Todd, Michael M. MD; Hindman, Bradley J. MD; King, Brian J. BA
Residual neuromuscular blockade in the postanesthesia care unit (PACU) is associated with clinical signs and symptoms of weakness,1 measurable respiratory and pharyngeal dysfunction,2–4 and an increased risk of respiratory complications in the postoperative period.5,6 Based on quantitative train-of-four (TOF) measurements, as many as 40% of patients arriving in the PACU may have evidence of residual blockade, depending on the TOF ratio criteria chosen.7,8 As a result, there have been increasing calls for more frequent use of quantitative assessments to guide both the administration and reversal of nondepolarizing drugs.9–14
However, despite the evidence, quantitative measurements have not been widely adopted. Most anesthesia providers, if they rely on any assessment at all, continue to depend on either clinical signs (head lift, negative inspiratory force) and/or visual/tactile assessment of the TOF to titrate and assess reversal of nondepolarizing neuromuscular blocking drugs (NMB).15 Clinical signs are of limited value (and can only be performed on awake patients), and even qualitative visual/tactile twitch assessments are questionable.16 Specifically, although visual/tactile observation of fade is a highly specific indicator of residual paralysis, it has a very low sensitivity; clinicians cannot reliably detect fade when the TOF ratio is >0.3 to 0.4.17
In this report, we describe one department’s successful efforts to implement universal quantitative monitoring, using an electromyographic (EMG) system. We specifically hypothesized that an ongoing program of education, monitoring, and repetitive provider feedback would reduce the incidence of inadequately reversed patients being admitted to the PACU. Our findings also discuss some of the reasons that make adoption of quantitative monitoring methods difficult.
The data presented in this article were gathered as part of a departmental quality-improvement project registered with the University of Iowa Hospitals and Clinics Clinical Quality, Safety, and Performance Improvement Office. Publication of these results was approved by the University of Iowa IRB.
In mid-2010, the senior authors (MMT and BJH) became concerned about the misuse of NMBs by clinical care providers. Examples include patients being taken to the PACU without reversal because it had been >2 hours since the last rocuronium dose, providers administering neostigmine, then tracheally extubating patients shortly thereafter without any effort to assess the degree of blockade, and providers stating that it was appropriate to extubate patients because “the patient is breathing spontaneously and has 4 twitches,” etc. At the same time, a review of our department’s Morbidity and Mortality database (approximately 1200 cases gathered over 5 years) identified 11 patients who required reintubation in the PACU for respiratory difficulties that were definitely or probably due to residual paralysis. Another 9 reintubations were deemed to be possibly related. Many of these cases were not recognized as being relaxant-related at the time of their presentation.
Based on the above, we reexamined our neuromuscular monitoring capabilities and alternatives. At the time (mid-2010), quantitative blockade monitoring modules and piezoelectric mechanotransducers (Datex-Ohmeda MechanoSensors™; Datex-Ohmeda Inc., Madison, WI) were available on request. However, these were not being used, in part because of perceived unreliability. As a result, all monitoring was based on qualitative twitch assessments using free-standing, battery-powered peripheral nerve stimulators that could be obtained from the anesthesia workroom on request, but were not available in every operating room (OR).
After an initial evaluation period in late 2010, a decision was made in January 2011 to equip all 30 main OR suites (now 32 rooms) with neuromuscular monitoring modules and Datex-Ohmeda EMG ElectroSensor™ systems (Datex-Ohmeda Inc., Madison, WI). The ElectroSensor system requires the placement of 5 standard electrocardiogram surface electrodes, 2 over the ulnar nerve, 1 over the adductor pollicis, 1 over the distal first digit, and a ground lead over the median nerve at the distal crease of the wrist (Fig. 1, top). The system displays a bar graph showing the amplitude of each twitch along with the ratio of the height of the 4th-to-1st twitch (TOF ratio). If monitoring is initiated before the initial administration of NMB, the system will also determine a supramaximal stimulus (in mamps) and displays the height of the first twitch of the TOF as a percentage of baseline. In addition, the system can be configured to display the EMG waveforms for each twitch.
Over the next 6 months, only modest progress was made. Although the introduction of the system was accompanied by an extensive educational program, including the circulation of multiple articles documenting the hazards of inadequate reversal,8,9,12,16,18,19 many providers found the system difficult to use and/or were unconvinced as to its value. Review of a sample of anesthesia records indicated that the quantitative TOF monitoring system was being used in <50% of patient given nondepolarizing NMBs.
