Peripheral nerve stimulation

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This is used in anaesthetized patients, the details of which are outside the remit of this book. The following is intended as no more than a brief outline.

A peripheral nerve supplying a discrete muscle group is stimulated transcutaneously with a current of 50 mA. The resulting contractions are observed or measured. One arrangement is to stimulate the ulnar nerve at the wrist whilst monitoring the contractions (twitch) of the adductor pollicis. Although most often done by looking at or feeling the response, measuring either the force of contraction or the compound action potential is more objective. Sequences of stimulation used include:

• four stimuli each of 0.2 s duration, at 2Hz for 2 s, referred to as a 'train-of-four' (TOF);

• One stimulus at 50 Hz of 5 s duration, that is, a tetanic stimulus;

Table 2.6 Non-depolarizing neuromuscular blocking drugs

Drug

Dose for intubation

Maintenance dose

Time to intubation

Duration of action

Systemic effects

Comments

Atracurium

0.5-0.6 mg/kg

0.1 5-0.2 mg/kg; 30-50 mg/h infusion

90-120 s

20-25 mins

Cutaneous histamine release, iBP

Spontaneous degradation in plasma. Cisatracurium a single isomer, more potent

Rocuronium

0.6-0.7 mg/kg

0.1 5-0.2 mg/kg; 30-50 mg/h infusion

90-100s; after 1 mg/kg, 60s

20-30 mins

Minimal

Alternative to suxamethonium for RSI

Vecuronium

0.1 mg/kg

0.02-0.03 mg/kg; 6-10 mg/h infusion

90-120 s

15-20 mins

Minimal, no histamine release

White powder, dissolved before use

Mivacurium

0.1 5-0.2 mg/kg

0.1 mg/kg

100-120s

10-15 mins

Histamine released if large dose injected rapidly

Metabolized by plasma cholinesterase. Rapid recovery, reversal often unnecessary

Pancuronium

0.1 mg/kg

0.015 mg/kg

120-150s

35-45 mins

t BP, t HR

Long-acting

BP: blood pressure; HR: heart rate; RSI: rapid sequence induction.

BP: blood pressure; HR: heart rate; RSI: rapid sequence induction.

• two groups of three tetanic bursts at 50 Hz, 750 ms apart.

During non-depolarizing neuromuscular blockade, there is a progressive decremental response to all the sequences, termed 'fade'. In the TOF, the ratio of the amplitude of the fourth twitch (T4) to the first twitch (T1) is used as an index of the degree of neuromuscular blockade. During depolarizing blockade, the response to all sequences of stimulation is reduced but consistent, that is, there is no fade.

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