Optimal Quantitative TOF Monitoring — Electrodes Matter Too

Why is neuromuscular transmission (NMT) monitoring (also known as TOF Monitoring) needed?

Neuromuscular blocking agents (NMBAs) are routinely used in general anaesthesia to facilitate intubation, ventilation and muscle relaxation.¹ The use of NMBAs is considered a double-edged sword, however, since although it facilitates anaesthesia and surgery, it is also associated with significant postoperative morbidity due to residual neuromuscular paralysis. NMBAs reversibly inhibit motor endplates and their effect weans over time but the incidence of residual neuromuscular blockade (NMB) after an operation can be as high as 45% in the absence of reversal agents even after a single dose of an NMBA.² Premature extubation, i.e. extubation in the presence of high levels of residual neuromuscular paralysis, is associated with aspiration, pneumonia, pharyngeal dysfunction, hypoxemia and airway obstruction.³ Neuromuscular transmission (NMT) monitoring, or Train-of-Four (TOF) monitoring as it’s commonly referred to, is the only recognised method of detecting residual block and is advised for every patient receiving NMBAs, both in the ICU, operating theatre and post anaesthesia care unit.

How is TOF monitoring performed?

TOF monitoring helps plan boluses and infusion rates of muscle relaxants to achieve optimal neuromuscular blockade.^4,5 Conventionally, the degree of NMB was evaluated by clinical assessments of muscle strength and respiratory function. However, quantitative assessment methods are superior as they provide a more reliable and accurate result. Today, acceleromyography (AMG) and electromyography (EMG) are popular measurement techniques used in quantitative NMT or TOF monitoring.⁶

Do electrodes matter in TOF monitoring?

Quantitative NMT monitoring techniques like TOF monitoring require the use of surface electrodes to stimulate peripheral nerves. Needle electrodes are discouraged as they can cause bleeding, infection, burns or nerve injury. Burns can occur due to high current density owing to limited contact surface and skin resistance. The choice of monitoring location depends on the nature of the surgery. Stimulating electrodes are commonly placed on the ulnar, posterior tibial or facial nerves, on the most superficial aspect of the specific nerve. The nerve stimulator generates Train-of-Four (TOF) and Post Tetanic Count (PTC) sequences on a periodic basis.⁶

Self-adhesive electrodes exclusively designed for TOF monitoring devices are preferred for use in NMT Monitoring applications but not routinely used in practice as cheaper ECG electrodes are readily available. Although standard ECG monitoring electrodes appear well-suited for use in neuromuscular stimulation applications, they are not specifically made for this purpose and their use may in fact pose a risk to patients.

Conventional ECG vs. Xavant’s proprietary NMT electrodes: is there a difference?

Physical and electrical characteristics of conventional ECG electrodes such as charge density, current density, impedance and resistance can play a role in producing heating effects and skin damage at the skin-electrode interface during electrical stimulation. ECG electrodes typically include a self-adhesive hydrogel formulation over a Ag-AgCl contact pad which is specifically designed for sensing and monitoring purposes. Stimulation electrodes, on the other hand, typically include a self-adhesive hydrogel formulation over a carbon-based contact pad. Due to this difference in design, fabrication and chemical composition of the hydrogel, the electrical properties of the two types of electrodes are expected to differ and react differently in the presence of electrical stimuli.

The impact of changing electrode impedance

Electrodes are designed to present the least possible impedance to the patient at the skin-electrode interface. This is true for both monitoring and stimulation electrodes. It’s not uncommon for monitoring electrodes, such as ECG electrodes, to present a lower initial impedance than stimulation electrodes. At face value monitoring electrodes, therefore, look like a suitable alternative to stimulation electrodes, but the electrode impedance is not guaranteed to remain stable during stimulation. Several hours of continuous use in the presence of appreciable current intensity may in fact result in a sharp increase in the electrode impedance.

It is the increase in impedance of monitoring electrodes during stimulation that poses an unseen risk to the uninitiated. At first glance it would appear that a nerve stimulator would heat up an electrode with an increased impedance, however, nerve stimulators such as the Stimpod NMS450X simply do not generate sufficient energy to do so. So where then does the risk lie?

The risk actually lies in the presence of high currents that are induced in the leads of the stimulator and in the electrode by external high-frequency electrosurgical medical equipment, commonly used in the operating theatre to cut or coagulate tissue during surgery.

The undeniable impact of surrounding conditions

• Electrosurgical units in close proximity

It is well-documented that high-frequency electromagnetic fields radiated by electrosurgical units (ESUs) induce currents in nearby devices, cables or patient leads that can become strong enough to cause burns at monitoring electrodes even with an intact dispersive pad function.^7,8

The dissipated power of a monitoring electrode increases at the skin/electrode interface as the impedance increases in the presence of an induced current, and due to the small surface area of its contact pads, the power density also increases. This results in ohmic heating of the electrode contact pad, creating a concentrated heat source that can result in burns. Induced currents can cause localized heating of the cathode electrode when the impedance is low but is compounded as impedance increases with time, and even more so in the presence of high surrounding temperatures.

• Forced-air convection heaters

The chemical composition of the hydrogel used in monitoring electrodes typically change and increase the impedance when exposed to elevated temperatures for extended periods of time. The use of convection heaters in OTs to maintain body temperature is a perfect example of a heat source that may elevate the immediate environmental temperature sufficiently to exacerbate the increase in impedance of monitoring electrodes used for stimulation purposes during long surgical procedures.

Contributors

Jacques Swart, Roche Janse van Rensburg, Lourie Höll

Enquiries

Michelle Strydom
Product Specialist
michelle@xavant.com