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Succinylcholine is the only available neuromuscular blocker with a rapid onset of effect and an ultrashort duration of action. The ED95 of succinylcholine (the dose causing on average 95% suppression of neuromuscular response) is 0.51 to 0.63 mg/kg.[62] [63] Using cumulative dose-response techniques, Smith and coworkers[64] and Kopman and associates[65] have estimated that its potency is far greater with an ED95 less than 0.3 mg/kg.
Administration of 1 mg/kg succinylcholine results in complete suppression of response to neuromuscular stimulation in approximately 60 seconds. [66] [67] [68] In patients with genotypically normal butyrylcholinesterase (also known as plasma cholinesterase or pseudocholinesterase) activity, recovery to 90% muscle strength after the administration of 1 mg/kg succinylcholine requires from 9 to 13 minutes. [69] [70]
The short duration of action of succinylcholine is due to its rapid hydrolysis by butyrylcholinesterase to succinylmonocholine and choline. Butyrylcholinesterase has an enormous capacity to hydrolyze succinylcholine, and only 10% of the administered drug reaches the neuromuscular junction.[71] The initial metabolite (succinylmonocholine) is a much weaker neuromuscular blocking agent than succinylcholine is[72] and is metabolized much more slowly to succinic acid and choline. In dogs,[73] after
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Acceptable Clinical Results to Suggest Normal Function | Approximate Percentage of Receptor Occupied When Response Returns to Normal Value | Comments/Disadvantages/Advantages |
|---|---|---|---|
| Tidal volume | At least 5 mL/kg | 80 | Insensitive as an indicator of peripheral neuromuscular function |
| Single twitch | Qualitatively as strong as baseline | 75–80 | Uncomfortable, need to know twitch strength before relaxant strength as baseline administration. Insensitive as an indicator of recovery, but useful as a gauge of deep neuromuscular blockade |
| Train-of-four (TOF) | No palpable fade | 70–75 | Still uncomfortable, but more sensitive as an indicator of recovery than single twitch is. Useful as a gauge of depth of block by counting the number of responses perceptible |
| Sustained tetanus at 50 Hz for 5 sec | No palpable fade | 70 | Very uncomfortable, but a reliable indicator of adequate recovery |
| Vital capacity | At least 20 mL/kg | 70 | Requires patient cooperation, but is the goal for achievement of full clinical recovery |
| Double burst | No palpable fade | 60–70 | Uncomfortable, but more sensitive than TOF as an indicator of stimulation of peripheral function. No perceptible fade indicates TOF recovery of at least 60% |
| Sustained tetanus at 100 Hz | No palpable fade | 50 | Very painful, a "stress test" for the neuromuscular junction. It is not always possible to achieve or demonstrate lack of fade at 100 Hz |
| Inspiratory force | At least -40 cm H2 O | 50 | Sometimes difficult to perform without endotracheal intubation, but a reliable gauge of normal diaphragmatic function |
| Head lift | Must be performed unaided with patient supine at 180 degrees and for 5 sec | 50 | Requires patient cooperation, but remains the standard test of normal clinical function. Must be performed with the patient in a completely supine position |
| Handgrip | Sustained at a level qualitatively similar to preinduction baseline | 50 | Sustained strong grip, though also requiring patient cooperation. It is another good gauge of normal function |
| Sustained bite | Sustained jaw clench on tongue blade | 50 | Very reliable with patient cooperation. Corresponds to TOF ratio of 0.85 |
Because little or no butyrylcholinesterase is present at the neuromuscular junction, the neuromuscular block induced by succinylcholine is terminated by its diffusion away from the neuromuscular junction back into the circulation. Butyrylcholinesterase therefore influences the onset and duration of action of succinylcholine by controlling the rate at which the drug is hydrolyzed before it reaches and after it leaves the neuromuscular junction.
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