25%-75% recovery index: an indicator of the rate of skeletal muscle recovery - essentially, the difference in time between the time to recovery to 25% and time to recovery to 75% of baseline value
T4:T1 ≥ 0.7: a 70% ratio of the fourth twitch to the first twitch in a TOF - provides a measure of the recovery of neuromuscular function
T4:T1 ≥ 0.9: a 90% ratio of the fourth twitch to the first twitch in a TOF - provides a measure of the full recovery of neuromuscular function

History
Atracurium besilate was first made in 1974 by George H. Dewar, a pharmacist and a medicinal chemistry doctoral candidate in John B. Stenlakes medicinal chemistry research group in the Department of Pharmacy at Strathclyde University, Scotland. Dewar first named this compound "33A74" before its eventual emergence in the clinic as atracurium. Atracurium was the culmination of a rational approach to drug design to produce the first non-depolarizing non-steroidal skeletal muscle relaxant that undergoes chemodegradation in vivo. The term chemodegradation was coined by Roger D. Waigh, Ph.D., also a pharmacist and a postdoctoral researcher in Stenlakes research group. Atracurium was licensed by Strathclyde University to the Wellcome Foundation UK, which developed the drug (then known as BW 33A) and its introduction to first human trials in 1979, and then eventually to its first introduction (as a mixture of all ten stereoisomers) into clinical anesthetic practice in the UK, in 1983, under the tradename of Tracrium.
Atracurium besilate, also known as atracurium besylate, is a medication used in addition to other medications to provide skeletal muscle relaxation during surgery or mechanical ventilation. It can also be used to help with endotracheal intubation but suxamethonium (succinylcholine) is generally preferred if this needs to be done quickly. It is given by injection into a vein. Effects are greatest at about 4 minutes and last for up to an hour.Common side effects include flushing of the skin and low blood pressure. Serious side effects may include allergic reactions; however, it has not been associated with malignant hyperthermia. Prolonged paralysis may occur in people with conditions like myasthenia gravis. It is unclear if use in pregnancy is safe for the baby. Atracurium is in the neuromuscular-blocker family of medications and is of the non-depolarizing type. It works by blocking the action of acetylcholine on skeletal muscles.Atracurium was approved for medical use in the United States in 1983. It is on the World Health Organizations List of Essential Medicines. Atracurium is available as a generic medication. Medical uses
Atracurium is a medication used in addition to other medications in to provide skeletal muscle relaxation during surgery or mechanical ventilation. It can be used to help with endotracheal intubation but takes up to 2.5 minutes to result in appropriate intubating conditions.