Background: 
Local anaesthetics (LA) can induce methaemoglobin (metHb) formation with variable severity. Because cyanosis in recovery have a plethora of causes, diagnosis of local anaesthetic-induced methaemoglobinemia could be quite challenging & therefore delayed, especially when safe doses are used.

Case report: 
An ASA II 45-yr-old female, who had upper limb day surgery under regional block, was desaturating in recovery. Having a history of brain tumor removal (20 yrs back) causing left hemiplegia, she recovered completely after 6 months of physiotherapy. 
She had a left US-guided axillary block (prilocaine 1%, 35 ml) augmented by mid-arm radial and forearm ulnar & median nerve blocks (L-bupivacaine 0.25%, 5 ml each). She was positioned right laterally and surgery was uneventful. In recovery, although pink initially, faint central and peripheral cyanosis started to develop. Air entry on left lung base was reduced. CXR revealed raised left hemi-diaphragm & increased hilar bronchovascular markings. ECG & arterial blood gas (ABG) results were normal, except for hypoxaemia. Co-oximetry showed metHb level of 13.1% which dropped to 9.1% after 4 hrs of high-flow oxygen & then 0.1% the following morning. At 1 month follow-up, CXR was normal.

Discussion:
Methaemoglobinemia, caused by exposure to various toxins including prilocaine, reduces RBC ability to release O2 to tissues & hence left shift of oxyHb dissociation curve. It is not generally appreciated that therapeutic doses of most local anaesthetics can produce this condition. 1 Prilocaine's metabolite, O-toluidine, is responsible for oxidation of oxyHb into metHb. Diagnosis is based on clinical findings (oxygen-resistant cyanosis with normal calculated oxygen saturation) & co-oximetry. Treatment is both symptomatic and specific (methylene blue and/or ascorbic acid). 2 Although prilocaine's maximum recommended dose is 6 mg/kg, 3 this patient developed significant methaemoglobinemia at around 4.5mg/kg. She had no personal or family history of G6PD deficiency. It was previously advised that prilocaine's dose should be limited to 2.5 mg/kg. 2

References: 
1.Abu-Laban RB et al. Severe methemoglobinemia from topical anesthetic spray:.. Can J Em Med. 2001; 3(1):51-56. 
2. Guay J. Methemoglobinemia related to local anesthetics:... Anesth Analg. 2009; 108(3):837-845. 
3. British National Formulary. Sep 2014: NICE.

Learning points: 
Prilocaine can induce methaemoglobinemia even in sub-maximal therapeutic doses, so exercise caution when using it in large volumes for major nerve blocks in small patients. Prompt recognition and treatment of methaemoglobinemia can be lifesaving.

Cyanotic in recovery: what seems safe maybe unsafe