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* From the Neuromuscular Research Group (NRG), Department of Anesthesiology, Montreal General Hospital, McGill University; and the
Institut de Génie Biomédical, Université de Montréal, Montreal, Quebec, Canada.
Address correspondence to: Dr. Thomas Hemmerling, Anaesthesia Department, McGill University Health Centre (MUHC), Montreal General Hospital, 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada. Phone: 514-934-1934, ext. 43795; Fax: 514-934-8249; E-mail: thomashemmerling{at}hotmail.com
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Abstract |
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 TOP Abstract IntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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Source: A PubMed search of relevant article for the period 1985–2005 was undertaken, and bibliographies were scanned for additional sources.
Principal findings: There is no substitute for objective neuromuscular monitoring; for research purposes, mechanomyography (MMG) is the gold standard; however, the most versatile method in the clinical setting is acceleromyography since it can be applied at various muscles and has a long track record of clinical utility. Kinemyography is valid to monitor recovery of neuromuscular transmission at the adductor pollicis muscle (AP), whereas phonomyography is easy to apply to various muscles and shows promising agreement with MMG. Monitoring of the corrugator supercilii muscle (CS) may be used to determine the earliest time for tracheal intubation as it reflects laryngeal relaxation better than monitoring at the AP. Recovery of neuromuscular transmission is best monitored at the AP, since it is the last muscle to recover from neuromuscular blockade (NMB). If train-of-four (TOF) stimulation is used, a TOF-ratio > 0.9 should be the target before awakening the patient. If surgery or the type of anesthesia necessitates NMB of a certain degree, e.g., TOF-ratio = 0.25, monitoring of muscles which best reflect the degree of NMB at the surgical site is preferable.
Conclusion: Objective methods should be used to monitor neuromuscular function in clinical anesthesia. Acceleromyography offers the best compromise with respect to ease of use, practicality, versatility, precision and applicability at various muscles. The CS is the optimal muscle to determine the earliest time for intubation, e.g., for rapid sequence induction.
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Introduction |
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TOPAbstractIntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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This review considers the benefits and limitations of currently available methods of monitoring neuromuscular function, and weighs the related technology and different stimulation possibilities. Finally, recommendations for monitoring neuromuscular function in routine daily practice are provided. Relevant articles were searched through PubMed for the period 1980–2005 using the following keywords and terms: "neuromuscular blockade"; "neuromuscular monitoring"; "mivacurium", "rocuronium", "cisatracurium", "succinylcholine", "vecuronium", "atracurium"; "neuromuscular blockade" + "larynx", "diaphragm", "adductor pollicis muscle", "corrugator supercilii muscle", "orbicularis oculi", "vastus medialis muscle", "flexor hallucis brevis muscle"; "mechanomyography", "acceleromyography", "electromyography", "kinemyography", "phonomyography", "acoustic myography"; "staircase effect". Screened articles which considered the pharmacodynamic variability of NMBDs and which recorded comparisons between the adductor pollicis muscle (AP) and the larynx, the corrugator supercilii muscle (CS), the orbicularis oculi muscle (OO) and/or the diaphragm were considered, and their bibliographies were scanned for additional sources.
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Rationale for routine monitoring of neuromuscular function |
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TOPAbstractIntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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Although clinical judgment of surgical conditions (e.g., adequate relaxation) can be subjective, recent studies have shown that NMBDs can improve surgical conditions, most specifically in abdominal surgery.10,11 Neuromuscular blockade can be used to maintain a lighter plane of anesthesia; profound NMB can be useful for procedures such as intracranial surgery or complex eye surgery where even slight movement could result in critical events.
Probably, the most critical time to monitor neuromuscular function is at the end of surgery, prior to emergence from anesthesia. Use of a peripheral nerve stimulator is common in order to document the extent of recovery of neuromuscular function by either tactile or visual evaluation – however limited this method might be.12,13 It has been shown that a train-of-four (TOF) ratio > 0.9 at the AP is necessary to achieve adequate airway protection in order to avoid postoperative atelectasis or pneumonia.14,15 Eriksson et al.14 found a significantly reduced upper esophageal sphincter resting tone at TOF-ratios < 0.9, reduced muscle coordination and shortened bolus transit times at a TOF-ratio of 0.6 in 14 awake volunteers. Berg et al.15 showed that TOF-ratios < 0.7 present a significant risk for the development of postoperative pulmonary complications following a variety of surgeries. To date, the absence of an ideal monitoring device has been one of the main reasons why many anesthesiologists do not routinely use an objective method to monitor neuromuscular function.
