American Society of Regional Anesthesia and Pain Medicine August 2018 - 38
Table 1: Summary of magnesium dosing strategies.10
Surgery
Pain
rating*
Bolus
(mg/kg)
Time before
induction
(minutes)
Infusion
(mg/kg/hr)
Magnesium
mean fentanyl
consumption
(mcg)
Control mean
fentanyl
consumption
(mcg)
Mean difference
in fentanyl
consumption
(mcg) 95% CI
Upper extremity
Severe
30
15
10
89
104.8
-15.8
Laparotomy
Severe
40
15
10
70.05
100.55
-30.5
Open
cholecystectomy
Severe
50
15
8
52.81
83.64
-30.83
Hysterectomy
Severe
40
15
0/10/20**
555.72
588.88
-33.16
Lumbar
discectomy
Moderate
50
10
20
100
147
-47
Abdominal
hernioplasty
Moderate
30
After
6
192.44
288.8
-96.36
Endoscopy sinus
surgery
Moderate
40
10
15
82.27
173.04
-90.87
Laparoscopy
cholecystectomy
Mild
50
15
25
254
323
-69
* According to surgical procedure
** 0, 10, and 20 mg/kg/hr infusions were studied. A dose of 10 mg/kg/hr was found to reduce narcotic requirements. Increased doses did not yield further
benefits and led to more hemodynamic compromise.
control, magnesium's neuroprotective capabilities, and decreases in
overall anesthetic requirements.
Furthermore, magnesium can counter remifentanil-induced
hyperalgesia in patients undergoing thyroidectomy. However,
it can also mask hypocalcemia, a potential complication in
thyroidectomies.9
A meta-analysis of 13 studies with 694 patients undergoing
a variety of surgical procedures found that a magnesium
bolus (30-50 mg/kg) and infusion (6-25 mg/kg/hr) decreased
opioid consumption without compromising hemodynamic
stability.10 Based on this information and excluding studies
that used alfentanil, morphine, and remifentanil for the sake of
standardization, a loading dose of magnesium (30-50 mg/kg
given 10-15 minutes prior or shortly after induction) followed
by an infusion (8-15 mg/kg/hr) significantly reduced fentanyl
consumption overall and in surgeries associated with significant
pain (see Table 1 Adapted from data in meta-analysis).10
OTHER PERIOPERATIVE ROLES AND ADVERSE EFFECTS
When using perioperative magnesium, providers must
understand its other properties and possible adverse effects.
Magnesium has documented central and peripheral neurologic
effects. It increases microcirculation in cerebral blood flow and
38
may prevent vasospasm in patients suffering from aneurysmal
subarachnoid hemorrhage. 11 In parturients with pre-eclampsia,
magnesium provides systemic, cerebral, and uterine vasodilation
coupled with neuroprotection and seizure prophylaxis. This
offers maternal and fetal neuroprotection and may lower the
risk of fetal cerebral palsy in women at risk for preterm birth. 12
Peripherally, magnesium potentiates nondepolarizing muscle
relaxants at both the pre- and postsynaptic sites. Magnesium
infusions (60 mg/kg over 15 minutes) administered prior to
induction can reduce rocuronium onset time by 65%. 13 Similarly,
magnesium may reduce succinylcholine induced fasciculations 14
and the incidence and intensity of myoclonus caused by
etomidate. 15
As a competitive antagonist of L-type calcium channels,
magnesium also impacts the cardiac and pulmonary systems.
Magnesium has both antiarrhythmic properties and is the
treatment for Torsades de Pointes (pulseless: 1-2 g IV over
5-20 minutes; pulse present: 1-2 g over 5-60 minutes).
Concomitantly, it promotes arterial vasodilation and prevents
catecholamine secretion.16 Those cardiovascular characteristics
may be desirable to assist in the perioperative management of
patients with pathologically elevated hemodynamics, such as
pheochromocytoma. Moreover, antagonism of calcium-induced
muscle contraction by magnesium potentiates bronchodilation, and
American Society of Regional Anesthesia and Pain Medicine
2018
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