An Evaluation of the Adding Magnesium Sulfate to Ropivacaine on Ultrasound-Guided Transverse Abdominis Plane Block After Abdominal Hysterectomy

After cesarean section, hysterectomy is considered the second most common gynecological surgery in females (1). Post-hysterectomy pain is one of the postoperative complications. Pain along with nausea, vomiting, and drowsiness, is one of the three most common causes of patients’ delayed discharge (2). A variety of techniques are used to provide better pain management outcome, including systemic analgesics, patient-controlled analgesia, and peripheral analgesic procedures (3, 4). Opioids administration is associated with several systemic complications. Thus, inclination to perform peripheral analgesic procedures such as transverse abdominis plane block is recently increased (5-7).

This type of block is studied for its effectiveness in pain management in various surgical procedures such as cesarean section, hysterectomy, prostatectomy, colonic surgery and laparoscopy, and even in surgical procedures in children (8-14). Transverse abdominis plane (TAP) block was even performed alone in a case of a high-risk elderly patient undergoing gastric operation (15). In addition to the proper technique, the duration of analgesia in this block is influenced by the pharmacokinetics of the administered drugs. Various measures, such as catheter placement in site and use of adjuvant drugs are raised to prolong the effect of the block (16-18). The impact of systemic magnesium sulfate on postoperative pain management is evaluated in various studies (19, 20).

The current study aimed at evaluating the effects of adding Magnesium sulfate to ropivacaine in the TAP block after hysterectomy and the possible ensuing complications and adverse effects.

In a randomized, double-blind, clinical trial, a total of 60 patients, 30 - 60 years old, ASA (American Society of Anesthesiologists) class I-II, candidates for hysterectomy under general anesthesia were enrolled in the study, following the verbal explanation of the procedure and details, and obtaining informed consent. Patients with coagulation disorders, infection, history of any addiction, sensitivity to the local anesthetics and magnesium sulfate were excluded. Patients were anesthetized with 2 mg midazolam and 2 µg/kg Fentanyl, 2 mg/kg propofol, and 0.5 mg/kg atracurium. Anesthesia was maintained with 100 µg/kg/minute propofol in addition to O₂ and N₂O. After the procedure, 2.5 mg neostigmine and 1.25 mg atropine were injected as reverse of neuromuscular relaxants. After recovering from anesthesia, the patients were randomly divided into two groups of study (RM) and control (R) and underwent bilateral ultrasound-guided TAP block. In the study group (RM), 19 mL ropivacaine 0.2% (Naropin 1%, AstraZeneca, France) in combination with 1 mL magnesium sulfate (500 mg) (Pasteur Institute, Iran), and in the control group (R) 19 mL ropivacaine 0.2% in combination with 1 mL normal saline 0.9% were injected. The block was performed in recovery room under cardiac, blood pressure, and heart rate monitoring with ultrasound guidance (SonoSite S-Nerve ultrasound machine) by a well-qualified physician, unaware of the type of the solution to be injected. To perform the block, the patients were placed in supine position. The needle (Pajunk, Germany, 22G, 80 mm, visible to ultrasound) insertion point was between the mid and anterior axillary line, below the rib line extending to the iliac prominence, at the same level of the abdominal wall incision. A linear probe of a frequency between 6 - 15 MHz and an in-plane needle insertion technique were used in this block.

Using the visual analog scale (VAS), the patients' pain both in rest and with movement was measured and recorded, from 0 to 10 (0 for no pain and 10 for severe unbearable pain), at 1, 2, 6, 12, and 24 hours after the surgery by another person, without knowledge of group assignment. If a patient reported a VAS pain score of greater than 3, rescue analgesic (diclofenac suppository 100 mg) was given. Rescue analgesic consumption, complications, patients’ characteristics, and results were recorded on a questionnaire. The data were analyzed with SPSS version 18. The quantitative and qualitative data were evaluated using independent t and chi-square tests, respectively, and the P value of less than 0.05 was considered statistically significant.

The patients’ characteristics in the two groups (age and weight) were not significantly different (Table 1). Based on the VAS scale, the mean pain score of the patients in both groups at the first hour after surgery (before the block) was above 5, which indicated no significant difference between the two groups, but decreased gradually thereafter and at the 24-hour it reached 3.1 and 2.8 in the control and study groups, respectively. Although the mean pain score was less in the study group than in the control group at all hours of the study, there was no significant difference regarding the rate of pain score between the two groups (Table 1). The amount of rescue analgesic consumed (diclofenac suppository) gradually increased during the study hours until it reached its highest level at 12 hours; no significant difference was observed between the two groups at any hours of the study, however (Table 1). None of the patients experienced any side effects, including nausea, vomiting, sedation, and burning sensation.

