Improved Amplitude of Myogenic Motor Evoked Responses after Paired Transcranial Electrical Stimulation during Sufentanil/Nitrous Oxide Anesthesia

Nine patients undergoing spinal surgery and two patients undergoing thoracic aortic aneurysm repair gave informed consent to participate in this institutionally approved study. The neurologic status of all patients was normal. The patients received diazepam, 10 mg orally, 1 h before surgery. Anesthesia was induced with etomidate 0.3 mg/kg and sufentanil 1.5 micro gram/kg and was maintained with sufentanil 0.5 micro gram *symbol* kg sup -1 *symbol* h sup -1 and N²O 50%. When there were clinical signs that the level of anesthesia was light, ketamine 0.3-0.5 mg/kg was administered intravenously. Muscle relaxation was monitored electromyographically at the hypothenar eminence with a Relaxograph (Datex, Finland), and the amplitude of the single-twitch response was maintained at 25% of control with vecuronium with a closed-loop infusion system. Monitoring included the electrocardiogram, hemoglobin blood Oxygen sub 2 saturation by pulse oximetry, central venous pressure, invasive arterial blood pressure, end-tidal CO²concentration, and nasopharyngeal temperature. Figure 1shows the apparatus used to record tc-MERs to single and paired TCS. Two identical transcranial electrical stimulators (D180A, Digitimer, Welwyn Garden City, UK) were used. The stimuli from both units were delivered to the scalp by two 9-mm silver electroencephalographic disc electrodes, attached to the skin with collodion, with the anode positioned at C^z[18]and the cathode at F^z(International 10-20 system). The units were triggered either simultaneously or sequentially. The ISI could be varied between 0 (single pulse) and 10 ms. Myogenic responses were recorded from the skin over the left and right tibialis anterior muscles with adhesive gel Ag-AgCl electrodes (Cleartrace, Medtronic Andover Medical, Haverhill, MA); the active electrode was placed over the muscle belly, referenced to an electrode placed over the muscle tendon. A ground electrode was placed on the left leg, proximal to the knee. The signal was amplified 5,000-20,000 times (adjusted to obtain maximum vertical resolution), and filtered between 30 and 1,500 Hz with a biologic amplifier (3T PS-800, Twente Technology Transfer, Twente, The Netherlands). These amplifiers have an extremely high-input impedance (> 10¹²Ohm), and the common mode rejection ratio is greater than 95 dB. The responses were displayed and stored on a Macintosh Quadra computer (Apple Computer, Cupertino, CA) with 12-bit analog-to-digital conversion and motor evoked response (MER) acquisition software written with the LabView data acquisition development system (National Instruments, Austin, TX).

After achieving a stable anesthetic state, at least 20 min after induction of anesthesia, stimulus intensity (0-100%, [nearly equal] 0-1,200 V) was adjusted to achieve maximal responses with single-pulse stimulation, typically 600-700 V. At least 20 min after skin incision, but before any surgical interventions that might have resulted in impaired spinal cord functioning, quadruplicate CMAPs in response to single and paired TCS were recorded. Responses to paired stimulation were acquired every 2 min, while ISI was increased from 1 to 2, 3, 5, 7, and 10 ms. The effect of paired stimulation was also assessed after reducing the stimulus intensity to a level that elicited threshold responses to a single transcranial stimulus.

Peak-to-peak amplitudes and onset latency, as measured from the beginning of the first pulse, were determined from the average of the four individual responses. tc-MER latencies were normally distributed and are expressed as mean plus/minus SD. The coefficient of variation was calculated for the amplitudes of four consecutive single-sweep tc-MERs acquired with single or paired (ISI 3 ms) stimulation. Because tc-MER amplitude data did not appear to be normally distributed, amplitudes are presented as medians, with the 10th and 90th percentiles. Differences in amplitude and latency between single and paired stimulation were compared using Wilcoxon's signed-rank test.

