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Department of Anesthesiology, Hôpital Maisonneuve-Rosemont and Université de Montréal, Québec, Canada
Address correspondence and reprint requests to François Donati, PhD, MD, Department of Anesthesiology, Hôpital Maisonneuve-Rosemont, 5415, boul. l'Asssomption, Montréal, Québec, Canada H1T 2M4. Address e-mail to francois.donati{at}umontreal.ca.
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Abstract |
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Introduction |
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One possible reason for the failure of observers to detect fade is movement of many muscles of the hand and forearm after stimulation of the ulnar nerve at the wrist. The individual assessing the contraction of the thumb might be distracted by simultaneous movement of the wrist or hypothenar eminence. Recently, it was found that stimulation over the adductor pollicis in the hand yielded similar acceleromyographic results to stimulation of the ulnar nerve at the wrist, without significant movement of digits other than the thumb (11). The ability to detect fade was not evaluated in that study.
Therefore, the purpose of the present study was to evaluate, in the same patient, all the currently available tests: the TOF ratio obtained with acceleromyography and the tactile evaluation of TOF, DBS, 50-Hz tetanic, and 100-Hz tetanic fade. These tests were compared with the mechanomyographic TOF ratio under sevoflurane anesthesia. Stimulation electrodes were applied either over the ulnar nerve at the wrist or over the adductor pollicis on both sides of the hand to evaluate the influence of this new site of stimulation on the performance of the different tests.
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Methods |
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32 kg/m2, pregnancy, being on medication that influences neuromuscular function, or a history of allergic reaction to drugs used in the study. Monitoring included electrocardiography, noninvasive arterial blood pressure, pulse oximetry, capnography, and a temperature probe. Anesthesia was induced with 2–3 µg/kg of fentanyl, 2–2.5 mg/kg of propofol, and 0.6 mg/kg of rocuronium, and tracheal intubation was performed. Anesthesia was maintained with sevoflurane (1.5%–3% expired fraction) with air/oxygen (50%/50%), and intermittent doses of fentanyl (1–2 µg/kg). Additional doses of rocuronium 0.2 mg/kg were given if required. Central temperature was maintained to more than 35°C using a warming blanket covering the upper body and both arms. The patients' lungs were ventilated, and the end-tidal partial pressure for carbon-dioxide (Pco2) was maintained between 32 and 36 mm Hg.
Mechanomyography of the adductor pollicis was applied at random to the patient's dominant or nondominant hand, whereas the other upper extremity was reserved to test the various stimulation modalities. The hand and forearm used for mechanomyography were immobilized firmly. A TOF-Watch SX® nerve stimulator (Organon Ltd., Swords, Ireland) was used for supramaximal TOF stimulation (4 pulses of 0.2 ms in duration at a frequency of 2 Hz every 15 s) on the ulnar nerve at the wrist. The forearm and hand were immobilized, and the isometric force of contraction of the adductor pollicis was measured with a Cobe (Maxxim Medical, Athens, TX) transducer and recorded on paper. The ratio of the fourth to the first twitch (mechanomyographic TOF ratio) was measured.
On the contralateral extremity, the position of the stimulating electrodes was randomized to either the wrist, over the ulnar nerve, or the hand, on the skin overlying the dorsal and palmar aspects of the adductor pollicis (Fig. 1) (11). The hand and the fingers were immobilized with tape, leaving only the thumb free. During recovery, when the mechanomyographic TOF ratio had recovered to at least 0.2, the following occurred:
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The observers were anesthesiologists, anesthesia residents, and anesthesia assistants who were familiar with neuromuscular monitoring and used it frequently.
For each series of measurements, the acceleromyographic TOF ratio was compared with the corresponding mechanomyographic TOF ratio. Also, the presence or absence of detected fade was correlated with the mechanomyographic TOF ratio. The acceleromyographic fade was considered to be present whenever the TOF ratio was <1.0 because acceleromyographic TOF ratio often exceeds corresponding mechanomyographic values, and a threshold of 1.0 is recommended for acceleromyography to exclude residual paralysis (12). In addition, threshold for fade detection, defined as the highest mechanomyographic TOF ratio at which fade was last detected, was obtained for each patient and for each mode of stimulation. Logistic regression analysis (7) was used to determine the relationship between probability of fade detection and actual mechanomyographic TOF ratio.
Patient characteristics were compared using the Student's t-test and were expressed as mean ± sd and range; P < 0.05 was considered to indicate statistically significant differences. To investigate the agreement between mechanomyography and acceleromyography in the assessment of neuromuscular recovery, a Bland and Altman analysis was performed (13). For each test, the median ± quartile was calculated for the threshold of disappearance of the fade.
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Results |
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A total of 339 assessments were made by 10 anesthesiologists, 6 anesthesia residents, and 11 anesthesia assistants. The percent of patients in whom fade was detected for each clinical test as a function of mechanomyographic TOF ratio is shown in Table 2.
