Optimal electrode placement for noninvasive electrical stimulation of human abdominal muscles

Regulation of Human Respiration by Electrical Stimulation

The review addresses modern methods of electrical stimulation used to regulate the function of external respiration in humans. The methods include abdominal functional stimulation of respiratory muscles, diaphragmatic stimulation, phrenic nerve stimulation, epidural and transcutaneous spinal cord stimulation. The physiological rationale of their application is described along with the examples of their use in clinical practice, including stimulation parameters and electrode placement diagrams for each of the methods. We analyze the effectiveness of each of the methods in patients with respiratory muscle paresis and the features of their use depending on the level of spinal cord injury. Special attention is paid to the method of epidural spinal cord stimulation because this technique is widely used in electrophysiological studies on animal models, providing deeper insight into the spinal levels of the functional control of external respiration. The review substantiates the great potential of using the method of transcutaneous electrical spinal cord stimulation both in fundamental studies of external respiration and in clinical practice.

Effects of restoration of cough via spinal cord stimulation on subject quality of life

Objectives

To determine participant quality of life before and after use of the cough stimulation system (Cough System).

Design

Prospective assessment of life quality at 4 timepoints via questionnaire responses.

Setting

Out-patient hospital, United States.

Participants

28 subjects with spinal cord injury (SCI) completed life quality assessment questionnaires before and at the 28- 40- and 52-week timepoints following use of the Cough System.

Results

Each subject demonstrated significant clinical improvements in terms of restoration of an effective cough and ability to manage airway secretions with use of the Cough System. Positive airway pressures and peak expiratory airflows approached values associated with a normal cough. Related to cough/secretion management, use of this system also resulted less interference with family life and daily activities, less financial difficulties, less requirement for caregiver assistance, less stress, less embarrassment and greater control of their breathing problems (p < 0.01), for each comparison). There also significant improvements in that their overall health and quality of life (p < 0.01, for each comparison). Subjects also reported greater ease in breathing, restored ability to sneeze and enhanced mobility. The incidence of acute respiratory tract infections fell from 1.3 ± 0.3 to 0.2 ± 0.1 events/subject year (p < 0.01). Ten subjects developed mild hemodynamic effects consistent with autonomic dysreflexia that abated completely with continued use of the Cough System. Some subjects experienced mild leg jerks during SCS, which were well tolerated and abated completely with reduction in stimulus amplitude, No subjects reported bowel or bladder leakage.

Conclusion

Use of the Cough System by SCI subjects is a safe and efficacious method which significantly improves life quality and has the potential to reduce the mortality and morbidity associated with SCI.

Spinal cord injury and diaphragm neuromotor control

Introduction: Neuromotor control of diaphragm muscle and the recovery of diaphragm activity following spinal cord injury have been narrowly focused on ventilation. By contrast, the understanding of neuromotor control for non-ventilatory expulsive/straining maneuvers (including coughing, defecation, and parturition) is relatively impoverished. This variety of behaviors are achieved via the recruitment of the diverse array of motor units that comprise the diaphragm muscle.Areas covered: The neuromotor control of ventilatory and non-ventilatory behaviors in health and in the context of spinal cord injury is explored. Particular attention is played to the neuroplasticity of phrenic motor neurons in various models of cervical spinal cord injury.Expert opinion: There is a remarkable paucity in our understanding of neuromotor control of maneuvers in spinal cord injury patients. Dysfunction of these expulsive/straining maneuvers reduces patient quality of life and contributes to severe morbidity and mortality. As spinal cord injury patient life expectancies continue to climb steadily, a nexus of spinal cord injury and age-associated comorbidities are likely to occur. While current research remains concerned only with the minutiae of ventilation, the major functional deficits of this clinical cohort will persist intractably. We posit some future research directions to avoid this scenario.

