Claude Meistelman,
Department of Anaesthesiology, School of medicine, Nancy, France
Several studies have demonstrated that the time course of neuromuscular block in various skeletal muscles is different from that in the adductor pollicis and discrepancies could exist both in onset time, duration of action and intensity of paralysis. Therefore the understanding and knowledge of the relationship between neuromuscular function at the monitored muscle and the other muscles are important in the interpretation of monitoring.
The diaphragm and the
laryngeal adductor muscles are more resistant to non depolarising muscle
relaxants (NDMR) than the adductor pollicis, complete paralysis of these
muscles is not expected with a dose that barely blocks the adductor pollicis.
The resistance of the respiratory muscles explain why the diaphragm and the
laryngeal adductor muscles recover from paralysis induced by muscle relaxants
more rapidly than does the adductor pollicis; Although respiratory muscles are
more resistant to NDMR, paralysis may occur before complete paralysis at the
adductor pollicis. This apparent discrepancy can be explained by considering
the factors which influence neuromuscular effect, particularly the rate of
equilibration between plasma and the effect compartment. After a bolus dose of
NDMR, equilibrium between plasma and the neuromuscular junction will be reached
sooner at the muscle group with a more rapid equilibration.. Even if the muscle
group with a more rapid equilibration is resistant, the greater peak
concentration will offset this to produce a more intense peak effect; Using a
pharmacodynamic modelling approach, it was shown that that the rate of
equilibration between plasma and effect was faster for the diaphragm and the
laryngeal adductor muscles than for the adductor pollicis 1. Another study has shown, following rocuronium
administration, that the rate constant of transport between plasma and the
effect compartment (ke0) was significantly greater at the laryngeal adductor muscles (0.260
minutes) than at the adductor pollicis (0.168 minutes) The EC50 was
significantly greater at the laryngeal muscles (1424 g/L) than at the adductor
pollicis (823 g/l) 2. Although the respiratory muscles are
resistant they are blocked before the adductor pollicis because concentration
at the neuromuscular junction increase more rapidly at the respiratory muscles
than at the adductor pollicis. The Ke0 of NDMR is mainly dependant of circulatory
factors (muscle blood flow) and the partitioning of the relaxant between blood
and muscle 3. Therefore, it is likely that the more rapid
equilibration of laryngeal muscles when compared with the adductor pollicis is
the result of the greater perfusion of the respiratory muscles. During
recovery, both rapid equilibration and lesser sensitivity of the respiratory
muscles contribute to earlier recovery when compared with the adductor
pollicis.
Surprisingly, ventilatory
weakness may still be detected until the adductor pollicis has recovered almost
completely. Small doses of pancuronium (0.02 mg/kg) depress the swallowing
reflex to a greater extent than the strength of peripheral muscles. This
indicates that muscles involved with maintenance of airway patency are more
sensitive to relaxants than other respiratory muscles and possibly peripheral
muscles. Thus following
administration of small doses of non depolarising muscle relaxants such as used
during the priming technique, there is a risk of aspirating gastric content by
paralysis of upper airway muscles. Recent studies have demonstrated that
impaired swallowing and pharyngeal dysfunction could be observed for TOF ratio
less than 90% at the adductor pollicis. Pharyngeal function is not normalised
until an TOF ratio above 90% at the adductor pollicis is reached 4.
As
suggested by Donati, although the effect of NMBA is different on the thumb, the
airway and respiratory muscles, we persist in using the adductor pollicis to monitor
neuromuscular block because it is convenient. However neuromuscular monitoring
can be improved by using different types and sites of stimulation.
During
onset, the use of the adductor pollicis to determine good intubating conditions
can be misleading because paralysis of the adductor pollicis lags behind onset
of neuromuscular block at the vocal cords and the diaphragm. Furthermore, the
adductor pollicis may be blocked with a dose insufficient to block these
respiratory muscles. Several studies have demonstrated that the sensitivity of
the orbicularis oculi to NDMR and the time course of onset was close to that of
respiratory and laryngeal muscles. It has been shown that when a NDMR was given
at a dose sufficient to block respiratory muscles (2xED95),
disappearance of the TOF at the orbicularis oculi could predict good intubating
conditions. Detection of good intubating conditions could be provided in all
the patients approximately 1 min before complete paralysis at the adductor
pollicis.
During surgery, the choice
of the site and the type of stimulation will depend of the degree of muscular
relaxation sought. When an intense neuromuscular blockade is required,
disappearance of the TOF at the adductor pollicis does not eliminate the
possibility of hiccups, cough or extrusion of abdominal contents. During that
phase of so called period of no response, two different techniques may allow
the evaluation of very intense blockade when complete paralysis of the
diaphragm is sought and predict the time to reappearance of the TOF at the
adductor pollicis. Post-tetanic-count (PTC) at the adductor pollicis was the
first technique of assessment of very intense neuromuscular blockade. Less than
5 responses indicate a deep neuromuscular block. For a given muscle relaxant,
time until spontaneous return of the first response of the TOF is related to
the PTC at a given time. Monitoring of the orbicularis oculi, using TOF
stimulus has also a role during surgery because it is a good reflection of
diaphragmatic paralysis. When profound blockade is not required, monitoring of
the adductor pollicis using TOF is sufficient and allows easier antagonism of
relaxation at the end of the case. No tetanic stimulation must be applied when
TOF monitoring is used in order to avoid post-tetanic facilitation and
artificial increase of the response to the TOF.
As previously discussed,
although orbicularis oculi may be valuable for monitoring onset of relaxation
or maintenance of profound blockade, the adductor pollicis is preferable for
the management of recovery because it represents one of the most sensitive
muscles. When the adductor pollicis has almost completely recovered, it can be
assumes that no residual paralysis exists at both the diaphragm and the
laryngeal adductor muscles. When the fourth response at the TOF reappears,
neuromuscular blockade is usually less than 75% and the TOF ratio may be
estimated. It has been demonstrated that a TOF ration of 90% was needed to
avoid any respiratory problems secondary to residual neuromuscular blockade 4. Unfortunately, visual or tactile assessment
of fade of the TOF when it is above 40% is very difficult, even for experienced
anaesthetists. The double-burst stimulation (DBS) has been introduced to
improve the clinicians ability to detect residual blockade by visual or
tactile means. Fade can be detected more easily and can be observed at degrees
of block corresponding to a TOF ratio under 60%. However postoperative
ventilatory weakness must be anticipated if the adductor pollicis activity has
not returned fully at the end of anaesthesia. It is therefore important that
monitoring can be used in association with clinical tests for recovery such as
the patient ability to swallow to stick out his tongue or to strongly appose
the incisor teeth 5
In conclusion the type and the site of
monitoring should be adapted to the requirement of paralysis.
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3. Stanski DR, Ham
J, Miller RD, Sheiner LB. Anesthesiology 1979;
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4. Eriksson LI, Sundman E, Olsson R, Nilsson L,
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5. Kopman AF, Yee PS, Neuman GG. Anesthesiology
1997; 86: 765-71