- Email: [email protected]
A recorder for off-line acquisition of brain stem auditory response data
Elec troencephalography and Clinical Neurophysiology, 1981, 51 : 117--118
117
© Elsevier/North-Holland Scientific Publishers, Ltd. Technical contribution
A RECORDER FOR OFF-LINE ACQUISITION OF BRAIN STEM A U D I T O R Y RESPONSE DATA R.R. MARSH and W.P. POTSIC
Department of Otorhinolaryngology and Human Communication, Children's Hospital of Philadelphia, 34th St. and Civic Center Blvd., Philadelphia, Pa. 19104 (U.S.A.) (Accepted for publication: September 15, 1980)
To make brain stem auditory response tests more readily available within the hospital, we have modified a stereo cassette recorder to collect data at bedside for later analysis when the signal-averaging computer becomes available. This tape recorder, though much less expensive and much more portable than laboratory-type recorders, gives us sufficient flexibility and precision for many clinical applications. The device is especially useful for long-term monitoring of comatose patients and for taking advantage of natural periods of sleep in testing children, without committing the computer for long periods of time. We are also able to test acutely ill patients on short notice when the computer has been previously scheduled for outpatient evoked-response audiometry or for research. In these cases the data, once collected, can be analyzed quickly when the computer becomes available for a few minutes. The tape recorder used in this system, a Hitachi D-220, has more than adequate frequency response and tape-speed stability for clinical applications. This model, incidentally, has Dolby noise-reduction circuits, which we do not use. Because the recorder is used only for neurological (as opposed to auditory threshold) assessment, only a single stimulus is used, generated by the circuit shown in Fig. 1. This circuit, which we have installed in the cabinet of the recorder, generates 0.1 msec square pulses with an intensity of 70 dB above normal threshold and an interpuise interval of 100 msec. The polarity of every second pulse is inverted so that the artifacts of positive and negative pulses cancel oneanother when the tape is analyzed. The Sync output of this circuit delivers a pulse to the Line input of one channel of the tape recorder, coinciding with the onset of each click. The other channel receives the output of an optically isolated preamplifier, which is connected to the patient in the usual way. An encapsulated DC power supply operates both the preamplifier and the stimulus circuit. When the tape is to be analyzed, the response channel is connected to the analog-digital converter
of the signal averager via a bandpass filter, just as the preamplifier would be in on-line measurement, and the synchronizing channel triggers the computer at points corresponding to the onset of each click during the original recording. The Line outputs of the recorder are used during playback, giving a gain of 10, which must be taken into account in scaling the display. Fig. 2 demonstrates the quality of reproduction to he expected. The wave form obtained by taping the response and later analyzing the data (bottom) is well-defined, with latencies comparable to those obtained in on-line analysis (top). Differences between the two responses are attributed to cross-talk between the channels of the tape recorder, and a -+1 dB departure from a flat frequency response. The former is reflected in the notchlike artifact at the onset of the display, corresponding to the synchronizing pulse, and the latter is seen in an enhancement of the slow
I
Fig. 1. Stimulus generator circuit. Interstimulus interval and click duration are determined by the left and right 555 timers, respectively. A flip-flop, 4027, and AND-gates, 74C08, deliver alternate pulses to one or another of two inputs in driver 4050.
118
R.R. M A R S H , W.P. POTSIC V /
I/I /
I 2 msec
/~ On-line
~/
Fig. 2. Brain s t e m r e s p o n s e t o 0.1 msec, 70 dB n S L click, N = 2 0 4 8 ; b a n d p a s s = 3 0 - - 3 0 0 0 Hz. T o p : data a n a l y z e d directly b y Nicolet 1 0 7 3 signal-averaging c o m p u t e r . B o t t o m : same r e s p o n s e ; d a t a were first r e c o r d e d o n tape a n d later analyzed.
