Sensitivities of rat primary sensory afferent nerves to magnesium: Implications for differential nerve blocks

摘要

Context: Contrasting findings have been published regarding the role of magnesium sulphate used as an additive to local anaesthetics in peripheral nerve blocks. Objective: To clarify the effect of magnesium sulphate on nerve excitability. Setting: C and Ab compound action potentials were recorded extracellularly in vitro in saphenous nerves from adult rats. Animals: Saphenous nerves (n=30) from male Wistar rats (n=19), 12 to 16 weeks old. Intervention: Primary sensory afferents were tested with a computerised threshold tracking program (QTRAC) with a supramaximal 1 ms current pulse either alone or after 300 ms of conditioning polarising ramp currents in the presence and absence of 10 mmol |-1 magnesium sulphate, 80μmol |-1 lidocaine and a combination of both. Main outcome measures: Changes in current thresholds to elicit compound action potential amplitudes of 40% of the maximal response. Results: Magnesium sulphate increased excitability thresholds to a greater extent in Ab fibres than in C fibres. It enhanced the effects of lidocaine in both Aβ fibres [mixture 0.470mA (SD 0.105) versus lidocaine 0.358mA (SD 0.080), P0.001] and C fibres [mixture 2.531mA (SD 0.752) versus lidocaine 2.385mA (SD 0.656), P=0.008]. Preconditioning experiments also showed that magnesium sulphate had an enhancing effect with lidocaine in Aβ fibres [mixture 0.620mA (SD 0.281) versus lidocaine 0.543mA (SD 0.315), P=0.005], but not in C fibres [mixture 2.412mA (SD 0.641), lidocaine 2.461mA (SD 0.693), P=0.17]. Conclusion: These results suggest that the binding of magnesium ions depends on both the type and conformational state of voltage-gated sodium channels. They also may help to explain the conflicting reports regarding the clinical effects of magnesium sulphate as an additive to lidocaine in peripheral nerve blocks.