Four trials (29% of all) reported on a significant decrease in postoperative pain intensity in patients treated with magnesium compared with placebo (42,44,49,51); observation times were between 7 and 24 h. From only one of those trials, could actual pain intensity data be extracted; with placebo, average pain intensity at 12 h was 15 mm on a 100 mm visual analog scale, with magnesium, it was reported to be 10 mm (42). Seven trials (50%) were unable to find any difference in pain intensities between magnesium and placebo (10–12,43,45,47,52). In one trial (7%), patients treated with magnesium laevulinate had significantly increased pain scores at 3 h postoperatively; on a 5-point verbal rating scale, pain intensity increased from 1 with placebo to 2 with magnesium (50). Two trials did not report on postoperative pain intensity.
Eight trials (57% of all) reported a significant decrease in postoperative analgesic requirements in patients treated with magnesium compared with those who received placebo. In 4 of those, cumulative morphine consumption at 24 or 48 h was decreased on average by 12% to 47% (median, 28%) (10,42,44,48); in 2, fentanyl consumption at 2 and 4 h postoperatively was decreased by 53% and 80% (46,47); and in 1 each, remifentanil consumption at 12 h and piritramide consumption at 24 h was decreased by 67% and 36%, respectively (49,51). Five trials (36%) were unable to find any difference in the postoperative consumption of epidural bupivacaine-fentanyl (45), or morphine (11,43,50), or different opioids (morphine, fentanyl, meperidine) and nonopioid analgesics (nonsteroidal antiinflammatory drugs, acetaminophen) (12). In one trial (7%), children treated with magnesium consumed significantly more codeine postoperatively, but not fentanyl or acetaminophen (52).
Four trials reported on both a significant decrease in pain intensity and analgesic requirements with magnesium (42,44,49,51). Four trials reported on absence of both a significant decrease in pain intensity and analgesic requirements (11,12,43,45). There was no obvious relationship between the tested magnesium regimens and pain outcomes.
Data on further, potentially magnesium-related, beneficial or harmful outcomes came from eight studies (Table 3). Shivering, vomiting, and dizziness occurred less often with magnesium. For shivering, the difference was statistically significant; the average incidence with placebo was 11.8%, with magnesium was 4.5% (NNT 14). Nausea, arterial hypotension, and bradycardia occurred more often with magnesium. None of the differences, however, was statistically significant. Some trials reported on decreased scores of discomfort (43), or insomnia during the first postoperative night (10,43,51), or increased scores of satisfaction (51) in patients who received magnesium. None of those results was statistically significant.
Seven trials reported on magnesium serum concentrations before and after the end of study drug administration (Table 4). Six tested an IV bolus and a subsequent continuous infusion of magnesium sulfate (10,41,43–45,47), and one tested an IV bolus and continuous infusion of magnesium gluconate (49). In five of the seven trials, average magnesium serum concentrations in control patients decreased significantly by −9% to −27%; the median of all average changes was −11%. In 6 of the 7 trials, average magnesium serum concentrations in patients receiving magnesium increased significantly by 31%–84%; the median of all average changes was +79%. No correlation between the administered cumulative doses of magnesium and the increase in the magnesium serum concentrations was apparent. The trials were too heterogeneous and data reporting was too inconsistent to allow for a relationship between the degree of hypomagnesemia and pain intensity in controls.
Three main results emerged from this systematic review. First, in some trials, magnesium had a beneficial effect on postoperative pain intensity and analgesic requirements. Second, magnesium treatment decreased the incidence of postoperative shivering. Third, in most trials, magnesium serum concentrations in control patients decreased.
The beneficial effects of magnesium were not unequivocal. In some trials, the benefit seemed to be obvious; others were unable to show any improvement or even showed some deterioration in patients treated with magnesium. Perhaps the clearest beneficial result was the decrease in postoperative analgesic consumption in patients who received magnesium. For instance, in four trials, the reported degree of morphine sparing (median, 28%) was similar to what has been reported with intraoperative ketamine (15) or postoperative nonsteroidal antiinflammatory drugs (53). However, morphine-sparing per se is of questionable clinical relevance. The main issue is whether or not the decrease in morphine consumption is followed by a decrease in the incidence of morphine-related adverse effects. This was not the case; the incidence of nausea or vomiting, for instance, was not reduced with magnesium treatment. Further potentially morphine-related adverse effects, such as sedation, pruritus, or respiratory depression, were not reported in these trials. Yet another reason that would justify the concomitant use of a nonopioid adjuvant for postoperative opioid-based analgesia, would be a decrease in postoperative pain intensity compared with the opioid alone. Four trials reported on a significant decrease in pain intensity in magnesium-treated patients. However, one of those trials only provided actual visual analog scale data, and the reported average decrease in pain intensity with magnesium (from 15 to 10 mm) could not necessarily be regarded as clinically relevant. Against these trials that provided some evidence that magnesium supplementation may have a beneficial effect on analgesic requirements or pain intensity, all the trials need to be weighted that were unable to find any benefit for both outcomes.
