Classify local anaesthetic agents and give examples. (30% of marks) Describe the pharmacology of lignocaine. (70% of marks)
The first part of this question was answered well by most candidates.
Generally, the second part of the question was poorly organised by many candidates, the
consequence being that many opportunities for picking up marks were lost. A brief statement
as to what lignocaine is, its presentations and dose, some facts about PD and PK followed by a
few lines on toxicity (CC/CNS ratio) was mostly what was required. Only a few candidates
mentioned lignocaine toxicity.
The classification of local anaesthetic agents is usually along chemical lines, into esters and amides
Lignocaine:
| Class | Class Ib antiarrhythmic |
| Chemistry | Aminoamide |
| Routes of administration | IV, inhaled, subcutaneous |
| Absorption | Oral bioavailability = 35% |
| Solubility | pKa = 7.9; about 25% is not ionised at pH 7.4 |
| Distribution | VOD= 0.9L/kg; 70% protein-bound |
| Target receptor | Nav1.5 subunit of the fast voltage-gated sodium channels |
| Metabolism | Hepatic metabolism (90-95%) |
| Elimination | Minimally renally excreted; half-life 10-20 minutes following IV bolus, closer to 45-90 minutes with subcutaneous infiltration |
| Time course of action | Duration of action is similar to half-life |
| Mechanism of action | Regional anaesthesia, by differential block (pain and temperature finres are blocked earlist). With higher doses, also motor block. In toxicity, CNS effects (visual disturbances, perioral mumbness, delirium,seizures, coma) and cardiovascular side effects (initially tachycardia and hypertension followed by bradycardia, negative inotropy, vasodilation and arrhythmias) Does not prolong the QRS, and actually shortens the QT. |
| Clinical effects | Antiarrhythmic effect, analgesic and local anaesthetic effects. Lowers seizure threshold, causes CNS excitation. Does not prolong the QRS, and actually shortens the QT. |
| Single best reference for further information | Weinberg et al (2015) |
HOLMDAHL, M. H: SON. "Xylocain (lidocaine, lignocaine), its discovery and Gordh's contribution to its clinical use." Acta Anaesthesiologica Scandinavica 42 (1998): 8-12.
Eipe, N., S. Gupta, and J. J. B. E. Penning. "Intravenous lidocaine for acute pain: an evidence-based clinical update." Bja Education 16.9 (2016): 292-298.
Weinberg, Laurence, et al. "Pharmacokinetics and pharmacodynamics of lignocaine: A review." World Journal of Anesthesiology 4.2 (2015): 17-29.