In July of 2011, a sentinel event occurred. A patient admitted to the PACU required emergent reintubation due to profound residual paralysis. This occurred despite the administration of neostigmine shortly before extubation and the provider stating that the patient had 4 twitches based on the use of a qualitative nerve stimulator. As a result, we embarked on a more systematic and aggressive program to encourage use of the quantitative system.
First, the sentinel case was discussed at our departmental Morbidity and Mortality conference, which is attended by all providers. Second, our Epic anesthesia information management system (AIMS) was modified to automatically record neuromuscular monitoring data (twitch count and TOF ratio). Third, the correct functioning of the monitoring systems was confirmed in every OR, and the default mode for all anesthesia monitors was reconfigured to display both the quantitative twitch information and the EMG waveform. Fourth, we began a series of PACU surveys to better document the incidence of inadequate reversal in patients receiving nondepolarizing NMBs.
The PACU survey program began in August of 2011. Five separate surveys were conducted through July 2013 (August 2011, November 2011, March 2012, December 2012, and July 2013). TOF ratio determinations were made by a single individual (BK), using the same Datex-Ohmeda EMG system as used in the OR. Measurements were made on 409 tracheally extubated, adult, largely elective patients who had received nondepolarizing relaxants (almost exclusively rocuronium). Seventy-three other patients who had not received any nondepolarizing relaxants were also studied. All measurements were made as soon as possible after patient arrival in the PACU, after initial vital signs had been obtained by nursing. Whenever possible, 4 separate sequential measurements of the TOF ratio (at approximately 20-second intervals) were made in each patient (4 measurements were successfully obtained in 93% of patients with 3% of patients having only 1 or 2 measurements). The TOF ratio was recorded as the mean of the available measurements. Although an effort was made to take measurements in a sequential fashion on all eligible patients entering the PACU, this was limited by having only 1 individual and 1 monitoring system. Hence, the sample did not include all patients. No effort was made to select specific types of patients or patients having specific operations. Also, due to limitations in the availability of BK, the sample sizes for each survey varied (Table 1). Because of these factors, our sample must be considered as one of convenience.
In addition to the quantitative measurements, the anesthetic records of all patients examined in the PACU in August 2011 and in December 2012 were reviewed by both MMT and BK to assess the documentation of (a) any form of neuromuscular monitoring to assess reversal (even monitoring recorded as a text note), (b) the use of the quantitative monitoring system in any fashion to assess reversal, and (c) the appropriate use of the quantitative monitoring system to document the adequacy of reversal during each case (with a target TOF ratio of 0.8 or higher). This later category required a recorded twitch count and TOF ratio before reversal, along with a progressive improvement in count/ratio after reversal. Documentation of progressive spontaneous reversal without the use of neostigmine was also acceptable.
After each survey session, the quantitative monitoring results were compiled and presented to the department by MMT at a weekly conference. This was accompanied by the continued distribution of added educational materials.
We also hypothesized that any possible changes in PACU reversal status might be due to a change in the patterns of use of either NMBs or neostigmine (incidence or dose). To determine whether such changes were present, the proportion of cases using rocuronium and/or neostigmine, as well as the total dose of each drug, was retrieved from our Epic AIMS system for 3-month periods, encompassing both the August 2011 and December 2012 survey periods. Doses of rocuronium and neostigmine were expressed in mg, mg/kg of body weight, and mg/kg/min of case duration.
Our statistical assessment was focused on a comparison of the PACU sampling survey data obtained in August 2011 and December 2012; survey information from November 2011 and May 2012 were used solely to provide feedback to the department on progress. Survey information from July 2013 was used only to determine whether or not any possible improvements observed between the August 2011 and December 2012 samples were sustained.
Although no formal a priori sample size calculations were performed, we did plan to obtain measurements on approximately 100 patients receiving NMBs during the August and December surveys. In retrospect, our power to detect a 50% reduction in the proportion of patients with a TOF ratio of ≤0.9 (e.g., from 30% to 15%) was approximately 66%. Comparisons between the proportion of patients with TOF ratios of ≤0.5, ≤0.8 and ≤0.9, as well as the proportion of patients in whom neuromuscular monitoring was used were performed by Fisher exact test. In addition, we compared the mean and median TOF ratios between patients with and without quantitative monitoring using a 2-tailed t test and by Mann-Whitney U. TOF ratios are expressed both as mean ± SD and median (interquartile range [IQR]). All analyses were performed with JMP 10.0 (SAS Institute, Cary, NC). A P value of <0.05 was considered significant.
