train-of-four

train-of-four

 

a test for measuring the level of neuromuscular blockade: four consecutive stimuli are delivered along the path of a nerve and the response of the muscle is measured in order to evaluate stimuli that are blocked versus those that are delivered. Four equal muscle contractions will result if there is no neuromuscular blockade, but if nondepolarizing blockade is present, there will be a loss of twitch height and number, which will indicate the degree of blockade. This test is commonly used in intensive care units. See illustration.

Ulnar nerve placement for train-of-four stimulation of the ulnar nerve. Electrodes are positioned in a direct line along the lateral side of the wrist. The thumb adducts to stimulation. From McQuillan et al., 2002.

train-of-four

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TOF

A monitoring protocol for counting the number of contractions produced by peripheral nerve stimulators in patients who have received neuromuscular blocking agents (NMBAs) such as Pavulon and vecuronium. When NMBAs are used, staff may be unable to use normal assessment techniques of neurological function.

Patient care

In TOF, electrodes are placed on the patient's wrist, and the number of thumb twitches is counted. After the NMBA infusion is begun, thumb twitches are measured every 30 min for 2 hr to ensure the appropriate level of paralysis has been reached. The absence of contractions indicates that too much NMBA is being given; 1 to 2 twitches indicate the appropriate level of drug is being administered, and 3 to 4 twitches indicate the need to increase the infusion rate. Once the desired level is reached, response to peripheral nerve stimulation is measured every 4 hr.

Patients retain sensory nerve function and awareness of their surroundings, so analgesics and sedatives are usually administered concurrently. Whether TOF augments clinical assessment of neuromuscular blockade is controversial; it may be more useful with some neuromuscular blocking agents (e.g., vecuronium) than others.