Effect of clonidine and magnesium sulphate on anaesthetic consumption, haemodynamics and postoperative recovery: A comparative study

Discussion                           

In our study, we observed the effects of clonidine and MgSO 4 as adjuvants of general anaesthesia.

 Our results demonstrate a significant reduction in consumption of propofol and fentanyl used for balanced anaesthesia with both clonidine and MgSO 4 .

Importantly, we used an objective, qualitative measure of anaesthetic state (BIS) to guide anaesthetic requirements and to determine endpoints.

Altan and Turgut [12] used clonidine 3 μg/kg intravenously over a period of 15 minutes before induction and 2 μg/kg/hour by continuous infusion intraoperatively. They observed significant incidences of bradycardia and hypotension in their study.

Reduced the infusion to 1 μg/kg/hour intraoperatively in our study. In spite of this reduced infusion rate of clonidine, we observed significant incidences of bradycardia and hypotension in our study. Further studies using lesser dose of clonidine may be necessary.

Elsharnouby and Elsharnouby [14] used MgSO 4 40 mg/kg intravenously over a period of 15 minutes before induction and 15 mg/kg/hour by continuous infusion intraoperatively.

They noticed more episodes of severe hypotension using this dose of MgSO4 . In our study, we reduced the dose of MgSO 4 to30 mg/kg before induction and 10 mg/kg/hour by continuous infusion intraoperatively.

The dose selected by us resulted in a steady and smooth reduction of MAP and heart rate, with no episodes of severe hypotension and bradycardia. Our finding was supported by a study conducted by Telci and Esen, [15] who used similar dose of MgSO 4 as ours.

In our study, propofol and fentanyl requirements were significantly lower in patients of both Group C and Group M in comparison to Group P. Studies with rat model showed that at clinical concentration, clonidine partially inhibits voltage-gated Na and K channels and suppresses the generation of action potentials in tonic firing spinal dorsal horn neurons. [13]

This may contribute to the reduction of propofol and fentanyl requirements. Fehr and Zalunardo [16] observed similar findings in their study.

MgSO 4 has been reported to produce general anaesthesia and enhance the activity of local anaesthetic agents. [2]

Depressant effects of MgSO 4 on the central nervous system (CNS) of animals has been reported too. [3] Magnesium antagonised NMDA receptors in the CNS. [4] Another mechanism could involve the reduction of catecholamine release through sympathetic stimulation by which magnesium might decrease peripheral nociceptor sensitisation or stress response to surgery.

However, these mechanisms do not explain the reduction in propofol requirements, independent of the reduction of the requirement of fentanyl. Clearly, further studies on the interaction between magnesium and propofol as sole agents need to be done. By acting as an antagonist of NMDA receptors, magnesium has the potential to prevent pain.

The effect of magnesium on perioperative analgesic requirement was first evaluated by Koinig and colleagues [6] in patients with identical level of surgical stimulation. This is also confirmed in a study done by Shulz-Stubher et al. [17]

Taittoven and colleagues [18] compared clonidine and midazolam as premedication agents and observed no differences in oxygen consumption, anxiolysis, energy expenditures and CO 2 production.

Administration of clonidine before induction and intraoperatively results in improved perioperative haemodynamic stability. Preoperative oral clonidine protects against the pressure response to intubation. [19]

Hypotension and bradycardia have been encountered with clonidine. [13] Clonidine can provide better perioperative haemodynamic stability in patients with mild to moderate hypertension. In laparoscopic surgical procedures where adverse cardiovascular change like increased arterial pressure is common, haemodynamic effect like hypotension may actually be beneficial.

Van Den Berg and colleagues [20] found that MgSO 4 attenuated the haemodynamic response to endotracheal intubation .

In our study, both clonidine and MgSO 4 lowered the haemodynamic response to intubation but clonidine was more effective in attenuating the sympathetic response.

In our study, recovery time was significantly prolonged in patients receiving MgSO 4 in comparison to other two groups. The delay in recovery may be due to CNS depressant effect of MgSO 4.

A narcotic state in human beings undergoing surgical operations was achieved in a study by Peck and Meltzer, [21] who attempted anaesthesia by MgSo 4 infusion in three patients of herniorhaphy. However, Aldrete and Vazeery [22] suggested this was actually a sleep-like state caused by cerebral hypoxia from progressive respiratory and cardiac depression.

When ventilation was maintained, even very high level of serum Mg produced no CNS depression.

To conclude, perioperative use of both clonidine and magnesium sulphate significantly reduced the requirement of propofol and fentanyl citrate. They were able to attenuate the haemodynamic response to tracheal intubation.

Both clonidine and magnesium sulphate caused bradycardia and hypotension. Besides, magnesium sulphate caused a delay in recovery. Therefore, both clonidine and magnesium sulphate need careful management, to be used as adjuvant agents to general anaesthetics.