4. Pharmacology

Right and left hands are mirror images of each other but cannot be superimposed when the palms are facing in the same direction These enantiomers have the capacity to rotate polarized light and so are also known as optical isomers. Drugs such as bupivacaine and prilocaine are racemic mixtures. The more favourable safety profile of single enantiomer preparations has given chirality more immediate anaesthetic relevance. Chiral drugs that are found in nature are usually single enantiomers because they are synthesized enzymatically in reactions that are stereospecific. Such drugs include adrenaline (epinephrine), atropine, cocaine, ephedrine, hyoscine, morphine and noradrenaline (norepinephrine). All are levorotatory and still have the designation (l).

Bupivacaine: the S (−) enantiomer has less affinity for, and dissociates quicker from, myocardial sodium channels. The risk of cardiovascular and CNS toxicity is reduced. The S (−) enantiomer also exerts some vasoconstrictor activity. • Ropivacaine: this is the pure S (−) enantiomer of propivacaine. It also has a safer cardiovascular profile in overdose • Lidocaine: this is achiral. • Ketamine: the S (+) enantiomer has a greater affinity for its main binding site (the NMDA receptor) and is up to four times as potent as the R (−) form. Its administration is also associated with fewer emergence and psychotomimetic phenomena.