Abdominal Functional Electrical Stimulation to Augment Respiratory Function in Spinal Cord Injury

Background: Functional electrical stimulation (FES) is the application of electrical pulses to a nerve to achieve a functional muscle contraction. Surface electrical stimulation of the nerves that innervate the abdominal muscles, termed abdominal FES, can cause the abdominal muscles to contract, even when paralysed after spinal cord injury. As the abdominal muscles are the major expiratory muscles, and commonly partially or completely paralysed in tetraplegia, abdominal FES offers a promising method of improving respiratory function for this patient group. Objective: The aim of the article is to provide readers with a better understanding of how abdominal FES can be used to improve the health of the spinal cord-injured population. Methods: A narrative review of the abdominal FES literature was performed. Results: Abdominal FES can achieve an immediate effective cough in patients with tetraplegia, while the repeated application over 6 weeks of abdominal FES can improve unassisted respiratory function. Ventilator duration and tracheostomy cannulation time can also be reduced with repeated abdominal FES. Conclusion: Abdominal FES is a noninvasive method to achieve functional improvements in cough and respiratory function in acute and chronically injured people with tetraplegia. Potential practical outcomes of this include reduced ventilation duration, assisted tracheostomy decannulation, and a reduction in respiratory complications. All of these outcomes can contribute to reduced morbidity and mortality, improved quality of life, and significant potential cost savings for local health care providers.

Optimal electrode position for abdominal functional electrical stimulation

Abdominal functional electrical stimulation (abdominal FES) improves respiratory function. Despite this, clinical use remains low, possibly due to lack of agreement on the optimal electrode position. This study aimed to ascertain the optimal electrode position for abdominal FES, assessed by expiratory twitch pressure. Ten able-bodied participants received abdominal FES using electrodes placed: 1) on the posterolateral abdominal wall and at the motor points of 2) the external oblique muscles plus rectus abdominis muscles, and 3) the external obliques alone. Gastric (P_ga) and esophageal (P_es) twitch pressures were measured using a gastroesophageal catheter. Single-stimulation pulses were applied at functional residual capacity during step increments in stimulation current to maximal tolerance or until P_ga plateaued. Stimulation applied on the posterolateral abdominal wall led to a 71% and 53% increase in P_ga and P_es, respectively, compared with stimulation of the external oblique and rectus abdominis muscles ( P < 0.001) and a 95% and 56% increase in P_ga and P_es, respectively, compared with stimulation of the external oblique muscles alone ( P < 0.001). Stimulation of both the external oblique and rectus abdominis muscles led to an 18.3% decrease in P_ga compared with stimulation of only the external oblique muscles ( P = 0.040), with inclusion of the rectus abdominis having no effect on P_es ( P = 0.809). Abdominal FES applied on the posterolateral abdominal wall generated the highest expiratory twitch pressures. As expiratory pressure is a good indicator of expiratory muscle strength and, thus, cough efficacy, we recommend this electrode position for all therapeutic applications of abdominal FES. NEW & NOTEWORTHY While abdominal functional electrical stimulation (abdominal FES) can improve respiratory function, clinical use remains low. This is at least partly due to lack of agreement on the optimal electrode position. Therefore, this study aimed to ascertain the optimal electrode position for abdominal FES. We show that electrodes placed on the posterolateral abdominal wall generated the highest expiratory twitch pressures. As such, we recommend this electrode position for all therapeutic applications of abdominal FES.

Stimulation of abdominal and upper thoracic muscles with surface electrodes for respiration and cough: Acute studies in adult canines

OBJECTIVE

To optimize maximal respiratory responses with surface stimulation over abdominal and upper thorax muscles and using a 12-Channel Neuroprosthetic Platform.

METHODS

Following instrumentation, six anesthetized adult canines were hyperventilated sufficiently to produce respiratory apnea. Six abdominal tests optimized electrode arrangements and stimulation parameters using bipolar sets of 4.5 cm square electrodes. Tests in the upper thorax optimized electrode locations, and forelimb moment was limited to slight-to-moderate. During combined muscle stimulation tests, the upper thoracic was followed immediately by abdominal stimulation. Finally, a model of glottal closure for cough was conducted with the goal of increased peak expiratory flow.