c o m p o n e n t of t h e t a p e d response. T h e s e deviations in n o way c o m p r o m i s e t h e clinical i n t e r p r e t a t i o n o f the data. Certain p r e c a u t i o n s are essential to reliable a n d safe o p e r a t i o n . T h e m o s t f u n d a m e n t a l relate t o the p r e v e n t i o n o f electrical h a z a r d : all m o d i f i c a t i o n s s h o u l d be d o n e b y qualified p e r s o n n e l ; t h e u n i t m u s t be prOperly f u s e d a n d g r o u n d e d ; a n d a n optically isolated p r e a m p l i f i e r is t o be r e c o m m e n d e d for inp a t i e n t use. Finally t h e u n i t m u s t be i n s p e c t e d by a c o m p e t e n t p e r s o n c o n v e r s a n t w i t h safe practices a n d s t a n d a r d s . T o r e d u c e c h a n c e s for o p e r a t o r e r r o r , we have, w h e r e possible, l o c k e d or spring-loaded c o n t r o l s in t h e p r o p e r p o s i t i o n . T h e s t i m u l u s g e n e r a t o r is o p e r a t e d a u t o m a t i c a l l y w h e n e v e r d a t a a r e b e i n g collected, b y m e a n s o f a s w i t c h l i n k e d t o t h e R e c o r d lever. To insure p r o p e r r e c o r d i n g c o n d i t i o n s , t h e
o p e r a t o r m o n i t o r s the r e s p o n s e c h a n n e l , using earp h o n e s plugged i n t o the o u t p u t jack o f t h e tape recorder. T h e various sources o f c o n t a m i n a t i o n -muscle activity, l i n e - f r e q u e n c y h u m , electrical noise f r o m life-support e q u i p m e n t , a n d t h e like - - are as readily identified b y ear as b y o b s e r v a t i o n o n a n oscilloscope. A t t h e same time, t h e o p e r a t o r m a y use t h e m i c r o p h o n e i n p u t to i n t e r r u p t a c q u i s i t i o n a n d a n n o t a t e t h e tape, to i d e n t i f y t h e p a t i e n t a n d t h e ear u n d e r test. Obviously, t h e r e are m a n y c i r c u m s t a n c e s w h e r e a tape r e c o r d e r c a n n o t s u b s t i t u t e for on-line analysis o f t h e brain s t e m response. A u d i t o r y t h r e s h o l d determ i n a t i o n is one, w h e r e the o p e r a t o r m u s t m a k e decisions i m m e d i a t e l y , based o n the r e s p o n s e t o t h e preceding stimulus. F o r m a n y p a t i e n t s , h o w e v e r , t h e r e c o r d e r p e r m i t s us to p e r f o r m tests o f r e t r o - c o c h l e a r f u n c t i o n p r o m p t l y a n d accurately.
Summary A m o d i f i e d cassette tape r e c o r d e r collects brain s t e m r e s p o n s e d a t a for later off-line analysis. This simple device p e r m i t s assessments o f b r a i n s t e m funct i o n w h e n a signal-averaging c o m p u t e r is n o t i m m e diately available, a n d at l o c a t i o n s n o t readily accessible to t h e c o m p u t e r .
R~sum~
E n r e g i s t r e m e n t p o u r acquisition off-line des d o n n e e s c o n c e r n a n t les rdponses auditives d u tronc cdrdbral U n e n r e g i s t r e u r a cassette modifi~ recueille les d o n n ~ e s c o n c e r n a n t les r~ponses du t r o n c c~r~bra] p o u r analyse ultSrieure off-line. Ce syst~me simple p e r m e t l ' ~ v a l u a t i o n de la f o n c t i o n du t r o n c c~r~bral l o r s q u ' u n o r d i n a t e u r p e r m e t t a n t de m o y e n n e r les sign a u x n ' e s t pas i m m ~ d i a t e m e n t d i s p o n i b l e , ou lorsque la prise de d o n n ~ e s est r4alis~e d a n s des lieux qui n ' o n t pas i m m ~ d i a t e m e n t acc~s ~ l ' o r d i n a t e u r .
