Magnesium (Mg) serves as an analgesic by controlling calcium influx into cells and acting as an NMDA receptor antagonist, effectively reducing neuropathic pain. It has been proven to decrease the necessity for postoperative opioids and enhance postoperative pain management, with a recommended loading dose of 30-50 mg/kg followed by a maintenance intravenous infusion at 10 mg/kg/hr.
Notably, magnesium can lead to hypotension and bradycardia, which are more prevalent at higher doses but can be managed with standard treatments. Intraoperative use of magnesium necessitates a reduction in muscle relaxants and anesthetic agents to avoid prolonged neuromuscular blockage and delayed anesthesia recovery. Magnesium amplifies the effects of neuromuscular blockers and CNS depressants, thus monitoring the depth of neuromuscular blocks and ensuring complete neuromuscular recovery before extubation is essential.
Research, including a 2013 systematic review by De Oliveira et al., has shown that magnesium significantly lowers both early and late postoperative pain, as well as nausea and vomiting. Patients receiving magnesium both during and after surgery experienced more significant pain score reductions and opioid usage decreases compared to controls or those only receiving intraoperative magnesium. On average, morphine consumption dropped by over 10 mg in the immediate postoperative phase, although the total dose of magnesium did not correlate with overall postoperative opioid use. This indicates the importance of intraoperative rather than postoperative magnesium administration in pain and opioid reduction, even though it did not directly lessen morphine's systemic side effects like nausea or vomiting.
| Agent | Dosing | Dose-limiting caution |
|---|---|---|
| Dexmedetomidine | IV bolus doses until the desired effect; 20 mcg can replace 2 mg midazolam for preoperative anxiolysis; two-hour half-life | Bradycardia is the major dose-limiting effect, manageable with atropine |
| IV acetaminophen | Do not exceed 15 mg/kg every 6 hours; daily ceiling 4 g (2 g in liver disease) | Liver toxicity |
| Ketamine | Sub-anesthetic boluses up to 0.25 mg/kg; 2–3 hour half-life; best not to exceed 0.5 mg/kg | Tachycardia and hypertension; myocardial ischemia risk in coronary artery disease; dysphoria and hallucinations at higher doses; high addictive potential |
| Lidocaine | Load 1–2 mg/kg then 1–2 mg/kg/hr, halved every 6 hours; a 1 mg/kg bolus with 1.5 mg/kg/hr gives about 2 mcg/mL | Reduce the rate in low cardiac output; perioral paresthesia, metallic taste, tinnitus, seizures; cardiac arrest above 5 mcg/mL |
| Gabapentin | 300 mg orally; perioperative doses of 300–1200 mg reduce opioid use | — |
| Pregabalin | 150 mg orally | — |
| Magnesium | Load 30–50 mg/kg then IV infusion at 10 mg/kg/hr | Hypotension and bradycardia at higher doses; potentiates neuromuscular blockers and CNS depressants — confirm full neuromuscular recovery before extubation |
