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Simultaneous recording of acceleration and brain waves
550
TECHNICAL
Electroencephalography and Clinical Neurophysioiogy. 1973, 34:550 552 ~' Elsevier Scientific Publishing Company, A m s t e r d a m Printed in The Netherlands
CONTRIBUTION
S I M U L T A N E O U S R E C O R D I N G OF A C C E L E R A T I O N A N D B R A I N WAVES x F R I T Z B U C H T H A L , KRISTIAN D A H L AND WERNER TROJABORG
Laboratoo' ~! Clinical Neuroph.vsiolo#v, Riffshospitalet, Copenhagen (Denmark~ (Accepted for publication: October 23, 1972)
When electroencephalograms are recorded at high gain from unconscious patients artefacts arising from mechanical disturbances are often difficult to identify. Vibrations of the electrode or its connections are transduced to electrical potentials that may resemble cortical activity. The source of these vibrations may be the respirator, the patient's heart beat or movements of doctors and nurses, especially on intensitive care wards since several patients are supervised at the same time. To recognize when and how this interference occurs we have routinely recorded mechanical vibrations together with the EEG by means of a transducer which responds solely to mechanical changes. To evaluate whether acceleration, rate of movement or displacement by the movement was the best indicator of mechanical artefact, acceleration was recorded directly and after single and double integration. METH OD To establish in what direction movement artefacts were most apt to be picked up, we at first used three small piezoelectric accelerometers mounted on a c o m m o n base (Brtiel and Kjaer, Copenhagen, 4344). However, since movements in the antero-posterior plane also appeared in the lateromedial or cranio-caudal recordings, we used routinely only two accelerometers, one sensitive to latero-medial and the other to cranio-caudal movements. The output from each accelerometer was amplified in a commercial pre-amplifier (Briiel and Kjaer 2623) with additional filters to give a lower limiting frequency of 0.7 c/sec (3 dB down) and an upper limiting frequency of 34 c/sec (3 dB down). The outputs of the two pro-amplifiers were connected to two channels of an electroencephalograph to record acceleration, and then integrated once on the same two channels to record the rate of movement. The resulting lower limiting frequency was 1 c/sec (40 dB) and integration was obtained above a frequency of 2 c/sec. The gain was unity at 5 c/sec. Since measurement of displacement obtained by inteSupported by a grant from the Danish Medical Research Council.
Fig. I. Placement of the box containing the two-directional accelerometer on the forehead of a patient in the mtensiw' care uniL grating twice did not give advantages in identifying mcchanical artefact and was apt to be disturbed by variations in temperature, we recorded routinely only acceleration and rate of movement. A sensitivity of the accelerometer of 2.5 mVg corresponded to an acceleration of 4 mm/sec'/HV. Integrating once (additional amplification 4.7 times) gave rates of movement of 0.02 mm/sec//~V. The piezoelectric pick-up was sensitive to changes in temperature, about 40g/:~(" resulting in a change in outpu! voltage of 100 mV,' ('. The drift due to changes in temperature was reduced by placing the piezoelectric pick-up in a box ( 5 0 x l 0 0 x 7 0 m m ) of polystyrene foam fitted tightly to the forehead of the patient by tape (Fig. I ). RESULTS When EEG activit} coincided with the accelerometcr recording, it could be identified as artefact. Sometimes the cause of the artefact could not be established (Fig. 2, /e/i): often it was due to movements of the personnel who were caring for other patients in the intensive care ward (Fig. 2.
551
VIBRATION DETECTOR
Fig. 2. Electrical activity in otherwise fiat EEGs identified as artefacts by their time relation to the accelerometric recording of mechanical changes. Three unconscious, artificially respirated patients: a 20-year-old male with cardiac arrest (left); a 46-year-old male, operated for a subdural h a e m a t o m a (middle); and a 3-year-old boy with respiratory arrest after a febrile convulsion (right). Left:A fiat E E G with trains of 10-11 c/sec activity that turned out to be mechanically induced artefacts, as indicated by records of acceleration in the upper two leads. The cause of this artefact was not identified. Middle and right : Trains of 3-4 c/sec (middle) and 10 c/sec activity (right) in otherwise fiat EEGs. The simultaneous accelerometric records show that the trains were mechanical artefacts, probably induced by movement of the personnel around other patients in the intensive care unit. (In this and following figures: A c c = acceleration: ] cranio-caudal direction; ~-* latero-medial direction; E O G = electro-oculogram; E K G = electrocardiogram; F = frontal; C = central ; T = temporal ; P = parietal ; O = occipital ; L = left ; R = right.)
middle and right). Artefacts caused by artificial respiration were frequent and could appear as slow waves simultaneous with respiratory m o v e m e n t s (Fig. 3) or as faster waves caused by vibration of the tracheotomy tubes (Fig. 4). Occasionally myoclonicjerks were recorded simultaneously as movements and on the E E G ; the paroxysmal activity could then be
verified only when it was still recorded after the patient had been paralysed by s u x a m e t h o n i u m (Fig. 5).
