Good Practice in Postoperative and Procedural Pain Management ...

Prilocaine has relatively low toxicity compared withmost amide-type local anesthetics. It is 55% bound toplasma proteins and is rapidly metabolized mainly inthe liver and kidneys and is excreted in the urine. Oneof the principal metabolites is o-toluidine, which isbelieved to cause the methemoglobinemia observedafter large doses. It crosses the placenta and duringprolonged epidural anesthesia may producemethemoglobinemia in the fetus. It is distributed intobreast milk. The peak serum concentration of prilocaineassociated with CNS toxicity is 20 mgÆml )1 .Symptoms usually occur when doses of prilocainehydrochloride exceed about 8 mgÆkg )1 but the veryyoung may be more susceptible. Methemoglobinemiahas been observed in neonates whose mothers receivedprilocaine shortly before delivery and it has also beenreported after prolonged topical application of a prilocaine/lidocaineeutectic mixture in children. Methemoglobinemiamay be treated by giving oxygen followed,if necessary, by IV methylthioninium chloride.Prilocaine should be used with caution in patientswith anemia, congenital or acquired methemoglobinemia,cardiac or ventilatory failure, or hypoxia. Prilocainehas been associated with acute attacks ofporphyria and is considered unsafe in porphyricpatients. Methemoglobinemia may occur at lowerdoses of prilocaine in patients receiving therapy withother drugs known to cause such conditions (e.g., sulfonamidessuch as sulfamethoxazole in co-trimoxazole).(iii) EMLALidocaine forms a mixture with prilocaine that has amelting point lower than that of either ingredient. Thiseutectic mixture containing lidocaine 2.5% and prilocaine2.5% can produce local anesthesia when appliedto intact skin as a cream. It is used extensively for proceduralpain including venepuncture, intravenous orarterial cannulation, lumbar puncture, minor dermatologicalprocedures, and others (see section 4.0). Theeutectic cream is usually applied to skin under anocclusive dressing for at least 60 min and a maximumof 5 h. Transient paleness, redness, and edema of theskin may occur following application.Eutectic mixtures of lidocaine and prilocaine areused in neonates and are safe in single doses. Therehas been concern that excessive absorption (particularlyof prilocaine) might lead to methemoglobinemiaparticularly after multiple applications. For this reason,the maximum number of doses per day should belimited in the neonate. In some countries, EMLA hasbeen licensed for use in neonates provided that theirgestational age is at least 37 weeks, and that methemoglobinvalues are monitored in those aged 3 months orless. In fact, systemic absorption of both drugs fromthe eutectic cream appears to be minimal across intactskin even after prolonged or extensive use. However,EMLA should not be used in infants under 1 yearwho are receiving methemoglobin-inducing drugs; itshould not be used on wounds or mucous membranesor for atopic dermatitis. EMLA should not be appliedto or near the eyes because it causes corneal irritation,and it should not be instilled into the middle ear. Itshould be used with caution in patients with anemia orcongenital or acquired methemoglobinemia.6.2.3 Tetracaine (amethocaine)(i) PreparationsTetracaine is a potent, para-aminobenzoic acid esterlocal anesthetic used for surface anesthesia and spinalblock. It is highly lipophilic and can penetrate intactskin. Its use in other local anesthetic techniques isrestricted by its systemic toxicity.For anesthesia of the eye, solutions containing 0.5–1% tetracaine hydrochloride and ointments containing0.5% tetracaine have been used. Instillation of a 0.5%solution produces anesthesia within 25 s that lasts for15 min or longer and is suitable for use before minorsurgical procedures.A 4% gel (Ametop) is used as a percutaneous localanesthetic. This formulation of 4% tetracaine producesmore rapid and prolonged surface anesthesia thanEMLA and is significantly better in reducing paincaused by laser treatment of port wine stains and forvenous cannulation. A transdermal patch is effective,and patches containing a mixture of lidocaine and tetracainehave also been tried. Tetracaine has beenincorporated into a mucosa-adhesive polymer film torelieve the pain of oral lesions resulting from radiationand antineoplastic therapy. Liposome-encapsulated tetracainecan provide adequate surface anesthesia.LAT (LET) 4% lidocaine, 0.1% epinephrine, and0.5% tetracaine have been combined in a gel andapplied as a surface anesthetic to lacerations of theskin especially the face and scalp. It is less a painfulalternative to LA infiltration prior to suture of lacerations.(ii) Dosage side effects and toxicityTetracaine: A stinging sensation may occur when tetracaineis used in the eye. Absorption of tetracaine frommucous membranes is rapid, and adverse reactions canª 2012 Blackwell Publishing Ltd, Pediatric Anesthesia, 22 (Suppl. 1), 1–79 69