RESULTS

Optimized stimulation of abdominal muscles included three sets of bilateral surface electrodes located 4.5 cm dorsal to the lateral line and from the 8^th intercostal space to caudal to the 13^th rib, 80 or 100 mA current, and 50 Hz stimulation frequency. The maximal expired volume was 343 ± 23 ml (n=3). Optimized upper thorax stimulation included a single bilateral set of electrodes located over the 2^nd interspace, 60 to 80 mA, and 50 Hz. The maximal inspired volume was 304 ± 54 ml (n=4). Sequential stimulation of the two muscles increased the volume to 600 ± 152 ml (n=2), and the glottal closure maneuver increased the flow.

CONCLUSIONS

Studies in an adult canine model identified optimal surface stimulation methods for upper thorax and abdominal muscles to induce sufficient volumes for ventilation and cough. Further study with this neuroprosthetic platform is warranted.

Electrical Neuromodulation of the Respiratory System After Spinal Cord Injury

Spinal cord injury (SCI) is a complex and devastating condition characterized by disruption of descending, ascending, and intrinsic spinal circuitry resulting in chronic neurologic deficits. In addition to limb and trunk sensorimotor deficits, SCI can impair autonomic neurocircuitry such as the motor networks that support respiration and cough. High cervical SCI can cause complete respiratory paralysis, and even lower cervical or thoracic lesions commonly result in partial respiratory impairment. Although electrophrenic respiration can restore ventilator-independent breathing in select candidates, only a small subset of affected individuals can benefit from this technology at this moment. Over the past decades, spinal cord stimulation has shown promise for augmentation and recovery of neurologic function including motor control, cough, and breathing. The present review discusses the challenges and potentials of spinal cord stimulation for restoring respiratory function by overcoming some of the limitations of conventional respiratory functional electrical stimulation systems.

Review of Epidural Spinal Cord Stimulation for Augmenting Cough after Spinal Cord Injury

Spinal cord injury (SCI) remains a debilitating condition for which there is no cure. In addition to loss of somatic sensorimotor functions, SCI is also commonly associated with impairment of autonomic function. Importantly, cough dysfunction due to paralysis of expiratory muscles in combination with respiratory insufficiency can render affected individuals vulnerable to respiratory morbidity. Failure to clear sputum can aggravate both risk for and severity of respiratory infections, accounting for frequent hospitalizations and even mortality. Recently, epidural stimulation of the lower thoracic spinal cord has been investigated as novel means for restoring cough by evoking expiratory muscle contraction to generate large positive airway pressures and expulsive air flow. This review article discusses available preclinical and clinical evidence, current challenges and clinical potential of lower thoracic spinal cord stimulation (SCS) for restoring cough in individuals with SCI.

Using a Sniff Controller to Self-Trigger Abdominal Functional Electrical Stimulation for Assisted Coughing Following Cervical Spinal Cord Lesions

Individuals with cervical spinal cord lesions (SCLs) typically depend on caregivers to manually assist in coughing by pressing against their abdominal wall. Coughing can also be assisted by functional electric stimulation (FES) applied to abdominal muscles via surface electrodes. Efficacy of FES, however, depends on precise temporal synchronization. The sniff controller is a trigger that enables paralyzed individuals to precisely control external devices through alterations in nasal airflow. We hypothesized that FES self-triggering by sniff controller may allow for effective cough timing. After optimizing parameters in 16 able-bodied subjects, we measured peak expiratory flow (PEF) in 14 subjects with SCL who coughed with or without assistance. Assistance was either manual assistance of a caregiver, caregiver activated FES, button self-activated FES (for SCL participants who could press a button), or sniff-controlled self-activated FES. We found that all assisted methods provided equally effective improvements, increasing PEF on average by 25 ± 27% (F[4,52] = 7.99, p = 0.00004 ). There was no difference in efficacy between methods of assistance ( F[3,39] = 0.41, p = 0.75 ). Notably, sniff-controlled FES was the only method of those tested that can be activated by all paralyzed patients alone. This provides for added independence that is a critical factor in quality of life following SCL.