Fig. 4. Records from a 36-year-old female who suddenly lost consciousness during dialysis because of acute anuria. Trains of 8 c/sec activity, mainly in the temporal regions, in an otherwise fiat EEG. The simultaneous accelerometric records indicate mechanical disturbances from the respirator. SUMMARY Fig. 3. Record from a 43-year-old male in hepatic coma. The slow waves in the left fronto-parietal and right centrooccipital leads occur at the same frequency as spontaneous respiration, indicated by the arrows, in the records of acceleration (upper trace) and of rate of movement (second trace).
The use of an accelerometer is described that records mechanical events apt to appear as artefacts in high-gain recording of a flat EEG. By simultaneous recording of acceleration and brain waves the mechanically induced artefacts can be identified and distinguished from cerebral
v. BUCHTHAL et al.
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Fig. 5. EEG and accelerometric records from a 14-year-old girl who had suffered cardiac arrest alter anaphylactic shock. She was unconscious, artificially respirated and had generalized myoclonic seizures during the recording. Left and rioht: The myoclonic jerks were associated with generalized slow waves in the EEG as well as in the direct accelerometric records (upper two traces). Middle: After intravenous administration of suxamethonium there was no activity in the accelerometric records whereas the EEG changes were still present and could thus be identified as paroxysmal discharges of cerebral origin. potentials. The method has proved to be indispensable in establishing brain death. RESUME E N R E G I S T R E M E N T SIMULTANE DE L'ACCELERATION ET DES ONDES CEREBRALES Les auteurs d6crivent l'utilisation d'un acc616rom6tre qui enregistre les 6v6nements m6caniques susceptibles d'apparaitre comme des art6facts au cours d'enregistrements gain 61ev~ d'un EEG plat. Par l'enregistrement simultan6
de l'acc616ration et des ondes c6r6brales, les art~facts induits m~caniquement peuvent ~tre identifiOs et distingu+s des potentiels c~r~braux. Cette mbthode s'est i'6v616e indispensable pour &ablir la mort c~r+brale. NOTE A D D E D IN PRESS Our attention has just been drawn to the aceelerometer (Brtiel and Kjaer, Copenhagen, has the advantages, over the type 4344 that we times greater sensitivity and 5 times lesser dependence.
piezoelectric 4333) which used, of an 8 temperature
TECHNICAL
Electroencephalography and Clinical Neurophysioiogy. 1973, 34:550 552 ~' Elsevier Scientific Publishing Company, A m s t e r d a m Printed in The Netherlands
CONTRIBUTION
S I M U L T A N E O U S R E C O R D I N G OF A C C E L E R A T I O N A N D B R A I N WAVES x F R I T Z B U C H T H A L , KRISTIAN D A H L AND WERNER TROJABORG
Laboratoo' ~! Clinical Neuroph.vsiolo#v, Riffshospitalet, Copenhagen (Denmark~ (Accepted for publication: October 23, 1972)
When electroencephalograms are recorded at high gain from unconscious patients artefacts arising from mechanical disturbances are often difficult to identify. Vibrations of the electrode or its connections are transduced to electrical potentials that may resemble cortical activity. The source of these vibrations may be the respirator, the patient's heart beat or movements of doctors and nurses, especially on intensitive care wards since several patients are supervised at the same time. To recognize when and how this interference occurs we have routinely recorded mechanical vibrations together with the EEG by means of a transducer which responds solely to mechanical changes. To evaluate whether acceleration, rate of movement or displacement by the movement was the best indicator of mechanical artefact, acceleration was recorded directly and after single and double integration. METH OD To establish in what direction movement artefacts were most apt to be picked up, we at first used three small piezoelectric accelerometers mounted on a c o m m o n base (Brtiel and Kjaer, Copenhagen, 4344). However, since movements in the antero-posterior plane also appeared in the lateromedial or cranio-caudal recordings, we used routinely only two accelerometers, one sensitive to latero-medial and the other to cranio-caudal movements. The output from each accelerometer was amplified in a commercial pre-amplifier (Briiel and Kjaer 2623) with additional filters to give a lower limiting frequency of 0.7 c/sec (3 dB down) and an upper limiting frequency of 34 c/sec (3 dB down). The outputs of the two pro-amplifiers were connected to two channels of an electroencephalograph to record acceleration, and then integrated once on the same two channels to record the rate of movement. The resulting lower limiting frequency was 1 c/sec (40 dB) and integration was obtained above a frequency of 2 c/sec. The gain was unity at 5 c/sec. Since measurement of displacement obtained by inteSupported by a grant from the Danish Medical Research Council.
