Pharmacodynamics of local anesthetics

The present review points out the present state of pharmacodynamics of local anesthetic agents, After briefly repeating the basic elements of the anatomy and physiology of neurons, the mode of action of local anesthetics is shown, The two possibilities for their action, the ionic form from the cells' interior side after passing the membrane and the uncharged form working by a swelling effect in the cells' membrane are discussed, The concept of the minimal blocking concentration Cm, which was found to be necessary, in vitro to block impulse conduction is explained, Thereafter some reasons are mentioned as to why it may be difficult in some circumstances to reach the Cm in a nerve in practice. The different substances in clinical use are listed and their onset and duration of action is mentioned, The dynamics of a nerve blockade is shown discussing the possibilities of altering them by adding a vasoconstrictor or carbon dioxide. The time passing from the first event of pain until the re-supply with local anesthetic seems to be the main determinant for development of tachyphylaxia. The main actions of the local anesthetics, their side-effects and their interaction with other xenobiotics are discussed in the next chapter. The role of bupivacaine- and etidocaine- related cardiotoxicity is emphasized. The reason for this phenomenon should be the long dissociation time constant from the heart's natrium channel. Finally a short outlook with respect to ropivacaine is made, also mentioning EMLA- cream and miscellanous application forms of local anaesthetics.