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There was good correlation between acceleromyographic and mechanomyographic TOF ratios, and the agreement was better with stimulation in the hand than at the wrist. When the ulnar nerve was stimulated at the wrist, bias was 5.3%, with 95% confidence limits between –21% and 31%. With stimulation at the hand, bias was less (0.5%; P < 0.001 compared with wrist stimulation), with 95% confidence limits between –19.8% and 20.7% (Fig. 2). There were fewer instances of acceleromyographic TOF ratios more than 1.0 when hand stimulation was used (Fig. 3). The acceleromyographic TOF ratio exceeded 1.0 at the last measurement in 13 of 32 patients. Of these 13, 9 had stimulation at the wrist, whereas 4 had stimulation at the hand.
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The mean mechanomyographic TOF ratio when acceleromyographic TOF ratio first exceeded 1.0 was 0.89 ± 0.06. This threshold did not depend on the site of stimulation but was more variable with stimulation at the wrist (Fig. 4). The tactile TOF fade became undetectable at a mean mechanomyographic TOF ratio of 0.31 ± 0.15. The corresponding threshold was significantly higher for DBS fade (0.76 ± 0.11). Tetanic stimulation at 50 Hz and 100 Hz demonstrated no fade at a mechanomyographic TOF ratio of 0.31 ± 0.15 and 0.88 ± 0.18, respectively. For 50-Hz tetanus, 14 of the 32 patients did not show any fade at the first assessment (0.13–0.4). For 100-Hz tetanus, 21 of the 32 patients still had a fade at the last assessment when the mechanomyographic TOF ratio was 0.87. For each of these tests, the mechanomyographic threshold at which fade became undetectable was similar whether stimulation was at the wrist or the hand (Fig. 4).
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The probability of detecting fade by tactile means after TOF, DBS, 50-Hz tetanus, and 100-Hz tetanus at a mechanomyographic TOF ratio of 0.9, derived from the logistic regression analysis, was 1%, 11%, 3%, and 66%, respectively (Fig. 5). If acceleromyographic fade is defined as a TOF value <1.0, then the probability to detect a mechanomyographic TOF ratio of 0.9 was 72%.
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Discussion |
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Tactile detection of fade is no better with stimulation in the hand than with stimulation at the wrist, irrespective of the stimulation mode used. Stimulation at the hand was proposed as an alternative to stimulating at the wrist and was shown not to produce direct muscle stimulation (11). Most likely, small terminal branches of the nerve are stimulated preferentially because current intensity and duration are insufficient to cause direct muscle stimulation. When electrical stimulation is applied directly over the hand, there is less movement of digits other than the thumb (11). This implies that the reason why fade may be missed with TOF stimulation is not the distraction produced by the movement of other fingers. However, the acceleromyographic TOF ratio exceeded 1.0 less often with hand stimulation than with wrist stimulation, which indicates that stimulation over the hand might provide more reliable acceleromyographic values during recovery than stimulation over the wrist.
In the present study, tactile evaluation of fade and acceleromyographic TOF were compared with the mechanomyographic value obtained in the other extremity. The assumption here is that neuromuscular blockade was the same in both extremities. Several studies have used a similar design, and there is no evidence that, on average, there is a difference in response between extremities because of the position of the IV access, hand dominance, or other factors (14,15). However, it is possible that some of the discrepancy measured between mechanomyographic and acceleromyographic TOF ratio might have been due to individual differences between arms. Measuring mechanomyography in the same extremity as acceleromyographic and assessment of fade is impractical because it would require setting up and dismantling mechanomyographic equipment frequently.
We chose to apply either 50-Hz or 100-Hz tetanus whenever a tactile evaluation was performed, but not both, because 2 consecutive tetanic stimulations could affect the result of the second tetanus. The time interval chosen between 2 consecutive measurements (3–10 min) is more than that required for the neuromuscular function to return to its pretetanic value (1–2 min) (16,17).
The individuals who were asked to evaluate fade were anesthesiologists, anesthesia technicians, or residents. All of them used neuromuscular monitoring routinely, and most were aware of the limitations of tactile detection of fade. Thresholds for detection of TOF and DBS fade in this study are consistent with previous investigations. On average, TOF fade was no longer detected when mechanomyographic TOF ratio exceeded 0.31. This compares well with a threshold of 
0.4 in 25 of 37 patients (68%) in Drenck et al.'s study (5). Tactile detection of fade when the measured mechanomyographic TOF ratio was 0.31–0.4 was reported to be 25%–80% (4,5,18–21). It was 35% in the present study. In the range of 0.41–0.5, fade was reported in only 24%–40% of cases (5,18–21) compared with 22% here. It seems that visual and tactile detections yield similar results (22). The threshold for DBS fade seems to be marginally higher in the present study (0.71) than in previous investigations. Drenck et al. (5) reported that for most patients, DBS fade threshold was in the 0.51–0.6 range. A DBS fade was reported in 40%–55% of patients when mechanomyographic TOF ratio was 0.61–0.7 (5,18–21,23), with one study reporting 0% (24). The similarity of the results across studies suggests that experience and awareness of the limitations of conventional monitoring do not alter one's ability to detect fade significantly.
