Evaluation of Respiratory Muscle Activation Using Respiratory Motor Control Assessment (RMCA) in Individuals with Chronic Spinal Cord Injury

Summary

The purpose of this publication is to present our original work on a multi-muscle surface electromyographic approach to quantitatively characterize respiratory muscle activation patterns in individuals with chronic spinal cord injury using vector-based analysis.

Abstract

During breathing, activation of respiratory muscles is coordinated by integrated input from the brain, brainstem, and spinal cord. When this coordination is disrupted by spinal cord injury (SCI), control of respiratory muscles innervated below the injury level is compromised^1,2 leading to respiratory muscle dysfunction and pulmonary complications. These conditions are among the leading causes of death in patients with SCI³. Standard pulmonary function tests that assess respiratory motor function include spirometrical and maximum airway pressure outcomes: Forced Vital Capacity (FVC), Forced Expiratory Volume in one second (FEV₁), Maximal Inspiratory Pressure (PI_max) and Maximal Expiratory Pressure (PE_max)^4,5. These values provide indirect measurements of respiratory muscle performance⁶. In clinical practice and research, a surface electromyography (sEMG) recorded from respiratory muscles can be used to assess respiratory motor function and help to diagnose neuromuscular pathology. However, variability in the sEMG amplitude inhibits efforts to develop objective and direct measures of respiratory motor function⁶. Based on a multi-muscle sEMG approach to characterize motor control of limb muscles⁷, known as the voluntary response index (VRI)⁸, we developed an analytical tool to characterize respiratory motor control directly from sEMG data recorded from multiple respiratory muscles during the voluntary respiratory tasks. We have termed this the Respiratory Motor Control Assessment (RMCA)⁹. This vector analysis method quantifies the amount and distribution of activity across muscles and presents it in the form of an index that relates the degree to which sEMG output within a test-subject resembles that from a group of healthy (non-injured) controls. The resulting index value has been shown to have high face validity, sensitivity and specificity^9-11. We showed previously⁹ that the RMCA outcomes significantly correlate with levels of SCI and pulmonary function measures. We are presenting here the method to quantitatively compare post-spinal cord injury respiratory multi-muscle activation patterns to those of healthy individuals.

Protocol

1. Settings

1. Surface electrode heads were placed over the muscle bellies of left (L) and right (R) respiratory muscles: sternocleidomastoid (SC), scalene (S), upper trapezius on midclavicular line (UT), clavicular portion of pectoralis on midclavicular line (P), diaphragm on parasternal line (D), intercostal at 6^th intercostal space on anterior axillary line (IC), rectus abdominus at umbilical level (RA), obliquus abdominis on midaxillary line (O), lower trapezius paraspinally at midscapular level (LT), and paraspinal paraspinally on iliac intercrestal line (PS)⁶. The ground electrodes were placed over the acromion processes. A Motion Lab System Back Pack Unit, with attached electrodes, was connected to a Motion Lab EMG Desk Top Unit and Powerlab System (Figure 1).
2. T-piece Monitoring Circuit to record the airway pressure was assembled as shown in Figure 2 and connected to the Low Pressure Transducer (MP45) using air tube.
3. MP45 was connected to CD15 and Powerlab System (Figure 1 and Table 1).

2. RMCA Protocol

1. The respiratory motor tasks consisted of Maximum Inspiratory Pressure Task (MIPT) and Maximum Expiratory Pressure Task (MEPT). To perform MIPT or MEPT, subjects were asked to produce maximum inspiratory effort from residual volume or expiratory efforts from total lung capacity for 5 sec using a T-piece Monitoring Circuit (Figures 1 and 2). Each maneuver was cued by an audible 5-sec long tone and repeated 3x. At least 1 min of rest was allowed between each effort.
2. EMG input was amplified with a gain of 2,000; filtered at 30-1,000 Hz and sampled at 2,000 Hz. Airway pressure input was calibrated at 100 cm of water and sampled at 2,000 Hz. The EMG and airway pressure inputs were converted by the Powerlab acquisition system using 16-bit full scale ADC resolution. Airway pressure, sEMG and marker signals were recorded simultaneously⁹.

3. Data Analysis

1. Multi-muscle activity distribution analysis windows of 5 sec each for MIPT or MEPT were determined from the event marker and airway pressure recorded with the cuing tone that signaled the subject when to begin and end the task (Figure 3). The sEMG activity for each muscle was calculated using a root mean square (RMS) algorithm^6,12 (Figure 4). Three repeated trials for each task were averaged¹³ for each muscle (channel).
2. The multi-muscle activation patterns were evaluated based on a vector analysis method known as the Voluntary Response Index (VRI)⁸ (Figures 4-6) using custom-made Matlab software (MathWorks). For each maneuver, the VRI calculation produces two values, a Magnitude and a Similarity Index (SI) (Figures 5-6). The Magnitude parameter, the amount of combined sEMG activity for all muscles within the specific time window, was calculated as a length of the Response Vector (RV) for specific task (Figure 7). The Similarity Index (SI) provides a value that expresses how similar the RV of SCI subject is to the Prototype Response Vector (PRV) obtained from healthy subjects during the same task. The SI value was computed for each task as a cosine of the angle between the SCI subject RV and PRV. The SI value ranges between 0 and 1.0 where value of 1.0 represents the best match for compared vectors⁹ (Figure 8) .

Results

Figure 3 represents the electromyogram and airway pressure (on top) simultaneously recorded during MEPT from a non-injured (left) and SCI (right) individuals. Note decreased airway pressure and absence of sEMG activity in expiratory muscles in an SCI subject when compared to a non-injured individual (marked with gray ellipses). Note also that start of the task, as marked on the bottom, is associated with increased sEMG activity and raising airway pressure.

Figure 4

Discussion

Standard clinical tests to evaluate respiratory motor function after SCI and other disorders include the pulmonary function tests and the American Spinal Injury Association Impairment Scale (AIS) evaluation^14,15. However, these tools are not designed for quantitative evaluation of the trunk and respiratory motor control. In our previously published work⁹, we have shown that the RMCA is a valid method to quantitatively evaluate the respiratory motor function affected by SCI. We have demonstrated that...

Materials

Name	Company	Catalog Number	Comments
PowerLab System 16/35	ADInstruments	PL3516	Number of units depends on number of channels recorded
EMG System MA 300	Motion Lab Systems	MA300-XVI	Number of units depends on number of channels recorded
Low Pressure Transducer MP45	Validyne	MP45-40-871
Basic Carrier Demodulator CD15	Validyne	CD15-A-2-A-1
Air Pressure Manometer	Boehringer	4103	Needed for MP45 calibration
Event Marker			Hand held switch that when pressed gives a DC voltage and sound output (including 5-sec long mark)
Alcohol Wipes	Henry Schein	1173771	Needed for electrodes placement
Electrode Gel	Lectron II	36-3000-25	Needed for electrodes placement
Tagaderm	Henry Schein	7779152	Needed for electrodes placement
Noseclip	Henry Schein	1089460
T-piece Ventilator Monitoring Circuit with One-way Valves	Alleglance (Airlife)	1504
Air Tube	UnoMedical	400E
Table 1. List of specific equipment and supplies used for the Respiratory Motor Control Assessment.