Neuromuscular blocking agents have differential effects on various groups of striated muscles such as the diaphragm, skeletal or laryngeal muscles. The onset and offset of their action are usually more rapid for the diaphragm and the muscles of the larynx compared with skeletal muscles [1,2]. The onset and duration of the effects of rapacuronium, a rapid-onset rapid-offset, amidosteroid, non-depolarizing neuromuscular blocking agent, have been described for the adductor pollicis and laryngeal muscles, but not for the diaphragm . While the response of the laryngeal muscles can be important for induction of anaesthesia and intubation, differences in the duration of action at the diaphragm and skeletal muscles are more important for surgery and the safety of recovery. In the present study, we simultaneously studied the duration of action of a 1.5 mg kg−1 dose of rapacuronium (1.5 × ED90) on the diaphragm, adductor pollicis and orbicularis oculi muscle. Most investigations of diaphragm function require the use of needle electrodes to stimulate the phrenic nerve and measure the diaphragmatic response as changes in gastric and oesophageal pressures or as an electromyographical response of the diaphragm. The present study used a new method, previously employed in postoperative patients, in which the phrenic nerve is stimulated by cervical magnetic stimulation and the diaphragmatic response is measured as changes in the occluded airway pressure . The differences between these methods are also addressed.
This study was conducted with the approval of our institutional Ethics Committee, and written informed consent was obtained from all patients. We recruited 20 patients, ASA I and II, undergoing elective maxillofacial surgery with general anaesthesia. All patients were free of neuromuscular, renal or hepatic disorders, and none were taking drugs that interfere with neuromuscular function. Patients with pacemakers or similar devices were excluded from the study. Patients were premedicated with midazolam 7.5 mg orally 30 min before induction of anaesthesia. Anaesthesia was induced with propofol 2 mg kg−1 and remifentanil 1 μg kg−1. The trachea was intubated after topical anaesthesia of the hypopharynx, larynx and trachea with lidocaine 2%, and the lungs were ventilated with 40% oxygen in air. The cuff of the endotracheal tube was inflated with enough air to assure a complete seal. Mechanical ventilation of the lungs was adjusted to maintain normocapnia. Since changes in bronchomotor tone would distort the results of the airway pressure twitch responses, the patients were observed carefully for signs of bronchoconstriction (capnometry curve, expiratory flow curve, compliance, auscultation).
Anaesthesia was maintained with propofol 6 mg kg−1h−1 and remifentanil 0.2 μg kg−1 min−1. Additional doses of propofol (50 mg) or remifentanil (1 μg kg−1) were given if necessary. The patients were carefully covered to maintain a core temperature >36°C and skin temperature of the study hand >32.5°C.
A pneumatic occlusion valve was inserted between the endotracheal tube and the Y-piece of the ventilator breathing hoses. Non-compliant tubing connected a side-port of the endotracheal tube connector with the pressure transducers. Occluded airway pressure (Pao) was measured with differential pressure transducers (Huba Control, Würenlos, Switzerland). The pressure signals were monitored and collected with a sampling frequency of 200 Hz using a PC and commercial software (TurboLab®; Bressner Technology, Gröbenzell, Germany).
Neuromuscular blockade of the diaphragm was quantified by measuring the twitch airway pressure responses (TwPao) after stimulation of the phrenic nerves  using a method developed for intubated patients . Bilateral phrenic stimulation was performed with a magnetic stimulator (Magstim200®; Magstim Co, Whitland, UK) using a 90 mm single coil placed over the posterior cervical spine. The neck was bent forwards and supported with cushions to achieve stimulation of the phrenic nerve roots [6,7]. The site of optimal stimulation was determined by moving the coil between C5 and C7 and comparing the responses in the Pao signal. The coil was affixed in the position that gave the maximum Pao response and was left there for the duration of the measurement. We did not test for supramaximal stimulation but set the magnet stimulator at the maximum output of 2 Tesla. Since a magnetic field of 1.5 Tesla gives supramaximal stimulation in five of six subjects if the coil position is optimized as done in this study , we expected a magnetic field of 2 Tesla to be supramaximal in at least the same percentage of subjects. The coefficient of variation for repeated measures in an individual subject during one experimental session is approximately 6% .
