Neuromuscular monitoring
As Relaxometry or Neuromuscular monitoring (NMM ) refers to the monitoring of neuromuscular transmission of impulses at the motor endplate in the use of muscle relaxants ( relaxation, neuromuscular blockade ) under anesthesia. By means of two electrodes while a peripheral nerve is stimulated by the Relaxometer and the resulting evoked muscle response measured qualitatively or quantitatively. Based on these values , the anesthetist can evaluate the effect of muscle relaxants and control the dosage accordingly.
- 6.1 Literature
- 6.2 Notes and references
Basics
For a striated skeletal muscle can be activated, must the product derived from the brain signal that is passed through a motor nerve, be transferred to the muscle. This happens at the neuromuscular junction, a chemical synapse, which forms the interface of nerve and muscle cells. By the action potential, which is passed through the nerve, the neurotransmitter acetylcholine (ACh) from the pre-synaptic membrane is released. It diffuses across the synaptic cleft and binds to acetylcholine receptors on the muscle cell and triggers by influx of sodium ions from a muscle contraction.
In the context of general anesthesia ( anesthesia ) will allow to improve the conditions for endotracheal intubation or surgery in the abdominal area, administered muscle relaxants. This block ACh receptors on the muscle cell, so that a transmission signal is no longer possible; the muscle relaxes ( neuromuscular blockade, relaxation ). To monitor this deliberate blockade relaxometry ( Neuromuscular monitoring) is used. This makes it possible to customize the need for muscle relaxants to the patient to ensure optimal operation conditions, while limiting the duration of action as precisely as possible.
In relaxometry two glued electrodes through the skin ( transcutaneous ) a peripheral nerve is stimulated and the response of the associated muscle is measured. By a supramaximal current ( 40-60 mA) ensures the complete excitation of the nerves and the reproducibility of the results. The optimal stimulation is achieved in addition to the current by a monophasic, rectangular shape of the pulse and a duration of about 0.2 ms.
As a test muscle usually serves the musculus adductor pollicis, is thereby stimulated the ulnar nerve. This differs in the course of neuromuscular blockade in its properties but from other clinically relevant muscle groups ( diaphragm, muscles of the larynx ). Better match indicates the orbicularis oculi muscle ( stimulated via the facial nerve ). Also possible is the use of the flexor hallucis brevis ( posterior tibial nerve ) when monitoring on the wrist is not possible.
Stimulation pattern
Single stimulation
A single stimulation (single twitch ) is the simplest form of stimulation. It can be used to verify correct electrode placement and setting the stimulus intensity.
Train -of -Four
Train- of-four ( TOF) is the most widely used stimulation patterns of relaxometry and standard in monitoring the effects of non - depolarizing muscle relaxants. Here, a series of four stimuli is delivered at a rate of 2 Hz. Besides the magnitude order, the nature of neuromuscular block can be determined. The TOF stimulation is suitable for monitoring during all phases of anesthesia.
In a relaxation with nondepolarizing muscle relaxants ( succinylcholine ) responses to stimulation are reduced equally, the use of non-depolarizing agents can be combined with the increased effects of a decrease (fatigue, fading) determine from the first to the fourth answer. This ratio ( T4/T1 ) is referred to as TOF ratio or TOF ratio and is the unrelaxed muscle 1.0. With increasing relaxation of this ratio becomes smaller at low relaxation can in certain circumstances only single or no more stimulus responses cause (TOF- number: For two remaining answers TOF Figure 2).
Tetanic stimulation
A high-frequency nerve stimulation leads to the fusion of the associated muscle contractions ( tetanus ). After a short series of stimuli at 50 Hz, it is due to the increased transmitter release at the motor endplate to a temporary reinforcement of the muscular responses, which can be determined with the following single stimulation. This method is referred to as a post tetanic Count (PTC) and enables monitoring at very low relaxation when a TOF stimulation may produce no response.
Double - burst stimulation
In the double- burst stimulation ( DBS ) is a double series of three and three (DPS 3.3 ) or three and two (DPS 3.2 ) stimuli is delivered. By this sequence is intended to facilitate tactile assessment during the recovery of the block, it is used during the Narkoseausleitung.
Registration methods
There are various methods to measure the stimulus response of the stimulated muscle. The visual or tactile assessment allows the verification of deep relaxation, but it is inaccurate in the recovery phase, a distinction of TOF ratio> 0.5 is not possible. In Mechanomyografie (MMG), the development of the muscle force is measured by a force transducer. It presents the scientific reference method, is for clinical practice, however, not suitable. The electromyography (EMG ) measures the induced over the test muscle action potentials, which is performed by means of two measuring electrodes. The disadvantage is the influence of movements, electrical artifacts and cold. The Akzeleromyografie (AMG ) measures the acceleration of a piezo sensor which is fixed, eg at end member of the thumb and presents the sufficient accuracy and good practicability a clinical routine method dar. In addition to these, there are other methods of measurement.
Clinical Use
The relaxometry is focused on interventions used, in which a muscle relaxation is obligatory ( abdominal surgery, eye surgery, etc.). It allows it to control the potency and duration of muscle relaxants that can be metered so that a sufficient neuromuscular blockade is present when needed, an overdose with prolonged duration of anesthesia can be avoided, however.
Through the routine use in all anesthesia with the use of muscle relaxants may be a Relaxanzüberhang (residual blockade of the muscles) are almost auszuschlossen after anesthesia. Although called by some authors, such a broad application in practice over a purely clinical assessment by the anesthetist has not yet been enforced.
Historical Aspects
Harvey and Masland 1941 led the first measurements of muscular blockade by means of nerve stimulation. With the mechanomyografischen and electromyographic registration Botelho 1955 developed methods for measuring muscle response. The first device for intraoperative monitoring designed Christie and Churchill -Davidson 1958.