Patient characteristics are presented in Table 1. Single-pulse TCS elicited tc-MERs in all but one patient. Large interpatient amplitude variability was observed. The median amplitude of the right tibialis anterior muscle response was 106 (23-1,042) micro Volt, and the onset latency was 33.2 plus/minus 1.4 ms. With paired TCS (ISI 2-3 ms), median tc-MER amplitude increased to 285 (79-1,605) micro Volt or 269% of the single-pulse response (P < 0.01) (Figure 2). With single-pulse stimulation the coefficient of variation for the amplitude of four consecutive responses within an individual patient was 43%. With paired stimulation, with an ISI of 3 ms, the coefficient of variation was 17%. When ISI was increased to 5 or 7 ms, no further augmentation occurred. An ISI of 10 ms often elicited two overlapping responses of lower amplitude.

Onset latency decreased from 33.2 plus/minus 1.4 to 31.4 plus/minus 3.2 ms (P < 0.05) for paired (ISI 3 ms) versus single TCS respectively. When stimulus intensity was reduced to a level that elicited a threshold response with single stimulation, the amplitude-augmenting effect of paired stimulation became more pronounced. Similarly, in patients in whom maximal single-pulse stimulation elicited only low-amplitude responses, the effect of paired stimulation was more pronounced than in patients who had high-amplitude responses to single-pulse TCS (Figure 3). Although not specifically studied, paired stimulation appeared to decrease the stimulus intensity needed to elicit a detectable response. One patient had only one detectable response to four separate single stimuli. With paired stimulation and an ISI of 2-5 ms, responses of 150-350 micro Volt could be recorded, whereas no facilitation was obtained when ISI was increased to 10 ms (Figure 4).

The data derived in the current study indicate that application of paired transcranial stimuli, with an ISI of 2-3 ms, in N²O-sufentanil-anesthetized patients results in increase in tc-MER amplitude just as in awake patients. Our findings suggest that paired stimulation may be preferable in terms of MER amplitudes and reproducibility to the more commonly used single-pulse TCS paradigms for intraoperative monitoring.

It is unknown whether facilitation by paired TCS occurs predominantly at the cortical or spinal level, and our data do not allow differentiation between effects at these two sites. However, there is evidence that at least a significant component of the facilitation occurs at the spinal level. Taylor et al. applied single or paired constant-voltage stimuli to the thoracic spinal cord with an epidural electrode in patients anesthetized with propofol and fentanyl. [19]Single-pulse stimulation failed to elicit responses with stimulus voltages up to 125 V, whereas paired stimulation with an ISI of 2-5 ms produced maximal responses (20-30 micro Volt). The responses gradually became smaller as ISI was increased to 10 ms.

It is also possible that paired TCS alters the pattern of efferent activity in the descending motor pathways. That pattern is, in general, characterized by an initial direct wave followed by a series of indirect waves. Multiple indirect waves can occur as the result of repetitive transsynaptic activation in the motor cortex [20]and, accordingly, it is possible that paired stimulation increases the number of indirect waves. Epidural recordings have shown that at least one anesthetic, isoflurane, decreases the number of indirect waves after a single transcranial electrical stimulus, whereas the initial direct wave is unaffected. [21]Paired stimulation may either increase the number of cortical motor neurons firing, increase the number of indirect waves travelling down the spinal cord, or both. Therefore, it is at least possible that paired stimulation produces facilitation at both the cortical and the spinal level.

A more likely explanation for the facilitation of tc-MERs by paired TCS is that the first stimulus lowers the excitation threshold of the cortical and spinal motor neurons, thereby facilitating the initiation of neuronal discharge by the second stimulus. This phenomenon is known as temporal summation. Each time a neuronal terminal depolarizes, sodium channels open for a period of 1-2 ms. After closure of the channels, the resulting excitatory postsynaptic potential decreases over the next 10-15 ms. A second opening of the same channels within this period will result in an augmentation (temporal summation) of the excitatory postsynaptic potential. [22]The more rapid the rate of repetitive depolarization, the greater the postsynaptic potential that develops. The counterpart of temporal summation is spatial summation, which is the summation of excitatory postsynaptic potentials from several synaptic terminals converging on one motor neuron. If paired TCS increases the number of cortical motor neurons firing then, in addition, spatial summation may occur at the spinal level. The occurrence of these phenomena, spatial or temporal summation, has not been demonstrated in response to paired TCS, however, there is sufficient evidence obtained in other circumstances to suspect its occurrence.

