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METHOD AND APPARATUS FOR
MEASURING STIMULUS-EVOKED
POTENTIALS OF THE BRAIN
This application claims the benefit of Provisional appli- 5 cation No. 60/020,135, filed Jun. 20, 1996.
This invention pertains to a method and a test assembly for measurement and display of stimulus-evoked potentials of the brain, especially for monitoring analgesia and anesthetic depth. 10
During surgery it has to be assured that the patient will not wake up from general anesthesia and especially that pains during surgery or other surgical manipulations will neither be perceived during surgery nor remembered postsurgically by the patient. The fright caused by an experience 15 like this may result in a so-called post-traumatic stress syndrome. For this reason, it is of great interest to measure and display suitable parameters which determine anesthetic depth and analgesia to enable anesthetists to control anesthesia more precisely than previously and to reduce patient 20 strain to a minimum. In particular, undesired awakenings of the patient during anesthesia have to be realized as early as possible.
We know different methods to detect wake stages during general anesthesia. The most important (1) are the so-called 25 PRST-score, calculated from changes of blood pressure, heart rate, sweating and tear production, and (2) the isolated forearm method, during which one of the patient's forearms is isolated against anesthesia by interrupting the blood flow by means of, for example, a hemomanometer cuff. To 30 monitor conciousness the patient will then be examined whether he is able to adhere to simple commands during surgery. Furthermore, an EEG processing method is known, which evaluates EEG frequency and amplitude changes occurring during wake and anesthetic stages. However, the 35 PRST score is not always a reliable method to detect intraoperative wake stages. The isolated forearm method can only be applied over a short period of time and is consequently not suited to indicate motor responses of the patient during long-lasting procedures. The processed EEG and its 40 resulting parameters (median frequency and spectral cut-off frequency) are not optimally suited for this purpose, too.
Stimulus-evoked EEG signals, such as visually evoked potentials, somato-sensory evoked potentials and auditory evoked potentials, which will undergo dose-dependent sup- 45 pression during general anesthesia, are better suited to fulfill this task. With midlatency auditory evoked potentials this dose-dependent effect becomes especially obvious. Auditory evoked potentials consist of a series of positive and negative voltages generated at different sites along the auditory 50 pathway which can be picked up by electrodes at the skull. They reflect collection, transmission and processing of acoustic information from the cochlea via the brain stem to the cerebral cortex. Early auditory evoked potentials are generated by structures of the peripheral auditory pathway 55 and the brain stem. They give evidence of stimulus transduction and primary stimulus transmission. It is known that early auditory evoked potentials remain almost stable during anesthesia in contrast to dose-dependent suppressed midlatency auditory evoked potentials. 60
In general, stimulus evoked potentials are well suited for monitoring anesthetic depth as well as for recording further neurophysiological functions. Change of the time course of the measured potentials in comparison to the unchanged potentials make it possible to draw conclusions for the 65 neurophysiologic function to be observed. Previously, it has been difficult to apply this method practically. This was due
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to the fact that there were no satisfying possibilities for measuring and evaluating the potentials, especially when the measurement could not be performed under laboratory conditions but, for example, in the operating room. Under suboptimal conditions many difficulties may occur. Maybe the staff is not familiar with recording of evoked potentials or the anesthetist does not know how to interpret the recorded curves. It needs experienced experts to assess the results and to operate the equipment mentioned. Furthermore, the electrodes have to be applied simply and quickly. Other problems may occur by interference due to specific instruments in the operation room and by signals of other equipment induced into the electrodes. Besides, attending measures on the patient may give rise to movement artifacts.
Amplifier circuits, normally used for recording cerebral potentials, may be a further source of problems. They normally consist of an instrumentation amplifier, which transfers the incoming signal (typically 0.5 to 10 fiV) via a high-pass filter, a mid-amplifier, a low-pass filter and a post-amplifier to an A/D converter coupled with an evaluation unit. Patient and analysis unit must be isolated from each other by, for example, an isolation amplifier, type BF or CF, to prevent inadmissible currents from passing through the electrodes if external voltages are applied.
In known circuits, this isolation unit is located either after the first amplifier stage, i.e. after a gain between 10 and less than 100, or after total gain of up to 106 or after the A/D converter. This results in various disadvantages. In the first case extremely small signals pass through lines which impose a capacitive load on the driving amplifier and consequently may give rise to signal distortion. Furthermore, interference decoupling is limited, even with low driver impedance. However, if isolation is intended to be introduced after total gain, decoupling of interference will be good; indeed, the required number of components does not allow for the desired miniaturization of the circuit. Consequently, the distances to the recording sites are longer than desired for optimal recording. Besides, coupling of the output signals into the input circuit can not definitely be avoided in small sized amplifiers with high gains (from about 5000 onwards). Gains, as high as those required for measurement of evoked potentials, may cause a feedback from the amplifier output to its input, either via direct capacitive coupling or via the supply voltage. This feedback is especially disturbing if the terminating impedance of the input amplifier is high or asymmetrical. A feedback loop will change the frequency vs. time behavior of the amplifier system so that instabilities may occur which may give rise to amplifier oscillations. This is aggravated by the fact that these effects will often only give rise to minimal changes of the entire amplified signal, but will have considerable effect on the portion to be measured, which is "hidden" in the entire signal. For example, the brainstem auditory evoked potentials (BAEP) having a signal amplitude of 1 fiVss cannot be recognized in the spontaneous EEG, which has an amplitude of 20 fiVss to 50 fiVss. The BAEP can best be measured by means of averaging, provided that the spontaneous portion of the EEG averaged over time does not correlate with BAEP and that the BAEP portions occur time-synchronous after the stimulation signal. This cannot be assured, if the time behavior of the amplifier is subject to change or non-linearities due to intermodulation distortion occur. The recording signals are distorted or disappear during averaging. Another side-effect is that common mode rejection deteriorates due to positive feedback on the input. Furthermore, the system can encounter an unfavorable
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