Ain-Shams Journal of Anaesthesiology

: 2014  |  Volume : 7  |  Issue : 1  |  Page : 59--64

The effect of adding magnesium sulfate to lidocaine in an interscalene plexus block for shoulder arthroscopic acromioplasty

Amr M Abdelfatah, Ahmed N Elshaer 
 Department of Anesthesia, Intensive Care, and Pain Management, Faculty of Medicine, Ain-Shams University, Cairo, Egypt

Correspondence Address:
Amr M Abdelfatah
MD, Department of Anesthesia, Intensive Care, and Pain Management, Faculty of Medicine, Ain-Shams University, Cairo


Background Incorporation of adjuvants into local anesthetics could potentially aid in producing satisfactory prolonged postoperative analgesic duration and reducing postoperative adverse reactions associated with excessive systemic analgesic intake. The aim of this trial was to study the effect of incorporating magnesium sulfate as an adjuvant into lidocaine in an interscalene plexus block (ISPB) for arthroscopic acromioplasty. Materials and methods Sixty ASA I and II grade patients scheduled for elective arthroscopic acromioplasty under ultrasound guidance and peripheral nerve stimulation technique were allocated randomly to two groups: group L, which included patients who received 20 ml of lidocaine 2% with epinephrine 1 : 200 000 plus 5 ml 0.9% NaCl, and group LM, which included patients who received 20 ml of lidocaine 2% with epinephrine 1 : 200 000 plus 5 ml of 10% MgSO 4 (500 mg MgSO 4 ). The following data were collected: the onset of sensory block, the occurrence of satisfactory motor block, sensory block duration, Numeric Rating Scale (NRS) at the first analgesic dose, motor block duration, total morphine consumption in the first 24 h, and the presence of any adverse reactions. Results Patients in the magnesium sulfate group showed a prolonged postoperative analgesic duration, the NRS records decreased significantly on the first analgesic dose, and morphine consumption was reduced, and no adverse events related to its use in ISPB were recorded. Conclusion The addition of magnesium sulfate to lidocaine in ISPB significantly prolongs the analgesic duration and reduces the NRS score and postoperative opioid requirements in shoulder arthroscopic acromioplasty without side effects.

How to cite this article:
Abdelfatah AM, Elshaer AN. The effect of adding magnesium sulfate to lidocaine in an interscalene plexus block for shoulder arthroscopic acromioplasty.Ain-Shams J Anaesthesiol 2014;7:59-64

How to cite this URL:
Abdelfatah AM, Elshaer AN. The effect of adding magnesium sulfate to lidocaine in an interscalene plexus block for shoulder arthroscopic acromioplasty. Ain-Shams J Anaesthesiol [serial online] 2014 [cited 2021 Dec 7 ];7:59-64
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Magnesium (Mg) has noncompetitive postsynaptic N-methyl D-aspartate (NMDA) receptors and calcium channel-blocking properties that have been used successfully to potentiate opioid analgesia and to treat neuropathic pain in animals [1],[2]. NMDA receptors are important components of pain processing. As additives to local anesthetics can potentially enhance the quality and duration of the block, magnesium is one of the additives that have been investigated thoroughly in the field of regional anesthesia. The intrathecal administration of NMDA antagonists, such as magnesium and dizocilpine maleate (MK-801), was effective in blocking ipsilateral hyperesthesia in rats with unilateral experimental peripheral mononeuropathy with no reported effect on thermal sensitivity on the operated side [3],[4]. Magnesium, when coadministered intrathecally with morphine, enhanced morphine-induced analgesia in normal rats and reduced mechanical allodynia after surgical incision [5].

Modulation of external magnesium concentration bathing a nerve bundle results in an enhancement of both frequency-dependent and non-frequency-dependent conduction block with various local anesthetics [6].

Arthroscopic acromioplasty for the treatment of shoulder impingement syndrome requires early resumption of motor activity for the shoulder with proper postoperative analgesia and the use of lidocaine - an intermediate-acting local anesthetic - would be valuable to achieve early shoulder motion. Interscalene plexus block (ISPB) is an ideal technique for performing shoulder surgical procedures. Recently, ultrasound-guided peripheral nerve block gained wide recognition as it reduced the complications associated with blind techniques in regional blocks, thus limiting the number of needle attempts for nerve localization, with better visualization for the local anesthetic deposited in the vicinity of nerves [7],[8].

