Tetanic fade

Long stimulus trains will also induce tetanic fade, observed as a decrease in force during a single contraction. Fade is measured as the ratio of final force to initial peak force. Diseased muscle/nerve may show a lower ratio. Accompanying measurement of EMG can reveal if fade is due to failure of the muscle or the nerve. In purely muscle defects (muscle dystrophy) force will drop without any change in EMG whereas reduced force accompanied by reduced EMG suggests a defect in transmission (NMJ) (e.g., (3)) or excitation-contraction coupling. 

Two additional cases of this syndrome were reported several years later both patients required ventilatory support during the paralytic phase.47 In another series, 29 of 90 patients with organophosphate poisoning had the intermediate syndrome.48 Tetanic fade with no posttetanic facilitation was maximal between days 4 and 6, and the response to electrical stimulation had returned to normal by 8 to 10 days. The author suggested that a neuromuscular junction defect was responsible for the lesion. 

Initial TOF should demonstrate each successive twitch decreasing in amplitude this is fade. The tetanic stimulus should fail to reach 100% response and should also demonstrate fade. The second TOF should still demonstrate fade but the twitches as a group should have increased amplitude. This is post-tetanic potentiation. 

While most of the muscle relaxants exert their predominant actions at the post-junctional nicotinic receptors, many also have variable pre-junctional effects. Although pre-junctional receptors have not been demonstrated there is putative evidence for their existence. The pre-junctional mechanisms are supposed to be responsible for the development of fade in response to tetanic or train-of-four (TOP) stimulation following administration of non-depolarising neuromuscular blocking drugs. 

Suxamethonium produces a typical depolarising block that is characterised by the appearance of fasciculations before the onset of block, absence of fade in response to tetanic and TOP stimulations, and potentiation of block by anticholinesterase drugs. 

Response to a tetanic stimulus Unsustained (fade) Sustained2 (no fade) Unsustained (fade)... 

The double-burst stimulation pattern is a newer mode of electrical nerve stimulation developed with the goal of allowing for manual detection of residual neuromuscular blockade when it is not possible to record the responses to single-twitch, TOF, or tetanic stimulation. In this pattern, three nerve stimuli are delivered at 50 Hz followed by a 700 ms rest period and then, by two or three additional stimuli at 50 Hz. It is easier to detect fade in the responses to double-burst stimulation than to TOF stimulation. The absence of fade in response to double-burst stimulation implies that clinically significant residual neuromuscular blockade does not exist. 

Train of four and tetanic stimulation No fade Eade ... 

Response to tetanic stimulus Unsustained ( fade") Sustainetf Unsustained... 

Tetanic contractions (Wedensky inhibition) induced by repetitive nerve stimulation in phrenic nerve-diaphragm preparations of mice was faded by 0.5-2 ijlM neostigmine (Chang ei al., 1986). The fade was brought about by failure to elicit muscle action potentials, which was due to end-plate depolarization and a decrease in transmitter release. Both effects were attributed to ACh accumulation as a result of ChE inhibition. 

The causative oiganophosphate was identified in nine patients four fenthion, two dimethoate. two monocrotophos, and one mclhamidophos. Standard biochemistry and cerebrospinal fluid examination were normal. Cholinesterase (ChE) assays were not available. Electromyography (EMG) showed normal motor and sensory nerve conduction velocities and normal needle myography. Tetanic stimulafion of the abductor pollicis brevis muscle 24-48 hr after the onset of IMS showed a marked fade at 20 and 50 Hz. A train of four stimuli applied at 2 Hz produced no changes in the amplitude of the compound muscle action potential (CMAP). CMAPs are the motor responses recorded with surface electrodes over a muscle after stimulation of its motor nerve. 

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