After injection of rocuronium 0.6 mg/kg, 17 of 20 patients showed no twitch response at both sites. Two patients showed four equal twitches of 4%–5% of control T1 only at the wrist site, with no response at the hand site. A movement of the hypothenar eminence was noted in the first case, and a movement just below the electrodes at the wrist was noted in the second case. Finally, one patient had four twitches with a fade response at both sites despite rocuronium 0.6 mg/kg and an additional dose of 0.2 mg/kg. Four patients had a delay of 4–6.5 min between the time no twitch response was present at the wrist and the time there was no response at the hand. All other patients had no delay between complete neuromuscular blockade at the hand and at the wrist.
During recovery, TOF ratios obtained at the hand site were similar to those obtained at the wrist site (Figs. 2 and 3). There was a bias of +0.5%, and 95% of the values obtained at the hand were within ±11.8% of the values measured at the wrist. Bias and limits of agreement were not dependent on the degree of recovery. Twitch heights measured at the hand and the wrist were also comparable (Fig. 4). The bias for T1 was larger (+6.9%; values at the hand site were larger than those at the wrist site), and 95% of the values obtained at the hand were within ±40.3% of the values measured at the wrist. However, for a given patient, the difference between hand and wrist values tended to remain relatively constant throughout recovery.
This study shows that neuromuscular monitoring at the hand does not produce direct muscle stimulation and that TOF ratios measured after stimulation at the hand are comparable to those obtained with stimulation at the wrist. Any response to TOF stimulation in the hand of more than 2% of control T1 in the absence of response at the wrist was assumed to represent direct muscle stimulation.
The electrodes on the hand were applied over the adductor pollicis muscle, which lies just medial and distal to the thenar eminence, between the thumb and the second finger. It was not placed over the thenar eminence because stimulation of muscles innervated by the median nerve—such as the flexor pollicis brevis, the opponens pollicis, or the abductor pollicis—could have occurred. However, the recurrent branch of the median nerve lies very close to the adductor pollicis before it enters the thenar eminence (4). It is therefore possible that a portion of the response we got while stimulating at the hand site came from contraction of muscle fibers innervated by the median nerve.
We decided to compare the two stimulation sites on the same arm, using the same accelerometer to minimize bias because there could have been a difference between baseline drift between both arms and between positioning of the accelerometer on each thumb (6–8). This way, calibration was done with only one accelerometer, and it allowed us to compare all T1 values with the control T1 at the wrist and thus estimate the relative force of contraction of the muscle at both sites. The disadvantage was that we could not stimulate both sites at the same time and had to switch the stimulating wires from one electrode pair to the other. To ensure that we compared TOF ratio and twitch height at the same time during recovery and to minimize variation between successive measurements, a mean of four TOFs was used for each stimulation site: four consecutive TOFs and T1 values for the hand site and the two immediately before and after these for the wrist site. This was done for every 10% increase in TOF or T1 at the wrist site in two minutes.
The wrist site and the hand site showed remarkable correlation with respect to TOF ratios, and TOF is the most commonly used modality in clinical practice. When both methods were compared, there was practically no bias (0.5%), and the 95% limits of agreement (±11.8%) could be within the accuracy limits of the accelerometer. In regard to T1, dispersion was more important, and there was a positive bias of 6.9% (the hand site showed higher T1 values than the wrist site). The greater T1 values are probably due to stimulation of muscle fibers of the thenar eminence innervated by the median nerve, thus increasing the force of contraction or changing the direction of the acceleration vector toward the accelerometer.