Fig. I. Placement of the box containing the two-directional accelerometer on the forehead of a patient in the mtensiw' care uniL grating twice did not give advantages in identifying mcchanical artefact and was apt to be disturbed by variations in temperature, we recorded routinely only acceleration and rate of movement. A sensitivity of the accelerometer of 2.5 mVg corresponded to an acceleration of 4 mm/sec'/HV. Integrating once (additional amplification 4.7 times) gave rates of movement of 0.02 mm/sec//~V. The piezoelectric pick-up was sensitive to changes in temperature, about 40g/:~(" resulting in a change in outpu! voltage of 100 mV,' ('. The drift due to changes in temperature was reduced by placing the piezoelectric pick-up in a box ( 5 0 x l 0 0 x 7 0 m m ) of polystyrene foam fitted tightly to the forehead of the patient by tape (Fig. I ). RESULTS When EEG activit} coincided with the accelerometcr recording, it could be identified as artefact. Sometimes the cause of the artefact could not be established (Fig. 2, /e/i): often it was due to movements of the personnel who were caring for other patients in the intensive care ward (Fig. 2.
551
VIBRATION DETECTOR
Fig. 2. Electrical activity in otherwise fiat EEGs identified as artefacts by their time relation to the accelerometric recording of mechanical changes. Three unconscious, artificially respirated patients: a 20-year-old male with cardiac arrest (left); a 46-year-old male, operated for a subdural h a e m a t o m a (middle); and a 3-year-old boy with respiratory arrest after a febrile convulsion (right). Left:A fiat E E G with trains of 10-11 c/sec activity that turned out to be mechanically induced artefacts, as indicated by records of acceleration in the upper two leads. The cause of this artefact was not identified. Middle and right : Trains of 3-4 c/sec (middle) and 10 c/sec activity (right) in otherwise fiat EEGs. The simultaneous accelerometric records show that the trains were mechanical artefacts, probably induced by movement of the personnel around other patients in the intensive care unit. (In this and following figures: A c c = acceleration: ] cranio-caudal direction; ~-* latero-medial direction; E O G = electro-oculogram; E K G = electrocardiogram; F = frontal; C = central ; T = temporal ; P = parietal ; O = occipital ; L = left ; R = right.)
middle and right). Artefacts caused by artificial respiration were frequent and could appear as slow waves simultaneous with respiratory m o v e m e n t s (Fig. 3) or as faster waves caused by vibration of the tracheotomy tubes (Fig. 4). Occasionally myoclonicjerks were recorded simultaneously as movements and on the E E G ; the paroxysmal activity could then be
verified only when it was still recorded after the patient had been paralysed by s u x a m e t h o n i u m (Fig. 5).
Fig. 4. Records from a 36-year-old female who suddenly lost consciousness during dialysis because of acute anuria. Trains of 8 c/sec activity, mainly in the temporal regions, in an otherwise fiat EEG. The simultaneous accelerometric records indicate mechanical disturbances from the respirator. SUMMARY Fig. 3. Record from a 43-year-old male in hepatic coma. The slow waves in the left fronto-parietal and right centrooccipital leads occur at the same frequency as spontaneous respiration, indicated by the arrows, in the records of acceleration (upper trace) and of rate of movement (second trace).
The use of an accelerometer is described that records mechanical events apt to appear as artefacts in high-gain recording of a flat EEG. By simultaneous recording of acceleration and brain waves the mechanically induced artefacts can be identified and distinguished from cerebral
v. BUCHTHAL et al.
552
eKe, ',,',a,A~f'~
/%'£
1
~v
,1%,,'~%/%~
~'~
/'"~
i
Fig. 5. EEG and accelerometric records from a 14-year-old girl who had suffered cardiac arrest alter anaphylactic shock. She was unconscious, artificially respirated and had generalized myoclonic seizures during the recording. Left and rioht: The myoclonic jerks were associated with generalized slow waves in the EEG as well as in the direct accelerometric records (upper two traces). Middle: After intravenous administration of suxamethonium there was no activity in the accelerometric records whereas the EEG changes were still present and could thus be identified as paroxysmal discharges of cerebral origin. potentials. The method has proved to be indispensable in establishing brain death. RESUME E N R E G I S T R E M E N T SIMULTANE DE L'ACCELERATION ET DES ONDES CEREBRALES Les auteurs d6crivent l'utilisation d'un acc616rom6tre qui enregistre les 6v6nements m6caniques susceptibles d'apparaitre comme des art6facts au cours d'enregistrements gain 61ev~ d'un EEG plat. Par l'enregistrement simultan6
de l'acc616ration et des ondes c6r6brales, les art~facts induits m~caniquement peuvent ~tre identifiOs et distingu+s des potentiels c~r~braux. Cette mbthode s'est i'6v616e indispensable pour &ablir la mort c~r+brale. NOTE A D D E D IN PRESS Our attention has just been drawn to the aceelerometer (Brtiel and Kjaer, Copenhagen, has the advantages, over the type 4344 that we times greater sensitivity and 5 times lesser dependence.
piezoelectric 4333) which used, of an 8 temperature