Re-establishment of paralysis using mivacurium following apparent full recovery from mivacurium-induced neuromuscular block.

Recent published data suggest that despite apparently satisfactory recovery from nondepolarising block (train-of-four ratios in excess of 0.90), even very small doses of additional relaxant may re-establish significant paralysis. We sought to verify this observation and quantify its magnitude. Twelve adult patients were studied under nitrous oxide-propofol-opioid anaesthesia and neuromuscular block was monitored electromyographically. Train-of-four stimuli were delivered to the ulnar nerve every 20 s throughout the period of observation. After baseline stabilisation, an initial bolus of mivacurium 25 micrograms.kg-1 was administered and the twitch depression noted. When the twitch was stable for two consecutive stimuli, a second bolus, calculated to produce approximately 90% twitch depression, was administered. Recovery was then allowed to proceed spontaneously until the train-of-four ratio reached 0.95. At that time a second 25 micrograms.kg-1 dose was administered and the effect on twitch height recorded. Using the slope for the log-dose/logit dose-response relationship of mivacurium (5.5), it was possible to estimate any change in the ED50 of mivacurium. The control ED50 of mivacurium (calculated from the initial dose of mivacurium) averaged 43 micrograms.kg-1. When the same dose of drug was given at 95% recovery of the train-of-four ratio, the ED50 was reduced to 19 micrograms.kg-1 (p < 0.0001). Hence, there remains a considerable reduction in the neuromuscular margin of safety even at a train-of-four ratio of 0.95.

The influence of changes in hand temperature on the indirectly evoked electromyogram of the first dorsal interosseous muscle.

The evoked EMG response commonly decreases in amplitude during the first few minutes of anaesthesia. The purpose of this study was to determine if a relationship exists between changes in hand temperature, which are known to occur with induction of anaesthesia, and drift in the EMG signal. The indirectly evoked response of the 1st dorsal interosseous muscle was measured using a Datex Relaxograph in 15 patients undergoing elective surgery. The test arm was wrapped in towels in order to minimize heat loss. Core body temperature, hand temperature, and T1 were recorded at two minute intervals for the next 30 min. Patients then received a bolus of mivacurium 0.08 mg.kg-1 and additional doses were given as needed. Complete recovery was defined as a TOF ratio > 0.90. Regression analysis plotting delta temperature against delta T1 was performed for each individual. The slope of the regression line for the relationship between delta degree C and delta T1 was then used to calculate a correction factor (CF) which might be used to "fine tune" the last measured T1. The initial hand temperature averaged 30.8 +/- 1.4 degrees C and this increased by 4.1 +/- 1.2 degrees C over the next 30 min. During this period T1 decreased by 24.8 +/- 5.9% or -6.05%/degrees C. The final mean T1 value at the end of anaesthesia (uncorrected) was 70.6 +/- 7% of control. The average corrected T1 value was 94.7 +/- 8.5% (range, 83-111%). It is concluded that there was a correlation between delta degree C and delta T1 during the first 30 min of anaesthesia (r2 = 0.77, P < 0.0001). However, in 5 of 15 individuals it was not possible to "temperature correct" the final T1 value to within +/- 10% of control. Hence, while changes in muscle temperature probably play a major role in the T1 drift seen with the Datex monitor, other factors remain to be identified.

Antagonism of mivacurium-induced neuromuscular blockade in humans. Edrophonium dose requirements at threshold train-of-four count of 4.

BACKGROUND: Mivacurium's rapid rate of recovery has led to the suggestion that routine reversal of its residual effects may be unnecessary once signs of spontaneous recovery are evident. When antagonism is attempted at 90% twitch depression, the time saved to return to train-of-four (TOF) ratios > 0.70 compared to control has been reported to average < or = 8 min. This study was an attempt to determine whether similar savings in time could be achieved once spontaneous recovery was well underway. Also investigated was the ability of a TOF count of 4 to serve as a marker that might predict the dose of edrophonium necessary for satisfactory antagonism of mivacurium. METHODS: Fifty-eight adult patients were studied under nitrous oxide/propofol/opioid anesthesia. Neuromuscular block was monitored electromyographically and maintained by infusion of mivacurium at a level sufficient to abolish any palpable response of the thumb. TOF stimuli were delivered to the ulnar nerve at the wrist every 20 s throughout the period of observation. When the infusion was terminated, an observer was asked to note the time when the 1st through the 4th twitches first became detectable. In group 1, recovery to a TOF ratio > 0.90 was allowed to proceed spontaneously. In groups 2, 3, and 4, 0.3, 0.5, and 0.75 mg/kg edrophonium, respectively, was administered when the 4th response to TOF stimulation first became palpable. Times to TOF ratios of 0.70 and 0.90 were recorded in all groups. RESULTS: TOF counts of 1, 2, 3, and 4 first became palpable at 8 +/- 4% (SD), 20 +/- 6%, 33 +/- 9%, and 44 +/- 10% of control twitch height. Fade on TOF stimulation could no longer be detected once the TOF ratio exceeded a value of 0.41 +/- 0.07 (range 0.25-0.51). Once the 1st evoked response was palpable, the 2nd, 3rd, and 4th responses could be detected 2.5 +/- 1.1 (SD), 4.6 +/- 1.6, and 6.1 +/- 1.6 min later. Spontaneous recovery to TOF fade ratios of 0.7 and 0.9 occurred on average 10.7 +/- 2.3 and 16.9 +/- 4.7 min, respectively, after a threshold count of 4. Administration of 0.3 mg/kg edrophonium shortened the recovery process by about 7.5 min. Increasing the dose of edrophonium beyond 0.3 mg/kg did not further accelerate recovery. CONCLUSIONS: After recovery from profound mivacurium-induced neuromuscular block, TOF counts of 1, 2, 3, and 4 approximate 10%, 20%, 30%, and 40% return to control twitch height, respectively. Finally, > or = 0.3 mg/kg edrophonium will accelerate recovery from mivacurium by approximately 7-8 min.