Spinal fusion surgery has become the gold standard treatment for a variety of musculoskeletal disorders affecting the spine. This surgical technique aims to stabilize the spinal column by limiting movement between unstable vertebrae. Fusion is achieved by connecting one or more vertebrae using bone grafts, often supported with stabilizing implants such as screws, rods, or intervertebral cages [1]. These grafts may be positioned between vertebral bodies to replace intervertebral discs or placed posteriorly to reinforce the vertebrae. Following successful fusion, the treated vertebrae function as a single, solid unit, effectively eliminating pathological motion and alleviating pain.

This approach is widely employed to manage vertebral fractures, spinal deformities, degenerative disc diseases, tumors, spinal cord injuries, and other structural abnormalities involving vertebrae or discs [2].

Magnesium, an essential intracellular cation, plays a crucial role in maintaining human homeostasis. It is involved in activating over 300 enzymatic pathways, particularly those related to energy metabolism [3,4]. While the exact mechanisms underlying magnesium’s analgesic effects remain incompletely understood, its modulation of calcium channels and N-methyl-D-aspartate (NMDA) receptors is believed to be central to its action [5,6]. NMDA receptors facilitate excitatory synaptic transmission and play a significant role in central sensitization, especially in postoperative pain syndromes. Thus, NMDA receptor antagonists like magnesium may help prevent the development of heightened nociceptive sensitivity and chronic pain states [7,8]. Furthermore, magnesium has been shown to reduce catecholamine release, which helps attenuate the exaggerated nociceptive responses triggered by surgical stress. The use of magnesium sulfate for postoperative pain management has been explored for several decades. The first randomized controlled trial (RCT) investigating its analgesic adjunct role was published in 1996, demonstrating its efficacy in decreasing intraoperative and postoperative analgesic requirements [9-11].

Subsequent studies have consistently reported that perioperative magnesium administration can significantly reduce fentanyl consumption and improve pain scores during the perioperative period [12,13]. Spine surgeries present unique anesthetic challenges, requiring stable hemodynamics, minimal intraoperative bleeding, and precise patient positioning, all of which complicate anesthesia management [14,15]. Several clinical trials have indicated that magnesium sulfate infusion during general anesthesia can reduce both intraoperative anesthetic consumption and postoperative analgesic needs. Nonetheless, some authors have argued that its effect on postoperative pain is variable and warrants further investigation. Few studies have examined magnesium sulfate administration in the context of regional anesthesia. One study reported that a single intravenous dose of 8 mg/kg magnesium sulfate significantly decreased postoperative pain scores at 1, 3, 6, and 12 hours after surgery and reduced morphine requirements in the first 24 hours [16]. Additionally, despite its limited ability to cross the blood-brain barrier, intrathecal administration of fentanyl combined with magnesium sulfate has been shown to prolong the duration of analgesia more effectively than fentanyl alone. This is likely due to magnesium’s potentiating effect on opioids via both central and peripheral mechanisms [17].

A systematic review conducted in 2007 concluded that the impact of perioperative magnesium on postoperative pain and analgesic requirements might be modest, highlighting the need for further high-quality research to clarify its role [9]. Beyond its analgesic properties, magnesium sulfate is well-established in obstetric practice for the prevention and treatment of eclampsia and pre-eclampsia-related seizures [18,19]. Additionally, it possesses cardiovascular effects, including vasodilation and reduction of systemic vascular resistance (SVR), which can lead to increased cardiac output [20,21]. Given these multifaceted pharmacological properties, magnesium sulfate holds promise as a perioperative adjunct to improve anesthetic outcomes, reduce analgesic requirements, and potentially enhance recovery profiles in patients undergoing major spinal surgeries.

Aim

To evaluate the effect of varying intravenous magnesium sulfate doses on intraoperative anesthetic requirements and postoperative pain control in patients undergoing posterior spinal fusion surgery.

Objectives

1. To compare intraoperative anesthetic (fentanyl) consumption among different magnesium sulfate infusion dose groups and a placebo group.
2. To assess intraoperative blood loss across the study groups receiving different magnesium sulfate doses.
3. To evaluate postoperative pain scores at various time intervals in patients receiving magnesium sulfate versus placebo.
4. To determine the hemodynamic stability (heart rate and blood pressure changes) associated with different doses of magnesium sulfate during surgery.
5. To identify the optimal dose of magnesium sulfate that provides effective intraoperative and postoperative analgesia without significant adverse effects.

Study Design and Setting

This was a double-blind, randomized clinical trial conducted at Nova Institute of Medical Sciences and Research Centre, Hyderabad, Telangana. The study was approved by the institutional ethics committee, and written informed consent was obtained from all participants prior to enrollment.

