Neuromuscular effects and intubating conditions following mivacurium: a comparison with suxamethonium

Onset time, duration of action, and intubating conditions after mivacurium in elderly and younger patients

BACKGROUND

The neuromuscular blocking agent mivacurium can be used during anesthesia to facilitate tracheal intubation. Data on onset time, duration of action, and effect on intubating conditions in patients 80 years and older are however limited. We hypothesized that onset time and duration of action of mivacurium would be longer in elderly patients than in younger adults.

METHODS

This prospective observational study included 35 elderly (≥80 years) and 35 younger (18-40 years) patients. Induction of anesthesia comprised fentanyl 1-3 μg kg-1 and propofol 1.5-2.5 mg kg-1 and propofol and remifentanil for maintenance. Acceleromyography was used for monitoring neuromuscular blockade. The primary outcome was onset time defined as time from injection of mivacurium 0.2 mg kg-1 to a train-of-four (TOF) count of zero. Other outcomes included duration of action (time to TOF ratio ≥0.9), intubating conditions using the Fuchs-Buder scale and the intubating difficulty scale (IDS), and occurrence of hoarseness and sore throat postoperatively.

RESULTS

No difference was found in onset time comparing elderly with younger patients; 219 s (SD 45) versus 203 s (SD 74) (difference: 16 s (95% CI: -45 to 14), p = .30). Duration of action was significantly longer in elderly patients compared with younger patients; 52 min (SD 17) versus 30 min (SD 8) (difference: 22 min [95% CI: 15 to 28], p < .001). No difference was found in the proportion of excellent intubating conditions (Fuchs-Buder); 31/35 (89%) versus 26/35 (74%) (p = .12) or IDS score (p = .13). A larger proportion of younger patients reported sore throat 24 h postoperatively; 34% versus 0%, p = .0002. No difference was found in hoarseness.

CONCLUSION

No difference in onset time of mivacurium 0.2 mg kg-1 was found comparing elderly and younger patients. However, elderly patients had significantly longer duration of action. No difference was found in intubating conditions.

The comparative effect of single dose mivacurium during sevoflurane or propofol anesthesia in children

BACKGROUND

We aimed to randomly compare intubating conditions, recovery characteristics and neuromuscular effects of single dose of mivacurium (0.2 mg.kg(-1)) during sevoflurane vs. propofol anesthesia in 60 healthy children, undergoing inguinal surgery.

METHODS

All children were randomly allocated to receive 2 mg.kg(-1) propofol iv or sevoflurane 8% inspired concentration for induction of anesthesia. Anaesthesia was maintained with 66% nitrous oxide in oxygen and 100-120 microg.kg(-1) propofol or sevoflurane approximately 2-3% inspired concentration with controlled ventilation. The ulnar nerve was stimulated at the wrist by a train-of four (TOF) stimulus every 20 s and neuromuscular function was measured at the adductor pollicis. When the response to TOF was stable, 0.2 mg.kg(-1) mivacurium was given. The trachea was intubated successfully at the first attempt in all patients.

RESULTS

Onset time following a single dose of mivacurium was shorter in the sevoflurane group (2.99 min), than in the propofol group (4.42 min). The times to 25, 50, 75, and 90% recovery were significantly longer in the sevoflurane group (13.1, 15.7, 18.6, and 21.2 min, respectively) than in the propofol group (11.4, 13.2, 14.4, and 17.2 min respectively). TOF ratios of 50, 70, and 90% were significantly occurred later in sevoflurane group than propofol group.

CONCLUSIONS

Our results indicate that when compared with propofol group, the sevoflurane group had an accelerated onset and a delayed recovery of neuromuscular block induced by mivacurium in children.

Equi-lasting doses of rocuronium, compared to mivacurium, result in improved neuromuscular blockade in patients undergoing gynecological laparoscopy : [Des doses de durée équivalente de rocuronium, comparé au mivacurium, améliorent la curarisation chez des patientes qui subissent une laparoscopie gynécologique]

PURPOSE

To compare equi-lasting doses of a short-acting (mivacurium) to an intermediate-acting (rocuronium) neuromuscular relaxant, with regard to intubating conditions, efficacy, number of maintenance doses, hemodynamic alterations, adverse events and costs, in patients undergoing laparoscopic gynecological surgery.

