What is neuromuscular transmission?
Neuromuscular transmission (NMT) is the transfer of an impulse between a nerve and a muscle in the neuromuscular junction. NMT can be blocked by neuromuscular blocking agents – drugs which cause transient muscle relaxation and prevent the patient from moving and breathing spontaneously.
Muscle relaxation is used during general anesthesia to enable endotracheal intubation and to provide the surgeon with optimal working conditions. In critical care, muscle relaxation is used during mechanical ventilation to minimize the patient’s work of breathing and to improve oxygenation.
How is the NMT block measured?
The level of neuromuscular block is routinely measured by stimulating a peripheral nerve, usually in the hand and by subjectively evaluating the muscle response either visually or tactilely with a device. Evidence suggests that despite use of subjective neuromuscular monitoring techniques, a large percentage of patients arrive to the PACU with residual paralysis.1
By comparison, GEHC offers the NMT module, which provides quantitative, automatic measurements of muscle response to stimulus and consequentially, the level of block. This objective measurement can be used to safely time extubation and avoid the occurrence of residual paralysis.
GE Healthcare offers two different sensor types for the NMT module. The innovative MechanoSensor measures the motion of the thumb with a piezoelectric sensor, which converts the physical motion to an electrical signal and quantifies the evoked mechanical response. The MechanoSensor sensor is available in adult and pediatric sizes. The ElectronSensor directly measures the electrical activity of the muscle with recording electrodes, quantifying the response to nerve stimulation. The ElectroSensor can be used on the patient's hand or foot in both adult and pediatric patients.
- Module with NMT measurement capability
- NMT sensor cable
- MechanoSensor or Pediatric MechanoSensor lead wire set
- Electrode, white lead connection site for nerve stimulation
- Electrode, brown lead connection site for nerve stimulation
- ElectroSensor leadwire set
- White stimulating electrode
- Brown stim ulating electrode
- Electrode, black lead connection site, ground
- Electrode, green lead connection site, recording muscle-contraction effect
- Electrode, red lead connection site, recording muscle-contraction effect
Starting nerve stimulus
Start the NMT measurement by defining a patient specific reference with a supramaximal stimulus current for the unrelaxed patient before administering neuromuscular blocking agents. This will ensure reliable measurements are achieved during all levels of neuromuscular blockade. Thereafter module automatically maintains the optimum current throughout the procedure.
For reliable NMT measurement use GE validated NMT electrodes to provide large conductive surface.
Train-of-four (TOF) is used as the default stimulation mode with four stimuli at 0.5 second intervals causing muscles to contract.
Quantitative muscle response
The muscle response can be quantified with different parameters depending on the type and the level of neuromuscular block.
TOF Count is the number of detected muscle responses. Train-of-four ratio (TOF%) is the ratio of the fourth muscle response to the first one. TOF% indicates fade in non-depolarizing block. Once the TOF Count drops below four responses or T1% is less than 10%, the TOF% is not shown.
When depolarizing agents are used, no fade occurs, and the height of the four responses indicates the level of block.
When no responses are detected to TOF stimulation, the post tetanic count (PTC) is the only way to measure the neuromuscular block. A tetanic stimulation (50 Hz) is generated for five seconds and post-tetanic responses to single twitch stimulation are counted. The larger the PTC, i.e., the number of detected responses, the sooner the normal TOF responses return. PTC is rarely used in critical care.
Monitoring of neuromuscular block in five steps
1. Properly secure the sensor of choice (as depicted on page 1). Press measurement start-up. The monitor will start the measurement by setting the stimulus current automatically and by performing a reference measurement. Depolarizing relaxants result in an equal drop in all four responses, without fade.
2. Non-depolarizing relaxants cause a fade in the responses, indicated by a lower TOF% and a slope in the bar graph.
3. Neuromuscular block can be used to facilitate endotracheal intubation. The physician can use the time when all responses disappear (i.e., TOF Count is 0) as a guide to determine when to intubate.
4. During surgery and in critical care, TOF Count is used to maintain steady optimal level of neuromuscular block. When TOF Count exceeds a level set by the user, the GE monitor will give a “Block recovery” message.
5. Antagonists, if used, should not be given before TOF count rises to 4. For safer extubation, TOF% should be higher than 90.
Nerve location for regional block
The NMT module can also be used to locate the nerve to be blocked in regional anesthesia.
A regional block needle is stimulated with small, repeated 2.0 mA stimuli while the nerve is approached. Each nerve stimulus should result in muscle contraction. The closer the needle is to the motor nerve, the lower the current is needed to give a response. When even a small stimulus current (e.g., < 0.5mA) results in a visible muscle contraction, the optimum site has been located and local anesthetic can be injected.
This method helps clinicians find the optimum site of the regional nerve to be blocked and, thus, optimizes the bolus of the anesthetic to be injected. The correct location also protects the patient against mechanical nerve and vessel lesions.
Adequacy of Anesthesia
Adequacy of Anesthesia consists of several interrelated components as depicted below.
One of the objectives of general anesthesia is immobility, i.e., the assurance that the patient does not move. Often neuromuscular blocking agents are used to achieve this goal. Anesthetic agents prolong and amplify the effects of neuromuscular blocking agents, therefore, it is recommended to use quantitative NMT monitoring when anesthetic agents are used together with neuromuscular blockade. NMT is an important part of adequacy of anesthesia monitoring and, when combined with other parameters such as the Entropy and hemodynamic measurements, it helps to achieve a more complete picture of the patient’s status.
Why use the NMT module?
Automatic and hands-free
Neuromuscular block measurements with the NMT module are easy. Simply attach the sensor and push the Start-up key.
The module will set the supramaximal current and automatically cycle according to the user-defined measurement interval.
Optimal dosage during anesthesia and in critical care
Quantitative NMT monitoring gives a clear picture of the individual dosage needs of the patient and facilitates optimal and cost-effective administration of neuromuscular blocking drugs.
Monitoring the level of neuromuscular block enables follow-up and prediction of recovery and helps in correct timing of the antagonists, which may decrease the incidence of residual paralysis.
Enhanced patient safety
Objective, quantitative monitoring is the only means to safely indicate recovery of neuromuscular block (TOF%>90%).2, 3 Therefore, utilizing the NMT module can help decrease the incidence of residual paralysis and the associated respiratory complications.
Fast patient throughput
Patients that arrive to the PACU with residual paralysis (TOF%<90) stay on average 90 minutes longer.4 Using NMT to guide extubation times can support shorter length of stay.
When the NMT measurement is integrated in a monitoring system, the measured values are displayed, trended and automatically documented together with all the other monitored parameters.
For white papers, guides and other instructive materials about our clinical measurements, technologies and applications, please visit http://clinicalview.gehealthcare.com/
- Murphy GS, Brull SJ. Residual neuromuscular block: Lessons unlearned. Part 1: Definitions, incidence, adverse psychological effects of residual neuromuscular block. Anesth Analg 2010;111:120-128
- ATOTW 290 – Residual Neuromuscular Blockade, 26/08/2013
- Lars I. Eriksson, M.D., Ph.D., Evidence-based Practice and Neuromuscular Monitoring: It's Time for Routine Quantitative Assessment. Anesthesiology 5 2003, Vol.98, 1037-1039.
- Butterly A, Bitner EA, George E, Sandberg WS, Eikermann M, Schmidt U. Postoperative residual curarization from intermediate-acting neuromuscular blocking agents delays recovery room discharge. Br J Anaesth. 2010; 105: 304-309.