Diagnostic methods, where the electrical response to an electrical stimulus is recorded, provoke charging processes in the electrode and tissue interface. In the recorded signal the stimulation artifact normally strongly dominates in relation to the electrical stimulation response. If the response signal and the artifact overlap, the response signal is lost due to overdrive of the recording amplifier. This especially occurs during Intraoperative Motor Evoked Potentials in brachial plexus surgery and scoliosis surgery, where the stimulation amplitude may reach several hundreds volts: the ratio of the stimulation amplitude and the evoked, efferent motor Electroneurogram (ENG) is in the range of 10⁸ together with a short latency ranging between 2 and 4ms. Similar problems occur during intraoperative localization of chronic facial nerve lesions, where the stimulation site (skull) and the recording site (facial nerve trunk) are especially close to each other, with latency down to 1ms.

Considering the fact that every rising edge of the stimulation impulse leads to a charging current and that every falling edge leads to a discharging current in the capacitive part of the load impedance, the two edges of a monophasic stimulation impulse produce a slowly decreasing discharge current through the tissue after the second (falling) edge. In order to minimize the remaining charge at the end of the impulse, we used a biphasic stimulation impulse with asymmetric length of the phases. By adjusting the ratio of the durations of the first and second phase, the remaining discharge current can theoretically be driven towards zero.

Reducing the electrode-tissue interface to an equivalent network of a resistor, the phase duration ratio can be calculated as described in the methods part. The resistor represents the tissue which is connected serially to a parallel circuit of a capacitor and a resistor, both representing the electrode-skin junction. This ratio depends on the time constant t of the equivalent network, which is in the range of 0.5 to 2ms, and the duration of the first stimulation phase T₁ that is normally used in the range of 50µs to 500µs.

For example: with a pulse width of the first phase of 50µs, the percentage of the second phase T₂ of the stimulation impulse needs to be 95% to optimally minimize the stimulus artifact at a time constant of 1ms and with a pulse width of 500µs this percentage needs to be 65%. This shows that longer time constants need shorter second phases for optimal compensation and vice versa. The theoretical findings were confirmed by series of animal and clinical experiments.

STATE OF THE ART

Intraoperative Electroneurodiagnostic (IOE) /1/ with high voltage (above 100V) stimulus often lead to technical problems if latencies of the recorded ENG are very small, i.e. between 1 and 4ms. This is the case with intraoperative MEP in brachial plexus surgery, scoliosis surgery and transcranial electrical stimulation (TES) of the facial nerve. Following electrical stimulation of the motoric cortex, the evoked efferent motor nerve action potential is recorded from the surface of the exposed peripheral nerve in order to conclude from the potential’s shape to the function of the nerve.

Electrical stimulation is applied by transcutaneous needle electrodes. The stimulus is a single, constant voltage, rectangular monophasic pulse up to 750V in amplitude and 100µs in duration (Digitimer 180) or up to 1000V and 50µs (Digitimer 185), respectively /2/. In spite of this short stimulus duration, the stimulation artifact will overdrive the recording amplifier and the evoked Electroneurogram (ENG) with a delay down to 1ms cannot be measured. One reason of the above mentioned is the recovery time of the instrumentation amplifier, and the second is the discharging process for charge balance of the load impedance. If fast recovery amplifiers are used, only discharging dominates overdriving the amplifier after the second (falling) edge of the monophasic pulse.

Biphasic stimulation pulses are well known in Functional Electric Stimulation (FES) /3/ for charge balancing over trains of pulses to avoid electrode and tissue damage. Usually rectangular biphasic stimulation pulses with up to 100V in amplitude and up to equally 1ms in duration of first and second phase are used to activate peripheral nerve structures via surface electrodes for causing muscle contraction. For IOE it is necessary to activate the central nervous system like the motoric cortex or the spinal root, respectively. Charge balance is not optimally provided by equally spaced phase duration, especially for higher stimulation amplitudes, which are needed to reach the nervous structures under bone tissue. The goal is to find the ratio between the first and the second phase of the biphasic rectangular stimulation pulse to get an optimally charge balanced situation immediately after the stimulus.

MATERIAL AND METHODS

Amplifier and Stimulator

The IOE-System consists of two computer controlled modules, a single-channel electrical stimulator and a six-channel ENG/EMG-recording unit /4/. The maximum amplitude of a single biphasic stimulation pulse is 1000V per phase at 50µs duration. The duration of the first phase is adjustable from 50µs to 1ms and the second phase can be tuned from 0% to 100% of the first (i.e. from monophasic to symmetric biphasic) due to optimal compensation of the stimulus artifact.

The amplifier consists of three parts. The first is a shielded part (i.e. the preamplifier) and has an amplification of 20. The second part consists of an isolation amplifier with two fixed amplifications, one for EMG- and the other for ENG-signals, 5 and 500, respectively. The third part is a multifunction I/O-PCMCIA-card for A/D-conversation of 12bit and an amplification of 1 to 100. Thus the amplification range is adjustable from 10² up to 10⁶.

