Methemoglobin Formation by a Local Anesthetic and Some Related Compounds

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Abstract
SUMMARY: Citanest (L 67, α‐n‐propylamino‐2‐methylpropionanilide) and some related compounds were studied for methemoglobin formationin vitroandin vivo.In vitro,Citanest did not induce met‐Hb formation in red cell suspension. Under the same conditions a related compound, o‐toluidine, in a dose of 40 mg/dl, did produce met‐Hb. Intramuscular injection of Citanest and o‐toluidine in cats induced met‐Hb in the circulating blood.Lidocaine and a related compound, 2,6‐xylidine, did not produce met‐Hb in the cat on intramuscular injection.In clinical experiments on 50 patients who received Citanest (200‐1,600 mg) by epidural injection, met‐Hb, exceeding 10% of the total hemoglobin, was observed in a few cases. The met‐Hb concentration increased roughly parallel with the dose of Citanest administered. The only complication encountered was slight cyanosis in one case, whose met‐Hb concentration was 2.79 g/dl. In most cases maximum met‐Hb concentration was attained three to five hours after administration of Citanest, and met‐Hb disappeared within 24 hours after the injection. On the basis of these results, it is suggested that the safe upper limit of Citanest for a single injection is 10 mg/kg body weight.