The aim of this study was to monitor the effect of magnesium sulfate on the action of lidocaine in an ISPB for arthroscopic acromioplasty. We hypothesized that magnesium may enhance the quality of neural block and produce an opioid-sparing effect when incorporated with lidocaine.

 Materials and methods

The study was carried out in Ain-Shams University Hospital's orthopedic surgery theater. The Institutional Ethics Committee approved this study and written informed consent for the use of the regional anesthetic technique was obtained from all patients before the surgical procedure. This randomized, double-blind study enrolled 60 ASA I-II grade patients scheduled for elective shoulder arthroscopic acromioplasty under ultrasound nerve stimulation-guided ISPB. Using a computer-generated randomization on the basis of patients' hospital number, they were allocated randomly to two groups. Patients in group L received 20 ml lidocaine 2% with epinephrine 1 : 200 000 plus 5 ml 0.9% NaCl, whereas patients in group LM received 20 ml lidocaine 2% with epinephrine 1 : 200 000 plus 5 ml of 10% MgSO 4 (500 mg MgSO 4 ) for ISPB.

Exclusion criteria included patients with known allergy to local anesthetics, severe chronic obstructive airway disease, hepatic and renal insufficiency, post-traumatic nerve injury in the upper limb, and infection at the site of injection.

All patients were premedicated in the induction room with 3 mg intravenous midazolam and on arrival to the operating theater, 500 ml lactated Ringer's solution was infused intravenously. Patients were monitored using a five-lead ECG, pulse oximeter, and noninvasive blood pressure was recorded. The block was achieved with the patient in the supine position, 45° table head up (beach chair position), and with the head rotated toward the nonoperative side. The lateral aspect of the neck was cleaned using an antiseptic iodine solution and draped. A linear array ultrasound transducer (LOGIQ e, 8L-RS 42 × 8 mm 4-12 MHz* broadband, multifrequency transducer; General Electric, WI, USA) was used in the study. A sterile cover was used over the transducer along with a sterile gel. The transducer was held in the nondominant hand locating the plexus roots through a short-axis view between the anterior and middle scalene muscles at the level of the sixth cervical vertebra deep to the sternocleidomastoid muscle. The needle entry point was infiltrated with 2 ml lidocaine 1%. Using an in-plane approach, a 5 cm insulated needle (Stimuplex D; B. Braun Melsungen AG, Bethlehem, PA, USA) was directed toward the plexus roots in the interscalene region. The needle was connected to the peripheral nerve stimulator (EzStim II ES 400 Dual-Purpose Peripheral Nerve Stimulator, Texas, USA), which was set on 1 mA. Once deltoid, biceps, or triceps contractions were attained, the stimulating current was reduced until the motor response was still detected between 0.2 and 0.4 mA. The needle was redirected under real-time ultrasound guidance to ensure proper deposition of solution around the nerve roots. Syringes containing the studied drugs were labeled from 1 to 60 and sealed in envelopes. The anesthesiologist performed the block and the physicians who evaluated the patients postoperatively were blinded to the drugs studied.

The onset of sensory block was the time elapsed since completion of injection till loss of sensation at the surgical site using noncrushing forceps at 5-min intervals; satisfactory motor block was the inability to elevate the upper limb over the operating table. Sensory block duration was the time from loss of sensation at the surgical site till the first analgesic required by the patient in the ward. Motor block duration was estimated as time from loss of motor power to elevate the arm above the operating table till resumption of arm movement. Morphine sulfate (3 mg intravenously) was used as postoperative analgesia by the nursing staff, followed by a patient-controlled intravenous analgesia (CADD-Legacy PCA Ambulatory Infusion Pump, Model 6300 Smith medical, Dublin, OH, USA) with a standard regimen of: 1 mg morphine sulfate bolus on pushing the demand button, no basal infusion rate, lockout interval 10 min, with a maximum of six doses per hour. The total quantity (mg) of morphine consumed over the first 24 h postoperatively was recorded. Numeric Rating Scale (NRS) between 0 and 10 was recorded at the time of the first analgesic dose administered to the patient, where 0 was no pain and 10 was worst imaginable pain. Hemodynamics, that is mean arterial blood pressure (MAP), heart rate (HR), and SpO 2 , were recorded throughout the procedure at 2, 6, and 12 h postoperatively.