METHODS

Sixty patients were randomly allocated to receive either 0.2 mg*kg(-1) (3 x ED(95)) mivacurium or 0.5 mg*kg(-1) (1.7 x ED(95)) rocuronium, under propofol/fentanyl anesthesia. T1, first twitch of the train-of-four (TOF) and TOF ratio (T4:T1) were used to evaluate neuromuscular block using the Relaxometer(R) mechanomyograph. The trachea was intubated when T1 was maximally suppressed. Neuromuscular block was maintained at 25% T1 with equi-lasting doses of 0.075 mg*kg(-1) mivacurium or 0.15 mg*kg(-1) rocuronium.

RESULTS

Mean (min) +/- SD mivacurium onset time (1.9 +/- 0.4) was longer than that of rocuronium (1.3 +/- 0.3). This did not yield a statistical difference in intubating conditions between the two groups. Interval 25-75% T1 recovery and time to 0.8 TOF recovery were prolonged following rocuronium (11.9 +/- 3.9, 52.6 +/- 15.5 respectively) compared to mivacurium (6.7 +/- 2.3, 39.2 +/- 8.1 respectively). More patients, 22/30, required mivacurium maintenance doses compared to 14/30 patients in the rocuronium group. Arterial blood pressure declined and 13/30 patients manifested erythema following mivacurium administration. The acquisition costs of rocuronium (6.93 Euro/patient) were 23% lower compared to mivacurium (8.96 Euro/patient).

CONCLUSION

Equi-lasting doses of rocuronium resulted in favourable intubating conditions more rapidly, improved hemodynamic stability, required less frequent administration of maintenance doses and were not associated with erythema, compared to mivacurium.

Comparison of recovery following rapacuronium, with and without neostigmine, and succinylcholine

The neuromuscular blocking effects of a single dose of rapacuronium 1.5 mg x kg(-1) with or without reversal with neostigmine have been examined in the present study and compared with a dose of succinylcholine 1.0 mg x kg(-1). Neuromuscular block was measured mechanomyographically using train-of-four stimulation. Complete block occurred within 1 min with both agents. Twenty-five per cent recovery of the first response of the train-of-four occurred in a median [range] time of 7.6 [5.7-11.3] min in the succinylcholine group and in 14.2 [8.8-23.6] and 15.1 [9.6-23.4] min in the rapacuronium groups with and without neostigmine reversal, respectively. Spontaneous recovery to a train-of-four ratio of 0.8 took 33.4 [20.0-79.0] min with rapacuronium but this was reduced to about 21.2 [13.9-33.7] min when neostigmine was administered at 25% recovery of first twitch of the train-of-four.

Intubation conditions and postoperative myalgia in outpatient dental surgery: a comparison of succinylcholine with mivacurium

Ninety-four patients undergoing elective outpatient third molar extraction were recruited into a double-blind, randomized, prospective trial comparing mivacurium (group M) with succinylcholine (Group S) for conditions for endotracheal intubation and the occurrence of postoperative myalgia. Anaesthesia was induced with fentanyl 1 microgram.kg-1 and propofol 2.5 micrograms.kg-1 in all patients. Group S patients were given gallamine 20 mg while group M patients were given mivacurium 0.2 mg.kg-1. Manual ventilation was commenced and anaesthesia maintained with nitrous oxide 70% and isoflurane 1 to 2% in oxygen. After two minutes, group S patients were given succinylcholine 1.5 mg.kg-1 and group M patients 0.9% saline. Nasotracheal intubation was performed 30 seconds later. Intubating conditions in group M were significantly better than those in group S (P < 0.001). The incidence of postoperative myalgia was 9.5% in group M and 26% in group S but this was not statistically significant (P = 0.09). We propose that mivacurium is a suitable neuromuscular blocker to use for endotracheal intubation in outpatient dental surgery.

