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Neuromuscular blockade can be assessed accelerographically over the vastus medialis muscle in patients positioned prone

[Le blocage neuromusculaire peut être évalué par accélérographie sur le muscle vaste interne chez des patients en décubitus ventral]

From the Department of Anesthesiology, Fukushima Medical University School of Medicine, Fukushima, Japan.

Address correspondence to: Dr. Yuhji Saitoh, Department of Anesthesiology, Fukushima Medical University School of Medicine, Hikarigaoka 1, Fukushima-City, Fukushima 960-1295, Japan. Phone: +81-24-548-2111; Fax: +81-24-548-0828; E-mail: ys{at}m6.people.or.jp

	Abstract

TOP Abstract Introduction Methods Results Discussion References

 
Purpose: To study accelerographic monitoring of neuromuscular blockade over the vastus medialis muscle in anesthetized patients in the prone position.

Methods: Sixty adult patients were divided into four groups of 15: prone (P)-post-tetanic count (PTC), P-train-of-four (TOF), supine (S)-PTC or S-TOF group. In the P-PTC and P-TOF groups, the muscular branch of the femoral nerve was stimulated and the movement of the vastus medialis muscle was measured accelerographically. In the S-PTC and S-TOF groups, the depth of neuromuscular blockade was assessed mechanically at the thumb. After administration of vecuronium 0.1 mg•kg^-1, TOF stimuli were delivered every 20 sec in all groups. Onset of neuromuscular blockade (time from vecuronium to the minimum level of T1 (first response of TOF) was compared between the P-PTC or P-TOF group and S-PTC or S-TOF group. Recoveries of PTC and those of TOF were compared between the P-PTC and S-PTC groups and P-TOF and S-TOF groups, respectively.

Results: Onset of neuromuscular blockade in the P-PTC or P-TOF group was shorter than in the S-PTC or S-TOF group (132 ± 43 vs 233 ± 40 sec, mean ± SD, P < 0.001). Recoveries of PTC in the P-PTC and S-PTC groups followed a similar time course. Recoveries of T1/control in the P-TOF and S-TOF groups were also comparable. In contrast, train-of-four ratio (T4/T1, TOFR) in the P-TOF group was higher than in the S-TOF group 60–120 min after vecuronium (P < 0.05).

Conclusions: The degree of neuromuscular blockade can be assessed accelerographically over the vastus medialis muscle even when patients are in the prone position. When evaluated accelerographically over the vastus medialis muscle, onset of neuromuscular blockade is quicker and TOFR is higher than that assessed at the thumb.

	Introduction

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THE degree of neuromuscular blockade during anesthesia is usually assessed mechanically or accelerographically at the hand or foot. During orthopedic surgery of the cervical spine or neurosurgery of the posterior cranial cavity, patients are positioned prone while the patients’ arms lie alongside the trunk and the patients’ feet are fixed on an operating bed. Therefore, the degree of neuromuscular blockade cannot be evaluated mechanically or accelerographically at the hand or foot since the movement of the hand or foot in these positions is limited.¹ In such cases the degree of neuromuscular blockade can be evaluated electromyographically at the gastrocnemius muscle.¹ To assess the depth of neuromuscular blockade electromyographically, however, a special monitoring device is needed. On the contrary, the degree of neuromuscular blockade can easily be monitored accelerographically using a small acceleration transducer. Stimulation of the muscular branches of the femoral nerve and quantification of the response of the vastus medialis muscle allows us to measure neuromuscular blockade in anesthetized patients during cervical spine or posterior fossa operations. Using this method, we assessed the level of neuromuscular blockade over the vastus medialis muscle of anesthetized patients in a prone position, and compared it with that evaluated mechanically at the thumb of anesthetized patients in a supine position.