Target Population and Sample

A total of 80 patients aged between 18 and 65 years, classified as ASA physical status class I or II, who were candidates for elective posterior spinal fusion (PSF) surgery involving 2–3 spinal levels, were initially included. Patients were selected using a simple random sampling method based on a table of random numbers. The sample size was determined using Krejcie and Morgan’s table, and confirmed by preliminary data and statistical consultation.

Group Allocation

Patients were randomly assigned into four groups of 20 each:

• Group M1: Magnesium sulfate infusion at 10 mg/kg/hr.
• Group M2: Magnesium sulfate infusion at 15 mg/kg/hr.
• Group M3: Magnesium sulfate infusion at 20 mg/kg/hr.
• Group S: Control group receiving an equivalent volume of normal saline.

Due to severe hypotension and bradycardia observed in Group M3, the infusion was discontinued in this group, and these patients were excluded from final analysis. Thus, 60 patients from Groups M1, M2, and S were included in the final study.

Blinding and Drug Administration

Each patient received a sealed envelope indicating group allocation, opened by a designated nurse in the operating room. Patients were blinded to the intervention, and all study medications were prepared and administered by the anesthesiologist, who was aware of group allocation. Other research team members remained blinded throughout the study. Fifteen minutes before induction of anesthesia, patients in Groups M1 and M2 received a bolus dose of magnesium sulfate 50 mg/kg IV over 15 minutes, followed by continuous infusion at the assigned rate (10 or 15 mg/kg/hr) until the end of surgery. Group S received a bolus and infusion of normal saline in identical volumes.

Anesthesia Protocol

All patients underwent a standardized anesthetic technique:

• Preoxygenation with 100% oxygen for 3 minutes.
• Premedication: Midazolam 0.02 mg/kg IV.
• Induction: Fentanyl, lidocaine (3–5 mg/kg), propofol (1.5–2 mg/kg), and atracurium 0.5 mg/kg IV.
• Maintenance: Propofol infusion, 50% oxygen in air, with BIS monitoring targeted at 40–50, and hemodynamic parameters maintained within ±25% of baseline values.
• Supplemental doses of atracurium were administered as needed, guided by TOF monitoring. Titrated fentanyl boluses were given intraoperatively based on heart rate and blood pressure fluctuations (maintained within 75–125% of baseline).

Intraoperative Monitoring

Standard monitoring included continuous ECG, SpO₂, BIS, invasive blood pressure (IBP), non-invasive blood pressure (NIBP), and TOF. An arterial line was placed for all patients. Central venous catheter placement was determined as needed based on individual clinical conditions.

Outcome Measures

• Primary outcomes: Intraoperative fentanyl consumption, intraoperative blood loss (measured by weighing surgical sponges and estimating suction canister volumes after subtracting irrigation fluids), and postoperative pain scores using the Numerical Rating Scale (NRS) at 1, 2, 6, 12, and 24 hours postoperatively.
• Secondary outcomes: Hemodynamic stability (heart rate and mean arterial pressure), need for postoperative rescue analgesics.

Postoperative Analgesia Protocol

For NRS scores of 4–6, patients received ketorolac 30 mg IV. For NRS scores of 7–10, pethidine 25 mg IV was administered. The total amount of analgesics used was documented for each patient.

Data Collection and Statistical Analysis

Baseline serum magnesium levels were measured preoperatively. Data were entered into EPI6 software and analyzed using SPSS. Statistical tests included independent t-test, chi-square test, one-way ANOVA, and Kruskal-Wallis test, as appropriate. A p-value <0.05 was considered statistically significant.

Surgical Technique

All patients underwent posterior spinal fusion surgery performed by the same experienced surgeon under general anesthesia in the prone position. No intraoperative neuromonitoring was used.

Table 1. Gender Distribution

Table 2. Age Descriptive Statistics

Table 3. Group Distribution

Table 4. Intraoperative Variables Descriptive Statistics

Table 5. Pain Scores and Anesthetic Data

In this double-blind clinical trial, 60 patients undergoing posterior spinal fusion were analyzed across three groups: M1 (10 mg/kg/hr magnesium sulfate), M2 (15 mg/kg/hr magnesium sulfate), and control group S (normal saline). The groups were comparable in terms of age and gender distribution, with no significant demographic differences observed.

Hemodynamic variables, including heart rate and mean arterial pressure, were generally stable across groups, though the M2 group exhibited slightly lower heart rates and MAP values, indicating better intraoperative stability. Blood volume loss was significantly lower in the magnesium groups, particularly in M2, compared to the control group.

Intraoperative fentanyl consumption was significantly reduced in the M2 group compared to M1 and S groups, reflecting a sparing effect on anesthetic requirements. Additionally, the M2 group showed a marked reduction in intraoperative bleeding.

Recovery time (time from cessation of anesthesia to extubation) was longer in the magnesium groups, with the M2 group experiencing slightly longer wake-up times than M1, but remaining clinically acceptable.