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Techniques to monitor neuromuscular function |
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TOPAbstractIntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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Subjective monitoring of NMB is still the most widely used method of neuromuscular monitoring. Subjective monitoring is mostly used at the hand to evaluate the response of the AP, but also at the eyebrow to record the response of the CS or the OO. Although agreement between these subjective and objective methods depends on observer experience17–19 (especially with AMG), at best, each method serves to count the number of TOF-responses. The ability to distinguish subjectively between TOF-ratios of 0.7, 0.8 or 0.9 using AMG, while important clinically, is limited. Although double-burst stimulation (DBS) (see below) might improve the clinical assessment, DBS cannot be a substitute for other objective methods, and does not facilitate precise titration of iterative doses of NMBDs to maintain a specified degree of paralysis during surgery.
Quantitative (objective) methods
MECHANOMYOGRAPHY (MMG)
If neuromuscular monitoring should reflect the degree of relaxation of a given muscle, then measuring the actual force is the best method to monitor NMB. Of all the monitoring methods, MMG requires the most stringent preparations and precautions: ideally, the muscle should be fixed in a specially moulded cast to prevent changes of position, and a constant pre-load should be applied according to the muscle site monitored and the size of the muscle (Figure 1
). However, MMG is not free of limitations: drift can be a problem, the response does not return to the control amplitude; and even a slight detachment of the hand from the monitoring device can change the amplitude of the evoked signal. In general, MMG devices are awkward and bulky to prepare, require meticulous control of hand positioning, and can only be used to measure the response at the AP.
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Electromyography (EMG)
Electromyography is the oldest method of neuromuscular monitoring22 and is based on recording of the compound action potential after evoked stimulation of a motor nerve (Figure 2). Whether integrated or peak-to-peak amplitude is used to determine the power of the potential, the ability to use this method for routine monitoring is not influenced.23 In general, EMG has been evaluated for most muscle sites of interest in research and clinical practice, such as the larynx,24–26 the diaphragm,27–29 the AP30,31 and, with restrictions, the CS and the OO.32 Electromyography of smaller muscles, however, is difficult because of the small action potentials created. Agreement with MMG can be a problem, and there are several studies which show that EMG and MMG cannot be used interchangeably. 33–37 Technical problems which arise with EMG include drift over time – the EMG potential does not return to control levels, failure to descend completely in fully relaxed muscles, and interference with other electronic devices. Despite these limitations, good correlation has been demonstrated in monitoring the effects of NMBDs at the diaphragm between mano-metrically measuring the pressure induced within the esophagus38,39 (although cumbersome and invasive) and external EMG monitoring. Refinements of this monitoring technique have evolved with monitoring of the posterior diaphragm.28
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). Acceleromyography shows good agreement with MMG or EMG when the set-up has been carefully established.40,41 However, its use at muscles other than the AP is limited: it cannot be used at the larynx or the the diaphragm, and only with difficulty at muscles which do not create a distinctive movement. Recently, it has been shown that use of AMG at the eye muscles, such as the CS, is limited, especially in detecting the maximum degree of NMB.42 In general, the accuracy of AMG decreases when less movement, or acceleration, occurs at the end-organ, such as at the CS or the OO. Accuracy is dependent on the mass of the acceleromyographic probe. In a recent study, Kopman et al.43 showed that the addition of an elastic preload to the thumb decreased the TOF variability without any effect on the relationship between twitch height and the TOF-ratio. At present, AMG is the most accurate and reliable monitoring method commercially available to measure NMB objectively in routine clinical settings.
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). Kinemyography employs a piezoelectric transducer and consists of a moulded plastic device which mirrors the contour of the outstretched thumb and index finger. One study has shown that this device agrees reasonably with MMG for monitoring TOF-ratios, but other pharmacodynamic responses do not agree well with the MMG.44 Although the Mechanosensor NMT device (Datex Ohmeda Inc, Madison, WI, USA) is practical in the clinical setting, its accuracy is not superior to that of AMG, and careful hand positioning is necessary to avoid artefacts. The response, especially during recovery of neuromuscular function, can be misleading.45
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). It is important to have sufficient sensitivity to record very low frequencies; frequencies below 50 Hz represent approximately 90% of the signal power spectrum. 42 The signal is biphasic, and the peak-to-peak amplitude is the most useful measured parameter.