Intubating conditions and blockade after mivacurium, rocuronium and their combination in young and elderly adults

PURPOSE

Mivacurium-rocuronium combinations have been demonstrated to be more potent than either drug given alone. Combinations were compared with mivacurium and rocuronium, with respect to onset, intubating conditions, and duration of action in young and elderly adults.

METHODS

Fentanyl-propofol-N2O-isoflurane anesthesia was given to ASA I and II adults aged 18-65 yr (45 patients) and over 66 yr (45 patients). In this blinded randomized study, we compared accelerographic adductor pollicis response and visual assessment of response to facial nerve stimulation after 0.25 mg x kg(-1) mivacurium, 0.6 mg x kg(-1) rocuronium, and a combination of 0.08 mg kg(-1) mivacurium plus 0.2 mg x kg(-1) rocuronium. Intubating conditions at 2.5 min were rated as excellent, good, fair or poor.

RESULTS

Onset times were similar for all drugs regimens and for both age groups (204-276 sec at the thumb; 142-196 sec at the eye) (P<0.05 between muscles). Intubating conditions were similar in all groups, and rated good or excellent, except in two subjects. In young patients duration to 25% recovery was longer (P<0.05) for rocuronium (mean +/- SD) (39+/-11 min) than for either mivacurium (23+/-6 min), or the combination (27+/-7 min). Duration was prolonged in the elderly for rocuronium (54+/-17 min), and the combination (35+/-11 min), but not for mivacurium (24+/-6 min).

CONCLUSIONS

Mivacurium-rocuronium combinations yield onset times and intubating conditions similar to either parent agent with only two thirds as much total drug. Duration for such a mixture is similar to that of mivacurium in young adults and slightly prolonged in the elderly.

Succinylcholine: adverse effects and alternatives in emergency medicine

Succinylcholine has long been the favored neuromuscular blocking agent for emergent airway management because of its rapid onset, dependable effect, and short duration. However, it has a plethora of undesirable side effects, ranging from the inconsequential to the catastrophic. When patients requiring tracheal intubation present with potential contraindications to succinylcholine use, the emergency physician will need to substitute a rapid-onset nondepolarizing neuromuscular blocking agent, such as rocuronium or mivacurium. An understanding of the pharmacology of these agents is essential.

Potency and time course of mivacurium block during sevoflurane, isoflurane and intravenous anesthesia

PURPOSE

To determine the potency and time course of action of mivacurium neuromuscular block under routine clinical conditions during sevoflurane, isoflurane and intravenous anesthesia.

METHOD

Patients were anesthetized with nitrous oxide 66% in oxygen and 1.5 MAC sevoflurane or isoflurane or a propofol infusion, neuromuscular block being monitored using mechanomyography. Potency was determined using administration of single doses of mivacurium of 40-100 micrograms.kg-1 and construction of dose-response curves (n = 72). The onset and duration of action were determined following a bolus dose of 0.2 mg.kg-1 of mivacurium (n = 30).

RESULTS

The ED50 and ED95 (with 95% confidence limits) were estimated to be 42 (35-51) and 86 (74-98) micrograms.kg-1, 52 (45-60) and 89 (72-110) micrograms.kg-1, and 53 (45-62) and 95 (81-112) micrograms.kg-1 during sevoflurane, isoflurane and propofol anesthesia respectively (P < 0.05 between sevoflurane and propofol). Following administration of the 0.2 mg.kg-1 dose, neither the times (mean +/- SD) to maximum block (1.6 +/- 0.31, 1.7 +/- 0.21 and 1.6 +/- 0.45 min, respectively) nor the times to 25 and 90% recovery of T1 (20 +/- 4.5 and 33 +/- 8.8 min, 21 +/- 3.8 and 33 +/- 6.5 min, and 18 +/- 4.1 and 28 +/- 5.8 min respectively) were different among groups. The times to recovery of TOF ratio to 0.8 were 40 +/- 10.0, 36 +/- 8.5 and 29 +/- 5.5 min in the sevoflurane, isoflurane and propofol groups respectively (P = 0.017 between the sevoflurane and propofol groups).

CONCLUSIONS

Under usual conditions of clinical anesthesia the potency of mivacurium was slightly enhanced during sevoflurane compared with intravenous anesthesia but the duration of action was only minimally prolonged during sevoflurane and isoflurane anesthesia.