Results of the current study demonstrated that the addition of magnesium sulfate to ropivacaine, in TAP block after hysterectomy, had no effect on the pain score and on the amount of rescue analgesic consumption. By blocking the T6-L1 nerve roots, TAP block provides effective analgesia for the abdominal wall (21). The exact location of the needle for injection is the fascial layer between the internal oblique and the transverse abdominis muscles, which is the major pathway of the abdominal wall nerves (22, 23).

According to usefulness of the TAP block in postoperative pain management, efforts are made to improve its effect in recent years. One of the medicines to enhance the effect of local anesthetic is magnesium sulfate. Regulation of calcium influx into the cell and antagonism of N-methyl-D-aspartate (NMDA) receptors in the central nervous system are introduced as the two mechanisms, which underlie its analgesic properties (24).

In a systematic review performed on nine studies to assess the efficacy of TAP block in decreasing opioid consumption and postoperative pain scores in caesarean delivery, its effectiveness was observed in all the studies. The only group that had better analgesia than the TAP block groups was the one that received intrathecal morphine, at the expense of an increased incidence of side effects (21). However, considering the quality of pain in patients undergoing caesarean section, which is a mixed somatic and visceral pain, better pain management with intrathecal morphine is warranted. TAP analgesia blocks only somatic pain. However, the current study was strongly influenced by the heterogeneity of the studies with regards to the dosage of drug, the quality of block, and the way of performance.

In another systematic review conducted on 34 studies, the superiority of this type of block over other postoperative pain management techniques was demonstrated. However, further studies were strongly recommended by the researchers on how the block is performed, the dosage of medications administered, and the use of adjuvant drugs in various surgical procedures (25). The beneficial effects of the transverse abdominis plane block are confirmed in many studies so far (8-13, 15-18). However, its duration of effect is challenging.

In a study on pharmacokinetics of the local anesthetic drug, the prolonged duration of analgesic effect of the block (36 - 48 hours), due to slow drug clearance, and minimal adverse effects of systemic absorption of the drug, owing to the relatively poor vascularization of the block site, was noted (26). Nevertheless, the clinical effectiveness of medications is far less than this amount.

In some studies, a number of questions are raised as to the precise time of block administration and finding methods to prolong the effect of the block (27).

In a study, TAP catheters were placed for 36 hours after abdominal surgery. In addition to the technical issues of TAP catheter insertion using the posterior approach, the small number of patients was of the main constraints of the study (16).

In another study, in comparison with lateral approach in TAP block, benefits of posterior approach were demonstrated (28).

The effect of local anesthetic infusion on the abdominal wall is investigated in connection with analgesia induction in healthy people and with postoperative pain management in cesarean section and inguinal hernia repair, which produced different results (29-32). Thus, employing other techniques, rather than catheterization, seems necessary to prolong the duration of the effect of TAP block.

In a meta-analysis to evaluate the analgesic effect of systemic magnesium sulfate on postoperative pain, reduction of opioid consumption in all groups receiving magnesium sulfate and reduction of pain score on the first day after surgery were observed, without any side effects. There was no difference between the two modes of magnesium sulfate administration; bolus or continuous infusion (24).

The synergistic effect of magnesium sulfate with intrathecal bupivacaine and lidocaine is investigated for postoperative pain and shivering (20, 33).

Moreover, addition of magnesium sulfate to lidocaine as continuous infusion in the axillary block reduced pain and opioid consumption (34).

In a comprehensive study, the researchers studied the different techniques of TAP block administration in various surgery procedures. By examining several existing studies in gynecological surgeries, Chin et al. considered the effect of TAP block only limited to the early time after surgery, due to its major effect on the somatic pain than on the visceral pain. Regarding pharmacological considerations, they found the addition of mexamethasone and dexmedetomidine to the local anesthetic mixture somewhat promising, but the addition of clonidine to the local anesthetic was ineffective (35).

In another study conducted by Rana et al. a bilateral TAP block was performed on patients undergoing abdominal hysterectomy under intrathecal anesthesia, with 18 mL bupivacaine 0.25% alone or in combination with 150 mg magnesium sulfate. Reduced pain score and rescue analgesic consumption as well as increased duration of analgesia were the outcome (36).

According to the pain score and rescue analgesic consumption, the result of the current study was not in agreement with those of Rana et al. The difference, both in the applied anesthesia techniques (general vs. intrathecal) for the surgery and in the types of the administered analgesics in the TAP block (bupivacaine vs. ropivacaine) between the two studies might explain the incongruity. In the current study, the two types of pain were measured both at rest and during movement, while in theirs the type of pain was not mentioned. In terms of technique, the location of needle insertion in the current study was close to the mid axillary line while it was in the anterior axillary line in their study.

Although in the current study the addition of auxiliary magnesium sulfate to ropivacaine did not exhibit any increased analgesic effect, it is recommended that further studies be conducted with different doses of the drug and with other auxiliary drugs as well.