Med.& Biol. Eng. & Comput., 1979,17,081--086

Intracranial pressure monitoring by means of a passive radiosonde V. Barbaro

V. M a c e l l a r i

Laboratorio di Tecnologie Biomediche Istituto Superiore di $anita, Roma, Italy

Abstract--The system described is based on the employment of a radiosonde energised from the outside in such a way that it w i l l not be subject to the limitations arising from the use of batteries. The main sources of error, consisting in the thermal drift of the electronic components and particularly o f the pressure transducer, have been eliminated by transmitting to the outside the information regarding temperature. Thus the data concerning the pressure can be corrected after reception by a simple manual calculation or by automatic computing, K e y w o r d s - - l n t r a c r a n i a l pressure, Radiosonde, Telemetry

1 Introduction THE determination of the value of intracranial pressure and of its trend in relation to time is of a remarkable importance in neurosurgery and neuroreanimation. F o r several years the need has been felt that such measurements be introduced into normal clinical practice. To reach this aim, the techniques have to be perfected, making such a monitoring system more trustworthy in its results and simpler in its use.

Monitoring of the intracranial pressure m a y furnish most important indications concerning the deterioration of the patient's condition, especially if he is a/ready in coma, so that the correct therapy may be established in time; reanimation, pharmacological and surgical methods. The cases of interest in this respect are cranial traumas, spontaneous haemorrhage, intracranial tumours, brain infarct and hydrocephalus. Aside from the immediate clinical problems, there is also the neurophysiologic aspect connected with an accurate study of the relationship between intracranial pressure, cerebral flux, haemogasanalysis and other pathologic and physiologic parameters, the information about which, although of paramount importance, is still scanty. The fact that since 1972 international conferences have been held every two years exclusively on this subject is a further proof of the importance of intracranial pressure measurements. F r o m the point of view o f the measurements' importance there are essentially two possibilities: First received 7th December 1977 and in final form I st March 1978

short-term measurements (two weeks at the most) and long-term measurements (from two weeks to about a year) (BE~:S et aL, 1976; BROCK and DIETZ, 1972; LUNDBER~ et al., 1975). As far as the first case is concerned, the problem is solved in a fairly satisfactory way by using catheters of various types (NuMoTo et al., 1973), or, better still, implantable electric transducers (CoHA~ON et aL, 1974; HANDA et aL, 1974; JACOBSON and ROTHBALLER, 1966; SCHETrINI et aL, 1971). These methods involve the transmission of the pressure data from the interior of the skull to the measuring system by way of the electrical a n d / o r mechanical connections, with subsequent dangers of infection. The risks of infection, which are kept within an acceptable range in the first case, grow substantially in the case of long-term measurements. Hence, the necessity to avoid any mechanical connection, a practice that leads at the same time to considerable advantages of a psychological order and to more comfort for the patient. One way for attaining our goal is to use radiosondes implanted subcutaneously, which transmit electromagnetically to the outside information concerning the pressure. We are still in the experimental stage as far as the solution of this problem is concerned. Although several prototypes have been produced in various parts of the world, none of them has yet been accepted for use in clinical practice. A measuring system is described in this paper, which employs a radiosonde that possesses some interesting functional characteristics and that could turn out to be a valuable contribution to the final solution of the problem.

0140--0118/79/010081 q- 06 $01 950/0 9 IFMBE: 1979

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2 Description of the technical problem The measuring system must be capable of supplying, in a long-term period, the value of the intracranial pressure with a precision of 0.3 kPa, although in the first phase a precision of 0" 5 kPa could be acceptable. In view of the mechanism of adaptation to the atmospheric pressure of the internal pressure of the human body, the pressure to be measured is a relative one, the maximum value of which is 10 kPa. The most important problem in the construction of the radiosonde consists in the difficulty of obtaining a long-term stability of the baseline sufficient to assure the required precision. The need for miniaturisation makes the problem even harder. The measurement becomes more difficult because a measurement of absolute pressure must be obtained by means of the radiosonde, as no reference atmospheric pressure can be available on the transducer. Thus, in order to obtain the relative pressure required, the actual atmospheric pressure has to be subtracted during measurements from the pressure value given by the radiosonde. The absolute values of the pressure measured this way are about ten times higher than their maximum possible difference (relative pressure); consequently, the precision requirements of the radiosonde and of the outside instruments are much stiffer. Assuming the possibility of neglecting the error due to the outside instruments, the precision of the radiosonde should be 0.3} / at full scale. The dynamics of the system must cover a rar~ge of 25 kPa, as one has to take into account the maximum variation of the atmospheric pressure that may be found in long-term measurements (which is about 9 kPa), and the possible changes in altitude of the

