The Effect of the Magnesium Sulfate in Ultrasound-Guided Quadratus Lumborum Block on Postoperative Analgesia: A Randomized Controlled Trial

The results of the trial showed that local use of magnesium as an adjuvant for QLB resulted in lower VAS scores at both 12 h and 24 h postoperatively, longer analgesia duration, and lesser morphine requirement and rescue analgesia.

The use of QLB as a part of a multimodal regimen for postoperative analgesia by virtue of simplicity and effectiveness has succeeded as an analgesic technique in various abdominal surgeries. The quadratus lumborum muscle is a posterior abdominal wall muscle originating from the posterior part of the iliac crest and the lower part of the 12th rib and ending at the upper edge of the iliac crest. The middle layer of the thoracolumbar fascia (TLF) lies between the erector spinae and quadratus lumborum muscles. The sympathetic nerve fibers and mechanoreceptors pass through the TLF, which plays an important role in the mechanism of action of the QLB. Moreover, several investigations have shown that local anesthetics can spread through the TLF to the paravertebral space; hence, they can relieve visceral pain.

According to the anatomic location of needle tip placement in relation to the quadratus lumborum muscle, there are three injection pathways for QLB, namely lateral QLB (QL1), posterior QLB (QL2), and transmuscular QLB, also called anterior QLB, (QL3). We chose QL2 for the study because this approach is in close proximity to the surface; therefore, it is safer and quicker to perform. With a more superficial approach, the needle tip is separated from the peritoneum, reducing the risk of intraperitoneal injection and visceral injury.

Several studies have shown that QLB plays an important role in the treatment of postoperative pain after lower abdominal surgery, demonstrating that QLB reduces postoperative pain and provides a significant reduction in postoperative opioid consumption and side effects [10,11,12]. However, in a recent trial, the addition of QLB to intrathecal morphine after cesarean section has not reduced postoperative opioid consumption or provided additional analgesic benefit beyond 6 h [13]. Our results showed that VAS scores at rest and during activity were reduced 6 h after surgery but not at any other time point up to 48 h. Additionally, we did not find that the addition of QLB could reduce morphine consumption in 48 h or reduce the frequency of nausea and vomiting.

We found that magnesium sulfate augmented the postoperative analgesic effect of QLB. This result coincides with multiple previous studies that have investigated magnesium sulfate as an adjuvant to local anesthetics in various regional techniques. The mechanism of action by which magnesium sulfate potentiates the analgesic effect of local anesthetics is still unclear. Magnesium is an N-methyl-d-aspartate (NMDA) receptor and calcium antagonist. Some studies have shown that the analgesic effects of magnesium are primarily based on the inhibition of calcium influx and the excitability of NMDA receptors, thus reducing the sensitivity to central or peripheral pain stimulation [14, 15]. A meta-analysis has proven that the combination of magnesium sulfate and local anesthetics in nerve blocks could result in longer postoperative analgesia [6]. Ammar et al. [7] reported that the addition of magnesium sulfate to bupivacaine during transversus abdominis plane (TAP) block significantly prolonged the duration of analgesia and reduced postoperative morphine requirements and frequency of nausea and vomiting. In another trial, adding 150 mg magnesium sulfate as an adjuvant to 0.5% ropivacaine during subclavian brachial plexus nerve block resulted in a longer duration of sensory and motor block and lesser demands for rescue analgesics without significant side effects [8].

In our study, we added 400 mg of magnesium sulfate to 0.375% ropivacaine during QLB, and there were significant differences in postoperative pain scores among the three groups. The duration of the QLB was unknown. We found that pain scores at rest and during activity were reduced at 4 h and 6 h postoperatively in groups M and N. Lu et al. [16] studied QLB on volunteers, and they have found that the duration of analgesia with ropivacaine for QL2 was about 18.5 h. We observed that VAS scores at rest and during activity were significantly lower in group M compared to group N at 12 and 24 h postoperatively, which was consistent with previous results reported by Ammar et al. [7]. We believe that patients in group M had a low VAS score because in 6 h postoperatively, the effect of ropivacaine gradually disappeared, while the analgesic action remained after the combination with magnesium sulfate.

The optimal dose of magnesium sulfate for a peripheral nerve block is also unclear. From the current research, the minimum dose is 100 mg [17], and the maximum dose is 1000 mg [18]. Intrathecal magnesium administered at a dose of 200 mg has increased the incidence of nausea by two to three times within 12 h after surgery [19]. However, this side effect has not been reported with magnesium at an equal or even higher dose in several studies. Animal studies have shown that intrathecal administration of magnesium can cause nerve damage [20, 21]. However, no related side effects have been observed clinically. In our study, 400 mg magnesium sulfate was used, and there were no recorded side effects. The potential neurotoxicity and side effects of intrathecal magnesium have not been adequately studied. Therefore, further high-quality clinical studies are needed.

There are some limitations to our study. We lacked the measurement of serum magnesium level or adding intravenous administration of magnesium sulfate to detect if the enhancement effect is related to the systemic action of magnesium. Like transversus abdominis plane block, QLB is a fascial plane block. Local anesthetics are not directly injected near large nerves, but, rather, into the surrounding areas that have a high density of small nerve endings. There have been no reports of nerve damage [22]. All the patients in the study were women, who are sensitive to pain [23]. QLB under sedation may create additional pain and diminish patient satisfaction. We performed QLB under general anesthesia. It is easier to evaluate the level of analgesia in an awake state.

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