In the current study we found maximal response augmentation with ISIs between 2 and 5 ms. Application of the second stimulus within the first 1.5 ms was less effective, perhaps because the membrane channels are still open. Because the sodium channels close 1-2 ms after stimulus and the excitatory postsynaptic potential generated by a single synapse thereafter decays, it might be predicted that the optimal frequency for obtaining facilitation would occur with an ISI in the vicinity of 2 ms. [16,17,19]Our findings were consistent with that prediction.

The instrumentation available for the current investigation provided the capacity for the delivery of only two successive stimuli. It is conceivable that stimulation with more than two successive pulses would further increase tc-MER amplitudes. Nadstawek et al., using conventional constant current stimulators reported that at least three successive pulses of 60 mA with an ISI of 2 ms were required to obtain recordable responses (40-60 micro Volt) during propofol-alfentanil total intravenous anesthesia. [23]Manufacturers of electrical and magnetic transcranial stimulators are currently developing stimulators that will include the option of performing multiple pulse stimulation. When these stimulators become available it will become possible to determine optimal multiple pulse stimulation paradigms. It should kept in mind, however, that multiple pulse stimulation increases the total energy delivered. Single and dual stimulation appear to be well tolerated, and dual stimulation need not necessarily result in a doubling of the net charge delivered if responses can be obtained at lower stimulus intensities. However, with multiple stimulus paradigms, both total energy delivered and duration of stimulus will increase. These protocols should be explored carefully with respect for the possibility of the epileptogenesis or direct neuronal injury that have not thus far been observed with single or dual stimuli.

We chose to evaluate latency to onset rather than latency to specific peaks because CMAPs do not have consistent, characteristic morphologic features and may exhibit significant variation both within and between patients. Onset latency appeared to decrease slightly with the application of paired stimuli in our investigation. However, we feel that there were significant limitations in our capacity to determine latency. Determination of onset latency for low-amplitude responses (which constituted many of the responses to single stimuli) was sometimes difficult. The slope of the initial CMAP deflection was occasionally sufficiently gradual that identification of the precise moment of "onset" may have been unreliable. The greater amplitudes of responses to paired stimuli, with the concomitantly more rapid deviation from baseline, may have introduced a bias toward shorter apparent latencies. A mathematical definition of onset latency (e.g., a greater than 2-SD deflection from the average baseline noise level) would aid in uniform determination of CMAP onset and facilitate comparison among published results.

Our data suggest that the relative amplitude increase associated with paired stimulation is dependent on the initial amplitude of the response to single stimulation. The smaller the initial response to a single transcranial stimulus, the greater the effect of paired stimulation. This is in agreement with the results of Inghilleri et al., [17]who observed that the increase of the abductor pollicis brevis MER after paired TCS in awake subjects was inversely correlated with the amplitude of the control response. In our study, paired TCS had only a minor effect in patients who had high-amplitude (> 300 micro Volt) tc-MERs in response to single-pulse TCS. Motoneuronal firing is a quantal response, and therefore tc-MER amplitudes are directly proportional to the number of motor neurons firing. If single-pulse TCS resulted in firing of all tibialis anterior muscle motor units in some of our patients, further augmentation with the application of a second stimulus would not be expected.

In conclusion, we have demonstrated that application of paired transcranial electrical stimuli significantly increases amplitudes of intraoperative MERs during anesthetic-induced depression of the motor system. The results of this study justify further clinical evaluation of the efficacy and safety of double-pulse TCS as an adjunct to monitoring during surgical procedures that place motor pathways at risk.