Local anaesthesia consists of reducing or eliminating the perception of pain by using drugs which interfere with nerve conduction.
The mechanism of action of local anaesthetics involves blocking nerve conduction by reducing the flow of sodium ions.
An electrical potential is generated across the cell membrane by the ion concentrations inside and outside the cell. There is alternating depolarisation and hyperpolarisation according to these concentrations. When the potential is –70 mV the sodium channel is closed; it opens for a short time, reacting to a change in potential, when its structure is modified. When the threshold value of –40 mV is reached, the channel opens and sodium ions can pass into the cell. At the same time the channel becomes inactive and it remains in this form until the membrane is completely repolarised. In this state the channel is completely refractory to changes in potential. The drugs in question mainly act on the open and inactive sodium channels by binding to specific sites within them.
Local anaesthetics: factors which affect their efficacy and absorption
The factors which influence the degree of penetration and the duration of local anaesthetics are therefore: tissue pH, the pKa of the drug, the diffusion time, the morphology of the nerve, the concentration of the drug and its liposolubility. Local anaesthetics are often given together with a vasoconstrictor in order to prolong their effects. Its use also reduces the possibility of systemic reactions, avoids absorption and reduces bleeding. However, it is important to remember that its use should be adjusted, if not avoided, in at-risk patients, with particular reference to patients who are diabetic, hypertensive, have heart disease or are pregnant.
The distribution of the drug depends on its solubility coefficient and the degree to which the anaesthetic is plasma-protein bound. Its metabolism is completely dependent on which type of anaesthetic it is: esters are metabolised in the plasma by plasma pseudocholinesterase while amines are broken down in the liver. Nerve fibres are not all sensitive to local anaesthetics to the same degree; this depends on the diameter of the fibre and its degree of myelination.
The fibres which carry pain stimuli are among the most sensitive to local anaesthetics which act by inhibiting the transmission of pain sensations.