	Methods

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Thirty adult patients, ASA physical status I or II, undergoing elective orthopedic surgery of the cervical spine or neurosurgery of the posterior cranial cavity in a prone position, and another 30 patients, ASA physical status I or II, scheduled for elective ear-nose-throat surgery (tympanoplasty) or ophthalmologic surgery (segmental buckling or vitrectomy) under general anesthesia in a supine position were studied. The protocol was approved by our local Ethics Committee, and written informed consent was obtained from each subject. No patient had any neuromuscular, cardiac, hepatic, or renal disorder, nor was receiving any drug known to interfere with neuromuscular transmission. The 30 patients scheduled for surgery in a prone position were randomly divided into two groups of 15: prone (P)-post-tetanic count (PTC) or P-train-of-four (TOF) groups. The other 30 patients scheduled for general anesthesia in a supine position were randomly allocated to two groups of 15 patients: supine (S)-PTC or S-TOF groups.

Premedication consisting of atropine 0.01 mg•kg^-1 and midazolam 0.05 mg•kg^-1 was given im 30 min before induction of anesthesia. In the operating theater, in the P-PTC and P-TOF groups, two surface stimulating electrodes (Vitrodes, Nihon-Kohden Inc., Tokyo, Japan), connected to a neuromuscular transmission monitor (TOF guard, Biometer Inc., Odense, Denmark), were positioned over the muscular branches of the femoral nerve and vastus medialis muscle of the patients. An acceleration transducer belonging to the neuromuscular transmission monitor was placed between the two stimulating electrodes. The acceleration transducer was positioned approximately 10 cm above the upper edge of the patella (Figure 1). In the S-PTC or S-TOF groups, two surface stimulating electrodes, connected to an electrical stimulator (Isolator, Nihon-Kohden Inc., Tokyo, Japan), were positioned over the ulnar nerve at the wrist, and a force transducer connected to a neuromuscular transmission analyzer (Myograph 2000, Biometer International Inc., Odense, Denmark) was attached to the thumb of the investigated arm of the patients. Peripheral temperature over the infero-medial part of the thigh and the adductor pollicis muscle was monitored using a surface skin thermometer (Terumo-Finer, Nihon-Kohden Inc., Tokyo, Japan).

In each group, anesthesia was induced by the iv administration of propofol 1.5 mg•kg^-1 and fentanyl 2 µg•kg^-1. After disappearance of the eyelash reflex, train-of-four (TOF) stimuli, 0.2 msec square-wave stimulus at a frequency of 2 Hz, were administered every 20 sec at supramaximal current in all groups. The method to determine the supramaximal current is not established clearly. However, we measured the accelerographic and mechanical T1 values at ten, 20, 30, 40, 50, and 60 mA in all patients. It has been reported that the value of T1 recorded at the supramaximal current is generally > 90% of the T1 value elicited at 60 mA.^2-4 Thus, the lowest current able to produce a value of T1 of more than 90% of the value of T1 recorded at 60 mA was regarded as the supramaximal current. This method of determining the supramaximal current is in accordance with previous reports.^2–4

Once the supramaximal current had been determined in each patient in the four groups, the TOF stimuli were delivered every 20 sec at the supramaximal current for approximately five minutes until the accelerographic and mechanical responses stabilized. Thereafter, the accelerographic value of T1 and the mechanical twitch height in response to T1 were regarded as controls.

After recording the control value, vecuronium 0.1 mg•kg^-1 was administered intravenously to facilitate tracheal intubation. Time from vecuronium injection to the disappearance of the mechanical or accelerographic response to T1 was defined as onset time. If the response to T1 was not abolished, the time when T1/control reached the minimum level was regarded as the onset time. The onset time measured in the P-PTC and P-TOF groups was compared with that in the S-PTC and S-TOF groups. In each group, anesthesia was maintained with nitrous oxide 66% in oxygen and isoflurane 0.5% end-tidal concentration, and the patients’ lungs were ventilated to maintain normocapnia (F_ÉCO₂ 4.2–5.0 kPa) throughout the surgical procedure. The end-tidal anesthetic concentrations and F_ÉCO₂ were measured using a multiple gas monitor (Capnomac Ultima, Datex Inc., Helsinki, Finland).

In the P-PTC and P-TOF groups, patients were positioned prone within ten minutes of the vecuronium injection. A towel was placed between the operating table and the knee of the patient so that the surface of the vastus medialis muscle could move freely.