117
© Elsevier/North-Holland Scientific Publishers, Ltd. Technical contribution
A RECORDER FOR OFF-LINE ACQUISITION OF BRAIN STEM A U D I T O R Y RESPONSE DATA R.R. MARSH and W.P. POTSIC
Department of Otorhinolaryngology and Human Communication, Children's Hospital of Philadelphia, 34th St. and Civic Center Blvd., Philadelphia, Pa. 19104 (U.S.A.) (Accepted for publication: September 15, 1980)
To make brain stem auditory response tests more readily available within the hospital, we have modified a stereo cassette recorder to collect data at bedside for later analysis when the signal-averaging computer becomes available. This tape recorder, though much less expensive and much more portable than laboratory-type recorders, gives us sufficient flexibility and precision for many clinical applications. The device is especially useful for long-term monitoring of comatose patients and for taking advantage of natural periods of sleep in testing children, without committing the computer for long periods of time. We are also able to test acutely ill patients on short notice when the computer has been previously scheduled for outpatient evoked-response audiometry or for research. In these cases the data, once collected, can be analyzed quickly when the computer becomes available for a few minutes. The tape recorder used in this system, a Hitachi D-220, has more than adequate frequency response and tape-speed stability for clinical applications. This model, incidentally, has Dolby noise-reduction circuits, which we do not use. Because the recorder is used only for neurological (as opposed to auditory threshold) assessment, only a single stimulus is used, generated by the circuit shown in Fig. 1. This circuit, which we have installed in the cabinet of the recorder, generates 0.1 msec square pulses with an intensity of 70 dB above normal threshold and an interpuise interval of 100 msec. The polarity of every second pulse is inverted so that the artifacts of positive and negative pulses cancel oneanother when the tape is analyzed. The Sync output of this circuit delivers a pulse to the Line input of one channel of the tape recorder, coinciding with the onset of each click. The other channel receives the output of an optically isolated preamplifier, which is connected to the patient in the usual way. An encapsulated DC power supply operates both the preamplifier and the stimulus circuit. When the tape is to be analyzed, the response channel is connected to the analog-digital converter
of the signal averager via a bandpass filter, just as the preamplifier would be in on-line measurement, and the synchronizing channel triggers the computer at points corresponding to the onset of each click during the original recording. The Line outputs of the recorder are used during playback, giving a gain of 10, which must be taken into account in scaling the display. Fig. 2 demonstrates the quality of reproduction to he expected. The wave form obtained by taping the response and later analyzing the data (bottom) is well-defined, with latencies comparable to those obtained in on-line analysis (top). Differences between the two responses are attributed to cross-talk between the channels of the tape recorder, and a -+1 dB departure from a flat frequency response. The former is reflected in the notchlike artifact at the onset of the display, corresponding to the synchronizing pulse, and the latter is seen in an enhancement of the slow
I
Fig. 1. Stimulus generator circuit. Interstimulus interval and click duration are determined by the left and right 555 timers, respectively. A flip-flop, 4027, and AND-gates, 74C08, deliver alternate pulses to one or another of two inputs in driver 4050.
118
R.R. M A R S H , W.P. POTSIC V /
I/I /
I 2 msec
/~ On-line
~/
Fig. 2. Brain s t e m r e s p o n s e t o 0.1 msec, 70 dB n S L click, N = 2 0 4 8 ; b a n d p a s s = 3 0 - - 3 0 0 0 Hz. T o p : data a n a l y z e d directly b y Nicolet 1 0 7 3 signal-averaging c o m p u t e r . B o t t o m : same r e s p o n s e ; d a t a were first r e c o r d e d o n tape a n d later analyzed.