When neuromuscular function is monitored, it is usually thought that the stimulation site must be over a peripheral nerve to elicit the most accurate response (5). It is believed that stimulation over or near a muscle could produce direct muscle contraction. If direct muscle stimulation were to occur, interpretation of neuromuscular function could be hampered because muscle contractions would be present even with adequate muscle relaxation. However, very few data are available to substantiate this concern, because it appears that a high voltage or current is required to stimulate a muscle directly. Nerve and muscle fibers are excitable and can depolarize when submitted to an electrical stimulation. However, maximal direct stimulation of the adductor pollicis in humans may require as much as 1000 V. A lower voltage stimulates nerve fibers only (9). With IM electrodes placed in cat tibialis anterior muscle, maximal muscle contraction required a current of 2 mA for 1 ms when stimulated via a nerve. When a neuromuscular blocking drug was given, direct muscle contraction required a current of 15 mA for 1 ms (10). In another experiment, when IM electrical stimulation was applied, direct muscle contraction contributed only to 3%–7% of the total contraction; the rest was achieved by stimulation of IM nerve fibers (11). Similarly, our results support the notion that the current used in neuromuscular monitoring (10–60 mA for 0.2 ms) through surface electrodes is usually not sufficient to produce direct muscle contraction.
As was done in previous studies, we chose the presence of response to TOF stimulation at the hand site in the absence of response at the wrist site as the criterion for direct stimulation (12). In a study on the significance of electrode position for the orbicularis oculi, Gätke et al. (8) found that 80% of patients showed one to four acceleromyographic responses to TOF stimulation even though there was no visible response of the orbicularis oculi. They stated that direct stimulation was a possibility, but technical limitations of the TOF-Guard appear more likely. At high gains, the TOF-Guard might display a response in the absence of actual muscle contraction. In another study on acceleromyographic monitoring over the vastus medialis muscle, Saitoh et al. (13) found that only 4 of 30 patients did not show a disappearance of T1 response after the administration of vecuronium 0.1 mg/kg. In this last study, the electrodes were placed over a muscle and not over a nerve, and most patients showed a normal acceleromyographic response to muscle relaxation. With regard to the hand, Rodiera et al. (14) found that selective nerve stimulation over the ulnar nerve produced similar results as nonselective stimulation delivered over the forearm supinator longus muscle. Finally, in a study on excitability properties of human median axons measured at the motor point, Kuwabara et al. (15) provided evidence that direct muscle activation was unlikely because the stimulus-response curves they measured did not change abruptly, as would be expected with muscle fiber activation.
In our study, only one of the patients showed a four-twitch muscular response at the hand after rocuronium 0.6 mg/kg. This patient had four twitches with fade at both the wrist and hand sites and could have been more resistant to muscle relaxants. All other patients showed no twitch response at the hand site after injection of muscle relaxant. However, 2 other patients showed 4 equal twitches only at the wrist site and none at the hand site. This is the opposite of what one would expect to find. Direct muscle stimulation near the wrist could have occurred in these cases, because muscle movement near the wrist electrodes, but none near the thumb, was present. Therefore, in this study, direct muscle stimulation was observed with stimulation at the wrist but not at the hand.
In four patients, there was a delay between total block at the wrist site and total block at the hand site: the latter was slower than the former. These patients tended to have a higher initial T1 at the hand site than at the wrist site. This could be explained by the fact that nerves innervating muscles other than the adductor pollicis course in the area under the hand electrodes. These muscles could be slightly more resistant than the adductor pollicis to the effects of rocuronium.
We also looked at the number of fingers that moved when each site was stimulated. All five digits almost always moved with the wrist site. After repeated stimulation, this could cause artifacts on an oximeter placed on those fingers and could even potentially dislodge it. In contrast, when the hand site was stimulated, only the first three digits moved, and therefore an oximeter could be placed conveniently on the fourth or fifth finger. Whether tactile or visual interpretation of the TOF ratio could be more accurate with this method is not known because it was beyond the scope of this study.
In conclusion, monitoring the adductor pollicis by positioning the stimulating electrodes in the hand is an acceptable alternative to applying electrodes at the wrist. Direct muscle stimulation does not seem to be as common a phenomenon as previously believed, because it occurred in none of our cases. Neuromuscular monitoring near or over a muscle is possible. Additional studies on the possible advantages of this method of stimulation are required.
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© 2005 International Anesthesia Research Society
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