Postoperative pain scores measured by NRS were consistently lower in the magnesium groups at all time points (1, 2, 6, 12, and 24 hours), with M2 showing the most significant reduction. Consequently, postoperative analgesic requirements were lower in the M2 group, demonstrating improved postoperative pain control.

Overall, the findings support the use of magnesium sulfate infusion, particularly at a dose of 15 mg/kg/hr, to reduce intraoperative anesthetic and analgesic needs, minimize blood loss, and enhance postoperative pain outcomes, while maintaining hemodynamic stability.

The present study evaluated the effects of different doses of intravenous magnesium sulfate on intraoperative anesthetic requirements, postoperative pain control, blood loss, and hemodynamic stability in patients undergoing posterior spinal fusion surgery.

Our results demonstrated that patients receiving magnesium sulfate, particularly at a dose of 15 mg/kg/hr (group M2), had significantly reduced intraoperative fentanyl consumption, lower postoperative pain scores, and reduced intraoperative blood loss compared to the control group. These findings align with several previous studies highlighting the analgesic and opioid-sparing effects of magnesium sulfate.

In 2021, Silva et al. conducted a prospective, double-blind study involving 50 patients undergoing abdominoplasty after bariatric surgery. Patients were randomized to receive either remifentanil or magnesium sulfate for intraoperative analgesia. The authors concluded that magnesium sulfate is a safe and effective alternative for managing intraoperative pain, particularly useful when opioid use is contraindicated [22]. This supports our finding of decreased fentanyl consumption with magnesium sulfate administration. Rahman Abbasi and colleagues (2010) investigated the effect of a single preoperative dose of gabapentin on propofol and remifentanil requirements during laparoscopic cholecystectomy. They observed significantly reduced anesthetic needs in patients receiving gabapentin [23], suggesting that NMDA receptor antagonists (like magnesium) can modulate perioperative analgesia and anesthetic sparing, consistent with our study.

 In a 2021 randomized, double-blind trial by Luiz Benevides et al., 86 patients undergoing abdominal hysterectomy under spinal anesthesia received either magnesium sulfate or saline infusion, along with intrathecal morphine and systemic analgesics. The magnesium sulfate group reported significantly lower pain intensity and reduced tramadol consumption postoperatively [24]. Our results similarly revealed lower pain scores and reduced postoperative analgesic requirements in the magnesium groups. Ebrahimi et al. (2020) evaluated magnesium sulfate in reducing intraoperative bleeding in 80 surgical patients. Although morphine consumption and VAS scores were significantly lower in the magnesium group, no reduction in intraoperative bleeding was noted [25]. In contrast, our study observed a significant reduction in blood loss in the magnesium groups, suggesting potential hemostatic benefits specific to spine surgeries and the higher magnesium doses used.

Regarding recovery profiles, our study found a longer time to extubation (wakeup time) in magnesium groups compared to the control group, with the M2 group showing the longest duration. This difference was statistically significant (p < 0.05). These findings align with observations by Srivastava et al. (2016), who compared dexmedetomidine and magnesium sulfate in spine surgeries. Their study reported that recovery was faster with dexmedetomidine compared to magnesium sulfate, while both agents reduced anesthetic requirements [26].

On hemodynamic stability, the present study showed that the magnesium sulfate groups maintained relatively stable intraoperative heart rates and mean arterial pressures. Although Srivastava et al. demonstrated that dexmedetomidine was more effective than magnesium sulfate for hemodynamic control [26], our findings indicate that magnesium at a dose of 15 mg/kg/hr still offers acceptable hemodynamic stability and overall clinical benefits.

In summary, our study supports the analgesic, anesthetic-sparing, and potential hemostatic benefits of intraoperative magnesium sulfate, especially at 15 mg/kg/hr. However, it also highlights the trade-off of a slightly longer recovery time.

This study demonstrated that intraoperative administration of magnesium sulfate can effectively enhance the quality of anesthesia and improve postoperative pain control in patients undergoing posterior spinal fusion surgery. A dosage of 15 mg/kg/hr was found to be optimal, resulting in the lowest intraoperative narcotic (fentanyl) consumption, reduced postoperative analgesic requirements, and satisfactory hemodynamic stability. Higher doses exceeding 15 mg/kg/hr are not recommended due to the risk of significant decreases in heart rate and mean arterial pressure. Magnesium sulfate infusion also contributed to reduced intraoperative bleeding, fewer opioid-related side effects, and potential cost savings, making it particularly advantageous for patients requiring prolonged anesthesia. Given that intraoperative blood loss is a critical factor influencing morbidity and mortality, the hemostatic benefits observed with magnesium sulfate highlight its additional clinical value. Future research should explore the role of magnesium sulfate in different surgical settings and patient populations, as well as the effects of preoperative and perioperative administration strategies, to further optimize perioperative pain management and improve surgical outcomes.

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