PACU survey results and use of monitoring for all 5 monitoring sessions are shown in Table 1.
In August 2011, the overall mean TOF ratio was 0.90 ± 0.18 (mean ± SD) with a median of 0.94 and IQR of 0.87 to 0.99. Four percent of patients had a TOF ratio ≤0.5, 17% ≤0.8, and 31% ≤0.9. By December 2012, the mean TOF ratio had increased to 0.95 ± 0.08 (median 0.98, IQR 0.94 to 1.00) with the proportion of patients with TOF ratios of ≤0.5, ≤0.8, and ≤0.9 having decreased to 0%, 5%, and 15%, respectively. All comparisons between the 2 monitoring sessions, except for the differences in the proportion of patients with TOF ratios ≤0.5, were statistically significant. Use of neuromuscular monitoring increased significantly for 2 of the 3 categories (patients with any documented use of quantitative TOF to assess reversal and patients with quantitative TOF used to appropriately assess reversal) by nonparametric testing.
The distribution of TOF ratios for all patients receiving NMB during the August 2011 and December 2012 survey periods is shown in Figure 2.
The lowest TOF ratio for any patient not receiving a nondepolarizing NMB was 0.92 (mean 1.00 ± 0.03, median 0.99, IQR 0.98–1.00).
Based on the survey for July 2013, the improvement seen between August 2011 and December 2012 appears to have been maintained (no differences versus December 2012).
During the July to September 2011 quarter, 65% of patients receiving a general anesthetic in the main operating suite received rocuronium. The mean total dose for the case was 67 ± 46 mg (SD). Adjusted doses were 0.91±0.82 mg/kg or 4.25 ± 2.97 ug/kg/min of case duration. Sixty-three percent of patients given rocuronium received neostigmine. For the October to December 2012 period, 63% of patients receiving a general anesthetic received rocuronium, with a mean total dose of 67 ± 43 mg, and adjusted doses of 0.87 ± 0.58 mg/kg and 4.18 ± 2.42 ug/kg/min of case duration. Sixty-three percent of patients given rocuronium received neostigmine. There were no significant differences between July to September 2011 and October to December 2012 in any of the above noted values. There were also no differences in either the fraction of patients receiving neostigmine or the doses administered.
In an additional analysis, we compared the measured PACU TOF ratios between patients who did and did not have documented evidence of twitch monitoring in the 3 categories. These results are shown in Table 2. For 2 of the 3 categories (patients with any documented use of quantitative TOF to assess reversal, patients with quantitative TOF used to appropriately assess reversal), the TOF ratios were higher in the monitored patients by both t test and Mann-Whitney U.
An updated review of our morbidity and mortality database in July 2013 revealed no new PACU reintubation events since the summer of 2011 that could be attributed to residual paralysis.
We describe one department’s effort to improve the use and monitoring of nondepolarizing NMBs. Our experiences demonstrate that substantial improvement is possible, but that there are a number of obstacles. These obstacles have been described,14 but there is a very limited literature on how to overcome them.
Traditional quantitative TOF monitoring methods using strain-gauge transducers are not readily applicable to routine clinical use. Accelerometry (e.g., as used in the TOF Watch™) does not work unless free movement of the thumb and hand is permitted. In addition, the currently available device is not integrated into the OR monitoring system and does not (at present) interface with an AIMS. The piezoelectric mechanotransducer provided by Datex-Ohmeda (MechanoSensor) is also problematic. Free movement of the thumb and first finger is required, and only a single-size transducer is available. At the time this effort was undertaken no other commercially available quantitative monitors (other than the Datex-Ohmeda 5-lead EMG system [ElectroSensor]) could be identified.
Based on this, and despite the lack of any literature formally evaluating it, we chose to install the ElectroSensor™ system in all our main OR suites. We discovered that training providers in the proper use of the system was much more difficult than anticipated. The system requires precise placement of 5 electrodes; even small errors in placement can sometimes result in spurious recordings. Artifactual recordings are also common, and users must be trained to recognize these. Accurate use was not possible when only the bar graph display was used (Fig. 1, bottom right). We hence reconfigured all our monitors to routinely display the EMG waveforms. We recognized that the T1% value frequently failed to recover to a 100% value even with a TOF ratio of 1.0. These difficulties generated a great deal of early cynicism on the part of providers, something that required a substantial educational effort to overcome. However, once providers became familiar with observing the EMG waveforms and learning to recognize the presence of artifacts and erroneous information, acceptance improved.
