METHODS V. P .

OF

RECORDING

OvsyanLk

THRESHOLD

AUDITORY

EVOKED

POTENTIALS

UDC 615,471.03:616.28-008.1-073.97

Great attention has been paid in r e c e n t y e a r s to the p r o b l e m of a s s e s s i n g the s t a t e of hearing by objective a u d i o m e t r y b a s e d on p a r a m e t e r s of Long-Latency auditory evoked potentials (LAEP) in man Ln r e s p o n s e to acoustic stimuli. A p a r t i c u l a r f e a t u r e of Lsolation o f the LAEP f r o m an additive m i x t u r e of them with the EEG signal in r e s p o n s e to acoustic stimulation at the threshold of audibility is d e t e r m i n e d by the fact that the ratLo (M) of the amplitudes of components of LAEP to the effective level of the EEG is significantly less than unity. Accordingly, to r e a c h a reliable concius ion r e g a r d i n g the p r e s e n c e of LAEP, the n u m b e r of poststimulus r e a l izations of bioelectricaL activity (BEA) a v e r a g e d m u s t be consLderably i n c r e a s e d . This naturally prolongs the investigation and c o r r e s p o n d i n g l y i n c r e a s e s the p r o b a b i l i t y of a p p e a r a n c e of noncoherent evoked signals in the a v e r a g e d poststimuius realizations of BEA~ due to different physioLogLcai f a c t o r s . This reduces both the a c c u r a c y of isolation and m e a s u r e m e n t of the L A E P p a r a m e t e r s and also makes the diagnosis of the s t a t e of the auditory a n a l y z e r Less reliable. To isolate LAEP suitable for use Ln objective a u d i o m e t r y , an urgent t a s k is t h e r e f o r e to i n c r e a s e the ratLo M actually in the r e a l i z a t i o n s of BEA a v e r a g e d at an ass igned Level of acoustLc stLmuLation. A solution to this p r o b l e m would g r e a t l y diminish the influence of the a d v e r s e f a c t o r s mentioned above. We know that LAEP Lie within the frequency band of about 0.1-20 Hz and a r e abolished if the active e l e c t r o d e is located in the region of the v e r t e x (the point C z on the 10-20 s y s t e m ) . If the active e l e c t r o d e is Located Ln the region of the sylvian f i s s u r e (points T 3 and T 4) Lt is vLrtuaLiy LmpossLble to distLngulsh the L A E P even at Levels of acoustLc s t i m u l a t i o n much above the threshold of audLbLLtty. The possLbLLLty of LncreasLng the value of M in the original poststimuLus reaLizatLons of BEA can accordLngly be s u g g e s t e d in the c a s e of definite compensation of the EEG p r e s e n t in an additive mLxture with LAEP by means of a s p e c i a l l y p r o c e s s e d EEG sLgnaL r e c o r d e d f r o m point T 3 o r T 4. The effectLveness of such c o m p e n s a t i o n depends on the Level of c o h e r e n c e of the EEG sLgnals at points Cz and T 3 (T4). Data c h a r a c t e r i z i n g the spatLai distribution of coeffLcients of c o r r e l a t i o n (CC) of the EEG of the human brain a r e given in [2]. F o r instance, at points C z and T 3 (T 4) in the a b s e n c e of a t i m e shift, CC between EEG signals reaches values of r 0 = 0.6-0.7. The p r e s e n t w r i t e r also conducted investigations b a s e d on m e a s u r e m e n t of CC f o r r 0 and r r for c a s e s of a t i m e shift of z e r o and r between EEG signals r e c o r d e d f r o m poLnts Cz and T 3 o r T 4. The r e f e r e n c e e l e c trode was located on the m a s t o i d p r o c e s s , and the ground e l e c t r o d e on the subjectts forehead. M e a s u r e m e n t s were made within the frequency band x~f = 0.1-20 Hz and the frequency band Afl-Af 4, c o r r e s p o n d i n g to $, v, ~, f l - r h y t h m s . The duration of realizations of the s ignals chosen f o r anaLys is was 0.5 sec. The m e a s u r e m e n t s showed that values of r 0 f o r signals within the Af frequency band w e r e between 0.5 and 0.7, whereas in the p r e s e n c e of a definite t i m e shift T between the s i g n a l s , r T could exceed r 0 and attain values of 0.8. F o r signals in the frequency band zkfI = 1-3 Hz9 ~f2 = 3.5-7 Hz, zkf3 = 7.5-12 Hz, and Af4 = 13-20 Hz, the mean value of r 0 was 0.68, and that of rT~ 0.94. It is e a s y to show by t h e o r e t i c a l calculations that at the s a m e effective Levels of two signals Xl(t) and X 2(t) and with CC between them of r 0 > 0.5, the signa I level Z (t) = :~l(t) - X~(t) is below the s igna[ level X(t). F o r instance, for r0 = 0.8-0.94, the signal Level Z(t) is 1.59-2.92 t i m e s lower than the signal Level X(t). Consequently, with c o r r e c t matching of levels and phases of the principal components of EEG signals at points Cz and T 3 (T 4) the method of EEG compensation at point Cz can be used sufficLentLy effectively so that the ratio M can be i n c r e a s e d in original a v e r a g e d poststimuLus r e a l i z a t i o n of BEA. The block d i a g r a m of the s y s t e m developed f o r isolating L A E P with compensation of the p r i n c i p a l c o m ponents of the EEG [1] is illustrated in Fig. 1. Special Design and Technology Office with E x p e r i m e n t a l Workshops, Institute of Superhard M a t e r i a l s , Academy of Sciences of the Ukrainian SSR, Kiev. T r a n s l a t e d f r o m Meditsinskaya Tekhnika, No. 3, pp. 12-14, May-June, 1986. Original article s u b m i t t e d May 5, 1985.

