Interaction of Volatile Anesthetics With Magnesium

Magnesium and volatiles anesthetics both have an effect on the neuromuscular transmission. The primary objective of the study is to quantify the effect of a perfusion of intravenous magnesium on neuromuscular transmission measured by electromyography device TetraGraph device in patients undergoing general anesthesia with volatile anesthetics (desflurane, sevoflurane and isoflurane) as compared to intravenous anesthesia with propofol.

Magnesium sulfate is regularly used during anesthesia, for instance for the reduction of postoperative pain. It reduces the liberation of acetylcholine at the neuromuscular junction. At high plasma concentrations it can induce muscle weakness, flaccid paralysis and in cases of intoxication lead to respiratory arrest. It enhances the effect of muscle relaxants.

Volatiles anesthetics influence neuromuscular transmission. They inhibit postsynaptic nicotinic acetylcholine receptors by causing open channel block, receptor desensitization and reducing exocytosis from pre-synaptic vesicles at the neuromuscular junction. The ranking order of these effects of volatile anesthetics on neuromuscular transmission is: desflurane \> sevoflurane \> isoflurane, depending on their blood-gas and tissue-gas solubility index.

Magnesium given intravenously during volatile anesthesia induces effects on neuromuscular transmission similar to that of neuromuscular blocking agents. This effect has never been investigated and quantified systematically and prospectively.

Propofol, an intravenous anesthetic, has very little effects on neuromuscular transmission. Therefore magnesium given intravenously during total intravenous anesthesia with propofol has no or only very little effect on neuromuscular transmission.

The primary objective of the study is to quantify the effect of a perfusion of intravenous magnesium on neuromuscular transmission measured by accelerometry with theTetraGraph device in patients undergoing general anesthesia with volatile anesthetics (desflurane, sevoflurane and isoflurane) as compared to intravenous anesthesia with propofol. The investigators expect a following rank order of the effect: desflurane \> sevoflurane \> isoflurane \> propofol.

Eligibility Criteria:
Inclusion Criteria:

* Patients, age 18 to 65 years inclusive
* American Society of Anesthesiology \[ASA\] status I or II
* Body mass index 19 - 30 kg/m2
* Patient scheduled for elective surgery lasting ≥ 60 minutes
* Patient is able to read and understand the information sheet and to sign and date the consent form.
* Negative urinary or serum pregnancy test (not applicable if status post hysterectomy or tubal ligation or menopausal woman)

Exclusion Criteria:

* Surgery with need for neuromuscular block
* Contraindication for general anesthesia with laryngeal mask airway, such as gastro-oesophageal reflux.
* Hypersensitivity or allergy to magnesium sulfate or propofol
* Contraindication to volatile anesthetics such as malignant hyperthermia
* Patients with neuromuscular disease
* Patients receiving medications known to influence neuromuscular function (for instance, aminoglycosides or phenytoine)
* Known electrolyte abnormalities (for instance, hypermagnesemia)
* Atrioventricular heart block
* Patients with magnesium treatment within 48 hours before start of study
* Liver insufficiency (bilirubine \<1.5x, ALAT/ASAT\<2.5x the upper limit of normal value)
* Renal insuffisancy (créatinine \<1.5x upper limit of normal value, clearance\<30ml/minute)
* Patient having participated in any clinical trial within 30 days, inclusive, of signing the informed consent form of the current trial.
* Pregnant or breast-feeding women.