found this system to be a reliable, low-risk method of recording intracranial pressure in patients. They also simultaneously recorded cerebral ventricular pressure in 11 patients and found the pressure recordings to be similar. The 2 main limitations with the method are that the animal must be restrained because of the catheter connection to the pressure transducer, and movement of the animal causes a
Copyright
o 1978 ANKHO
International
fluid
Rabbit
change in the reference level of the transducer. We have attempted to minimize these 2 problems by using a relatively long flexible catheter, minimal restraint of the animal, and by utilizing a second open-ended pressure measuring catheter to give an index of the vertical position of the skull with respect to the fixed pressure transducer. The rabbit was chosen as the experimental animal because considerable research has been done with the rabbit on cerebrospinal fluid (CSF) secretion and absorption [3,5], as well as many other investigators. The rabbit is easily trained to sit quietly for periods of many hours. When placed in only minimal restraint, the body is maintained in 1 position. The head is almost always kept in alignment with the body and the body is maintained in a sitting posture. Animals of uniform age and weight can be easily obtained. The cranial bones are of sufficient thickness to provide a secure anchorage. Intravenous injections are easily made through the marginal ear vein, especially so if a temporary catheter is positioned within the vein. METHOD
A diagramatic representation of the modified subarachnoid screw assembly and its component parts used to measure intracranial pressure is illustrated in Fig. 1. The assembly is machined from stainless steel. The diaphragm is made of silastic. The experimental cap (1-B) is used when ICP is being measured, the protective cap (1-A) is used at all other times. Figure 1-C illustrates the assembly when measuring intracranial pressures. Six adult male New Zealand white rabbits (Oryctolagus cuniculus) were each anesthetized with pentobarbitone sodium (30 mg/kg) via a marginal ear vein. Each animal’s head was shaved and then positioned in a surgical head holder (David Kopf Instruments, Tujunga, CA). A midline incision was then made in
Inc.-0361-9230/78/060635-04$00.90/O
MALKINSON.
636
-
TO PRESSURE
VEALE AND COOPER
TRANSDUCER
IICPI
TO PRESSURE
-
TRANSDUCER
DIAPHRAGM
SUBARACHNOIO
A.
B.
FIG. 1. Illustration of the subarachnoid screw system used to monitor Intracranial Pressure in the unanesthetized rabbit. (I-A) The base and protective cap: (1-B) the base and experimental cap; (I-C) the base and experimental cap when measuring intracranial pressures.
the skin, the skin and subcutaneous tissue were retracted to either side and the periostium was scraped from the bone. A twist drill hole was made through the skull in the area where it was desired to monitor the pressure. We selected a site 6 mm posterior to the coronal suture and 4 mm lateral to the sagittal suture. After the hole was made, the depth of the dura from the skull surface was measured. The dura was then opened as was the arachnoid membrane and a small area slightly smaller than the diameter of the lumen of the base of the screw was excised. The base was then screwed into position so that its tip rested on the dura. A secure placement was necessary to ensure that there were no CSF leaks between the threads of the base and surrounding bone. By doing so, a closed compartment was formed between the subarachnoid space and the interior of the base. Two stainless steel screws were inserted into the skull nearby to serve as an anchor for the base and dental cement which was placed around the screws and base assembly. When the dental cement was dry, the lumen of the base was filled with saline. In most cases the pulsations caused by breathing could now be observed as movement of the saline. The protective cap was affixed and a small incision in the skin approximating the position of the base was made. As the midline incision was closed, the base and cap were passed through this lateral incision. Antibiotics were administered post-operatively for 4 days. The fur in the immediate vicinity of the implant was kept closely shaven in order to help keep the preparation clean. Animals were given 7 days to recover from surgery after which conditioning to the experimental setting was accomplished. Experiments were carried out in a quiet, minimally occupied, restricted access room at a temperature of