84

0006-3398/86/2003- 0084 $ 1 2 . 5 0 0 1987 Plenum Publishing C o r p o r a t i o n

J

Fig. 1. Block diagram of s y s t e m for isolating LAEP with compensation of principal components of EEG signais. 1) C o m p u t e r - c o n t r o l l e d acoustic stimulus g e n e r a t o r ; 2) acoustic chamber; 3) active e l e c t r o d e s ; 4) earphones; 5) r e f e r e n c e e l e c trode; 6) c i r c u i t for reception and p r o c e s s i n g of additive m i x t u r e of EEG and LAEP r e c o r d e d from point Cz; 7) c i r c u i t for reception and p r o c e s s i n g of EEG r e c o r d e d f r o m point T 3 (T4); 8~ 12) amplit i e r s ; 9) f i l t e r with t r a n s m i s s i o n band at level 0.7Af = 0.1-20 Hz; 10) subtraction unit; 11)l~lektronika 60 M computer, by means of which g e n e r a t o r 1 is controlled and poststimuius realizations of BEA with an assigned duration T = 300-500 m s e c are averaged; 13) filters with t r a n s m i s s i o n bands at level of 0.7Aft-All; 14) controlled phase shifters; 15) controlled amplifiers; 16) s u m m a t o r .

A =10 dB

A=60 dB

a

-.-.,, ~,4"., _ 0,1

/%, O,Z

0,3

Fig. 2

0,4

,.,, ~P',-..,_,~ sec

0,I

"O,a

0,,.?

0,4

see

Fig. 3

Fig. 2. Typical appearance of normalized poststimulus signal of BEA~ averaged for 32 r e a l i z a t i o n s , for patients with normal hearing: a) using the s y s t e m of isolation of I'~AEP with compensation of EEG; b) without compensation of EEG; A) level of acoustic p r e s s u r e of acoustic stimulus. Fig. 3. Typical appearance of n o r m a l i z e d poststimulus signal of BEAp averaged for 32 realizations for patients with auditory pathology (conduction deafness - loss of hearing 50 dB): a) us ing a s y s t e m of isolation of LAEP with compensation of EEG; b) without compensation of EEG.