The ventilatory mode was switched to spontaneous at the end of expiration, the airway automatically closed by the occlusion valve and four magnetic stimuli (2 Tesla, 0.1 ms) were applied at a frequency of 2 Hz. The measurements were repeated at 15 s intervals until maximum relaxation was obtained and thereafter at 30 s intervals.
The effect of rapacuronium on the adductor pollicis and orbicularis oculi muscles was quantified by acceleromyography (TOF-Guard®; Organon Teknika, Oss, The Netherlands). Stimulation electrodes were positioned over the ulnar nerve at the wrist and over the facial nerve in front of the tragus. The acceleromyometer probes were attached at the tip of the thumb and on the mediolateral portion of the upper eyelid just proximal to the eyelashes. The supramaximal stimulation current was determined and the muscles were then preconditioned with single-twitch stimulation at a frequency of 0.1 Hz for at least 10 min until a stable response was obtained. The stimulation mode was then set to train-of-four (TOF) stimulation at 15 s intervals throughout the study. Recovery of neuromuscular function was assessed by the guidelines described by Viby-Mogensen and colleagues .
Rapacuronium bromide was purchased by our hospital pharmacy in the commercially available formulation Raplon® (Organon, Inc, West Orange, NJ, USA). The content of each 100 mg vial was dissolved in 5 mL sterile water giving a concentration of 20 mg mL−1. Rapacuronium was given in a dose of 1.5 mg kg−1 body weight (approximately 1.5 × ED90) injected into a rapidly flowing infusion. Surgery did not begin until neuromuscular function had returned to a TOF quotient of 0.8 in the adductor pollicis muscle.
All data were tested for normality and then analyzed further with parametric or non-parametric methods as indicated using commercial statistics software (GraphPad InStat®, v.3.01 for Windows 95®; GraphPad Software, Inc, San Diego, CA, USA). The clinical duration, recovery and demographic data were evaluated by analysis of variance and tested post hoc with the Bonferroni multiple comparison test. The descriptive data are given as mean and SD. The depth of relaxation and TOF0.8 data did not follow a Gaussian distribution and were analysed with the Kruskal-Wallis test. P < 0.05 was considered as significant.
A power analysis of existing data showed that 11 patients would probably be sufficient to detect a reduction of clinical duration of action on the diaphragm by 40% compared with the adductor pollicis muscle. Twenty patients were studied to detect a smaller effect.
Fifteen male and five female patients (27 ± 8 yr, 73 ± 13 kg; mean ± SD) were recruited for the study. Laboratory data showed no impairment of renal or hepatic function.
The median maximum degree of relaxation using the selected dose of 1.5 mg kg−1 rapacuronium was 89% (range 63-98%) for the diaphragm, which was significantly (P < 0.001) less than for the adductor pollicis (median 100%, range 94-100%) or orbicularis oculi muscles (median 100%, range 79-100%). The twitch response of the diaphragm was suppressed to < 10% of baseline in only 11 of 20 patients (Fig. 1).
The onset time for maximum relaxation was identical for the diaphragm, adductor pollicis and orbicularis oculi (Table 1). The clinical duration of action (T1 25%) was significantly shorter for the diaphragm than for the adductor pollicis or the orbicularis oculi (P < 0.001). The recovery index (T1 25-75%) was virtually identical for the diaphragm and adductor pollicis but was significantly longer (P < 0.01) for the orbicularis oculi. The time for recovery to TOF0.8 was significantly shorter for the diaphragm than for the adductor pollicis or the orbicularis oculi (P < 0.05).
The results show that the diaphragm is less sensitive to the effects of rapacuronium than the adductor pollicis and orbicularis oculi muscles, both with regard to maximum effect as well as to the duration of action as measured with our method. This finding resembles the results of studies with other non-depolarizing neuromuscular blocking drugs, such as vecuronium or rocuronium, using needle electrode stimulation in which both a higher ED95 as well as a more rapid recovery was determined for the diaphragm [11-14].