19.0 * l.O”C. It was important to have a quiet environment in which the animals were comfortable as parameters such as respiration and general awareness, vary considerably with environmental stimuli. Animals were placed gently in an open top box, the top half of the front of which was cut away so that there was no possibility of the animal’s neck or chin resting on a firm surface which might impede blood flow through the vessels in the neck. This box provided only minimal restraint and all animals accepted it readily. The pressure transducer (Statham Instruments Type P23V, Statham Instruments Inc., Oxnard, CA) was calibrated with a saline manometer before and after the experiment. The signal output was displayed on a Beckman Dynograph Recorder (Beckman Instruments Inc.). The calibration range was 0.0 to 10.0 cm saline. Using sterile technique, the protective cap was removed and the lumen of the base was filled with sterile saline. The connecting tubing to the experimental cap was filled with sterile saline and the cap was then rotated into position on the base assembly. It was important to keep the distal end of the connecting tubing open in order to allow pressure and temperature equilibria to be established as the cap was tightened and for a few minutes thereafter. After 2 or 3 minutes, the connecting polyethylene tubing (PE 100: 0.034 i.d., 0.060 o.d.) 45 cm long was connected to the pressure transducer and the pressure waveforms could then be recorded. At the start of the recording period, tests to ensure the patency of the CSF pressure measuring system were performed. These included mild abdominal compression, jugular vein compression, and observation of the resting CSF pressure wave forms. In order to give an index of the vertical position of the
INTRACRANIAL
PRESSURE
637
IN THE RABBIT
animal’s head with reference to the fixed pressure transducer, an open-ended liquid filled catheter was positioned so that the open end was fixed rigidly to the experimental cap; the other end of the catheter was connected via a 45 cm length of tubing to a calibrated pressure transducer (P23V Statham Instruments Inc.) and the signal displayed on the Beckman Dynograph. This method allowed for the remote monitoring of the pressure as the investigator did not have to be in close proximity to the animal in order to note the level of the animal with reference to the fixed pressure transducer. Intracranial pressure data were interpreted according to the following conditions. Each animal quickly adopted a preferred position in which he was comfortable and remained for throughout most of the recording. The ICP and pulsations for any 1 rabbit were used for comparative analysis only when the rabbit was in this position. The data was then point-plotted on graph paper. By using this method, comparative pressures were obtained over the period of recording when the animal was in the same position. By use of the level indicator and close observation, the position of the animal was determined. The reference level of the transducer was chosen as the skull surface at the point of attachment of the base assembly and represented 0.0 mm saline.
RESULTS
Successful recordings were obtained in all 6 rabbits with the subarachnoid screw system and no infection occurred in any of the animals. In one animal, the lumen of the base had a clot of blood, this clot was removed as a single mass and the base was then irrigated with saline and good recordings were subsequently obtained. The subarachnoid screw system proved to be reliable for monitoring ICP in unanesthetized rabbits for continuous periods of 8 hr or more. The surgical procedure was relatively simple and very few complications occurred. The system remained patent for 3 to 4 weeks. After this time, some closure of the lumen of the base occurred and the waveforms became damped. Responses of the ICP recordings to manipulations such as abdominal compression, jugular vein compression, carotid artery compression were similar to those obtained by other methods of monitoring. Because the CSF pulsations in ICP are partially respiratory in origin, this was a method of determining respiratory rates. Inspiration causes a reduction in ICP; expiration causes an increase in ICP. In most animals, cardiac pulsations could also be seen superimposed upon the respiratory pulsations.
A
-
30 30
set
1
FIG. 2. Illustration of representative intracranial pressure waveforms obtained in 1 rabbit. (2-A) An enlarged portion of the recording; (2-B) the effects of mild abdominal compression on the ICP; (2-C) the effect of bilateral jugular vein compression on the ICP. The duration of compression is indicated by the solid bar.