As we embarked on this program, we were surprised to discover that providers had a very poor understanding of the pharmacology of the nondepolarizing relaxant used most commonly in our institution, rocuronium. The general belief was that rocuronium was a short-acting drug that could be redosed with some impunity and that was easily and quickly reversed with neostigmine, even in the presence of only a single visible twitch. In addition, providers had a poor understanding of the clinical signs of adequate reversal or, more importantly, their limitations. To address this, we collected and circulated a library of twitch recordings from carefully monitored patients in whom each of these “beliefs” could be demonstrated to be false. Nevertheless, even 14 months after making quantitative monitoring available to all providers in all rooms (e.g., in May 2012), we still encountered cases of inadequate reversal associated with either a failure to use the available monitor or failure to use the collected information in an appropriate fashion (e.g., extubating patients and transporting them to the PACU despite displayed TOF ratios indicating substantial residual paralysis). Only after almost 2 years of continued effort did we see an improvement in the use of the system and a concomitant reduction in the incidence of inadequately reversed patients arriving in the PACU, although we continue to encounter sporadic failures (see July 2013).
To the best of our knowledge, there is only one other published summary of a department-wide effort to implement quantitative monitoring. Baillard et al.20 implemented quantitative TOF monitoring in their ORs between 1995 and 2004, using the TOF-Guard™ or TOF-Watch™ accelerometry systems (Bluestar Enterprises, Inc., Omaha, NE). Like us, they saw no change in the use of NMBs. However, they did see a substantial increase in the use of reversal drugs (from only 5.6% to 42%). They also observed an increase in TOF monitoring use from 2% to 60% and a reduction in the fraction of PACU patients with at TOF ratio <0.9 from 62% to only 3.5%. It is difficult to directly compare their experience with that of ours, in particular because of the rarity of neuromuscular reversal during their baseline period. It is unclear whether or not the improvement they saw in PACU reversal status was due to monitoring, improved education, or the more frequent use of reversal drugs. By contrast over the much shorter observation period of our study, neither the use of rocuronium nor neostigmine changed, suggesting the observed improvement was more likely due to better monitoring. Nevertheless, we cannot determine the precise reason for the improvement. For example, we cannot determine whether the improvement was due to a change in the timing of NMB administration toward the end of the cases, longer waiting times after the end of surgery (although we saw no global changes in overall case durations), or even a larger fraction of patients being taken still intubated to the PACU. It is also entirely possible that the improvement was not directly due to monitoring but rather was related to a general improvement in the understanding of nondepolarizing drug pharmacology by our providers, related to both our educational efforts and the direct knowledge gained by observing (via monitoring) the patterns of response to drug administration.
A number of lessons can be gleaned from the above.
First, the Datex/Ohmeda electromyographic Electro Sensor™ system can be successfully used as a routine intraoperative monitoring system. One of its major advantages is that it is an integrated part of a complete OR monitoring system that easily communicates with an AIMS. This allows for administrative monitoring of the use of the device and permitting easy review of anesthetic records to determine reversal status. Nevertheless, we believe that there is a need for an easier-to-use system. The ElectroSensor™ system is reliable and robust in well-trained hands. It does fail in a few patients, particularly those in whom good electrode contact is difficult (e.g., patients with very thick skin) and cannot be used when access to the hand or wrist is impossible (e.g., patients with casts or dressings); we have been unsuccessful in applying the system at alternative sights (e.g., the posterior tibial nerve). Monitoring will also sometimes fail over time, particularly when access to the leads is impossible (e.g., when arms are tucked), although it can usually be reestablished after removal of the drapes (before reversal). However, use of the system requires a much greater educational effort to implement that we originally anticipated. Without providing a visual representation of the EMG waveform, providers frequently misinterpreted the bar graph/digital display. In particular, they failed to recognize major artifacts and hence often concluded that the system did not work, rather than working to improve monitoring conditions (which can almost always be achieved by more careful placement of electrodes, efforts to reduce skin impedance and, on occasion, resetting the monitoring module). Even after providing the EMG waveform display, it took more than a year to achieve near-universal adoption and correct use of the system.
