Features of local anaesthetic toxicity

The following statements regarding the features of local anaesthetic toxicity are true

EXCEPT which one?

a) Prilocaine toxicity may cause the pulse oximeter to read 85%

b) At a plasma lidocaine concentration of 5 mg/mL, tinnitus may be present

c) As toxicity develops, inhibitory pathway inhibition at first causes excitation

d) Unconsciousness may precede convulsions

e) Cardiac resting membrane potential is made more negative

Answer: b

Explanation

At a plasma concentration of 5 mcg/mL, tinnitus may well be present, but at a

thousand times this level (as in Option (b)) the patient would not survive to report

this symptom. Published data relates the increasing severity of manifestations of local

anaesthetic toxicity to plasma concentrations of lidocaine. At 5mcg/mL circumoral

paresthesia, light-headedness and numbness of the tongue would also be expected. At

5 to 10 mcg/mL, unconsciousness, muscle twitches or visual disturbance might be

seen; at 10 to 20 mcg/mL we might see convulsions, myocardial depression and coma;

and at >20 mcg/mL we would expect arrhythmias, respiratory arrest and cardiac

arrest.

Prilocaine, when metabolised to O-toluidine, can produce methaemoglobinaemia,

which distorts pulse oximetry (but not the co-oximeter in a standard arterial blood gas

analyser). Cardiac toxicity is via a number of mechanisms, not least of which is as a

membrane stabiliser, local anaesthetics will hyperpolarise myocardial and conduction

cells. This causes a prolonged PR interval, widened QRS complex and a prolonged ST

segment before tachyarrhythmias supervene.

Reference

Smith T. Systemic toxic effects of local anaesthetics. Anaesth Intens Care 2007; 8(4):155–8.