Vital Times 2017 - 68
Muscle Relaxants
Monitoring of Neuromuscular Blockade
Rocuronium is the most commonly used muscle
relaxant. Its half-life is 73 min. The intubating dose is 0.6
mg/kg. This corresponds to 40 mg for a 70-kg patient.
The dose for rapid sequence intubation is 1.2 mg/kg.
Intraoperatively, the maintenance dose is 0.15 mg/kg,
or 10 mg for a 70-kg patient. To minimize postoperative
residual NMB, avoid giving higher doses than these.
Other commonly used relaxants include succinylcholine
1 mg/kg total body weight and cis-atracurium.
There is substantial patient to patient variability in the
effect of muscle relaxants as well as reversal agents.
Within the same patient, there is substantial variability
in the response of various muscle groups to these agents.
Recovery of pharyngeal muscles is necessary for avoiding
complications such as airway obstruction and aspiration.
It occurs later than recovery of the diaphragm. Tidal
volume under anesthesia is determined by diaphragmatic
function. Hence a good tidal volume does not necessarily
indicate adequate recovery. Monitoring of NMB is useful
for determining effect. Physical signs such as five-second
head lift may overestimate recovery.
Moderate block is defined as the presence of 2-3 twitches
in a train of four (TOF). Deep block is defined as the
presence of 1-2 twitches on the post-tetanic count
(PTC). If deep block is required for surgical needs and
sugammadex is not available, reversal of deep rocuronium
block with neostigmine & glycopyrrolate will require the
deep block to recover to moderate block before reversal
can be safely administered.
Alternatively, maintenance with cis-atracurium, followed
by reversal with neostigmine & glycopyrrolate may
be utilized. The half-life of cis-atracurium is 29 min in
healthy persons and 32 min in those with end-stage renal
disease (ESRD). Hence, cis-atracurium is suitable for
patients with renal failure. Mixing bis-benzylisoquinoline
relaxants such as cis-atracurium with steroidal relaxants
such as rocuronium can lead to unpredictable and
prolonged block.
The required depth of block is determined by many
factors. Often no neuromuscular blockade is necessary,
even for retroperitoneal radical prostatectomy.6
And despite what some surgeons may think, even
laparoscopic bariatric surgery, does not require deep
levels of neuromuscular blockade, and often moderate
block is adequate.7 Intraoperative maintenance doses of
relaxant should be administered if the depth of muscle
relaxant is inadequate for surgical needs. Monitoring is
mandatory. Relaxant should not be administered purely
on a predetermined schedule or in order to suppress
spontaneous ventilation. Relaxant should not be
administered to compensate for inadequate depth
of anesthesia.
68 | CSA Vital Times
American Society of Anesthesiologists (ASA) standards
for basic monitoring do not include the use of NMB
monitoring. According to ASA practice guidelines for
post anesthetic care, "assessment of neuromuscular
function primarily includes physical examination and
on occasion, may include neuromuscular blockade
monitoring".8
The response of the adductor pollicis muscle to TOF
stimulation of ulnar nerve is the standard for qualitative
as well as quantitative monitoring. It predicts the
recovery of the upper airway muscles responsible for
preventing airway collapse and aspiration. If during
surgery, the arms cannot be accessed, qualitative
monitoring may not be possible. The response of eye
muscles, corrugator supercilii & orbicularis oculi to
stimulation of facial nerve recovers earlier than the
response of adductor pollicis muscle.9 There is a risk
of stimulating the eye muscles directly rather than
through the facial nerve. The monitoring guidelines are
based on the response of the adductor pollicis muscle.
Hence, eye muscles should not be monitored for clinical
applications.
Qualitative monitoring can overestimate recovery.
Commercial quantitative monitoring with
electromyography is suitable for accurate monitoring,
but it is not available commercially. Other quantitative
monitors that are accurate, such as kinemyograph or
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