Abstract:
OBJECTIVE : To compare the antinociceptive effects of
magnesium sulphate (MgSO4) when administered
epidurally alone and in combination with morphine.
STUDY DESIGN : Experimental, randomized, ‘blinded’,
crossover study.
ANIMALS : Six healthy adult Beagle dogs.
METHODS : Evaluated treatments were MgSO4
(2.5 mg kg
1) alone (Mg), morphine (0.1 mg kg
1)
alone (Mo), MgSO4 in combination with morphine
(Mm), and sterile water (0.115 mL kg
1; Co) that
were injected in the lumbosacral epidural space
using an epidural catheter. Antinociception was
measured using the von Frey mechanical threshold
device applied to the carpal pads, both sides of the
thorax and metatarsi. Measurements were obtained
at time points: before treatment (baseline) and 0.5,
1, 2, 4, 6, 12, 18 and 24 hours after the epidural
injection. Sedation, behaviour score and presence of
motor deficits were assessed. Data were analyzed
using a linear mixed model and Bonferroni adjustments,
with significance set at p < 0.05.
RESULTS : There were significant effects of treatment
and time in all regions. Overall threshold values in
grammes force [median (interquartile range)]
when stimulation regions were combined were
significantly higher in Mg [164 (135–200)], Mo 156 (129–195)] and Mm [158 (131–192)] compared
to Co [145 (120–179)]. Thresholds were
significantly higher compared to Co in Mg, Mo and
Mm at the thorax and metatarsi, but only in Mg and
Mo at the carpal pads. No motor deficits were
observed at any time point. Thresholds (combined
regions) were increased from baseline at one or more
time points with all treatments, including control.
CONCLUSION AND CLINICAL RELEVANCE : Epidural MgSO4
produced an antinociceptive effect characterised by
an increase in the mechanical thresholds of similar
magnitude to that produced by epidural morphine,
compared with the control group, without causing
any motor deficits. No potentiation of morphine
antinociception was observed. The onset and offset
times of antinociception could not be clearly
established. To what extent these results can be
extrapolated to clinical cases requires further
investigation.
