The median TOF count at reversal was 4 (range, 1–4). The average time interval between neostigmine administration and TOF measurements before tracheal extubation was 8 ± 6 min. Immediately before tracheal extubation, the mean TOF ratio was 0.67 ± 0.2. Among the 120 patients, 70 (58%) had a TOF ratio <0.7 and 105 (88%) had a TOF ratio <0.9 at this time (Fig. 1). Overall, the average time interval between neostigmine administration and tracheal extubation was 11 ± 7 min (12 ± 8 min in patients with TOF ratios <0.7 and 8 ± 5 min in patients with TOF ratios ≥0.7).
The mean TOF ratio measured on arrival to the PACU was 0.95 ± 0.15. The average time from neostigmine administration to TOF measurements in the PACU was 19 ± 7 min. Significantly fewer patients had TOF ratios <0.7 (9 subjects, 8%) and <0.9 (38 subjects, 32%) in the PACU compared with the OR (70 and 105 subjects, respectively) (P < 0.001) (Fig. 1).
No association was observed between severe residual paresis (TOF ratio <0.7) and any patient demographic or intraoperative variable.
No patients recalled supramaximal nerve stimulation when performed in the OR immediately before tracheal extubation. Only 9 patients (8%) remembered TOF measurements that were recorded in the PACU. The mean VAS score in these 9 subjects was 25 ± 13 mm. No patient reported VAS scores >50 mm on a 100-mm scale.
Residual neuromuscular blockade is a well recognized problem in the PACU. The incidence and severity of residual paresis at the time of tracheal extubation have not been examined. In the present investigation, we observed that acceptable neuromuscular recovery (TOF ratio >0.9) was present in only a small percentage (12%) of patients immediately before removal of the endotracheal tube. Quantitative neuromuscular monitoring is required to detect residual blockade. We also determined that supramaximal stimulating currents can be used in patients emerging from anesthesia; only 8% of patients recalled TOF-Watch monitoring when performed in the OR or PACU using 50-mA stimulation currents.
Complete recovery of neuromuscular function should be present at the time of tracheal extubation to reduce the risk of adverse respiratory events. Recent studies have demonstrated that respiratory and pharyngeal function do not normalize until TOF ratios of 0.8–1.0 are obtained. Eikermann et al. (5) observed that impaired inspiratory flow and upper airway obstruction occurred frequently at a TOF ratio of 0.83. Pharyngeal function and airway protection were impaired at TOF ratios of 0.9 in awake volunteers (4). The hypoxic ventilatory response was reduced by approximately 30% in awake volunteers with TOF ratios of 0.7 (6), and an association between mild residual neuromuscular block and postoperative hypoxemia has been described (7). These findings suggest that removal of an endotracheal tube in the presence of minimal levels of residual block can potentially contribute to adverse pulmonary outcomes. The results from a large, prospective, randomized outcome study support this hypothesis. In a clinical study of 691 patients, residual block caused by pancuronium was associated with a 3-fold increased risk of postoperative pulmonary complications (9).
There is a growing consensus that the new benchmark for the adequacy of neuromuscular recovery should be a TOF ratio of ≥0.9. Few patients (15 of 120 subjects, or 12%) in our study achieved this benchmark at the time of tracheal extubation. Severe residual paresis (TOF <0.7) was noted in 70 patients (58%) at the time the anesthesia care provider had judged the block to have recovered sufficiently to exclude residual paralysis. The inability of clinicians in our investigation to detect residual block in the OR is not unexpected. A 5-second head lift or hand grip (used by all clinicians in our study) can be maintained in some postoperative patients with TOF ratios as low as 0.25–0.4 (10,11). The use of a peripheral nerve stimulator in the OR may reduce, but does not eliminate, the problem of postoperative paresis (11). Detection of incomplete reversal of neuromuscular blockade is difficult with standard TOF or tetanic stimulation. Experienced observers are unable to detect fade when the TOF ratio is >0.4 (12). Our results demonstrate that few patients had achieved TOF ratios ≥0.9 at the time of tracheal extubation, even when careful clinical examinations were performed and peripheral nerve stimulators were routinely used.
There is a “period of vulnerability” between the time of tracheal extubation and that of complete recovery of neuromuscular function during which a patient may be at risk for adverse respiratory events (13). During this immediate postoperative period, airway obstruction, aspiration of gastric contents, and ventilatory depression are the three most common, severe anesthetic-related complications (14). Previous investigators have examined the incidence of postoperative residual paralysis in the PACU. In these studies, TOF measurements were typically obtained 15–30 minutes after neuromuscular reversal and tracheal extubation (1,9,15). The incidence of incomplete neuromuscular recovery during the immediate recovery period (from tracheal extubation until stabilization in the PACU) has not been previously determined. Our results demonstrate that neuromuscular recovery is seldom complete in the OR and during patient transport to the PACU. A significant reduction in the incidence of residual paresis was noted by the time TOF measurements were obtained in the PACU. Only 9 patients (8%) had TOF ratios <0.7 and 38 patients (32%) had TOF <0.9 at this time. These findings suggest that the “period of vulnerability” is relatively short, and neuromuscular recovery rapid, when intermediate-acting muscle relaxants are used and routinely reversed. However, nearly one-third of patients failed to achieve our clinical benchmark (TOF ≥0.9) in the PACU, approximately 20 minutes after neostigmine administration. The number of patients with residual paralysis in the PACU would likely have been even more if clinicians had not been instructed to delay tracheal extubation in patients with TOF ratios <0.6–0.7.