Is the recovery profile of mivacurium independent of the rate of decay of its plasma concentration in patients with normal plasma cholinesterase activity?

BACKGROUND

The duration of action of muscle relaxants is poorly correlated to the rate of decay of their plasma concentration. The plasma concentration of mivacurium may rapidly decrease below its active concentration because of the extensive hydrolysis of mivacurium. By inflating a tourniquet on one upper limb for 3 min after the administration of atracurium, mivacurium or vecuronium, we studied the influence of the initial decline of their plasma concentration on their effect.

METHODS

In 50 patients anaesthetised with thiopental, isoflurane and fentanyl, the effect of bolus doses of 0.15 or 0.25 mg.kg-1 mivacurium (MIV 15, MIV 25), 0.3 or 0.5 mg.kg-1 atracurium (ATR 30, ATR 50) and 0.06 or 0.1 mg.kg-1 vecuronium (VEC 06, VEC 10) were measured on both arms (evoked response of the adductor pollicis to train-of-four stimulation every 12 s), a tourniquet being applied on one arm just before and during 3 min after the muscle relaxant bolus.

RESULTS

Tourniquet inflation of 3 min almost abolished the neuromuscular effect of mivacurium. In the vecuronium groups and in the ATR 50 group, tourniquet inflation did not modify the maximum degree of depression of the twitch response. Also, the duration of action of vecuronium was unaffected by the tourniquet. In the ATR 30 group, times to return of the twitch response to 25% (duration 25%) and 75% (duration 75%) of control response were significantly shorter in the cuffed arm, 23 min vs 27 min, and 41 min vs 45 min, respectively. In the ATR 50 group, only duration 25% was significantly shorter in the cuffed arm (41 min vs 45 min).

CONCLUSION

The results suggest that the rate of decline of the plasma concentration of mivacurium is so rapid, that a very low and almost clinically ineffective concentration is present as soon as 3 min after its administration. The results also indicate that the recovery from a mivacurium-induced neuromuscular blockade is not influenced by the rate of decay of its plasma concentration in patients with genotypically normal plasma cholinesterase.

Rocuronium prevents succinylcholine-induced fasciculations

PURPOSE

The aim of this study was to assess the effect of rocuronium pretreatment at 3 and 1.5 min before succinylcholine administration on fasciculations, neuromuscular blockade and intubating conditions.

METHODS

Sixty ASA I or II adults scheduled for elective surgery were anaesthetised with midazolam, fentanyl, propofol, N2O and isoflurane. They were randomised in a double blind manner into three groups: group ROC-3 min (n = 22) received 0.05 mg.kg-1 rocuronium, 3 min before 2 mg.kg-1 succinylcholine; group ROC-1.5 min (n = 20) received 0.05 mg.kg-1 rocuronium 1.5 min before 2 mg.kg-1 succinylcholine; and group NO ROC (n = 18) had no rocuronium before injection of 2 mg.kg-1 succinylcholine. Fasciculations and intubating conditions were evaluated by the same physician who was unaware of the randomisation. Neuromuscular block was measured at the adductor pollicis with an accelerometer.

RESULTS

The incidence of fasciculations was lower in the ROC-3 min (9%) and ROC-1.5 min (30%) groups than in the NO ROC group (83%; P < 0.001). The intensity of fasciculations was also less in both pretreatment groups. No statistical difference was noted between pretreatment at 3 and 1.5 min. Intubating conditions, onset time and duration of succinylcholine blockade were comparable in all three groups.

CONCLUSION

The incidence and severity of succinylcholine fasciculations can be reduced by giving 0.05 mg.kg-1 rocuronium either 1.5 min or 3 min before succinylcholine. The effects of 2 mg.kg-1 succinylcholine with rocuronium pretreatment, and 1 mg.kg-1 succinylcholine, without pretreatment, are similar with respect to intubating conditions, onset of paralysis and duration of blockade.