The method used to assess the effects of the neuromuscular blocker on the diaphragm is relatively new with regard both to the method of stimulation and to that used to quantify diaphragmatic contractility. One confounding factor when using cervical magnet stimulation of the phrenic nerves to assess diaphragm function as was done in this study is that the diaphragm is not selectively stimulated. Cervical motor nerve roots are simultaneously stimulated and can enhance the TwPao response by stabilizing the chest wall [6,15]. This is not a problem when using the method to assess the maximum inspiratory force in patients with muscular impairment of ventilatory function . However, in studies such as the present one in which the diaphragm and skeletal muscles are expected to be affected differently by the neuromuscular blocking agent, it could prove difficult to determine accurately the effect that this factor will have on recovery data. The baseline airway pressure twitch response includes the contribution of the extrathoracic muscles and is higher than the response of the diaphragm alone . This would lead one to overestimate the clinical duration of action of muscle relaxants on the diaphragm since the response of skeletal muscles was still suppressed to <10% of baseline when the TwPao had already recovered to 25%. The same reasoning holds true for the recovery index and the recovery to a TOF quotient of 0.8 for the diaphragm. The duration of action measured by bilateral electrical stimulation of the phrenic nerves would therefore tend to be shorter and not longer than those measured in this study, amplifying the difference between the effects of rapacuronium on the diaphragm and skeletal muscles.
While it is now agreed that cervical magnet stimulation of the phrenic nerves can be used instead of direct needle electrode stimulation, there is no such agreement on the method used to quantify the diaphragm contractility. The standard method used to quantify the response to diaphragmatic stimulation, aside from electromyography of the diaphragm, is to determine the transdiaphragmatic pressures from gastric and oesophageal pressure recordings. However, it has been shown that the mouth twitch pressure as a measure of diaphragm contractility can correlate very well with these transdiaphragmatic pressures [5,17]. Nevertheless, the measurement can be easily disturbed by volitional activity of the diaphragm or by glottic closure , but this is not a problem in intubated and sedated subjects. A study conducted in our institution demonstrated that airway pressure responses to phrenic nerve stimulation were a suitable substitute for oesophageal and gastric pressures in intubated patients and were technically much easier to obtain . This method, adapted and employed in a slightly altered form, also gave acceptable results in intensive care patients with considerable lung pathology . The reservations put forth by Watson and colleagues pertaining to the variable interindividual differences between Pao and transdiaphragmatic pressures are of no import for the aims of the present study, since not absolute values but only the changes of the Pao response over time were compared with baseline values in individual patients. The intraindividual variation using cervical magnet stimulation and transdiaphragmatic pressures has been determined by Mills and colleagues  and by Wragg and colleagues  to be approximately 6-8% and would be similar for the airway occlusion pressure method.
We have no explanation for the observation that the recovery index was longer for the orbicularis oculi muscle than for the other two muscles tested, but this is essentially the same behaviour as observed by Plaud and colleagues  for rocuronium. Conflicting results such as those of Lagneau and colleagues  or Rimaniol and colleagues , who found that the orbicularis oculi muscle recovered more rapidly than did the adductor pollicis, might be due to differences in the placement of the acceleromyometry probe . On the other hand, the difference might depend on the type of neuromuscular blocking drug studied: only studies using neuromuscular blocking drugs with a benzylisoquinoline structure demonstrated a shorter duration of action on the orbicularis oculi [20,21], whereas studies investigating drugs with a steroid structure found either no difference or - as in the present study - a longer duration of action on the orbicularis oculi [19,21]. A consistent observation in other studies was that the orbicularis oculi muscle responded more rapidly to neuromuscular blocking drugs than the adductor pollicis muscle [19,21-24]. We could not confirm this observation in our study, but this may be due to the measurement error inherent in the chosen stimulation frequency of four per minute.
Rapacuronium was recently voluntarily with drawn from the market by the manufacturer for an undetermined period. This step was taken after several cases of severe bronchospasm with fatal outcome occurred in patients who had been given it as one of the anaesthesia induction drugs. This is an unfortunate fate for an interesting drug, but it does not detract from the validity of this study, where a novel method was used to stimulate the diaphragm and diaphragm contractility was assessed.
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