Pharmacology for Anaesthesia and Intensive Care

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

Section IICoredrugs in anaesthetic practice

TOF Count 4 4

Train of four

T 4 :T 1 Ratio 1 075 05025

Double burst

stimulus

DBS Ratio 1 075 05025

Single twitch

Height % 100 95 80 50 25 15 5

%Receptor blockade

70 80 90 100

4

4

2

3

1

Figure 11.4.Patterns of non-depolarizing muscle-relaxant block against increasing NMJ,

receptor blockade.

Tetanic stimulation

When individual stimuli are applied at a frequency >30 Hz the twitches observed in

the muscle become fused into a sustained muscle contraction – tetany. The response

may be larger in magnitude than a single stimulus as the elastic forces of the muscle

do not need to be overcome for each twitch. Most stimulators deliver stimuli of 0.1

msec duration at a frequency of 50 Hz, which provides maximum sensitivity. In the

presence of a partial NDMR the tetanic stimulation fades with time. This is due to

blockade by the NDMR of presynaptic ACh receptors, thereby preventing the positive

feedback (see above) used to mobilize ACh at times of peak activity. Partial DMR block

reduces but does not exhibit fade in response to tetanic stimulation.

Post-tetanic potentiation and count

Following tetanic stimulation, subsequent twitches are seen to be larger. This may

be due to increased synthesis and mobilization of ACh and or increased Ca^2 +in

the synaptic terminal. Post-tetanic potentiation forms the basis of the post-tetanic

count where stimuli at 1 Hz are started 3 seconds after a tetanic stimulation. The

number of twitches is inversely related to the depth of block. It is best used when the

degree of receptor blockade is >95%, that is, when single twitch or train-of-four are