85

Before the LAEP isolation procedttre, the system Ls calibrated individually for each subject by means of an effective value voltmeter and phase meter. During calibration, instead of the filter 9, a circuit identical with circuit 7 but without the s am m at or 16 LS connected. At the output of the corresponding filters 13 of the additional cLrcutt and of circuit 7, Ln the absence of acoustic stimulation, the mean effective Levels and phases of the EEG signals are equalized. After caLLbratLon~ which takes not m ore than 3-5 mLn, the effective Level of the EEG recorded from point Cz, when circuit 7 LS connected, Ls reduced by 1.8-2.5 times. Investigations into isolation of LAEP in response to threshold Levels of acoustic stimuli Ln patients with normal hearing and with different forms of pathology of hearing showed that the system as developed Ls hLghly effective. For instance, Fig. 2 shows typical poststimuLus realizations of BEA obtained with a patient with normal hearing, and at acoustic p r e s s u r e Levels of the stimulus of 10 dB relative to 2.10 -5 Pa. Corresponding eharacterLstLcs obtained from a patLent with hearing Lose of 50 dB, at frequencies corresponding to an acoustic stimulus with an acoustic pr e s s ur e Level of 60 dB., are shown in Fig. 3. It will be evident that the use of the suggested system for isolation of LAEP not only increase the accuracy of isolation and measurement of LAEP param et ers, wtth a corresponding increase in the reliability of auditory diagnosis, but it also shortens the time required for objective audLometry (this is particularly important in the case of examination of children), It mast also be pointed out that the suggested system of isolation can be completely set up ustng Soviet computers. LITERATURE

11 2.

CITED

V. P. OvsyanLk, AuthorVs Certificate 1055471 (USSR), Otkrytiya, No. 43 (1983). A. N. Shepovaltnikov, M. N. Tsitseroshin, and V. S. Apanas ionok, Formation of the BLopotentiaL Field of the Human Brain [in Russian], Leningrad (1979).

SELECTING

DRYING METHODS

FOR MEDICAL

OBJECTS

IN S T E A M S T E R I L I Z A T I O N V. B. T s i b L k o v

a n d B. Y a .

RabLnVkLL

UDC 615.47.014.451

Sterilization with saturated steam is a classical method widely used in medical practice. However, the phase state of s aturated s team is unstable, and condens atLon occurs Ln contact with cold objects, which c an Lead to infection or adverse effects on the hygroscopic parameters on use or storage. Therefore, drying (water desorption) is essential to high-grade sterilization. At present, medical materials are dried during sterilization in the medical industry by producing a vacuum in the sterilization chamber for 15 min; the p r e s sure does not exceed 0.08 MPa. The range of drying methods is much wider in other branches of the economy. The best methods in steam sterilization are drying by the use of vacuum and superheated steam, since these do not require complicated des ign changes. They can be implemented with evacuation and steam-generated systems present Ln the sterilizing equipment. A promising method of vacuum drying is with pulsating evacuation [1, 2], which can be provided either by periodically pumping out the steam and then supplying superheated and st eri l e air or by pumping off the steam without air supply. The number of pulsations is determined by the residual-water specifications. These methods reduce the electrical power consumption and provide fuller and more rapid water removal because of t h e p r e s s u r e d i f f e r e n c e s b e t w e e n material and chamber. The p r e s s u r e in the material is higher than that in the chamber, which causes the water in the material to boil and transports the vapor from the object. Superheated-steam drying also has some advantages, since one can use steam superheated in the s t e r i l izer generator. This method increases the water volume and provides partial removal from the object; when the steam is released, the water boils and the material is additionally dewatered. "Medoborudovanie" Scientific-Production Cooperative, Moscow. Translated from Meditsinskaya Tekhnika, No. 3, pp. 15-18, May-June, 1986. Original article submitted July 17, 1985.

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0006-3398/86/2003_ 0086 $12.50 © 1987 Plenum Publishing Corporation