Respiratory responses to stimulation of abdominal and upper-thorax intercostal muscles using multiple Permaloc electrodes

Stimulation of abdominal and upper-thoracic muscles was studied with the long-term goal of improved respiratory care for spinal cord injury (SCI) patients. A 12-channel stimulator and multiple surface and implanted Permaloc electrodes were evaluated in five anesthetized canines. Abdominal stimulation with 100 mA using four bilateral sets of surface electrodes placed on the midaxillary line at the 7th through 13th intercostal spaces and with a closed airway at a large lung volume produced an expiratory tracheal pressure of 109 +/- 29 cm H2O (n = 2, mean +/- standard error of the mean). Similar high pressures were induced with implanted electrodes at the same locations. Upper-thoracic stimulation with 40 mA and four sets of implanted electrodes ventral to the axilla induced inspiratory pressures of -12 +/- 2 cm H2O (n = 5). Combined extradiaphragmatic pacing with an open airway produced a tidal volume of 440 +/- 45 mL (n = 4). The robust respiratory volumes and pressures suggest applications in SCI respiratory care.

Electrical Stimulation of Abdominal Muscles to Produce Cough in Spinal Cord Injury

Background. Surface electrical stimulation of the abdominal muscles, with electrodes placed in the posterolateral position, combined with a voluntary cough can assist clearance of airway secretions in individuals with high-level spinal cord injury (SCI). Objective. To determine whether an increase in stimulus intensity of the trains of electrical stimuli delivered to the expiratory muscles has an increasing effect on a stimulated voluntary cough and to determine at which stimulus intensity a plateau of cough peak expiratory flow occurs. Methods. In 7 healthy individuals with a SCI at and above C7, gastric pressure ( Pga), esophageal pressure ( Pes), peak expiratory cough flow (PEFcough), and expiratory volume were measured as participants coughed voluntarily with simultaneous trains of electrical stimuli delivered over the abdominal muscles (50 Hz, 1-s duration). The intensity of the stimulation was increased incrementally. Results: A plateau in PEFcough occurred in all 7 individuals at a mean of 211 ± 29 mA (range 120-360 mA). Peak values reached for Pga, Pes, and PEFcough were 83.0 ± 8.0 cm H2O, 66.1 ± 5.6 cm H2O, and 4.0 ± 0.4 l/s respectively. Conclusions. The plateau in expiratory cough flow that was associated with increasing expiratory pressures is indicative of dynamic airway compression. This suggests that the evoked cough will be effective in creating more turbulent airflow to further assist in dislodging mucus and secretions.

Abdominal Muscle Training Can Enhance Cough After Spinal Cord Injury

Background. Respiratory complications in people with high-level spinal cord injury (SCI) are a major cause of morbidity and mortality, particularly because of a reduced ability to cough as a result of abdominal muscle paralysis. Objective. We investigated the effect of cough training combined with functional electrical stimulation (FES) over the abdominal muscles for 6 weeks to observe whether training could improve cough strength. Methods. Fifteen SCI subjects (C4-T5) trained for 6 weeks, 5 days per week (5 sets of 10 coughs per day) in a randomized crossover design study. Subjects coughed voluntarily at the same time as a train of electrical stimulation was delivered over the abdominal muscles via posterolaterally positioned electrodes (50 Hz, 3 seconds). Measurements were made of esophageal (Pes) and gastric (Pga) expiratory pressures and the peak expiratory flow (PEFcough) produced at the 3 time points of before, during, and after the training. Results. During voluntary coughs, FES cough stimulation improved Pga, Pes, and PEFcough acutely, 20-fold, 4-fold, and 50%, respectively. Six weeks of cough training significantly increased Pga (37.1 ± 2.0 to 46.5 ± 2.9 cm H2O), Pes (35.4 ± 2.7 to 48.1 ± 2.9 cm H2O), and PEFcough (3.1 ± 0.1 to 3.6 ± 0.1 L/s). Cough training also improved pressures and flow during voluntary unstimulated coughs. Conclusions. FES of abdominal muscles acutely increases mechanical output in coughing in high-level SCI subjects. Six weeks of cough training further increases gastric and esophageal cough pressures and expiratory cough flow during stimulated cough maneuvers.

