Local anaesthetics

Question 1

Which LA are associated with methemoglobinemia?

Answer 1

Prilocaine and benzocaine are the two local anesthetics responsible for most cases of local anesthetic–related methemoglobinemia, although in rare instances other agents have been implicated. Prilocaine is metabolized in the liver to O-toluidine, which is capable of oxidizing hemoglobin to methemoglobin. Prilocaine in a dose greater than 600 mg can produce clinical methemoglobinemia, making the patient appear cyanotic. Benzocaine, used as a spray for topical anesthesia of mouth and throat, can result in methemoglobinemia if excessive amounts are used in the form of multiple sprays or spraying the area for a longer duration than recommended. Methemoglobin is reduced through methemoglobin reductase, and this process is accelerated by intravenous methylene blue (1 to 2 mg/kg).

Question 2

Symptoms of LA toxiocioty

Answer 2

Premonitory CNS symptoms

• dizziness
• tinnitus
• perioral tingling
• unresponsiveness
• agitation
• nystagmus
• muscle twitches
• potentiated by hypercapnia and acidosis

Seizures

•usually self-limiting due to drug re-distribution
-premedication with benzodiazepines provides significant prophylaxis

CNS depression

Cardiovascular
•hypotension
•arrhythmias
•bupivacaine most cardiotoxic
•treatment of complications involves
-HCO3
-standard ACLS measures
-IV lipid emulsion (intralipid 20%) may be effective in resuscitation of circulatory collapse once after ACLS measures and seizures controlled

•outcome may be good following prolonged resuscitation

Question 3

Amides

Answer 3

Lignocaine
Prilocaine
Bupivicaine
Ropivicaine

Question 4

Esters

Answer 4

Procaine
Cocaine
Tetracaine
Benzocaine