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Stimulation patterns for monitoring neuromuscular function |
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TOPAbstractIntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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The most important and widely used stimulation patterns are single-twitch stimulation, TOF-stimulation and post-tetanic facilitation (Figure 6).
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Train-of-four stimulation
The delivery of square wave stimuli of 0.2 msec duration at 0.5 Hz has become the most commonly used stimulation mode. Typical fade of the TOF response defines competitive NMB by non-depolarizing NMBDs. Train-of-four monitoring is also very useful to evaluate the extent of intraoperative relaxation, with TOF-ratios between 0.15–0.25 usually indicating adequate surgical relaxation. A TOF-ratio > 0.9 has been shown to guarantee sufficient recovery of neuromuscular transmission for safe extubation of the trachea following surgery.14
Double-burst stimulation
Subjective evaluation of the TOF response at the AP has limited sensitivity to distinguish between a TOF-ratio of 0.7 or 0.9. To address this limitation, doubleburst stimulation was introduced.52,53 The preferred form of DBS (DBS 3, 3) consists of three stimuli of 0.2 msec duration at 50 Hz (20 msec intervals followed by three equal stimuli each lasting 750 msec). Although initially thought to improve the detection of fade, DBS might give the clinician a false sense of security in its ability to manually detect fade,12 and DBS should not be considered a substitute for an objective quantitative method.
Tetanus and post-tetanic count
Before complete recovery of the TOF response, tetanic stimulation (stimulation at 50 Hz), followed more than three seconds later by single twitch stimulation at one second intervals, can be used to estimate the extent of deep NMB and time to recovery.54 Several charts are available to correlate the numbers of single twitches to recovery from profound NMB.55 Generally, a count > 10 is needed before recurrence of the first twitch of TOF stimulation can be assumed.
In general, objective monitoring methods use single twitch stimulation to determine onset of NMB; TOF stimulation every 12 sec is used to monitor the extent of NMB during surgery and recovery from general anesthesia.
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Influence of monitoring site on the clinical response |
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groups several studies according to the pharmacodynamic variability of NMBDs. The following muscles are compared: AP vs larynx,24,56–59 AP vs diaphragm,38,60,61 AP vs OO,62,63 AP vs larynx vs diaphragm,27 AP vs CS vs larynx64 and AP vs OO vs diaphragm32 using the following NMBDs: atracurium,38,59,62 rocuronium, 24,27,56,57,60,61,64 vecuronium,32,62 succinylcholine, 27,38,56 mivacurium62,63,65 and cisatracurium.58,59 Monitoring solely muscles of the hand provides only a partial image of neuromuscular function of other major muscle groups. Therefore, it is important to establish which muscle to monitor for what purpose – monitoring NMB corresponding to a specific surgical site, monitoring NMB of the larynx, diaphragm or several peripheral muscles.
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Monitoring at sites other than the hand muscles
Several studies show that muscles around the eye, the CS and the OO, reflect accurately the response of laryngeal muscles or the diaphragm and recover faster from NMB than the AP.77 Monitoring at the CS is especially helpful in types of surgery where relaxation of the diaphragm and more central muscles (e.g., abdominal muscles) are necessary. Nonetheless, more peripheral muscles (e.g., AP) need to be monitored in order to determine timely recovery of NMB. Alternatives to hand muscles include monitoring of the great toe or the vastus medialis muscle. However, there are limitations of these sites as described in the following sections.
Neuromuscular monitoring at the larynx
A monitoring method based on either MMG, EMG or PMG has been applied to the larynx. A mechanomyography-like method for the larynx consists of placing the cuff of the endotracheal tube between the vocal cords and measuring the force of the adducting laryngeal muscles via the degree of the pressure changes within the cuff.20 A recent study has pointed out the importance of maintaining resting cuff pressure within the cuff in order to truly reflect the force of the adduction throughout NMB.78 Electromyography of the larynx consists of using either a specialized endotracheal tube with incorporated wire electrodes 27 or a superficial electrode26 attached circumferentially around the tube and placed between the vocal cords. Electromyography records the evoked compound action potential of several intrinsic laryngeal muscles, reflecting the neuromuscular action potential of adductors and the abductor of the larynx. A pharmacodynamic comparison of laryngeal EMG and the cuff pressure technique has not been done as yet.
