Pharmacology for Anaesthesia and Intensive Care

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9780521704632c11 CUFX213A/Peck 9780521618168 December 28, 2007 12:5

11 Muscle relaxants and anticholinesterases

unable to evoke muscle twitches. It should be remembered that the effects of tetanic

stimulation may last for up to 6 minutes and may therefore give a false impression of

inadequate block to single twitch or train-of-four analysis. Partial DMR block does

not exhibit post tetanic potentiation.

Train-of-four

The train-of-four (TOF) is four 0.1 msec stimuli delivered at 2 Hz. The ratio of the

fourth twitch height to the first twitch height (T 4 :T 1 ), or the number of twitches may

be recorded leading to the TOF ratio or the TOF count respectively (Figure11.4). It

does not require a baseline twitch height.

Inamanner similar to that seen with tetanic stimulation the rapid stimuli lead to

areducing twitch height in the presence of partial non-depolarizing muscle relaxant

blockade, that is, T 4 <T 1 .Asreceptor occupancy rises above 70% T 4 will start to

decrease in size. When T 4 has decreased by 25% T 1 starts to decrease, corresponding

to 75–80% receptor occupancy. T 4 disappears when T 1 is approximately 25% of its

original height. TOF ratio is difficult to assess in practise.

The TOF count records the number of twitches in response to a TOF. As recep-

tor ocupancy exceeds 90% T 4 disappears and only T 1 is present at 95% receptor

ocupancy. So the TOF count assesses the degree of deep NDMR block.

TOFratio in the presence of partial DMR block is 1.

Double burst stimulation

Double burst stimulation (DBS) describes the delivery of two bursts of stimulation

separated by 0.75 sec. Each burst consists of three 0.2 msec stimuli separated by

20 msec (i.e. at a 50 Hz). Double burst stimulation was developed to allow easy manual

detection of small amounts of residual NMJ blockade so that when the magnitude

of the two stmuli are equal clinically significant residual NMJ blockade does not

exist. However, when assessed mechanically DBS is no more sensitive than TOF

(Figure11.4).

Depolarizing muscle relaxants

Suxamethonium

CH 2 CH 2 CH 2 N+(CH 3 ) 3

N+(CH 3 ) 3

——— —C O —

C CH 2 CH 2

O

O

CH 2 ———— O

——

—

—— —

Suxamethonium was first introduced in 1952 and provided a significant advantage

over tubocurarine as profound muscle relaxation of short duration was achieved