The Ramsay Sedation Score 9 was used to measure the degree of sedation in both groups, where 1, anxious patient; 2, tranquil; 3, responding to commands; 4, brisk response to glabellar tap; 5, sluggish response to glabellar tap; and 6, no response. The Optimum Sedation Score was 2 or 3. Before discharge from the hospital, all patients were asked to indicate their degree of satisfaction with the anesthetic technique using a scale from 1 to 3, whereas 1 meant 'not satisfied', 2 'satisfied,' and 3 'find the experience pleasant.' Patients were asked whether they would like to have the same block should they need another shoulder surgery.

Any complication during the study was recorded, a decrease in the MAP greater than 25% of the preblock figure was treated using/by intravenous ephedrine (6 mg), respiratory depression (respiratory rate < 9/min or SpO 2 < 92%) was treated by supplemental oxygen, and respiratory support if needed, HR less than 50 bpm was controlled by 0.6 mg intravenous atropine, and nausea and vomiting, if any, was treated with 10 mg intravenous metoclopramide.

Results are expressed as mean and SD, median, and range or numbers. Analysis of data between the groups was carried out using the Student t-test for independent samples for parametric data or the Mann-Whitney U-test for nonparametric data. Categorical data between study groups were compared using the χ2 -test or Fisher's exact test. A P value of less than 0.05 was considered statistically significant.


Sixty patients undergoing shoulder arthroscopic acromioplasty were successfully enrolled in the study, 30 in each group; the variables in demographic data did not show a statistically significant difference between both groups with respect to age, sex, and the patient's weight [Table 1].

Patients in both groups had a successful sensory and satisfactory motor block throughout the operation and continued through discharge from the postanesthesia care unit. Two patients in the L group and one in the LM group reported mild pain at the surgical site during the operation that was controlled by 100 μg intravenous fentanyl. The onset of the block was 18.8 ± 4.7 min in group L and 18.5 ± 4.4 min in group LM, which showed no statistically significant difference between both groups (P > 0.05). The sensory block duration was 267.3 ± 28.1 min in group L compared with 442.3 ± 40.5 min in group LM, with a P value of less than 0.05 that was statistically significant, and indicated a prolongation in block duration in the lidocaine magnesium group [Figure 1], whereas the motor block duration was 212.8 ± 23.8 min in the L group and 244.3 ± 27.1 min in the LM group, with no statistically significant difference between the two groups.{Figure 1}

NRS upon the first analgesic requirement was significantly higher in the L group (4.7 ± 1.9) compared with that in the LM group (3.2 ± 1.3) (P < 0.05, [Table 2]).

The total morphine consumption in the L group was 5.7 ± 1.2 mg compared with 4.6 ± 0.8 mg that in the LM group, with a P value of less than 0.05, thus implying that significantly less morphine was consumed in the LM group in 24 h [Table 2] and [Figure 2].{Figure 2}

Four patients in the L group and two in the LM group experienced nausea and vomiting that were controlled by 10 mg intravenous metoclopramide; this was not statistically significant (P = 0.087). No neurological sensory or motor complications were recorded in the study. The Ramsay Sedation Score did not show any statistically significant difference in both groups.

With respect to the hemodynamics (MAP, HR) and SpO 2 , there was no statistically significant difference between both groups [Table 3]. All patients were discharged home on the next day of surgery. Forty-six patients rated their regional block as 'satisfactory' and the remaining 14 rated it as 'find the experience pleasant.' There was no significant difference in the satisfaction scores between the two groups. All patients answered that they would like to have the same block should they need a similar surgery.


This study documented the effectiveness of magnesium sulfate in enhancing lidocaine action in the setting of ISPB for shoulder arthroscopic acromioplasty; there was a significantly prolonged analgesic duration as well as a reduction in the opioid consumption postoperatively, with no obvious side effects, NRS was reduced upon the first request of rescue analgesia in the magnesium group. The sensory onset and motor block duration were similar in both groups, showing that the addition of magnesium to lidocaine neither enhanced the sensory onset nor the motor block duration.

The ISPB has an established advantage for shoulder surgery over general anesthesia as it results in less nonsurgical intraoperative time, postanesthesia care unit stay, fewer unplanned hospital admission for pain therapy, sedation, and postoperative nausea and vomiting [10].

