Lars I. Eriksson, MD, PhD
Associate Professor of
Anaesthesiology and Intensive Care Medicine
Dept of Anaesthesiology and
Intensive Care
Karolinska Hospital and Institute
SE-171 76 Stockholm
Sweden
Residual neuromuscular is a major
risk factor behind critical events in the immediate postoperative period.
Residual weakness due to muscle relaxants is seen in more than two thirds of
postoperative patients with ventilatory failure and hypoxia. Residual neuromuscular
block should therefore be regarded as a serious adverse event in the same way
as we regard ventilatory depression due to opioids and other anaesthetic
agents. This presentation aim to clarify our present knowledge and shortcomings
in the field of residual neuromuscular block.
The incidence of residual
neuromuscular block is dependent on how we define rest effect of muscle
relaxants. Using the traditional TOF value of 0.70, several investigator found
an incidence of about 30 % after pancuronium and 5-10 % after medium-acting
compounds. These studies are, however performed in mixed patients groups with
younger and older adults and with both long- and short-lasting procedures.
Thus, the incidence in various patient populations remain to be studied. Moreover,
we only have partial knowledge about the consequences of residual block in
various patients groups.
Return to normal muscle function after neuromuscular block varies
markedly between different muscle groups. Early recovery occurs in central respiratory
muscle such as the diaphragm and the larynx whereas muscles of the pharynx and
the eyes recovers considerably slower. Residual neuromuscular block will
therefore be more pronounced in the pharynx and the face muscles. Previously,
return to an adductor pollicis train-of-four (TOF) ratio of more than 0.70 has
been regarded as safe recovery to allow extubation and spontaneous breathing.
Recent finding, however, suggests that even residual TOF fade of 0.90 or less
in the hand is associated with a pronounced dysfunction of the pharynx and the
striated muscle of the upper esophagus. This lead to a considerable risk of
aspiration at a adductor pollicis TOF ratio of < 0.90. Moreover, residual
neuromuscular block is a risk factor for development of postoperative pulmonary
complications. In patients receiving long-acting drugs where residual block was
noted (a TOF ratio of < 0.70) the risk for developing postoperative
pulmonary complications where markedly increased. Finally, residual block
impair normal chemosensitivity of the carotid bodies by an interaction with the
cholinergic transmission of the chemoreceptor in the glomus caroticum.
Hence, at an adductor pollicis TOF
ratio of less than 0.90, patients will have an impaired pharyngeal function
with risk for aspiration in case of regurgitation and at a TOF ratio of 0.70 or
less they will additionally have an impaired hypoxic ventilatory response.
Based on the above, return to normal
vital muscle function and normal ventilatory regulation cannot be guaranteed
unless an adductor pollicis TOF ratio of ≥ 0.90 has been recorded. This
level of neuromuscular recovery can only be assessed by direct measuring
mechanical or electrical responses of the thumb (i.e. accelography, TOF Watch or Datex NMT Modul or by EMG, Datex EMG Modul) since tactile or
visual monitoring of the TOF of double burst (DBS) response has been shown to
be unreliable at TOF ratios > 0.60. At the end of the anaesthetic procedure
patients should remain intubated until a TOF ratio of 0.90 is reached. In
patients with TOF ratios below 0.90 and where pharmacological reversal is
beneficial, an anticholinesterase should be given in appropriate dosage (e.g.
neostigmine 30-70 µg/kg bodyweight). Further prevention of residual block is
best achieved by avoiding body hypothermia and/or combination with high
end-tidal concentrations of inhalational agents. Correct monitoring of
neuromuscular function is at present best achieved by direct methods such as
accelography or EMG..
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