Phonomyography shows good agreement with the standard cuff pressure method for monitoring NMB at the larynx.50 By placing microphones on the laryngeal muscles, this technique is more convenient because of a more stable baseline, ease of signal analysis, absence of respiratory artefacts and little risk of accidental extubation or cuff displacement.
There are basically two sites for transcutaneous stimulation of the recurrent laryngeal nerve. The nerve can be stimulated either medially, between the jugular notch and the thyroid cartilage or, especially with use of a bipolar stimulation probe, just lateral to the sternocleidomastoid muscle in the tracheoesophageal groove.61 In comparison to stimulation of the phrenic nerve, concomitant stimulation of the brachial plexus or the vagus nerve is rare.
In comparison to NMB at the AP, most studies confirm that onset and recovery of NMB at the larynx is faster. It seems that when sub-paralyzing doses of NMBDs are used, the peak effect at the larynx is less than at the AP. However, in doses which are generally used for endotracheal intubation (> 2 x ED95), there is no significant difference between the peak effects observed at the larynx and at the AP. Whereas faster onset of NMBDs at the larynx is most probably due to a more rapid "central" distribution of the drug, the reason for more rapid recovery is related primarily to morphological differences between the larynx and the AP (Table
).
Neuromuscular monitoring of the diaphragm
Some studies have used EMG and MMG for monitoring the diaphragm. Electromyography using transcutaneous needles or superficial skin electrodes 27,29 has been applied to recording of NMB at the diaphragm after phrenic nerve stimulation.61 The conventional site to record the electromyographic signals has been the seventh or eighth intercostal space, between the anterior axillary and the mid-clavicular line. A novel site at the patient’s back has recently been used to monitor NMB 28,61 and has shown good agreement with im needle EMG of the diaphragm. Measurement of evoked transdiaphragmatic pressure as a form of indirect MMG of the diaphragm has also been used to evaluate NMB.79 Two balloons are inserted into the esophagus to record pleural pressure and into the stomach to record intra-abdominal pressure. The balloons are connected with air-filled catheters to identical transducers. The transdiaphragmatic pressure (Pdi) is then obtained by electronic subtraction of gastric (alias intra-abdominal) and esophageal (alias pleural) pressure. This technique, however, is more invasive, more difficult to apply (it has to be recorded during resting end-expiration) and it requires bilateral phrenic nerve stimulation. Another disadvantage is that it cannot be used with an open abdominal cavity. Its advantage is the more MMG-alike approach, the transdiaphragmatic pressure being measured in the lower esophagus as a function of the force of diaphragmatic, yet non-isometric contraction. Two studies have shown good correlation between the transdiaphragmatic pressure method and EMG at the diaphragm.80,81
In general, most studies evaluating pharmacodynamic responses of NMBDs at the diaphragm have shown a similar time course and degree of NMB as responses at the larynx. The most difficult aspect of neuromuscular monitoring at the diaphragm is stimulation of the phrenic nerve. Stimulation must be done using either needles or a hand-held stimulator since the branches of the vagus nerve are deep; stabilization of the position of the stimulator is the main problem during continuous measurement evaluating onset or offset of NMB.
Neuromuscular monitoring of the CS
The CS is a small muscle around the eyebrow, responsible for vertical frowning. Recent studies have used AMG,64,82 and PMG21,42 to record the muscle response. Acceleromyography, although well established for the AP, might be problematic at this muscle because of the limited capability of the conventional acceleromyographic probe to detect acceleration created by this small muscle. The original acceleromyographic probe used with commercially available devices was originally designed for detecting acceleration created by the much larger AP. The TOF-watch SX (now distributed by Philips company, New York, NY, USA) has been designed with a different sensitivity set-up; the sensitivity can now be increased to 500 µC. A recent comparison of AMG and PMG to detect NMB at the CS has raised questions as to whether the TOF-watch SX is suitable to properly detect the neuromuscular response at this muscle.42 Another recent study has tried to measure the actual force created by the CS using an air-filled balloon as a pressure transducer, and found good agreement between PMG and this MMG-like method.21
Monitoring NMB at the flexor hallucis brevis muscle (great toe)
Several studies have investigated the pharmacodynamic behaviour of the flexor hallucis brevis muscle.83–87 with inconsistent results. The best agreement with the AP was observed in a study in infants85 where there was no significant difference in onset time, peak effect or recovery from NMB of vecuronium 0.1 mg·kg–1 comparing the AP and the flexor hallucis brevis muscle. Both muscles were monitored using AMG. Most studies in adults have found similar peak effects but significantly longer onset times and shorter recovery of NMB at the great toe in comparison to the hand muscles.83,84,86,87 Whereas higher blood flow to the hand88 may possibly explain a faster onset at the hand muscles, morphological differences between the AP and the flexor hallucis brevis muscle might be responsible for the difference in recovery time. The AP consists of a higher content of type I (slow twitch) fibres than the flexor hallucis brevis muscle, which are known to be more sensitive to non-depolarizing NMBDs.89 The pharmacodynamic differences between the great toe and the hand muscles limit the usefulness of the great toe as a replacement for monitoring NMB of the hand muscles in surgeries where access to the hand is impossible or movement of the hand is impaired.