The quantity and quality of the trials did not allow for the necessary subgroup analyses to evaluate the degree of efficacy with various regimens of magnesium. There was a large variability in doses; some trials tested a single bolus, and in the majority, a subsequent infusion was added. The largest trial tested a single bolus dose of magnesium sulfate 4 g and was unable to find any benefit in favor of magnesium (12). Zarauza et al. (11) infused more than 16 g of magnesium sulfate in their patients and were unable to find a beneficial effect. On the other hand, Levaux et al. (51) injected a bolus dose of magnesium sulfate 3.6 g and reported a significant decrease in opioid consumption and pain intensity. As a consequence, we still do not know whether there is dose-responsiveness for the analgesic efficacy with magnesium. Similarly, it remains unknown whether there is a difference between magnesium sulfate, laevulinate, and gluconate. One trial tested a cumulative dose of magnesium laevulinate 1.2 g for 5 h (50). Cumulative morphine consumption up to 4 h after surgery, and the number of episodes without any pain, did not differ between patients treated with magnesium laevulinate and those receiving placebo. However, those who had received magnesium laevulinate reported significantly more episodes with severe or unbearable pain and they had higher pain scores. In another trial, a cumulative dose of magnesium gluconate that exceeded 25 g for 12 h was given (49). The number of patients with inadequate pain control before tracheal extubation, average pain intensity at 7 and 8 h after surgery, and remifentanil consumption during the first 12 h after surgery, were all significantly decreased in patients who had received magnesium gluconate. Thus, it remains unclear whether these differences in efficacy were due to differences in doses and regimens (1.2 g during 5 h vs 25 g during 12 h), or due to differences in the chemical form of magnesium (gluconate versus laevulinate), or due to other factors.
Not unexpectedly, magnesium treatment was shown to decrease the risk of postoperative shivering. This was perhaps the clearest beneficial effect of magnesium supplementation. In healthy volunteers, magnesium sulfate (80 mg kg−1 bolus followed by an infusion at 2 g h−1) was shown to reduce the shivering threshold (54). In clinical practice, magnesium’s beneficial effect on shivering is of minor importance (NNT about 14) compared with, for instance, the efficacy that was reported with meperidine or clonidine (NNT 3–4) (19).
In trials including patients undergoing abdominal hysterectomies and hernioplasties, average magnesium serum concentrations in control patients who did not receive magnesium supplementation decreased by 11%–27%. Serum levels, which are those generally measured, reflect only a small part of the total content of magnesium; the intracellular magnesium content can be low, despite normal serum levels (55). However, we may infer from the data reported in these trials and from observations from others (9) that longer and larger surgical interventions are more likely to lead to hypomagnesemia. It remains unclear what the clinical relevance of low magnesium serum levels in surgical patients is. There is evidence that the response of the NMDA receptor is greatly enhanced by reducing the extracellular magnesium concentration below the physiological level (56). It may be hypothesized that the reported outcomes in these trials, for instance, decrease in pain intensity or in morphine consumption, were not due to a direct analgesic effect of magnesium but, rather, to the prevention of hypomagnesemia, and thus prevention of subsequent NMDA activation.
Our analysis has two major limitations. First, most trials were small. Small trials may detect a beneficial treatment effect by random chance (57). The largest trial included 200 patients (12); that trial was negative but it tested only a bolus dose of magnesium sulfate. Thus, we do not know whether that trial was negative since it was large, and therefore the risk of random variation was minimal, or because the magnesium regimen in that trial was inadequate. Second, some trials reported on low pain scores in controls. For instance, in the trial by Ko et al. (45), patients postoperatively received epidural fentanyl and bupivacaine with or without systemic magnesium and pain scores were below 40 mm at all time points in both groups. That trial was part of those that were unable to show any benefit with magnesium. However, if there is not enough baseline pain, an experimental intervention has no scope to show analgesic efficacy (58).
In conclusion, these randomized trials do not provide convincing evidence that perioperative magnesium supplementation has favorable effects on postoperative pain intensity and analgesic requirements. Nevertheless, it may be worthwhile to further study the role of magnesium as a supplement to postoperative analgesia, since this molecule is inexpensive, relatively harmless, and the biological basis for its potential antinociceptive effect is promising. There is the possibility that magnesium might have an additive or even synergistic effect with other NMDA antagonists, specifically ketamine or nitrous oxide. However, pharmacological interactions need to be formally tested. Future trials should be large enough to avoid random variations in outcomes, and they should report on clinically relevant end-points, such as pain intensity at short- and at long-term, and opioid-related adverse effects.
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