NMBDs interact with the ACh receptor either by depolarizing the end plate (depolarizing agents) or by competing with ACh for binding sites (nondepolarizing agents) (Table 20-1: Definition of Neuromuscular Blocking Drugs According to The Onset and Duration of Block At The Adductor Pollicis). The only depolarizing agent in clinical use is SCh.
- Pharmacologic Characteristics of Neuromuscular Blocking Agents. The effect of NMBDs is measured as the depression of adductor muscle contraction (twitch) after electrical stimulation of the ulnar nerve.
- Potency is determined by constructing the dose-response curves, which describe the relationship between twitch depression and dose. The effective dose 95 (ED95) is a clinically relevant value that corresponds to 95% block of single twitch (half of patients will reach 95% block, and half will reach a lower percentage) (Fig. 20-2: Example of a dose–response relationship).
- Onset time or time to maximum blockade can be shortened if the dose is increased (2 × ED95).
- Duration of action is the time from injection of the NMBD to return of 25% twitch height (comparisons are usually made at 2 × ED95). Categories of NMBDs may be based on their durations of action.
- Recovery index is the time interval between 25% and 75% twitch height (this reflects speed of recovery after return of twitch is manifest). The adductor pollicis is the most commonly monitored muscle for determining the onset and duration of action of NMBDs.