The use of local and ‘regional’ anaesthesia, where local anaesthetic (LA) is infiltrated around a specific nerve to produce numbness in the region it supplies, is gaining popularity in anaesthesia because patients can enjoy the benefits of pain-free surgery without being exposed to the risks of general anaesthesia. While the scope for regional anaesthesia is fairly limited in the field of ENT, soaking of the nasal mucosa in Moffat’s solution (which contains the local anaesthetic agent, cocaine), infiltration of skin with local anaesthetic agents to facilitate the removal of ‘lumps and bumps’ and infiltrating the surgical site with local anaesthetic to provide post-operative analgesia are all commonplace. For this reason, a working knowledge of commonly used local anaesthetic agents is useful to the ENT surgeon. Despite their relatively good safety profile, fatalities have resulted from LA overdose, so all doctors administering these drugs must be aware of the signs and symptoms of toxicity and how to manage it.
The Association of Anaesthetists of Great Britain and Ireland (AAGBI) advises that standard monitoring (blood pressure, electrocardiography [ECG] and pulse oximetry) should be employed when administering LA. Consideration should be given, when appropriate, to inserting an intravenous cannula, to allow for treatment of any LA toxicity that might occur.
4.1 Local Anaesthetic Agents
Pharmacologically, local anaesthetics can be classified into ester and amide types. Esters have an increased likelihood of precipitating hypersensitivity reactions when compared with amides (Table 4.1).
Maximum dosages are adjusted for lean body weight as described by Specialists in Obesity and Bariatric Anaesthesia (SOBAUK) with up to a maxi mum of 100 kg for males and 70 kg for females.
4.2 Mechanism of Action
Local anaesthetic drugs enter the nerve fibre and bind to sodium channels located on the internal surface of the membrane. Once bound, they inhibit the movement of sodium across the membrane and so prevent the propagation of the nerve impulse. LAs bind more avidly to sodium channels that are open or inactivated and so preferentially affect nerves that have a rapid discharge rate. This means sensory nerve fibres are more susceptible than motor nerves, because they fire at a higher frequency.
In order to pass through cell membranes, any drug must be in its unionised form. The degree of ionisation of any drug depends on its pKa (drugs with a lower pKa—close to pH 7.4—will have a higher unionised fraction than those with a higher pKa) and the pH of the local environment. For this reason, LAs are less effective at producing satisfactory operating conditions in infected areas, such as abscesses, because the reduction in pH in these areas results in a higher proportion of ionised drug, making it unable to cross into the nerve cells to exert its effect.