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place in which it is used ( ~ 0 . 0 1 2 k P a / m at sea level). The main cause of error lies in the thermal drift of the characteristics of the various components and mainly of the transducer. This error was neutralised by measuring not only the pressure but also the temperature that is used outside to correct the pressure data. Other causes of error are the stability of the power supply voltage for the electronic circuits and the changes occurring with time in the characteristics of the transducer and of the electronics, due to the normal aging of the components and to changes in the environmental conditions.

3 Description of the system The measuring system is characterised by the employment of a passive radiosonde powered from the outside through electromagnetic coupling. This procedure allows the solution of arty energetic problem (need for autonomy) and permits, size being equal, higher power supply voltages than those that can be obtained using batteries, making it consequently easier to design the circuits. In a radiosonde previously produced in the same laboratories (BRAZZODUROand MACELLARI, 1974), the demand for autonomy was satisfied with the 'on demand' technique. In this type of radiosonde, furthermore, the problems connected with the need for limiting the absorbed power are eliminated, as the only restriction is imposed by the necessity o f avoiding affecting t o o ~'auch the temperature of the surrounding tissues. The measuring system, shown in Fig. 1, consists of a radio frequency power source, the frequency of which, established by a voltage controlled oscillator

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82

Medical & Biological Engineering & Computing

January 1979

(v.c.o.), is periodically changed according to a timer (period ~ 1 s). The signals transmitted by the radiosonde are received by the receiver and channelled to a measuring system controlled by the same timer, so that the values of both pressure and temperature can be obtained. The power source works at frequencies of about 100 kHz, high enough to permit an efficient transmission in spite of the reduced size (35 mm diameter) of the transmitting and receiving coils and low enough to enable us to consider as negligible the losses in the tissues and to keep the losses within the coils themselves within reasonable limits. As already mentioned in the introduction, the main causes of error are the thermal drifts. Therefore, after having accurately reduced to a minimum the other causes of error, we turned to the thermal drift of the pressure transducer, as previously mentioned, the problem was circumvented by

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transmitting to the outside not only the information regarding the pressure but also that regarding the temperature in the vicinity of the same transducer. When the temperature of the device is known, it is easy to arrive at the correct value of the pressure either manually, using calibration tables or curves, or automatically, by turning to electronic means. The transmission of the two data values is carried out by frequency modulating the same subearrier audiofrequency oscillator, with the time subdivision system according to the timing suggested by the timer. The subcarrier oscillator then amplitude modulates a radio frequency carrier of 1050 kHz, selected for reception by the most common amplitude modulation commercial receivers. The audio output of such receivers may be directly connected to the measuring and recording system. A straingauge semiconductor transducer (Akers A E 830) was used as a pressure transducer implanted epidurally and connected in a flexible manner to the body of the radiosonde. In fact, it was shown (CORONEOSe t aL, 1973; DE ROUGEMONT et ak, 1974; DORSCH and SIMON, 1974; MAJORS et al., 1972; SUNDBARG and NO1NES, 1972; TURNER et al., 1974) that the pressure measurable by placing the transducer properly over the dura mater is directly proportional to the subdural pressure. The chief advantage obtained by carrying out the measurement epidurally is that of avoiding the danger of infection, to which the dura mater acts as a barrier. However, such an advantage becomes less relevant

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device as the various parts can be firmly glued with relative ease. The ability of the human tissues to tolerate this material has been further improved by covering it with a film of silicone rubber (Silastic 382). The functional block diagram of the radiosonde is shown in Fig. 3 and the schematic appears in Fig. 4. The power supply system (pA723 and transistor Q1) was designed in such a way that the output voltage occurs only if the voltage feeding the stabilising circuit is large enough to assure the correct operation of the circuit itself. Power supply voltages that are lower than a pre-established reference voltage keep a level detector circuit (Schmitt trigger formed by transistor Qx and by the currentlimiting transistor of/~A723) in such a way that it blocks the stabilising circuit. In this way the risk of wrong information, caused by an insufficient power supply voltage, is eliminated. Furthermore, this scheme permits a system of operation by which the radiosonde receives only the energy it needs and nothing more, avoiding useless

