Predictive Modelling of the Intraoperative Train-of-Four (TOF) Ratio

Neuromuscular Blocking Agents (NMBAs) are routinely administered to patients in a multiplicity o anaesthetic settings in order to paralyze and impede (re)active muscular contraction. The availability of monitoring devices allowing an accurate measurement fo the degree of neuromuscular block during anesthesia has raised the standards for a proper evidence-based use of NMBAs.

For this purpose, one of the most widely used methods is the Train of Four (TOF): transcutaneous application of a series of 4 square-wave supra-maximal electrical stimuli over the course of a nerve of choice (most commonly the ulnar nerve). These are applied at a frequency of 2Hz, and each with a duration of 0.2ms. These stimuli elicit a motor response on the adductor pollicis muscle, which on its turn dictates the adduction of the thumb. The acceleration of this movement can be followed by means of an uni/multi-directional accelerometer attached to the thumb. The ratio of the acceleration of the 4th and 1st elicited contractions is called the TOF-Ratio - a clinically and scientifically established method of assessing neuromuscular block recovery. A value of 1 translates a full recovery of the muscular function of a patient. In modern Anesthesia, the bar for deeming a recovery as adequate has been set at a minimum of a TOF-ratio of >0.9, with some authors advocating a ratio of 1 as the only acceptable and complications avoiding result.