Kopman et al. (16) have proposed that reports of residual weakness may merely represent an “artifact of improper anesthetic management.” In many previous studies, the use of neuromuscular monitoring and reversal drugs was at the discretion of the managing anesthesiologist and inconsistently applied (2,7,15). In other investigations, monitoring (1) and reversal (3) of neuromuscular blockade was avoided in the perioperative setting. Our protocol incorporated several well known methods to reduce the incidence of postoperative paralysis, which included: 1) use of peripheral nerve stimulators in the OR, 2) avoidance of total twitch suppression, 3) use of intermediate-acting muscle relaxants, and 4) administration of anticholinesterases at a TOF count of 2–4 (16). Despite adherence to a rigid protocol designed to limit postoperative paralysis, Kopman et al. (16) noted that 17% of patients had TOF ratios <0.9 in the PACU 30 minutes after reversal of a rocuronium neuromuscular blockade. In the present study, careful intraoperative management of muscle relaxant administration, monitoring, and reversal did not result in clinically acceptable levels of neuromuscular recovery at the time of tracheal extubation in the majority of our patients. Thus, the goal of a TOF ratio >0.9 may be difficult to achieve in all patients in the immediate recovery period.
Small, portable acceleromyography instruments have been developed for routine use in the perioperative setting. These devices can deliver stimulating currents ranging from 1 to 60 mA. Although TOF ratios may remain stable over a wide range of stimulating currents, supramaximal nerve stimulation is required to produce consistently accurate measurements in all subjects (17). Clinicians may be reluctant to use supramaximal stimulating currents in patients emerging from anesthesia, because electrical stimulation of nerves can result in painful muscle contractions. In awake, unmedicated volunteers, TOF stimulating currents of 50 mA resulted in median VAS scores of 5.0–6.0 on a scale of 10 (18). In the postoperative surgical patient, the amnestic and analgesic effects of potent inhaled drugs persist into the immediate recovery period. Therefore, we hypothesized that few patients would recall supramaximal nerve stimulation as painful when performed in the OR or PACU. No subjects recalled TOF stimulation when performed immediately before tracheal extubation. Only 9 patients (8%) remembered TOF measurements that were obtained in the PACU. Mean VAS scores in these 9 patients were low (25 ± 13), and no patients reported pain scores >50 mm on a 100-mm scale. Our findings suggest that 50-mA stimulating currents can be used during acceleromyography monitoring to detect residual paresis; few patients recall TOF measurements obtained in the early recovery period.
There are limitations to the current investigation. First, acceleromyography was used to quantify residual neuromuscular block. Several studies have noted a close correlation between TOF values obtained by acceleromyography and mechanomyography, a “gold standard” technique (19,20). Recent clinical investigations have demonstrated that acceleromyography may overestimate neuromuscular recovery, and that TOF ratios must recover to 0.95–1.0 to exclude residual paralysis (21,22). Therefore, the true incidence of impaired neuromuscular recovery may have been underestimated in our study. Second, there was a relatively short time interval between neostigmine administration and TOF measurements/tracheal extubation in the OR. The risk of residual block may be reduced if antagonism of rocuronium is initiated 20–30 minutes before tracheal extubation (16,23). Our protocol was designed to reflect standard clinical practices at our institution, however (neostigmine administered at the completion of the surgical procedure). Third, the significant reduction in residual paresis measured in the PACU may have been influenced by our study design, because clinicians were instructed to reverse neuromuscular blockade only at a TOF count of 2–3 and to delay tracheal extubation if the TOF ratio was <0.6–0.7. Fourth, only one set of TOF measurements was obtained in the OR. Complete neuromuscular recovery was not documented with acceleromyography before tracheal extubation. We believe patient safety was not compromised, because clinicians were instructed to delay tracheal extubation if severe residual paresis was noted. Finally, we used a 50-mA stimulating current in all subjects. In a small percentage of patients, a 50-mA stimulus may not represent a supramaximal current.
In conclusion, we determined that significant residual paralysis was present in the majority of patients at the anticipated time of tracheal extubation. Despite the use of a protocol directing strict monitoring and reversal of an intermediate-acting muscle relaxant, and the performance of a careful clinical examination for signs of muscle weakness, clinicians were consistently unable to achieve acceptable levels of neuromuscular recovery in the OR. In order for anesthesiologists to be assured that neuromuscular recovery is complete and that respiratory and pharyngeal muscle function has returned to normal, quantitative neuromuscular monitoring is required.
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© 2005 International Anesthesia Research Society
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