A Quantitative Comparison Study of Electrode Positions for Respiratory Surface Electromyography

Academic Background

Respiratory surface electromyography (SEMG) is a non-invasive technique used to record the electrical activity of respiratory muscles. It is widely applied in analyzing respiratory effort, patient-ventilator asynchrony, and respiratory training. However, due to the lack of standardized electrode placement protocols, there are significant variations in the positions and configurations used across different studies, limiting the comparability of results and hindering clinical application. Therefore, determining the optimal electrode position is crucial for improving the clinical acceptance of respiratory SEMG. This study aims to provide scientific evidence for the standardization of respiratory SEMG by quantitatively comparing the performance of unilateral and bilateral bipolar leads.

Paper Source

This paper was co-authored by Andra Oltmann, Jan Graßhoff, Nils Lange, Tobias Knopp, and Philipp Rostalski, all affiliated with the Fraunhofer Institute (Fraunhofer IMTE) and the University of Lübeck in Germany. The study was published in the journal IEEE Transactions on Biomedical Engineering and received funding from the European Regional Development Fund, the German Federal Government, and the state of Schleswig-Holstein in 2023.

Research Process

Study Population

The study included 20 healthy adults (10 males and 10 females), aged between 26 and 30 years, with a body mass index (BMI) below 50 kg/m². All participants signed informed consent forms prior to the study, which was approved by the Ethics Committee of the University of Lübeck.

Electrode Placement

The study utilized 64 gel electrodes (Kendall H124SG), placed according to an electrode scheme based on anatomical landmarks. Electrode positions included the midaxillary line (MAL), anterior axillary line (AAL), midclavicular line (MCL), and parasternal line (PSL). The inter-electrode distance was 25 millimeters, covering the areas of diaphragmatic and intercostal muscle activity. Additionally, 12-lead electrocardiogram (ECG) electrodes and a pair of sternocleidomastoid electrodes were placed.

Signal Acquisition

The 64-channel unipolar SEMG signals were recorded using the TMSI SAGA 64+ amplifier at a sampling frequency of 2000 Hz. Simultaneously, airway flow and pressure data were measured using the Biopac system, with signal synchronization achieved via a microcontroller.

Study Protocol

The study included three breathing tasks:
1. Quiet Breathing: Lasting 300 seconds without additional load.
2. Maximum Inspiratory Pressure (MIP) Test: Five maximum inspiratory efforts were performed to assess overall respiratory muscle strength.
3. Resistance Breathing: Fifteen deep breaths were performed at 20% of MIP resistance.

Signal Processing

Signal processing included the following steps:
1. Smoothing airway pressure and flow signals using a 100-millisecond moving average filter.
2. Detecting R-wave positions in the ECG using the Pan-Tompkins algorithm.
3. Removing power line interference using a Butterworth band-stop filter.
4. Suppressing cardiac interference using a wavelet denoising algorithm.
5. Calculating signal envelopes for unipolar or bipolar leads.

Data Analysis

The study used three performance metrics to quantitatively compare electrode leads:
1. Signal-to-Noise Ratio (SNRbase): The ratio of inspiratory muscle activity to baseline noise.
2. Expiratory Muscle Activity Signal-to-Noise Ratio (SNRexp): The ratio of inspiratory muscle activity to expiratory muscle activity.
3. ECG Interference Signal-to-Noise Ratio (SNR EMG-ECG): The ratio of R-wave power to inspiratory muscle activity power.

Key Results

Diaphragm Activity

1. Superior Performance of Bilateral Leads: Bilateral MCL leads (especially those positioned 2.5 cm above the costal margin) performed best across all performance metrics.
   
2. Poorer Performance of Unilateral Leads: Unilateral leads showed significantly lower SNRbase and SNRexp values compared to bilateral leads, with less ECG interference.
   
3. Optimal Lead Position: The MCL lead performed best for measuring diaphragmatic activity, while MAL and AAL leads showed poorer performance.

Intercostal Muscle Activity

1. Best Performance of Bilateral PSL Leads: Bilateral PSL leads positioned in the second intercostal space performed best.
   
2. Flexibility in Electrode Placement: Electrodes can be moved near the parasternal line to accommodate clinical needs.
   
3. Poorer Performance of Unilateral Leads: Unilateral leads showed lower SNRbase and SNRexp values compared to bilateral leads.

Conclusion

Through a quantitative comparison of different electrode positions, this study recommends using bilateral MCL leads (2.5 cm above the costal margin) to measure diaphragmatic activity and bilateral PSL leads (second intercostal space) to measure intercostal muscle activity. These findings provide important evidence for the standardization of respiratory SEMG measurements, helping to improve the clinical acceptance and application value of this technology.

Research Highlights

1. Systematic Comparison: The first systematic comparison of the performance of unilateral and bilateral leads in measuring diaphragmatic and intercostal muscle activity.
   
2. Quantitative Metrics: Introduction of three performance metrics to comprehensively evaluate the impact of electrode position on signal quality.
   
3. Clinical Application Value: Provides scientific evidence for the standardization of respiratory SEMG, promoting its clinical application.

Additional Information

The study also found that the flexibility of electrode placement is clinically significant, especially in cases where the patient’s chest wall is restricted or surgical trauma is present. Additionally, the results suggest that the degree of ECG interference can be optimized by adjusting electrode positions.