Local anaesthetics create a chemical roadblock between the source of pain and the brain by interfering with the ability of a nerve to transmit electrical signals or action potentials. The local anaesthetic blocks the operation of a specialized gate called the sodium potential. When the sodium channel of a nerve is blocked, the nerve signals cannot be transmitted. The only location at which the local anaesthetic molecules have access to the nerve membrane is at the nodes of Ranvier, where there is an abundance of sodium channels. The interruption of a nerve signal in a myelinated nerve (such as a dental nerve) occurs when nerve depolarization (the nerve signal) is blocked at three consecutive nodes of Ranvier.
Local anaesthetics are vasodilators and are eventually absorbed into the circulation. They have systemic effects that are directly related to their blood plasma level. Overdose with local anaesthetics can result in CNS depression, convulsions, elevated heart rate, and blood pressure.
Vasoconstrictors (epinephrine and levonordefrin) are added to local anaesthetics to counteract the vasodilatory action, slowing the removal of the anaesthetic from the area of the nerve and thus prolonging its action. Different anaesthetics have different rates of onset of symptoms and duration of action. The more acidic a local anaesthetic solution is the slower the onset of action. The more closely the equilibrium pH for a given anaesthetic approximates physiologic pH, the more rapid the onset of anaesthetic action. The better the local anaesthetic molecule binds to the protein in the nerve’s sodium channel, the longer the duration of anaesthesia. It is important to note if a local anaesthetic is injected into an area of infection, its onset will be delayed or even prevented. The inflammatory process in an area of infection lowers the pH of the extracellular tissue from its normal value (7.4) to six or lower. The low pH inhibits anaesthetic action because little of the free base form of the anaesthetic is allowed to cross into the nerve sheath to prevent conduction of nerve impulses.3