In the current study, the addition of ultrasound to the peripheral nerve stimulation was made to enhance the accuracy of the block to ensure proper deposition of 25 ml of the local anesthetic solution around the plexus roots [Figure 3].{Figure 3}

Ultrasound-guided nerve blocks were first described as early as 1978 and the advancement in ultrasound technology in the 1990s led to growing interest in this field [11]. In the present study, all cases had a successful block with the described technique, with satisfactory operative conditions for both the surgeon and the patient. The analgesic effect of ISPB was almost doubled by the addition of 500 mg of magnesium sulfate to lidocaine. The use of an intermediate-acting local anesthetic (e.g. lidocaine) aided early shoulder mobilization, plus early neurological testing, to exclude intraoperative surgical nerve injury.

The reduced NRS on the first analgesic requirements in the magnesium group was a good sign observed in the study, indicating a reduction in the nociception postoperatively by the addition of magnesium to the local anesthetic.

The response of NMDA receptors can be inhibited by magnesium as well as by ketamine, which have a superadditive effect in combination. This may explain in part why analgesia is more effective for the combination than for either compound alone [12]. Morphologic studies in animals have identified NMDA receptors on both myelinated and unmyelinated axons in peripheral somatic tissues [13],[14]. The peripheral administration of MK-801, a noncompetitive NMDA receptor antagonist, produces local anesthetic-like effects plus the peripheral administration of ketamine enhanced the local anesthetic and analgesic actions of bupivacaine used for infiltration anesthesia [15]. The systemic magnesium sulfate was found to decrease postoperative opioid requirements, suggesting a potentiating effect of magnesium on opioid analgesia [16],[17], and also to decrease pain associated with propofol injection [18]. Weissberg et al. [19] found an inverse relationship between the severity of pain and serum magnesium concentration. The antinociceptive effect was not confirmed in the study, that is, whether it is locally mediated by the action of magnesium on the brachial plexus fibers or related to the absorption of magnesium systemically. Turan et al. [20] reported enhanced quality of anesthesia and analgesia in intravenous regional anesthesia when magnesium was used as an adjunct to lidocaine.

Also, intrathecal magnesium prolonged spinal opioid analgesic duration in humans and this suggests that the availability of an intrathecal NMDA antagonist could be of clinical importance for pain management [21].

Lee et al. [22] found that the addition of 2 ml of 10% magnesium sulfate to a 0.5% bupivacaine-epinephrine mixture for an interscalene nerve block prolonged the analgesic duration from 553 ± 155 to 664 ± 188 min and reduced postoperative pain in arthroscopic shoulder procedures, with no recorded adverse effects. In the current study, 5 ml of 10% MgSO 4 added to a lidocaine-adrenaline mixture resulted in an extended analgesic duration without prolonged motor block that was favorable for shoulder acromioplasty that requires early shoulder mobilization, and this was superior to the use of long-acting local anesthetics.

In contrast, a study by Hung et al. [23] unexpectedly found that three different concentrations of magnesium had shortened the duration of the block by lidocaine, bupivacaine, and ropivacaine, when added to rat sciatic nerve, and concluded that magnesium is not a useful adjuvant to amide local anesthetics in peripheral nerve blocks. This observed antagonism was unclear and appeared to be independent of the action of local anesthetic and MgSO 4 at the local anesthetic receptor within the Na(+) channel.

Goyal et al. [24] concluded that 200 mg magnesium sulfate was superior to 100 mg for postoperative analgesia when injected independently into the axillary sheath, with reduced morphine consumption and no recorded adverse neurological outcome. In a study carried out by Bilir et al. [25], magnesium sulfate, when infused epidurally, showed a significant reduction in postoperative pain scores and fentanyl intake, with no residual neurological adverse effects, which was consistent with the results of the current study.

Further studies using higher doses of magnesium sulfate need to be carried out with various local anesthetics in different peripheral nerves.


This randomized double-blind study showed that the addition of magnesium sulfate 500 mg to 20 ml of 2% lidocaine with epinephrine 1 : 200 000 in ultrasound nerve stimulation-guided ISPB for shoulder arthroscopic acromioplasty surgery significantly prolonged the block offset duration, reduced both NRS on the first analgesic request, and intravenous morphine intake postoperatively without recording sensory or motor neurological side effects.


The authors acknowledge the anesthesia residents, nursing staff, and both the Anesthesia and the Orthopedics Department at Ain-Shams University Hospital for all their effort and support for the fulfillment of this study.

Conflicts of interest

None declared.


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