Second, changing culture is exceptionally difficult. Convincing practitioners who have never used quantitative monitoring that such measurements are important to the well-being of their patients was time consuming. For example, we encountered experienced practitioners who, even despite the circulation of scientific studies and review articles, questioned the need for quantitative monitoring and expressed their belief that qualitative methods were sufficient. This may not be surprising. Most of the complications of inadequate reversal are not dramatic; only 2 of 53 monitored PACU patients with TOF ratios ≤0.8 reported any respiratory difficulties (one of whom had only 2 twitches, the other had a TOF ratio = 0.31) and neither required reintubation. Our hospital performs approximately 30,000 anesthetics per year. The 20 cases of probable/possible paralysis-related PACU reintubations recorded in our database hence is an event rate of approximately 20 in 150,000 cases (from 2006 through 2010). Assuming that these events were distributed evenly over 60 faculty members, less than one-third of providers might have personally encountered such an event during this 5-year period. It is also important to note that many of these cases were not recognized as being relaxant-related at the time, making it even less likely that an individual provider would knowingly connect “reversal status” with a reintubation. This is consistent with the observations of Naguib et al.,15 whose survey indicated that 88% of respondents “had never observed patients in the postanesthesia care unit with residual neuromuscular weakness after intraoperative administration of a muscle relaxant.” It is therefore not surprising that the status quo was unchallenged by most providers.
Recognition of this cultural reality was what prompted us to move beyond simple education and new equipment. Repeatedly demonstrating that an unacceptably large fraction of patients in the PACU were inadequately reversed (sometimes dangerously so), presenting the details of management/mismanagement of those patients, reviewing the results of our case reviews and PACU surveys, along with circulating each new publication in the area was needed.
While we report on a successful implementation program, it is important to recognize some of the practical realities. The first is cost. The cost to equip an OR with the module, cable, and 5-lead sensor was approximately $1900, or a total cost of approximately $57,000 for 30 rooms. The second is that implementation required repeated rounds of postoperative monitoring and feedback of the resultant information to all providers. This, in turn, requires manpower to gather the information, and it requires the cooperation of providers and PACU personnel. We were fortunate to encounter no resistance from either; in fact, several PACU nurses voiced the opinion that the effort was overdue. The third, and most critical, is that persistent effort is required on the part of departmental leadership (and senior staff). As noted above, simply providing equipment, sending out a few messages, and expecting change to occur are unlikely to succeed.
We made no attempt to expand this effort to a pediatric surgical population. Although individual providers have successfully used the EMG system in children as young as 2 years of age, we have had little success with younger children or with infants. We should note, however, that none of the cases of PACU reintubations found in the departmental database involved children, nor is there meaningful literature regarding quantitative monitoring, residual paralysis, and postoperative problems in children and infants. In part, this may stem from the less frequent use of nondepolarizing relaxants in the very young. As noted, we have also not succeeded in reliably using the EMG system in anatomical locations other than the ulnar nerve (e.g., posterior tibial nerve), which, on occasions, makes monitoring difficult if not impossible until the end of surgery. We attempted to capture data on as many patients arriving in the PACU as possible and made no effort to select any specific types of patients or procedures, but we must also recognize that some inadvertent selection bias may have occurred. For example, we tended to miss more patients during the midday when PACU arrivals tend to peak, and we did not include many emergency cases or those transported to the intensive care unit, a group that may include many higher-risk patients. The amount of information collected under the auspices of this quality-improvement project was also limited, and as a result, we were unable to perform any in-depth analysis of patient, procedure, or provider-specific factors that might have been associated with PACU reversal status. Finally, it is important for the reader to understand that this project was not undertaken as a formal, preplanned scientific experiment. For example, we never planned to do >2 PACU surveys because we originally failed to recognize the difficulties in achieving an improvement. As a result, we acknowledge that many important questions regarding NMB use and impact simply cannot be addressed.