A comparison of the effects of isoflurane and desflurane on the neuromuscular effects of mivacurium

The effects of 1 MAC of desflurane and isoflurane (in 66% nitrous oxide) on the potency and duration of action of mivacurium were studied in 80 patients. The ED95 of mivacurium was 86 micrograms.kg-1 (74-100) and 88 micrograms.kg-1 (76-103) (mean and 95% confidence intervals) during anaesthesia with desflurane and isoflurane respectively. The onset and duration of recovery to 25, 75 and 90% of T1 (first response in the TOF) of 200 micrograms.kg-1 of mivacurium were 1.4 (0.3) and 1.5 (0.3) min (mean and SD), 22 (4.9) and 19 (4.0), 29 (6.6) and 26 (5.8), and 32 (7.3) and 29 (6.6) min respectively. There was no significant difference in any of the variables between desflurane and isoflurane. It is concluded that the neuromuscular effects of mivacurium are similar during anaesthesia with 1 MAC of desflurane and isoflurane.

Comparison of rocuronium and mivacurium to succinylcholine during outpatient laparoscopic surgery

Tracheal intubating conditions and neuromuscular effects of succinylcholine, rocuronium, and mivacurium were studied in 100 healthy women undergoing outpatient laparoscopic surgery. After a standardized fentanyl-thiopental induction, tracheal intubation was facilitated with succinylcholine 1 mg/kg in Groups I (n = 23) and II (n = 25), rocuronium 0.6 mg/kg in Group III (n = 27), or mivacurium 0.2 mg/kg in Group IV (n = 25). If clinically indicated, bolus doses of rocuronium 5-10 mg (Groups I and III) or mivacurium 2-4 mg (Groups II and IV) were administered during the maintenance period. Anesthesia was maintained with desflurane and nitrous oxide 60% in oxygen. At the end of the surgery, residual neuromuscular block was reversed with edrophonium 0.5 mg/kg and atropine 10 micrograms/kg, if needed. The neuromuscular function was assessed using electromyography with a train-of-four mode of stimulation every 10 s at the wrist. Intubating conditions 90 s after succinylcholine and rocuronium were significantly better than after mivacurium. The onset time (from the end of injection until 95% suppression of the first twitch [T1]) for succinylcholine (63 +/- 21 s and 62 +/- 17 s in Groups I and II, respectively) were significantly shorter than for rocuronium (158 +/- 76 s) or mivacurium (210 +/- 93 s). Moreover, the onset times for rocuronium were significantly shorter than mivacurium. The recovery times (of T1 to 25% of the control value) were significantly shorter with succinylcholine and mivacurium than rocuronium. Significantly fewer patients needed reversal of residual neuromuscular blockade after mivacurium compared to rocuronium. One patient in Group I and six patients in Group IV displayed erythema on the upper body. Postoperative myalgia were experienced by 16% of the patients in Groups I and II compared to none in Groups III and IV. There was on difference in the incidence of postoperative nausea and vomiting among the four groups. In conclusion, rocuronium appears to be an acceptable alternative to succinylcholine for tracheal intubation. However, rocuronium's longer duration of action increases the need for reversal drugs. When rapid tracheal intubation is unnecessary, mivacurium is also an acceptable alternative to succinylcholine and is associated with a more rapid spontaneous recovery than rocuronium.

Dose-response and minimum time to satisfactory intubation conditions after mivacurium in children

We assessed neuromuscular blocking effects and tracheal intubation conditions following mivacurium in 121 anaesthetised children aged 1-10 years. The study was conducted in three parts. Parts 1 and 2 were undertaken during thiopentone-alfentanil-nitrous oxide anaesthesia; neuromuscular blockade was evaluated by recording the force of contraction of the adductor pollicis in response to train-of-four stimulation at 0.1 Hz. In part 1 the potency of mivacurium was determined in 15 children using a single dose-response technique; in part 2 onset and recovery times were determined in six children following a dose of mivacurium 0.2 mg.kg-1. In part 3 of the study, clinical intubation conditions were assessed in two groups of 50 children whose tracheas were intubated 60 or 90 s after injection of mivacurium 0.2 mg.kg-1 during thiopentone-nitrous oxide anaesthesia. The ED50 and ED95 of mivacurium were 54 and 105 micrograms.kg-1 respectively. The times to 90% and 100% depression of control twitch were 1.3 (0.3) and 1.9 (0.5) min; times to 5%, 25%, 75% and 90% recovery were 6.4 (1.0), 8.4 (1.1), 12.5 (1.1) and 14.4 (1.9) min, respectively. Intubation conditions were rated satisfactory in 33/50 children (0.66, 95% confidence interval 0.51-0.79) at 60 s and in 49/50 children (0.98, 95% confidence interval 0.89-1.0) at 90 s (p = 0.0001). Intubation conditions 90 s after mivacurium 0.2 mg.kg-1 were significantly better than those obtained in 10 patients given anaesthetic drugs alone (p = 0.002).

