Asik and colleagues recently reported on the efficacy of metoprolol in relieving the pain on injection of propofol  besides others who have proposed mechanisms for this effect. Although the mechanism for this action of metoprolol is still open to conjecture, that proposed by the authors, i.e vasodilatation, in view of its concomitant β2-antagonist action, seems inappropriate. Metoprolol is a β1-selective (cardioselective) adrenergic antagonist. However, this selectively is not absolute and β2-antagonism is seen at higher doses [2,3]. In neurologically intact limbs in animal models, the intravenous infusion of metoprolol in larger doses has rather been found to increase peripheral vascular resistance and produce vasoconstriction [4,5]. Cardioselective β1-antagonists have minimal effect on vascular tone and they do not possess vasodilator action in their own mode. Furthermore, metoprolol does not possess intrinsic sympathomimetic action which could explain the vasodilating effect as proposed by the authors [1-3].
Albeit weak, metoprolol is known to possess a membrane stabilization action . This action may have assumed significant proportions in view of the high concentration of the drug achieved locally - distal to the site of venous occlusion - and may in fact explain the beneficial effect of metoprolol in reducing propofol-induced pain rather than its direct effect on vascular tone.
1ASCOMS, Jammu, Jammu and Kashmir, India
1. Asik I, Yorukoglu D, Gulay I, Tulunay M. Pain on injection of propofol: comparison of metoprolol with lidocaine. Eur J Anaesthesiol
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, 5th edn. Philadelphia, USA: Churchill Livingstone, 2000: 523-577.
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