The 50-Hz tetanus is probably the least useful clinical test. Half the patients had no detectable fade for a mechanomyographic TOF ratio 0.3. The results are similar to those of Dupuis et al. (6), who tested only 50-Hz tetanus in patients under isoflurane anesthesia. Therefore, this test should be abandoned for detecting residual paralysis. Only two previous studies have dealt with assessment of fade using 100-Hz tetanus. Baurain et al. (7) detected fade in all cases when the mechanomyographic TOF value was <0.85. This finding was not reproduced by Samet et al. (24), who reported no fade at mechanomyographic TOF values as low as 0.47. Both studies report many patients who still displayed fade when mechanomyographic TOF fade was >0.9. The findings of the present study agree with both studies with respect to the large proportion (64%) of fade detected when mechanomyographic TOF is >0.9. However, absence of fade was detected, even when TOF ratio was low, and the 100-Hz tetanus test in the present study seems to be very unreliable, confirming Samet et al.'s findings (24). In 14 of the 32 patients, detectable fade disappeared when the mechanomyographic TOF ratio was <0.9 (range, 0.14–0.89).
There are two important differences between this investigation and the other two studies. First, visual evaluation was made in both previous studies (7,24), whereas tactile assessment was made here. It is possible that, although visual and tactile assessments of TOF response are essentially the same, visual and tactile evaluations of tetanic stimulation are different. Second, both previous studies were conducted without a volatile anesthetic, whereas we used sevoflurane. High-frequency stimulation is associated with fade, even in the absence of a neuromuscular blocking drug (8). This fade is most likely accentuated by very small degrees of neuromuscular block. In the present study, both sensitivity and specificity of 100-Hz tetanus were poor because fade could be detected in some patients with a mechanomyographic TOF ratio >0.9, whereas in some other individuals, it was not detected with a significantly measured mechanomyographic TOF fade (0.14 in one patient). Samet et al. (24), using a noninhaled anesthetic, also found large variations in the threshold for reappearance of detectable 100-Hz tetanic fade. Thus, it seems that the background anesthetic is not the only factor that could explain the unreliability of the 100-Hz tetanus.
The most reliable test to detect residual paralysis, defined as a mechanomyographic TOF >0.9, is acceleromyography. Stimulation in the hand with acceleromyography was superior to stimulation at the wrist. There was less bias (0.5%) than with stimulation at the wrist (5.3%), and the bias was more apparent when mechanomyographic TOF ratio recovered to >0.9, i.e., in the range where the measurement is most useful. Limits of agreement were also narrower for hand stimulation. Measurements for acceleromyographic TOF exceeding 1.0 were less frequent with stimulation in the hand, which indicates that stimulation in the hand might be preferred if recovery is assessed by acceleromyography. This was not observed in a previous study on hand stimulation (11) because few measurements were made at high TOF ratios. For both sites of stimulation, the threshold at which an acceleromyographic value of 1.0, as recommended by Capron et al. (12), was close to a mechanomyographic TOF value of 0.9. Acceleromyography requires specialized equipment and is not available on every nerve stimulator. However, the additional information it provides makes it valuable.
In conclusion, during recovery from neuromuscular blockade under sevoflurane anesthesia, tactile evaluation of 50-Hz fade is no more sensitive than TOF fade. Neither of these tests can be used to exclude residual paralysis. Detection of DBS fade is more sensitive, but it cannot exclude residual paralysis. Fade after a 100-Hz tetanus is unreliable; it can be present when recovery is complete and can be absent when there is residual paralysis. For all these clinical tests, stimulation at the hand yields the same results as stimulation at the wrist. The acceleromyographic TOF, even uncalibrated, remains the most accurate test to exclude a residual paralysis when using an acceleromyographic TOF ratio of 1.0 as a threshold. Applying stimulation in the hand yields more accurate information than stimulation at the wrist when acceleromyography is used.
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This article has been cited by other articles:
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  M. Eikermann Does Fade with 100-Hz Tetanic Stimulation Reliably Detect Residual Neuromuscular Blockade? Anesth. Analg., January 1, 2007; 104(1): 215 - 215. [Full Text] [PDF]
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F. Donati Does Fade with 100-Hz Tetanic Stimulation Reliably Detect Residual Neuromuscular Blockade? Anesth. Analg., January 1, 2007; 104(1): 215 - 216. [Full Text] [PDF]
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