Background and Goal of Study: Magnesium potentiates the effect of nondepolarizing muscle relaxants. However, studies which have been conducted by using magnesium chloride (MgCl2) are rare. Sugammadex reverses neuromuscular block by steroidal nondepolarizing muscle relaxants. The purpose of this study is to investigate effects of MgCl2 on rocuronium-induced neuromuscular blockade and its reversal by sugammadex.
Materials and methods: Left phrenic nerve-hemidiaphragms from 40 Sprague-Dawley rats were randomly allocated to four groups (1 mM, 2 mM, 3 mM, and 4 mM MgCl2 group, each n = 10). Rocuronium was administered cumulatively until the first twitch (T1) of train-of-four (TOF) completely disappeared for obtaining the effective concentration (EC) of rocuronium in each group. Thereafter, equimolar sugammadex was administered and recovery of T1 height and TOF ratio were measured for 30minutes.
Results and discussion: EC50, EC90, and EC95 are significantly decreased, as each concentration of MgCl2 is increased (all P ≤ 0.001), except the comparison of 3 mM, and 4 mM MgCl2 group. After administration of sugammadex, maximal TOF ratio (%) was decreased in 4 mM MgCl2 group compared to 1 mM MgCl2 group (89.6 ± 8.7 vs. 98.7 ± 8.9, P = 0.049). Time (sec) from sugammadex injection to achieving of maximal T1 was prolonged significantly in 4mM MgCl2 group compared to 1 and 2 mM MgCl2 group (1483.9 ± 237.0 vs. 1039.0 ± 351.8 and 926.0 ± 278.1, P = 0.022 and 0.002, respectively). 
Conclusion(s): The increase of MgCl2 concentration reduces the ECs of rocuronium. In addition, equimolar sugammadex for administered rocuronium shows the limited efficacy as the change of MgCl2 concentration. 
References: Czarnetzki C, Tassonyi E, Lysakowski C, Elia N, Tramer MR. Efficacy of sugammadex for the reversal of moderate and deep rocuronium-induced neuromuscular block in patients pretreated with intravenous magnesium: a randomized controlled trial. Anesthesiology 2014; 121: 59-67

Effects of magnesium chloride on rocuronium-induced neuromuscular blockade and sugammadex reversal in the isolated rat phrenic nerve-hemidiaphragm preparation