Although our PACU survey data indicate substantial improvement in our practice, we still have not achieved a 100% incidence of adequate reversal (defined as a TOF ratio of either >0.8 or >0.9). In part, this is an expected consequence of our educational effort and stated departmental goals and targets. That effort focused on (1) avoiding foolish errors (e.g., tracheally extubating patients without knowing anything about their blockade status) and (2) on the use of “intelligent judgment” regarding reversal and extubation, rather than rigidly focusing on a specific TOF ratio. Specifically, we have not insisted that patients who are reacting vigorously to the endotracheal tube remain intubated until the TOF ratio exceeds some given threshold. In such situations, and when otherwise appropriate, (e.g., young patients, unremarkable airways, minimal underlying respiratory disease, no history of sleep apnea, etc.), removal of the endotracheal tube before complete reversal is acceptable. Providers are encouraged to either remain in the OR until reversal is complete or to transport the patient to the PACU, inform the PACU nurses of possible incomplete reversal, and insure that a physician can remain in close attendance until adequate airway patency and respiratory function are verified. Accordingly, we would expect a small percentage of patients with modest residual paralysis to arrive in the PACU, particularly patients with a TOF ratio between 0.8 and 0.9. In addition, it is not obvious that 100% complete reversal is clinically feasible. In the well-controlled study of Murphy et al.,21 patients were randomized to groups with and without intraoperative quantitative monitoring. All aspects of anesthetic management were standardized. It was recommended that extubation occur at a TOF ratio ≥0.8, although this could be performed earlier if the patients were unable to tolerate the endotracheal tube. Even with this degree of careful control, 14.5% of patients in the quantitative monitoring group had TOF ratios in the PACU of <0.9 (as compared with 15.1% in our completely uncontrolled final sample). This contrasts dramatically with the work of Baillard et al.,20 who showed that fewer than 5% of patients in the latter stages of their work had TOF ratios <0.9. However, the time frame of the work by Baillard et al.20 was much longer than either our work or that of Murphy et al.,21 and perhaps prolonged experience will result in even lower rates of inadequate reversal. In addition, the recent study by Liang et al.22 suggests that acceleromyometry overestimates recovery as compared with EMG.
Finally, we made no efforts to document any impact of our monitoring initiative on conditions in the PACU (e.g., SpO2) other than the subjective expression of respiratory difficulties and the incidence of emergent reintubations. We have not examined the relationships between reversal status and end points such as postoperative pneumonia or other respiratory problems, hospital length of stay, or mortality. While we are comfortable in concluding that a reduced incidence of inadequate reversal is a valuable achievement, our profession would benefit from a larger, prospective, multicenter effort to demonstrate the relationship between PACU reversal status and longer-term outcomes. Such a study would also make it possible to better define clinically meaningful reversal targets in specific patient conditions. We also believe that the use of neuromuscular blockade monitoring deserves greater attention by organizations charged with the development of professional standards and guidelines. It is perhaps surprising that the most recent (2011) American Society of Anesthesiologists Standards for Basic Anesthesia Monitoring makes no mention of any form (qualitative or quantitative) of neuromuscular blockade monitoring or assessment, and, to the best of our knowledge, no national anesthesia organization addresses the value of quantitative monitoring.
We have demonstrated that the introduction of an EMG-based quantitative twitch-monitoring system, in combination with extensive educational efforts, is feasible and can result in a significant reduction in the number of inadequately reversed patients arriving in the PACU. However, the introductory process can be daunting, particularly when the daily experiences of individual providers do not routinely demonstrate the existence or severity of a problem. The solution requires a diligent effort with repeated feedback to providers. However, once achieved, the change in practice culture can be sustained.
Name: Michael M. Todd, MD.
Contribution: This author, along with Bradley J. Hindman, was the primary instigator of the project, reviewed all data, performed the statistical analyses, and was the primary author of the manuscript. Along with Brian J. King, he performed the required chart reviews. He is also responsible for the analysis of data retrieved from the Departmental AIMS system (e.g., rocuronium and neostigmine use).
Attestation: Michael M. Todd attests to the accuracy of all information provided in this document and served as both the corresponding author and the archival author.
Name: Bradley J. Hindman, MD.
Contribution: This author worked closely with Michael M. Todd on all aspects of this project, including its organization and execution. He authored much of the Department-wide educational effort, reviewed all data, and extensively edited the final manuscript.
Attestation: Bradley J. Hindman attests to the accuracy of all information provided in this document.
Name: Brian J. King, BA.
Contribution: This author personally performed almost 90% of the PACU monitoring, collected the original twitch-monitoring data, reviewed and edited all these data along with Michael M. Todd, participated in the needed chart reviews, and reviewed and edited the final manuscript.
Attestation: Brian J. King attests to the accuracy of all PACU monitoring data provided in this document.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
The authors would like to thank Nick Lepa, medical student, who assisted with the final round of PACU monitoring (July 2013), Franklin Dexter, MD, PhD, for editorial comments regarding the manuscript, Emine Bayman, PhD, for her statistical assistance, and David Papworth, MB, FRCA, Departmental Quality Management Officer for his ongoing suggestions and support.