when the radiosonde is used, as the danger of infections is lessened by the fact that there is no open wound; in such a case, therefore, the transducer could well be placed in the subdural area without much harm. 4 Description of the radiosonde The dimensions of the radiosonde and the arrangement of its various parts are shown in Fig. 2. The aerial receiving the power required for the operation as well as the temperature and pressure transducers joined through a flexible connection to the body of the radiosonde can be clearly seen. The arrangement makes it easier to implant the radiosonde which does not require any tight fastening, as the slight shifts that are permissible are not transmitted to the transducer. The case of the radiosonde was manufactured from high-density polypropylene (Moplen of Montedison, Rome, Italy), a material easy to work with, which assures the hermetic sealing of the

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Medical & Biological Engineering & Computing

January 1979

power consumption and overheating. To achieve this result, the energising aerial must be placed on the head of the patient and the supply of power must be started and then increased up to the point at which the answering signal is received. The stabilisation of the power supply voltage was established so accurately that the errors that can be ascribed to it, taking into account that the useful input voltage can vary between 15 and 25 V, is better than 10 Pa and therefore can be neglected in the total calculation of errors. The tuning coil of the carrier oscillator (L2) also acts as the transmitting aerial. Its transmitting efficiency (actually such as to a'low the correct reception at a distance of 2 m) presents no problems, as the radiosonde, being of a passive type, always needs a very close external device in order to function. The subcarrier oscillator consists essentially of an unstable multivibrator (XR 2307), with a frequency of about 4 kHz, the time constant of which is determined, alternately, by the pressure transducer (p.t.) and by the temperature transducer (n.t.c.). The switching of the transducers is obtained by discriminating the energising frequency, which can be either 100 kHz or 60 kHz. Special care was devoted to the thermal drift of the whole electronic circuit; in a test performed when the transducers were disconnected, a drift equal to 0.02 kPa]~ was recorded. This entirely satisfactory result allowed us to place the temperature transducer, consisting of a thermistor, next to the pressure transducer in order to measure more precisely the latter's temperature, as it is the element most liable to be influenced by thermal drift.

both passive and active, take place within the first few hundred hours of operation. The radiosonde can, reasonably, be made to work for 2 h a week on average, i.e. about 100 h a year. This period is short enough for the normal aging of the components not to produce significant drift. Furthermore, the case and the covering materials of the device, the polypropylene and the silicone rubber, are such bad absorbers of the surrounding fluids (HARPER,1970; MACKAY,1970)that negligible degradations can be expected due to environmental conditions. Calibration was effected using a thermostated barometric chamber; the radiosonde was then subjected to variable pressures and temperatures precisely measured. The frequencies related to the pressure and temperature values were measured by way of a digital frequency meter connected to the audio output of a commercial-type receiver. The calibration curves are shown in Figs. 5 and 6. The temperature/frequency curve appears in Fig. 5; its trend within the limits of the temperatures of interest can be considered linear. Fig. 6 shows the frequency~pressure curves for various values of the temperature oscillator's frequency. As may be noted, the information concerning temperature is of fundamental importance as there is a drift of about 2kPa/~ The pressure, expressed as abscissae, is obviously an absolute pressure from which the atmospheric pressure has to be subtracted; thus, in order to obtain the value of interest, the atmospheric pressure

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5 Results and conclusions

After setting and tuning up, the radiosonde was kept in operation for 500 h, so that the aging effect of the components on t h e calibration would be limhed. In fact, it is well known that the most remarkable drifts in the values of the components,

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has to be accurately measured. A mercury barometer of the Fortin type is used for this purpose. To carry out a measurement, one must read the frequency value in relation to the pressure, and, by using the curves of Fig. 6, one can arrive at the value of the absolute pressure measured by the transducer. In order to obtain the value of the endocranial pressure (i.p.), the value of the present atmospheric pressure has to be subtracted from it. All these steps, even if basically easy to perform, may lead to some difficulties i f a continuous monitoring of the intracranial pressure is desired or when the performance of the measurements is entrusted, as is understandable, to nonspecialist personnel. Therefore, the authors are planning to perfect a system of automatic elaboration that will furnish directly the actual value of the temperature and of the real intracranial pressure; it will thus be possible to use it, in both digital and analogue form, for recording the pressure on paper.

