Adrenaline acts on two classes of receptors, beta and alpha: it is the action on the latter (to be precise on the α1, postsynaptic excitatory receptors) which induces contraction of vascular smooth muscle, i.e., vasoconstriction.
In the past, the use of a non-catecholamine compound, vasopressin was widespread. Chemically similar to the hormone vasopressin, it was administered in combination with the amide prilocaine.
The vasoconstrictor’s action it is not limited to counteracting the opposing effect of the local anaesthetic. By decreasing the blood supply to the affected site, the local anaesthetic’s passage into the vascular system is slowed down, so prolonging the drug’s duration of action and reducing the risk of its toxicity.
As regards the dosage, each cartridge containing local anaesthetic combined with a vasoconstrictor clearly shows the degree of dilution of the latter, expressed as a proportion. For example, a concentration of 1:1,000 indicates the presence of 1.0 mg of solute per 1 ml of solution. The concentrations of vasoconstrictors for dental use are normally of much smaller orders of magnitude (between 1:200,000 and 1:50,000).
Once its pharmacological action is over, adrenaline is rapidly metabolised by two different enzymatic systems, both of which are hepatic: monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).
The effects of overdose mainly include neurological symptoms (anxiety, tremor, weakness, headache etc.) and cardiovascular symptoms (increased blood pressure, and arrhythmias, mainly ventricular, rarely fibrillation). According to the protocols, the use of vasoconstrictors is actually contraindicated mainly — in either a relative or an absolute fashion —in patients with cardiological problems.
In terms of the general clinical recommendations, the maximum recommended dosage for a single session in healthy patients (ASA I) is 0.2 mg (200 μg) of adrenaline, corresponding to 11 cartridges at a concentration of 1:100,000 (or 22 of 1:200,000; the maximum quantity actually refers to the dosage of the local anaesthetic). In patients with significant cardiovascular disease (ASA III or IV), the dose is 0.04 mg (40 μg), corresponding to 2 (or 4) cartridges.
Basic biological and clinical principles for using single tooth anaesthesia
The local anaesthesia method most used today in dentistry is the nerve block, that is, the administration by injection of a compound which that can reach the nerve fibres carrying the sensitive afferents. Put simply, the main distinction, that between anaesthesia of the nerve plexus and the nerve trunk is based on the dimensions of the fibres themselves. However, it would appear to be useful to stress that there are some alternative techniques available to the dentist. These can be used alongside the classical method or replace it completely. Epiperiosteal (or subperiosteal) administration may have disadvantages when delivering the compound to intramedullary fibres, especially if there is thick cortical bone: this may lead to ineffectiveness or, at least, to a slowing of the anaesthetic action.
Single tooth anaesthesia (STA), a procedure introduced relatively recently, is of benefit in this regard. As a concept, it provides for a concentrated action at the level of a single dental element and requires the use of a computerised control system (also known as CCLAD).