In the P-PTC and S-PTC groups, TOF stimuli were discontinued after the onset of neuromuscular blockade was confirmed, and post-tetanic count (PTC) was measured ten, 15, 20, 25, 30, 35, and 40 min after the vecuronium injection. For PTC, a 50 Hz tetanic stimulation of a five-second duration was applied at 50 mA. After a pause of three seconds, 15 single twitch square-wave stimuli of 0.2 msec duration were given every one second. The number of detectable accelerographic or mechanical responses to the single twitch stimuli delivered after the tetanic stimulation was regarded as PTC. PTC was compared ten, 15, 20, 25, 30, 35, and 40 min after the vecuronium injection between the P-PTC and S-PTC groups.

In the P-TOF and S-TOF groups, during spontaneous recovery from neuromuscular blockade, TOF stimuli were given every 20 sec. The time course of recovery of T1/control and the TOFR (T4/T1) were compared between the two groups.

Patient characteristics were compared among the four groups using analysis of variance (ANOVA) followed by Scheffe’s F test. For comparison of the onset times of neuromuscular blockade, an unpaired t test was applied. Time courses of recovery of PTC were compared between the P-PTC and S-PTC groups using Kruskal-Wallis test followed by Mann-Whitney U test with Bonferroni’s adjustment. Time courses of recovery of T1/control or TOFR were compared between the P-TOF and S-TOF groups using ANOVA followed by unpaired t test with Bonferroni’s adjustment. A P value < 0.05 was considered statistically significant. Statistical analyses were performed using the SYSTAT statistical package (SYSTAT 8.0, SPSS Inc., Chicago, IL, USA). Data were expressed as mean ± SD.

	Results

TOP Abstract Introduction Methods Results Discussion References

 
Patient characteristics were comparable among the four groups (Table). In no patient did the peripheral temperature decrease below 32°C.

The onset of neuromuscular blockade in the P-PTC and P-TOF groups was faster than that in the S-PTC and S-TOF groups (132 ± 43 vs 233 ± 40 sec, P < 0.001). In two patients in the P-PTC group and two patients in the P-TOF group, the response to T1 was not abolished after administration of vecuronium 0.1 mg•kg^-1. In these patients, the minimum T1/control after administration of vecuronium 0.1 mg•kg^-1 ranged from 0.03 to 0.09.

During recovery from neuromuscular blockade, PTC in the P-PTC group did not differ from that in the S-PTC group (Figure 2). Similarly, during recovery from neuromuscular blockade, T1/control did not differ between the P-TOF and S-TOF groups (Figure 3), but TOFR in the P-TOF group was higher than in the S-TOF group 60 to 120 min after the administration of vecuronium (P < 0.05; Figure 4).

View larger version (16K): [in this window] [in a new window]   FIGURE 3 T1/control obtained in the P-TOF (•) and S-TOF (°) groups. P = prone, S = supine, TOF = train-of-four. No difference was observed between the groups.

View larger version (16K): [in this window] [in a new window]   FIGURE 4 TOFR obtained in the P-TOF (•) and S-TOF (°) groups. P = prone, S = supine, TOFR = train-of-four ratio. *P < 0.05 between groups.

	Discussion

TOP Abstract Introduction Methods Results Discussion References

It has been demonstrated that when the degree of neuromuscular blockade is assessed mechanically or accelerographically at the thumb, a current of 30 mA can produce supramaximal nerve stimulation in a majority of patients.^2,3 Similarly, in the present study, when the level of neuromuscular blockade was evaluated mechanically at the thumb and accelerographically over the vastus medialis muscle, supramaximal currents were 30.7 and 34.7 mA, respectively.

During ear-nose-throat or ophthalmologic procedure, although the patients’ arms lie alongside the trunk so that the movement of the thumb is restricted, the movement of the first toe in response to tibial nerve stimulation can be measured accelerographically.^5–8 During surgery of the cervical spine or neurosurgical procedures in a prone position, neuromuscular blockade is difficult to assess accelerographically at the first toe since the patient’s feet and arms are fixed on the operating table. In such cases, the degree of neuromuscular blockade can be evaluated electromyographically at the gastrocnemius muscle.¹ Electromyographic monitoring is, however, relatively troublesome since it requires special equipment. In contrast, accelerographic monitoring is easy to use clinically, requiring only a small acceleration transducer which can easily be attached to the patients.

