We read with great interest the April 2022 Monitor article by Hardman et al. that evaluated the use and reliability of rocuronium and succinylcholine as rapid-onset neuromuscular blocking agents (NMBAs) during induction (ASA Monitor 2022;86:42). The authors appropriately discuss the inverse relationship between the molar potency of NMBAs and their speed of onset. However, we question the authors’ statement that given the low potency of rocuronium, there appears to be a significant variability in its onset and duration.
First, can the variability of rocuronium onset and duration be explained by a cause-and-effect relationship with its low potency? The authors mention that succinylcholine appears to share this relationship as well. We are not aware of evidence to back either claim. The two cited papers purporting to support this idea do not relate low potency to variability of action of NMBAs. Indeed, a potency-onset effect for NMBAs is not easily explained by simple extension of the intuitive correlation between potency and onset time (Anesth Analg 2013;117:757-9). Potency is but one of many factors that can contribute to variability after administration of NMBAs (Korean J Anesthesiol 2017 Oct;70:500-10).
Second, it was mentioned that rocuronium is not optimally potent. We disagree with this statement. Rocuronium and succinylcholine are the two least potent NMBAs – compared with vecuronium, pancuronium, mivacurium, atracurium, and cisatracurium – but have the most rapid onset times (asamonitor.pub/3z3WU6u). Therefore, low potency can be advantageous and thus it is likely that any new, rapidly acting, nondepolarizing NMBA will have low potency (Br J Anaesth 2011;107:i60-71; ChemMedChem 2019;14:1108-14).
George R. Kracke, PhD
Adjunct Associate Professor, Department of Anesthesiology and Perioperative Medicine, University of Missouri School of Medicine, Columbia, Missouri Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri
Noah J. Timko, BS, BHS
Research Specialist, Department of Anesthesiology and Perioperative Medicine, University of Missouri School of Medicine, Columbia, Missouri
Quinn L. Johnson, MD, MBA
Professor of Clinical Anesthesiology, Department of Anesthesiology and Perioperative Medicine, University of Missouri School of Medicine, Columbia, Missouri
Disclosure: Mr. Timko reports prior research grants from Merck & Co. to the institution. No other disclosures are reported.
Authors’ Reply:
We appreciate Dr. Kracke and his colleagues’ interest in our article and the opportunity to respond. Our article hoped to emphasize that despite rocuronium’s potential as an optimal rapid-onset non-steroidal neuromuscular blocking drug designed to replace succinylcholine, it does not always meet these ambitions. The variability of rocuronium’s onset and offset is not in question. The reasons for its variability can be multifactorial, including inappropriate storage, its shelf life when stored at room temperature, and inherently variable pharmacokinetics. Although there may not be a “cause-and-effect relationship with its low potency,” we do not want such theoretical aspects to deviate from our clinically important message. To reiterate, it is important to consider more closely the manner in which rocuronium is used and stored. Similar clinical practice guidance should be applied to the use of succinylcholine. Additionally, efforts should continue toward development of a safe non-depolarizing neuromuscular blocking drug with ultra-rapid-onset and ultra-short duration with minimal variability.
Bailor L. Hardman, MD
Assistant Professor Department of Anesthesiology and Pain Management, Quality Improvement Director, and Safety Officer for University Hospitals Anesthesia, University of Texas Southwestern Medical Center, Dallas
Kapil Anand, MD, MBA
Associate Professor, Department of Anesthesiology and Pain Management, General Anesthesiology Section Chief, Clements University Hospital, University of Texas Southwestern Medical Center, Dallas
Girish P. Joshi, MB, BS, MD, FCAI, FASA
Professor of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, Dallas