Introduction
Quantitative train-of-four, or QTOF monitoring accuracy is key to managing neuromuscular blockades during anesthesia. Studies show that up to 180 million patients receive neuromuscular blocking agents (NMBAs) annually, with 72 million experiencing post-operative complications due to residual neuromuscular blockade (RNMB) [4]. These complications highlight the need for QTOF monitoring accuracy and reliability to increase patient safety and mitigate risks associated with anesthesia.
The leading Quantitative TOF monitors and their technologies are:
- Stimpod 450X+, (Xavant Technology), Accelerometry(AMG) and Electromyography(EMG)
- TwitchView (Blink Device Company), Electromyography(EMG) only
- TetraGraph, (Senzime), Electromyography(EMG) only
This article explores their QTOF monitoring accuracy, methodologies, and comparative advantages for Quantitative TOF Monitoring.
QTOF Calculation and Measuring Methods using EMG:
There are two main ways of calculating a patient’s TOF ratio using electromyography (EMG).
- Area Under the Curve Measurement
- Peak-to-Peak or Peak to Valley Voltage Measurement
Area Under the Curve (AUC)
- AUC calculates the compound motor action potential (cMAP) by integrating the signal over time, capturing the total electrical activity of the muscle response [6]. This method provides a more stable measurement and is less affected by temporary fluctuations that can misrepresent QTOF monitoring accuracy.
- The BJA, British Journal of Anesthesia, Open study found that AUC-based calculations are clinically interchangeable with peak-to-peak voltage measurement but provide a more complete view of muscle response [6].
Peak-to-Peak Voltage Measurement
- Measures the voltage difference between the highest and lowest points of the cMAP waveform [6]. While compelling, this method only captures a single waveform characteristic, making it more susceptible to external artifacts, electrical noise, and variations in patient physiology.
- Electrode placement and external interference can influence peak-to-peak voltage measurement, which can lead to variability in TOF ratio measurements [6].
A recent study confirmed that AUC and peak-to-peak voltage measurements provide clinically interchangeable information, with no preference for one method over the other in ideal conditions [6]. However, relying solely on peal-to-peak voltage calculations increases vulnerability to external influences affecting signal integrity, making it less resilient in environments with high electrical noise.
The Stimpod 450X+ combines AUC and peak-to-peak voltage for enhanced QTOF monitoring accuracy, adaptability, and reliability. TwitchView relies solely on AUC, while TetraGraph uses only peak-to-peak voltage measurement, which may limit its QTOF monitoring accuracy under challenging clinical conditions. While both methods are clinically valid, combining AUC and peak-to-peak amplitude adds a layer of accuracy, consistency, and reliability, reinforcing why Stimpod’s dual-method approach provides enhanced QTOF monitoring accuracy and a clinical advantage.
Devices and QTOF Calculation Method Summary:
| Feature | Stimpod 450X+ | TwitchView | TetraGraph |
| TOF Calculation Method | AUC and Peak-to-Peak Voltage Measurements | Area Under the Curve AUC Voltage Measurements | Peak-to-Peak Voltage Measurements |
| TOF Technology Method | Dual (EMG & AMG) | EMG only | EMG only |
| Clinical Validation Against MMG | Yes (Wedemeyer et al.) | Yes (Blink Device Company study [2]) | Limited (Bussey et al., 2021 [3]) |
| Self-Calibrating AMG Sensor | Yes, 80% of Surgeries | No | No |
| Best for Tucked Arms & Robotics Cases | Yes, EMG | Yes, EMG | Yes, EMG |
QTOF Monitoring Accuracy
Electromyography (EMG)
According to a study published in the Journal of Clinical Monitoring and Computing, the Stimpod 450X+ demonstrated accuracy comparable to mechanomyography (MMG), the gold standard for neuromuscular monitoring [1]. Combining EMG and AMG in a single device further enhances the Stimpod’s clinical versatility. Meanwhile, TwitchView has been validated against MMG, confirming its reliability [2], at the time of this document TetraGraph requires further research to ensure consistency in neuromuscular monitoring [3].
Acceleromyography (AMG)
Xavant’s 5th-generation AMG sensor for the Stimpod 450X+ features:
- 12x higher resolution than previous versions
- 50% increased sensitivity for improved detection of neuromuscular responses
- Self-calibrating 3-dimensional accelerometry technology, eliminating the need for manual adjustments
A comparative analysis suggests that AMG sensors in the Stimpod 450X+ closely align with EMG readings.[5]
The Stimpod 450X+ AMG and EMG sensors
The graphic below shows how close the AMG sensor reading is to the reading of an EMG electrode, which shows the QTOF monitoring accuracy of the Stimpod AMG sensor. The EMG sensor is preferred during tucked arm or robotics cases when access to the thumb is limited. Meanwhile, the AMG sensor performs the best in all other clinical situations where the thumb is unrestricted.
Summary
The American Society of Anesthesia guideline recommends monitoring muscle relaxants using a quantitative TOF monitor.
The Stimpod, TetraGraph, and TwitchView provide good clinical choices for monitoring paralytics. All three monitors offer EMG monitoring with a proprietary electrode. The Stimpod is the only QTOF monitor that offers both EMG and AMG in the same device, offering versatility and cost savings.
Stimpod 450X+:
- Dual EMG and AMG capabilities with the simple change of the cable
- Documented QTOF monitoring accuracy compared to MMG
- Self-calibrating three-dimensional AMG sensor technology with 12 times higher resolution and 50% enhanced sensitivity
- Cost-efficient hospital solution with AMG useful in up to 80% of general anesthetics
References
[1] Wedemeyer, Z., et al. Journal of Clinical Monitoring and Computing, 2023. DOI: 10.1007/s10877-023-01087-1
[2] Blink Device Company. “TwitchView Clinical Validation Data.” [Online]. Available: https://www.blinkdc.com/twitchview-validation-data.
[3] Bussey, L., Jelacic, S., Togashi, K., Bowdle, A. “A Pilot Study of Train-of-Four and Post-Tetanic Count Monitoring with the TetraGraph Electromyograph Compared to the TwitchView Monitor Electromyograph.” medRxiv. 2021. DOI: 10.1101/2021.03.19.21249475
[4] Xavant Technology. “Stimpod NMS450X Neuromuscular Transmission Monitor.” [Online]. Available: https://www.xavant.com.
[5] Xavant Technology. “Why Quantitative NMT Monitoring is Critical in Surgical Patients and How Best to Do It.” [Online]. Available: https://www.xavant.com.
[6] British Journal of Anaesthesia Open. “Comparison of AUC and Amplitude in EMG-Based Neuromuscular Blockade Monitoring.” 2024. DOI: https://www.bjaopen.org/article/S2772-6096(24)00037-6/fulltext