c o m p o n e n t of t h e t a p e d response. T h e s e deviations in n o way c o m p r o m i s e t h e clinical i n t e r p r e t a t i o n o f the data. Certain p r e c a u t i o n s are essential to reliable a n d safe o p e r a t i o n . T h e m o s t f u n d a m e n t a l relate t o the p r e v e n t i o n o f electrical h a z a r d : all m o d i f i c a t i o n s s h o u l d be d o n e b y qualified p e r s o n n e l ; t h e u n i t m u s t be prOperly f u s e d a n d g r o u n d e d ; a n d a n optically isolated p r e a m p l i f i e r is t o be r e c o m m e n d e d for inp a t i e n t use. Finally t h e u n i t m u s t be i n s p e c t e d by a c o m p e t e n t p e r s o n c o n v e r s a n t w i t h safe practices a n d s t a n d a r d s . T o r e d u c e c h a n c e s for o p e r a t o r e r r o r , we have, w h e r e possible, l o c k e d or spring-loaded c o n t r o l s in t h e p r o p e r p o s i t i o n . T h e s t i m u l u s g e n e r a t o r is o p e r a t e d a u t o m a t i c a l l y w h e n e v e r d a t a a r e b e i n g collected, b y m e a n s o f a s w i t c h l i n k e d t o t h e R e c o r d lever. To insure p r o p e r r e c o r d i n g c o n d i t i o n s , t h e
o p e r a t o r m o n i t o r s the r e s p o n s e c h a n n e l , using earp h o n e s plugged i n t o the o u t p u t jack o f t h e tape recorder. T h e various sources o f c o n t a m i n a t i o n -muscle activity, l i n e - f r e q u e n c y h u m , electrical noise f r o m life-support e q u i p m e n t , a n d t h e like - - are as readily identified b y ear as b y o b s e r v a t i o n o n a n oscilloscope. A t t h e same time, t h e o p e r a t o r m a y use t h e m i c r o p h o n e i n p u t to i n t e r r u p t a c q u i s i t i o n a n d a n n o t a t e t h e tape, to i d e n t i f y t h e p a t i e n t a n d t h e ear u n d e r test. Obviously, t h e r e are m a n y c i r c u m s t a n c e s w h e r e a tape r e c o r d e r c a n n o t s u b s t i t u t e for on-line analysis o f t h e brain s t e m response. A u d i t o r y t h r e s h o l d determ i n a t i o n is one, w h e r e the o p e r a t o r m u s t m a k e decisions i m m e d i a t e l y , based o n the r e s p o n s e t o t h e preceding stimulus. F o r m a n y p a t i e n t s , h o w e v e r , t h e r e c o r d e r p e r m i t s us to p e r f o r m tests o f r e t r o - c o c h l e a r f u n c t i o n p r o m p t l y a n d accurately.
Summary A m o d i f i e d cassette tape r e c o r d e r collects brain s t e m r e s p o n s e d a t a for later off-line analysis. This simple device p e r m i t s assessments o f b r a i n s t e m funct i o n w h e n a signal-averaging c o m p u t e r is n o t i m m e diately available, a n d at l o c a t i o n s n o t readily accessible to t h e c o m p u t e r .
R~sum~
E n r e g i s t r e m e n t p o u r acquisition off-line des d o n n e e s c o n c e r n a n t les rdponses auditives d u tronc cdrdbral U n e n r e g i s t r e u r a cassette modifi~ recueille les d o n n ~ e s c o n c e r n a n t les r~ponses du t r o n c c~r~bra] p o u r analyse ultSrieure off-line. Ce syst~me simple p e r m e t l ' ~ v a l u a t i o n de la f o n c t i o n du t r o n c c~r~bral l o r s q u ' u n o r d i n a t e u r p e r m e t t a n t de m o y e n n e r les sign a u x n ' e s t pas i m m ~ d i a t e m e n t d i s p o n i b l e , ou lorsque la prise de d o n n ~ e s est r4alis~e d a n s des lieux qui n ' o n t pas i m m ~ d i a t e m e n t acc~s ~ l ' o r d i n a t e u r .