Comparison of intubating conditions after rocuronium and suxamethonium following "rapid-sequence induction" with thiopentone in elective cases

BACKGROUND

Rocuronium (Org 9426) was shown to have the fastest onset of action of all currently available non-depolarizing neuromuscular blocking drugs and to provide intubating conditions similar to those of suxamethonium 60 to 90 s after administration. We compared the intubating conditions after rocuronium and suxamethonium following rapid-sequence induction of anaesthesia.

METHODS

Fifty unpremedicated patients of ASA physical status I or II, scheduled for elective surgery were studied. Anaesthesia was induced with thiopentone 6 mg kg-1 followed randomly by suxamethonium 1 mg kg-1 or rocuronium 0.6 mg kg-1 and, 45 s later, intubation was commenced. Muscle fasciculations, intubating conditions and intubation time, haemodynamic variables and oxygenation were assessed.

RESULTS

Intubation time did not differ between suxamethonium (9.8 +/- 2.2 s) (mean +/- SD) and rocuronium (10.5 +/- 2.9 s), respectively. Intubating conditions were clinically acceptable (good or excellent) in all patients given suxamethonium and in 96% of the patients given rocuronium. However, the condition of the vocal cords was better (P < 0.05) and diaphragmatic response to intubation was less pronounced with suxamethonium (P < 0.05). Changes in heart rate and arterial blood pressure were similar in both groups.

CONCLUSION

The authors conclude that rocuronium is a suitable alternative to suxamethonium for rapid tracheal intubation even under unsupplemented thiopentone anaesthesia, at least in elective, otherwise healthy patients. Its use for rapid-sequence induction under emergency conditions, however, needs further investigation.

A comparison of mivacurium infusion requirements between young and elderly adult patients

Forty-one patients of ASA classes I or II, undergoing elective surgery, were divided into two groups: young, 18-41 years (mean 31), and elderly, 64-79 years (mean 71). The integrated evoked compound electromyogram of the adductor pollicis muscle elicited by stimulation of the ulnar nerve was used to monitor the neuromuscular block of the non-depolarizing muscle relaxant mivacurium. An initial dose of mivacurium 0.15 mg kg-1 allowed six excellent, nine good, three adequate and three poor intubations in the young group, and nine excellent, eight good, three adequate and no poor intubations in the elderly group. Patients recovered until 1-2 stimuli of the train-of-four were visible, and an infusion of mivacurium was started (0.5 mg kg-1 h-1). At 3 min intervals the rate was adjusted by +/- 0.05 mg kg-1 h-1 (+/- 10% initial rate), as indicated during anaesthesia which was provided by nitrous oxide in oxygen, infusion of propofol, and fentanyl supplements. In the first 30 min, the young group had their mivacurium requirement increased to 111.4% (0.56 mg kg-1 h-1), which was reached in the first 15 min, after which it gradually decreased to 92.9% (0.46 mg kg-1 h-1). The elderly group's requirements decreased from the start, to 78.5% (0.39 mg kg-1 h-1). The difference between the two groups was significant (P < 0.05). After the first 30 min, both groups requirements decreased, with time, but with no statistically significant differences.