MALKINSON,
638
It is desirable to have both a protective as well as an experimental cap for the screw in order to provide access to the lumen of the base, to flush it out occasionally and to ensure that no air bubbles are trapped within the system which would tend to damp out or otherwise alter the pressure waveforms. The protective cap, because of its design, is much more suitable for the housing of the animal when experiments are not being carried out. The open-ended catheter provides a reliable index of when the animal’s head is at the same height above the fixed pressure transducer. Because of the behavioral characteristics of the rabbit, the level of the animal’s back did not change significantly during the course of a recording with the level of the head being the main concern. Figure 2-A illustrates a representative portion of the recording obtained in 1 rabbit. Both the respiratory pulsations and the superimposed cardiac pulsations are visible. Figure 2-B illustrates the change in ICP produced before, during, and after mild abdominal compression. Figure 2-C illustrates the changes in ICP produced by bilateral jugular vein compression. The mean intracranial pressure in 6 rabbits was 57.69 + 10.10 mm of saline. The mean pulsation pressure in 6 rabbits was 17.84 * 3.68 mm of saline. The mean pulsation rate in 6 rabbits was 118.00 * 27.95 pulses/min. There appeared to be a periodic fluctuation in the intracranial pressure in some rabbits. This sinusoidal fluctuation has been observed by other investigators [2] who point out the importance of considering these periodic fluctuations before interpreting the results of experiments.
VEALE AND COOPER
setting since little damage can be done to expensive pressure transducers which are some distance away. The surgery is simple and sterility is easily maintained. The brain is not traversed and there is no difficulty in maintaining a free-fluid communication. The recording system is easy to calibrate before and after the experimental protocol and can be very sensitive, depending on the characteristics of the pressure transducer used. The subarachnoid screw can be positioned at any convenient position on the skull and measure the pressure at that point. The implant can easily be removed under anesthesia at the end of the experimental procedure, and the animal could possibly be used for other experiments or the screw could be positioned at another place on the skull. The wound can be allowed to granulate to closure or covered with dental cement. As in any recording system of this type, the fixed location of the pressure transducer relative to the body of the unanesthetized animal which is free to move sometimes makes it difficult to obtain a continuous recording under the same hydrostatic conditions. The system remains patent for approximately 3 to 4 weeks after which time infiltration of the lumen of the screw with bone and scar tissue occurs and the recording is less distinct. Another limitation is that the animal is physically connected to the transducer by the catheter and therefore must have minimal restraint. There is some evidence from experience in humans that the subarachnoid screw preparation may not be suitable for measurement of increases in ICP over relatively long periods of time because of herniation of the brain into the screw opening with subsequent loss of subarachnoid space continuity with the column of fluid.
DISCUSSION
The recording system described can be used on a wide variety of anesthetized or unanesthetized animals. The animal is free to move within the restraints of the experimental
ACKNOWLEDGEMENT
This work Canada.
was supported
by
the Medical Research Council
of
REFERENCES Edvinsson, L., K. C. Nielsen, C. H. Owman and K. A. West. Alterations in intracranial pressure, blood-brain barrier, and brain edema after sub-chronic implantation of a cannula into the brain of conscious animals. Acta Physiol. .scrrncl. 82: 527-531, 1971. Hayden, P. W., D. B. Shurtleff and E. L. Foltz. Ventricular fluid pressure recordings in hydrocephalic patients. Arch. Nrurol. 23: 147-154, 1970. Pollay, M. and F. Curl. Secretion of cerebrospinal fluid by the ventricular ependyma of the rabbit. Am. J. Ph~siol. 213: 10311038, 1967.
Vries, .I. K., D. P. Becker and H. F. Young. A subarachnoid screw for monitoring intracranial pressure. J. Ne~ro.vur,y. 39: 41G419, 1973. 5. Welch, K. Secretion of cerebrospinal fluid by the choroid plexus of the rabbit. Am. J. Physiol. 205: 617-624, 1963. 6. Winn, H. R., R. G. Dacey and J. A. Jane. Intracranial subarachnoid pressure recording: experience with 650 patients. Strr-K. Neurnl. 8: 41-47, 1977. 4.