At the end of a balanced anaesthesia technique with non-depolarizing muscle relaxant, a patient recovered spontaneously from the effect of muscle relaxant without any reversal. Which is the most probable relaxant the patient had received?

This patient has received a non depolarizing muscle relaxant and has recovered spontaneously from its effect without any reversal, so he would have received atracurium. Atracurium is a sho acting muscle relaxant. It is inactivated spontaneously by nonenzymatic degradation (Hofmann elimination) in addition to that by cholinesterase. So reversal is mostly not required when it is used. Hofmann elimination is the spontaneous fragmentation of atracurium at the bond between the quaternary nitrogen and the central chain. Ref: Essentials of Medical Pharmacology By K D Tripathi, 5th Edition, Page 314 ; Fundamentals of Anaesthesia By Tim Smith, Colin Pinnock, Ted Lin, 3rd Edition, Pages 617-18