The vastus medialis muscle
The vastus medialis muscle has been presented as an alternative monitoring site to the muscles of hand or foot. Saitoh et al.90 showed that neuromuscular monitoring is possible at the vastus medialis muscle by stimulating the muscular branches of the femoral nerve, and quantifying the response of this muscle. In this study, the vastus medialis muscle, monitored by AMG, showed a faster onset of NMB than the AP. A possible explanation for these observations is a greater proportion of type II muscle fibres within the vastus medialis compared with the AP. A more recent study,91 using PMG, confirmed the shorter onset time, a less pronounced maximum effect, and a more rapid recovery of NMB after mivacurium 0.2 mg·kg–1 at the vastus medialis muscle than at the AP. However, the vastus medialis muscle should be considered an alternative monitoring site for those surgeries where there is no access to hand or foot muscles, taking into account the considerable pharmacodynamic differences compared with the AP.
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Recommendations for optimal monitoring of neuromuscular function |
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TOPAbstractIntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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Since muscles react differently with respect to onset, recovery, and the degree of NMB, an important means of providing ‘optimal’ relaxation for the surgical site should consider monitoring muscles of the surgical site, or alternatively, muscles which adequately reflect NMB at the surgical site. There are few studies which have investigated this concept in practice. One might consider for example, monitoring the eye muscles of the opposite eye, or at least the facial muscle of the contralateral site, to ensure profound NMB during retino-vitreal surgery. Monitoring of the AP does not reflect NMB at the eye muscles. Monitoring the AP as an indicator for relaxation of the thorax is also an inadequate. As described above, complete NMB at the AP does not necessarily mean complete NMB at the diaphragm. Monitoring NMB at the CS better reflects the degree of NMB at the surgical site during thoracic surgery.
The following concept is proposed: if surgery necessitates NMB of a certain degree, e.g., TOF 0.25, monitoring the muscle (or muscles) which best reflects the degree of NMB at the surgical site should be used. In general, for surgery of the upper or lower extremities, monitoring of the AP – or any other hand muscle - should be preferable. For surgery within the chest or abdomen where relaxation of the diaphragm is necessary, monitoring of the CS could be used.
Monitoring CS for intubation time
Current best evidence suggests that monitoring of the CS should be used to establish the earliest time for optimal conditions for tracheal intubation (e.g., for rapid sequence induction) as the CS reflects laryngeal relaxation better than monitoring of the AP.
Monitoring recovery from NMB
Recovery of neuromuscular transmission is best monitored at the AP since, in general, this is the last muscle group to recover from NMB. Train-of-four stimulation with a TOF-ratio > 0.9 indicates sufficient recovery of neuromuscular transmission for awakening the patient and ensuring safe tracheal extubation.
Subjective evaluation of NMB
Evaluation of clinical signs of recovered neuromuscular transmission after general anesthesia can be helpful, but requires that the patient is well awake, and should not replace objective neuromuscular monitoring.
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Conclusion |
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APPENDIX – List of definitions: |
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TOPAbstractIntroductionRationale for routine monitoring...Techniques to monitor...Stimulation patterns for...Influence of monitoring site...Recommendations for optimal...ConclusionAPPENDIX - List of...References |
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Footnotes |
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Accepted for publication May 18, 2005. Revision accepted August 17, 2006. Final revision accepted September 13, 2006.
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