Theory of Compensation

The load impedance, including tissue, skin and electrodes, can simply approximated by an equivalent network of a resistor R_G, representing the tissue, serially connected to a parallel circuit of a capacitor C_E and a resistor R_E, both representing the electrode-skin junction (Fig. 1a). Rectangular, constant voltage stimulation pulses produce a current flow over the electrodes and the tissue. In Fig. 1b three different pulses applied to the equivalent circuit are compared, where the amplitude of the current can be scaled because of independency regarding to minimize the artifact. Thus the monophasic pulse produces 20% of the maximum amplitude after its falling edge and has an exponential decay with a time constant of 1ms afterwards. The symmetric (100%) biphasic pulse leads to amplitude of 8% after its final rising edge, which is lower than the 12% of the monophasic at this point of time (1ms). The biphasic pulse with different length of phase 1 (100%) and phase 2 (67%) results in amplitude at almost 0% of the maximum current amplitude. The optimized ratio of the second phase T₂/T₁ depends on the pulse width of phase 1 T₁ and the time constant t, which results from the equivalent circuit in Fig. 1a and can be expressed as

Equ. 1

This means that for a pulse width of the first phase of 500µs a pulse width of the second phase of 335µs (67% of the first) is the optimal duration for minimizing stimulation artifact.

Fig. 1: a) Equivalent stimulation circuit with a time constant of 1ms and b) corresponding scaled stimulation current, calculated for rectangular (0.5ms), monophasic as well as 67% and 100% biphasic stimulation pulse of constant voltage U_ST.

RESULTS

An example in Fig. 2a points out the difference between three stimulation signals with amplitude of 500V and a pulse duration of 100µs, where first is monophasic, second and third are biphasic symmetric (100%) as well as asymmetric (90%). Due to a high pass filter, implemented in the recording amplifier, the recorded signals do not have a DC‑part and the shape look different to the calculated shape in Fig. 1b.

Fig. 2: a) ENG-recordings, corresponding to a time constant of about 0.9ms and a stimulation with an amplitude of 500V and a pulse duration of 100µs for monophasic, biphasic 100% and biphasic 90% pulses, where biphasic 90% has minimum artifact within 0.5ms after stimulation onset; b) percentage of T₂ over T₁ for minimized artifact depending on time constant t; calculated for 0.5ms, 1ms and 2ms (solid lines) and recorded for 0.7ms (squares), 0.9ms (dots) and 1.4ms (crosses).

Also the monophasic stimulation pulse overdrives the amplifier for about 1.5ms, which means that no ENG-signal can be recorded in‑between 1.5ms after the onset of the stimulation pulse. The recorded signal after the 100% biphasic pulse shows no overdriving of the amplifier, but it takes about 2ms of discharging exponential decay to reach an amplitude in the range of the amplitude of the ENG-signal itself (20µV). Minimized artifact, even at 0.5ms after stimulation onset, occurs after a asymmetric biphasic stimulation pulse with duration of the second phase of 90% from the first. Recordings confirm to the theory of Equ. 1, which is shown in Fig. 2b. The continuous lines are drawn for the calculated ratio of T₂/T₁ over T₁ to minimize stimulation artifact at time constants of 0.5, 1 and 2ms. The squares, dots and crosses represent the percentage of T₂ at time constants of about 0.7, 0.9 and 1.4ms for recordings with minimized stimulation artifact.

DISCUSSION

In the course of IOE, where the electrical response to an electrical stimulus is recorded, charging processes in the electrode and tissue interface are provoked. The results show (Fig. 2a), that the discharging processes in the recorded signal can be shortened in time due to different pulse shape of the stimulation signal. A monophasic rectangular pulse of constant voltage has two disadvantages. First the recording amplifier is overdriven for several miliseconds and second the discharging exponential decay starts at high amplitude. This means, that lower amplification is required and the ENG-signal is very small or even cannot be detected. In comparison, biphasic stimulation signals do not (or only for some microseconds) overdrive the recording amplifier and the exponential decay starts at lower amplitude. This starting point of the discharging exponential decay can be adjusted by appropriate discharging due to vary the duration of the second phase of the biphasic stimulation pulse. The duration of the second phase for minimizing stimulation artifact is dependent on the duration of the first phase. For this reason, longer pulse width require shorter width of second phase for minimizing stimulation artifact and vice versa.

The required duration of the second phase can be predicted by Equ. 1, if the time constant of stimulation circuit is known. But due to noise and the essential filtering of the recording system, the time constant cannot be determined exactly. In some cases, where optimally minimized artifact is required caused by very short delay of ENG-signal, the duration of the second phase has to be adjusted manually. In most other cases, minimizing of stimulation artifact can be done automatically after determining of the time constant of the stimulation circuit.

REFERENCES

/1/ Turkof E., Millesi H., Pfundner P., Mayr N., Intraoperative Electroneurodiagnostics (Transkranial Electrical Motor Evoked Potentials) to Evaluate the Functional Status of Anterior Spinal Roots and Spinal Nerves During Plexus Surgery, Plastic and Reconstruction Surgery, 1997, 99(6): 1632-1641

/2/ Turkof E., Tambwekar S., Kamal S., El-Dahrawi M., Mansukhani K., Soliman H., Ciovica R., Mayr N., Leprosy Affects Facial Nerves at the Main Trunk and Neurolysis Can Possibly Avoid Transfere Procedure, Plastic and Reconstruction Surgery, 1998, 102(5):1565-1573

/3/ Kralj A. and Bajd T., Functional Electrical Stimulation: Standing and Walking after Spinal Cord Injury, 1989, CRC Press, Inc.

/4/ Reichel M., Bijak M., Mayr W., Lanmüller H., Rafolt D., Rakos M., Sauermann S. , Unger E., Turkof E., Mobile PC-System for Intraoperative Electroneurodiagnostics, 7th Vienna International Workshop on Functional Electrical Stimulation, Vienna 2001, Proceedings, 2001, currently in press

ACKNOWLEDGEMENTS

Supported by the Austrian National Bank. Projects 6946, 7937 and 8661.

SUMMARY