Duration of action
Neuromuscular-blocking agents can be classified in accordance to their duration of pharmacological action, defined as follows:

Side effects
Cardiovascular
The tetrahydroisoquinolinium class of neuromuscular blocking agents, in general, is associated with histamine release upon rapid administration of a bolus intravenous injection. There are some exceptions to this rule; e.g., cisatracurium (Nimbex) is one such agent that does not elicit histamine release even up to 5xED95 doses. The liberation of histamine is a dose-dependent phenomenon such that, with increasing doses administered at the same rate, there is a greater propensity for eliciting histamine release and its ensuing sequelae. Most commonly, the histamine release following administration of these agents is associated with observable cutaneous flushing (facial face and arms, commonly), hypotension and a consequent reflex tachycardia. These sequelae are very transient effects: The total duration of the cardiovascular effects is no more than one to two minutes, while the facial flush may take around 3–4 minutes to dissipate. Because these effects are so transient, there is no reason to administer adjunctive therapy to ameliorate either the cutaneous or the cardiovascular effects. Bronchospasm
Bronchospasm has been reported on occasion with the use of atracurium. However, this particular undesirable effect does not appear to be observed nearly as often as that seen with rapacuronium, which led to the latters withdrawal of approval for clinical use worldwide.
The proportions of cis−cis, cis−trans, and trans−trans isomers are in the ratio of 10.5 :6.2 :1. [cis-cis isomers ≈ 58% cis-trans isomers ≈ 36% trans-trans isomers ≈ 6%]. One of the three cis-cis structures is marketed as a single-isomer preparation, cisatracurium (trade name Nimbex); it has the configuration 1R, 2R, 1′R, 2′R at the four stereocentres. The beta-blocking drug Nebivolol has ten similar structures with 4 stereocentres and a plane of symmetry, but only two are presented in the pharmaceutical preparation. Intramuscular function parameters
ED95: the dose of any given intramuscular-blocking agent required to produce 95% suppression of muscle twitch (e.g., the abductor pollicis) response with balanced anesthesia
Clinical duration: difference in time between time of injection and time to 25% recovery from neuromuscular block
Train-of-Four (TOF) response: stimulated muscle twitch response in trains of four when stimuli are applied in a burst of four as opposed to a single stimulus, equal depression in depolarizing and fading response with non-depolarizing blocker.