Acknowledgment--The authors take this opportunity to express their thanks to E. Mari for his co-operation in this work.

References

ATKINSON, J. R., SHUKTLEFF,D. B. and FOLTZ, E. L. (1967) Radiotelemetry for the measurement of intracranial pressure. J. Neurosurg. 27, 428-432. BEKS, J. W. F., BOSCH, D. A. and BROCK, M. (Eds.) (1976) Intracranial pressure Ili, Springer-Verlag, Berlin. BRAZZODURO, G. and MACELLARI, V. (1974) Realizzazione di un prototipo di radiosonda per la misura della pressione del fluido cerebrospinale. Ann. Ist. Super. Sanit~ 10, 268-274. BROCK, M. and DIEFENTHALER,K. (1972) A modified equipment for the continuous telemetric monitoring of epidural or subdural pressure. In lntracranialpressure, Springer-Verlag, Berlin, 21-26. BROCK, M. and DIETZ, H. (Eds.) (1972) Intracranial pressure, Springer-Verlag, Berlin. COHADON, F., LA BALME, i . , CASTEL, J. P. and VANOENDR1ESSCHE (1974) ICP microprobes series microfet, lntracranial pressure II, Springer-Verlag, Berlin, 375-376.

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COKONEOS, Y. J., McDDWALL, D. G., GIBSON, R. M., PICKERODT, V. W. A. and KEANEY, N. P (1973) Measurement of extradural pressure and its relationship to other intracranial pressures J. Neurol. Neurosurg. Psychiat. 36, 514-522. DE ROUGEMENT,J., BENABID,A. L. and BARGE,M. (1974) Intracranial pressure measurement by epidural technique--A simple solution. In Intracranial pressure II, Springer-Verlag, Berlin, 384-385. DORSCH, N. W. C. and SYMON,L. (1974) The validity of extradural measurement of the intracranial pressure. In Intracranial pressure II, Springer-Verlag, Berlin, 403-408. FOROGLOU, G., ZANDER, E., FAURE, R. and BESSE, R. (1974) Telemetric measurement of intracranial pressure with an electromagnetic detector. In Intracranial pressure II, Springer-Verlag, Berlin, 377. HANDA, H., YONEDA,S., MATSUDA,M. and HANDA,J. (1974) A miniature transducer for continuous monitoring of intracraniaI pressure. In Intracranial pressure H, Springer-Verlag, Berlin, 378-380 HARPER, C. A. (Ed.) (1970) Handbook of mawrials and processes for electronics, MacGraw-Hill, New York, 1-35. JACOBSON, S. A. and ROTHBALLER,A. B. (1966) Prolonged measurement of experimental intracranial pressure using a subminiature absolute pressure transducer. J. Neurosurg. 25, 603-608. LUNDBERG,N., PONTEN~U. and BROCK, M. (Eds.) (1975) Intracranial pressure II, Springer-Verlag, Berlin. MACKAY, S. R. (1970) Bin-medical telemetry, Wiley, New York, I01. MAJORS, R., ~CHETTtNI,A., ~/[AHIG,J. and NEVIS,A. H. (1972) Intracranial pressure measured with the coplanar pressure transducer. Med. & Biol. Eng. 10, 724-733. NUMOTO, M., WALLMAN,J. K. and DONAGHY,R. M. P. (1973) Pressure indicating bag for monitoring intracranial pressure. J. Neurosurg. 39, 784-787. SCHETTINI,A., McKAY, L., MAJORS, R., MAHIG, J. and NEVlS, A. H. (1971) Experimental approach for monitoring surface brain pressure, ibid. 34, 38-4-47. SUNOB~gG, G. and NOINES, H. (1972) Simultaneous recording of the epidural and ventricular fluid pressure. In Intracranial pressure, Springer-Verlag, Berlin, 46-50. TURNER, J. M., GIBSON,R. M., McDoWALL, D. G. and NAHHAS, F. (1974) Further experiences with extradural pressure monitoring. In Intracranial pressure H, Springer-Verlag, Berlin, 397-402.

Medical & Biological Engineering & Computing

January 1979

Intracranial pressure monitoring by means of a passive radiosonde.

Med.& Biol. Eng. & Comput., 1979,17,081--086 Intracranial pressure monitoring by means of a passive radiosonde V. Barbaro V. M a c e l l a r i Labo...
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