Temperature
Hypothermia prolongs the duration of action of nondepolarizing
neuromuscular blockers.[477]
[478]
[479]
The force of contraction of the adductor
pollicis
decreases by 10% to 16% per degree centigrade decrease in muscle temperature below
35.2°C.[480]
[481]
To maintain muscle temperature above 35.2°C, central temperature must be maintained
at 36.0°C.[477]
The recovery to 10% twitch
height with 0.1 mg/kg vecuronium increases from 28 minutes at a mean central temperature
of 36.4°C to 64 minutes at 34.4°C.[477]
The mechanism or mechanisms underlying this prolongation may be
pharmacodynamic, pharmacokinetic, or both[479]
and
include diminished renal and hepatic excretion, changing drug volumes of distribution,
altered local diffusion receptor affinity, changes in pH at the neuromuscular junction,
and the net effect of cooling on the various components of neuromuscular transmission.
[477]
[482]
[483]
[484]
[485]
Hypothermia
decreases plasma clearance and prolongs the duration of action of rocuronium and
vecuronium.[479]
[486]
Temperature-related differences in the pharmacodynamics of vecuronium have been
reported. The ke0
decreased (0.023/min/°C) with lower temperature,
thus suggesting slightly delayed equilibration of drug between the circulation and
the neuromuscular junction during hypothermia.[479]
The Hofmann elimination process of atracurium is slowed by a decrease in pH and
especially by a decrease in temperature.[9]
[487]
In fact, atracurium's duration of action is markedly prolonged by hypothermia.[478]
For instance, the duration of action of 0.5 mg/kg atracurium is 44 minutes at 37°C
and 68 minutes at 34.0°C.[478]
Decreases in
temperature also decrease the speed of neural conduction in humans.[488]
A decrease in muscle temperature from 35°C to 23.5°C resulted in a 50% reduction
in nerve conduction velocity.[488]
Changes in temperature will affect interpretation of the results
of monitoring neuromuscular blockade. The duration of action of vecuronium measured
in an arm cooled to a skin temperature of 27°C is prolonged, and monitoring by
post-tetanic count in that arm is unreliable.[489]
In the same patient, TOF responses are different if the arms are at different temperatures,
and correlation of responses in the two arms becomes progressively poorer as the
temperature difference between the arms increases.[490]
[491]
The efficacy of neostigmine is not altered by mild hypothermia.
[492]
[493]
[494]
Hypothermia does not change the clearance, maximum effect, or duration of action
of neostigmine in volunteers.[494]
