Sat27November0556PM 11

A 34-year-old woman had anaesthesia for an urgent laparoscopic appendicectomy. She was given 1.5 mg/kg suxamethonium as part of a rapid sequence induction. Following surgery, the patient failed to regain muscle twitch responses for a period of four hours during which time sedation and ventilation on the critical care unit was continued.

A provisional diagnosis of suxamethonium apnoea was made and appropriate biochemical investigations were sent to the laboratory.

The results were as follows:

The butrylcholinesterase (BChE) activity was 57 U/L (reference interval, 3300-10 300 U/L) and the dibucaine number was <5% (reference interval (83%-88%).

What is the single most likely BChE phenotype in this patient?

(Please select 1 option)

F (fluoride resistant) Incorrect answer selected

S (silent) This is the correct answer

FS (fluoride resistant/silent)

A (atypical)

AS (atypical/silent)

Explanation

There are a number of phenotypes that influence BChE activity in humans:

U - usual phenotype

A - atypical phenotype

F - fluoride-resistant phenotype, and

S - silent phenotype.

Other phenotypes are heterozygous combinations of the U, A, F, and S phenotypes.

Ninety six per cent of the population has the U phenotype with a normal dibucaine number of 83-88%, with normal plasma BChE levels and no susceptibility to suxamethonium.

Patients with the S phenotype have the lowest levels of BChE which makes them very susceptible to long periods of apneoa. The dibucaine number is often very low.

BChE phenotype BChE activity (U/L) DN(%) Susceptibility Frequency

U 3300-10300 83-88 None 96%

A 1600-4100 24-31 Very 1 in 3000

F 1600-4100 79-81 Somewhat 1 in 150000

S 0-650 - Very 1 in 40000

AS 540-1800 24-31 Very 1 in 8000

FS 1000-3800 78-84 Somewhat Rare

Answer Statistics

1

24%

2

33%

3

13%

4

14%

5

17%

Times answered: 276