Practical Points in using local anaesthetics:
1. Adding epinephrine (e.g, 1:200,000 to 1:100,000) can be particularly helpful in increasing the duration of local anaesthetic effect and in reducing bleding. There is, however, a downside to using epinephrine. First, when absorbed systematically it can cause hypertension, tachycardia and even arrhythmias, and this is obviously a potential concern in hypertensive patients or patients with coronary artery disease. Secondly, using epinephrine may disguise bleding sites that later become apparent once the epinephrine wears off. Thirdly, epinephrine can reduce uteroplacental blood flow.2. When using local anaesthesia in large amounts, always ensure that the patient has an intravenous line in place in case of local anaesthetic toxicity. Drugs to treat convulsions should be readily available(e.g, injectablediazepam) as well as equipment and drugs for airway management.
3. Know the relationship between drug concentration in percent and concentration in mg/ml. Remember that a 1% solution (1gm of solute in 100mls of solution) has a concentration of 10mg/ml. In a 60 kg woman, for example the maximum dose of lidocaine without epinephrine is 5mg/kg = 300mg = 30 ml of 1% lidocaine **.
4. Poor anaesthesia is frequently obtained when local anaesthetics are injected into infected or inflamed areas (e.g. Bartholin's abscess). This has to do with the fact that although a normal tissue pH is around 7.4, it may drop to 5 or 6 with inflammation, rendering most of the anaesthetic in the ionized form (which crosses cell membranes poorly), thereby reducing its effectiveness.
** Editors note: The maximum recommended dose of plain lidocaine is variously reported at between 3 and 5 mgs/kg.