The onset of neuromuscular blockade caused by vecuronium 0.1 mg•kg^-1 measured over the vastus medialis muscle was much faster than that evaluated at the thumb. This is probably because movement of the surface of the vastus medialis muscle is much less than that of the thumb and, consequently, more difficult to detect. In other words, after the administration of vecuronium, even when small movements persist in response to TOF stimulation, the response to T1 will be regarded as "zero". We believe this is why the onset of neuromuscular blockade in the P-PTC and P-TOF groups was faster than in the S-PTC and S-TOF groups.

The mechanical response to T1 was abolished after the administration of vecuronium 0.1 mg•kg^-1 in all patients in the S-PTC and S-TOF groups. In contrast, the accelerographic response to T1 did not disappear after vecuronium 0.1 mg•kg^-1 in four patients. In this study not only the muscular branches of the femoral nerve but also the vastus medialis muscle itself may have been stimulated directly. Even when neuromuscular transmission is blocked completely, direct stimulation of the muscle often produces a contraction.⁹ We believe this may explain why the response to T1 did not disappear in all subjects.

TOFR in the P-TOF group was higher than in the S-TOF group 60 to 120 min after administration of vecuronium. It has been reported that the vastus medialis muscle contains more type II muscle fibres than the adductor pollicis muscle.¹⁰ As the percentage of type II muscle fibres increases, the skeletal muscle becomes more resistant to a non-depolarizing neuromuscular relaxant and recovery from neuromuscular blockade is hastened.^1,5–8 Because the vastus medialis muscle is thought to be more resistant to non-depolarizing neuromuscular blocking drugs than the adductor pollicis muscle, TOFR assessed over the vastus medialis muscle was higher than that measured at the thumb. On the other hand, accelerographic TOFR is comparable to mechanical TOFR in the mechanical TOFR range of 0 to 0.70.¹¹ However, accelerographic TOFR tends to be higher than mechanical TOFR when mechanical TOFR > 0.70.¹¹ This finding would also concur with the present results that the accelerographic TOFR in the P-TOF group was higher than the mechanical TOFR in the S-TOF group. It has been shown that a mechanical TOFR > 0.9 correlates well with signs of adequate recovery of neuromuscular blockade.¹² Hence, when measured accelerographically, it may be difficult to diagnose adequate recovery from neuromuscular blockade since the accelerographic TOFR is prone to be higher than the mechanical TOFR. May et al.¹³ noted that T1/control measured accelerographically was not different from that measured mechanically. Additionally, Silverman et al.³ have reported that the accelerograph does not display a twitch response until T1/control becomes 0.03. Similarly, Kirkegaard-Nielsen et al.¹⁴ showed that when the force transducer was used to measure a twitch response, the minimum detectable level on the digital display was about 0.03 to 0.04. Thus, when assessed accelerographically or mechanically, twitch response is regarded as "absent" if T1/control is less than 0.03.

In this study PTC and T1/control evaluated over the vastus medialis muscle were not different from those measured at the thumb. Especially, as shown in Figure 3, T1/control measured over the vastus medialis muscle is similar to that measured at the adductor pollicis muscle. Hence, T1/control evaluated accelerographically over the vastus medialis muscle may represent that measured mechanically at the thumb.

In conclusion, the degree of neuromuscular blockade can be evaluated accelerographically over the muscular branches of the femoral nerve and the vastus medialis muscle in patients in the prone position. However, onset of neuromuscular blockade measured over the vastus medialis muscle is faster than that evaluated at the thumb. Additionally, during recovery, accelerographic TOFR assessed over the vastus medialis muscle is higher than that measured mechanically at the thumb.

View larger version (16K): [in this window] [in a new window]   FIGURE 1 Two surface stimulating electrodes and an acceleration transducer positioned over the muscular branches of the femoral nerve and vastus medialis muscle.

View larger version (14K): [in this window] [in a new window]   FIGURE 2 PTC obtained in the P-PTC (•) and S-PTC (°) groups. P = prone, S = supine, PTC = post-tetanic count. No difference was observed between the groups.

	References

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