Fifty years of muscle relaxants

The introduction of curare improved surgical relaxation and encouraged anaesthetists to enlarge their vision. They became interested in the pharmacologic properties of their drugs and the physiologic changes associated with paralysis, and this led naturally to their involvement in intensive care and respiratory physiology. Since 1942, more than 50 muscle relaxants have been introduced: the current emphasis is on the short- to intermediate-duration agents that allow rapid recovery and avoid the problems associated with residual curarization, but we still await a nondepolarizing replacement for suxamethonium.

Mivacurium chloride--a comparative profile

Mivacurium is a new nondepolarizing muscle relaxant of the benzylisoquinoline type. Its short duration of action is due to rapid breakdown by plasma cholinesterase. The dose of mivacurium which produces 95% inhibition of twitch response (ED95) is between 60 and 80 micrograms/kg. Thus, mivacurium is 0.8 times and four times as potent as vecuronium and atracurium, respectively. With 2-3 x ED95, tracheal intubation can be accomplished within 2.5 min of intravenous injection. The ensuing DUR25% (time from injection to 25% recovery of control twitch tension) is twice as long as with suxamethonium and about half as long as with equipotent doses of atracurium or vecuronium. For muscle relaxation during long surgical procedures, mivacurium has been used as a continuous infusion. The average 6-min recovery index after infusion of mivacurium is particularly favourable for flexible control of muscle paralysis, whereas the recovery indices after infusion of atracurium or vecuronium are 15-30 min. In conclusion, mivacurium will close the pharmacodynamic gap between suxamethonium and the nondepolarizing muscle relaxants of intermediate duration of action. It will probably also be a suitable alternative to suxamethonium in elective cases.

Intubating conditions and neuromuscular effects of mivacurium during propofol-alfentanil anaesthesia

In three groups of 20 patients, anaesthetized with propofol and alfentanil, tracheal intubation conditions and the onset of neuromuscular blockade after administration of three different doses of mivacurium chloride (0.11, 0.15, and 0.19 mg/kg = 1.5 x ED95, 2 x ED95, and 2.5 x ED95) were assessed. Intubation conditions were found to be clinically acceptable (good or excellent) in 83% of patients. Eighty-two per cent of patients were successfully intubated on the first attempt after 60-90 s. No difference in intubation scores or number of intubation attempts among the three dosage groups were found. We conclude that mivacurium chloride allows smooth intubation in most patients within 60-90 s, even with the lowest dose (0.11 mg/kg), after a propofol-alfentanil induction of anaesthesia. However, because there were a few patients in whom intubating conditions were inadequate at 60-90 s, we are reluctant to advocate the preference of mivacurium chloride over suxamethonium for rapid sequence induction in emergency situations.

Intubating conditions following 1R CIS, 1'R CIS atracurium (51W89). A comparison with atracurium

51W89 besylate, a new intermediate acting non-depolarising muscle relaxant, consisting of a single stereo-isomer of the commercial preparation of atracurium has been assessed in respect of intubating conditions during nitrous oxide-propofol-isoflurane anaesthesia. Intubating conditions at 2 min were acceptable in 67% of patients following a dose of 0.1 mg.kg-1 and in 90% of patients following a dose of 0.15 mg.kg-1. Intubating conditions at 1.5 min were acceptable in 76% of patients following a dose of 0.2 mg.kg-1. In comparison, intubating conditions at 2 min were acceptable in 95% of patients following 0.5 mg.kg-1 of atracurium. The intubating conditions at 2 min following a dose of 0.1 mg.kg-1 51W89 besylate were significantly worse than the other three groups (p < 0.05); however, there was no significant differences between the scores in the other three groups. There was no clinical evidence of histamine release in the groups receiving 51W89 besylate compared to two out of the 19 patients who had cutaneous flushing following the administration of atracurium. Our results suggest that 51W89 besylate provides acceptable intubating conditions at 2 min following a dose of 0.15 mg.kg-1 and may prove to be an acceptable alternative to atracurium if studies in progress confirm its greater cardiovascular stability and reduced propensity to release histamine than its parent compound.

