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Dibucaine number

Dibucaine is an ester type local anesthestic which can inhibit the normal plasma cholinesterase enzyme (PChE) or pseudocholinesterase. Dibucaine is not used much in clinical practice these days. In 1957, Kalow and Genest found several varients of plasma ChE which will respond to dibucaine differently from normal PChE. This lead to the test called Dibucaine Number (DN) which is based on the fact that dibucaine inhibits the normal enzyme about 80 % and the abnormal enzymes about 20 %

Relationship between DN and Duration of Succinyl Choline or Mivacurium

Although the dibucaine number can identify the genetic makeup of an individual patient, it does not measure the actual enzyme’s concentration, or the efficacy. The best test is the measurement of PChE activity, which express in IU. [substrate molecules (mumol) hydrolyzed per unit of time]

Succinylcholine and Mivacurium are destroyed by PChE. The genetic defect that causes a low level normal PChE or abnormal enzymes will affect the clinical duration of these muscle relaxants. Other drugs that are also destroyed by PChE but less cllinically importance include ester-local anesthetics and heroine.

PChE is synthesized by the liver, serum half life of 8-12 days. The concentration of PChE in plasma is about 5 mg/L. Physiologic variences such as age 0-6 month (50 % of normal adult level) and pregnancy (25-30% decreased) have no clinical significance. Other acquired PChE defects includes hepatitis, cirrhosis, malnutrition, cancer, myxedema, acute infection and MI, drugs e.g. organophosphage, echothiophate, neostigmine, pyridostigmine, phenelzine, cyclophosphamide, trimethaphan, etc.