The influence of the duration of control stimulation on the onset and recovery of neuromuscular block

The onset of action of atracurium 450 micrograms/kg, mivacurium 160 micrograms/kg, and vecuronium 80 micrograms/kg was measured after train-of-four (TOF) stimulation had been applied for 1, 5, 10, 15, or 20 min in groups of 10 patients each during anesthesia with thiopental, nitrous oxide-oxygen, and fentanyl. TOF stimulation was applied to the ulnar nerve at 2 Hz every 12 s and the force of contraction of the adductor pollicis muscle recorded. There was a progressive and significant reduction in the time to onset of maximum block with increasing times of control stimulation with all three relaxants (P < 0.0001). The mean +/- SD times to onset of maximum block decreased from 224 +/- 103 to 73 +/- 28 s with atracurium, 239 +/- 81 to 101 +/- 33 s with vecuronium, and 198 +/- 72 to 106 +/- 23 s with mivacurium as the period of control stimulation increased from 1 to 20 min. The time to recovery of T1 (first response in the TOF stimulation) to 25% of control (duration of clinical relaxation) increased from 33 +/- 5.7 to 52 +/- 13.4 min with atracurium, 25 +/- 7.6 to 38 +/- 9.4 min with vecuronium, and 13 +/- 2.5 to 18 +/- 3.5 min with mivacurium with the period of control stimulation increasing from 1 to 20 min. The differences were significant for atracurium and vecuronium (P < 0.05-0.0001). We conclude that increasing periods of control stimulation are associated with decreasing time to onset of neuromuscular block with atracurium, vecuronium, and mivacurium at the adductor pollicis muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

[Continuous administration of mivacurium for short procedures. Delayed onset and recovery from neuromuscular blockade]

OBJECTIVE

To assess the delays of onset and spontaneous recovery from neuromuscular block produced by mivacurium administered by continuous infusion for short procedure requiring a deep relaxation.

STUDY DESIGN

Prospective open non comparative study.

PATIENTS

Twenty-nine class ASA I and II adults undergoing a stomatological procedure of short duration were included in the study.

METHOD

General anaesthesia was obtained with a continuous infusion of propofol, supplemented with alfentanil and N2O-O2 mixture. Neuromuscular blockade, assessed with electromyography of the adductor pollicis muscle, was obtained with mivacurium (150 micrograms.kg-1). After restoration of 5% of neuromuscular transmission, mivacurium was administered by continuous infusion in order to maintain a blockade between 91 and 99%.

RESULTS

The delay for decreasing twitch height by 95% was 2.9 +/- 1.0 min. The mean dose for maintenance of blockade was 10.9 +/- 1.5 micrograms.kg-1.min-1. The delay of spontaneous recovery from blockade was 10.2 min, 16.6 min and 21.3 min for obtaining 25, 75 and 95% twitchs respectively. The delay for the twitch increase from 25 to 75% was 6.6 min.

DISCUSSION

Mivacurium in continuous infusion provides rapidly a deep and stable neuromuscular blockade followed by a rapid spontaneous restoration of neuromuscular transmission in patients with normal pseudocholinesterases.

Recovery of mivacurium block with or without anticholinesterases following administration by continuous infusion

Thirty patients received a bolus dose of 0.2 mg.kg-1 of mivacurium followed by an infusion during anaesthesia with thiopentone, fentanyl and halothane. Neuromuscular block was monitored using train-of-four stimulation and mechanomyography and the block maintained to keep the first response in the train-of-four (T1) at 10% of control. At the end of surgery the patients were randomly allocated to reversal with neostigmine or edrophonium or to spontaneous recovery. The mean dosage of mivacurium for maintaining the T1 at 10% was 5.7 micrograms.kg-1.min-1. There was a significant (r = -0.81, p < 0.001) negative correlation between time to recovery of T1 to 10% after the bolus dose and infusion rate. The times taken for T1 to reach 25, 75 and 90% of control and for the train-of-four ratio to reach 0.7 were significantly shorter (p < 0.05 to 0.001) with neostigmine and edrophonium compared to the spontaneously recovering group. The average (SD) times for attaining the train-of-four ratio of 0.7 were 7.0 (1.2), 6.8 (1.4) and 13.5 (2.3) min respectively for neostigmine, edrophonium and spontaneously recovering groups. There were no differences between endrophonium and neostigmine in any of the recovery times.