Pulse energy as a reliable reference for twitch forces induced by transcutaneous neuromuscular electrical stimulation

Voltage-controlled neuromuscular electrical stimulation has been considered to be safer in noninvasive applications notwithstanding the fact that voltage-controlled devices purportedly generate forces less predictable than their current-controlled equivalents. This prompted us to evaluate relevant electrical parameters to determine whether forces induced by voltage-controlled stimuli were able to match to those induced by current-controlled ones, which tend to evoke forces that were more predictable. Force magnitudes corresponding to current- and voltage-controlled stimuli were aligned with respect to electric charge (equivalent to average current intensity) and electrical energy (equivalent to average power) of the same stimulation pulse to determine which provided a better coherence. Consistency of forces evaluated with energy was significantly (p < 0.001) better than that evaluated with electric charges, suggesting that electrically stimulated forces can be reliably predicted by monitoring the energy parameter of stimulation pulses. The above results appear to show that electrode-tissue impedance, a factor that makes charge and energy evaluations different, redefined the actual effects of current intensities in generating favorable results. Accordingly, novel schemes that track the energy (or average power) of a stimulation pulse may be used as a reliable benchmark to associate mechanical (force) and electrical (stimulation pulse) characteristics in transcutaneous applications of electrical stimulation.

Treatments to restore respiratory function after spinal cord injury and their implications for regeneration, plasticity and adaptation

Spinal cord injury (SCI) often leads to impaired breathing. In most cases, such severe respiratory complications lead to morbidity and death. However, in the last few years there has been extensive work examining ways to restore this vital function after experimental spinal cord injury. In addition to finding strategies to rescue breathing activity, many of these experiments have also yielded a great deal of information about the innate plasticity and capacity for adaptation in the respiratory system and its associated circuitry in the spinal cord. This review article will highlight experimental SCI resulting in compromised breathing, the various methods of restoring function after such injury, and some recent findings from our own laboratory. Additionally, it will discuss findings about motor and CNS respiratory plasticity and adaptation with potential clinical and translational implications.

Posterolateral Surface Electrical Stimulation of Abdominal Expiratory Muscles to Enhance Cough in Spinal Cord Injury

Background. Spinal cord injury (SCI) patients have respiratory complications because of abdominal muscle weakness and paralysis, which impair the ability to cough. Objective. This study aims to enhance cough in high-level SCI subjects (n = 11, SCI at or above T6) using surface electrical stimulation of the abdominal muscles via 2 pairs of posterolaterally placed electrodes. Methods. From total lung capacity, subjects performed maximum expiratory pressure (MEP) efforts against a closed airway and voluntary cough efforts. Both efforts were performed with and without superimposed trains of electrical stimulation (50 Hz, 1 second) at a submaximal intensity set to evoke a gastric pressure ( Pga) of 40 cm H2O at functional residual capacity. Results. In the MEP effort, stimulation increased the maximal Pga (from 21.4 ± 7.0 to 59.0 ± 5.7 cm H2O) and esophageal pressure ( Pes; 47.2 ± 11.7 to 65.6 ± 13.6 cm H2O). During the cough efforts, stimulation increased Pga (19.5 ± 6.0 to 57.9 ± 7.0 cm H2O) and Pes (31.2 ± 8.7 to 56.6 ± 10.5 cm H2O). The increased expiratory pressures during cough efforts with stimulation increased peak expiratory flow (PEF, by 36% ± 5%), mean expiratory flow (by 80% ± 8%), and expired lung volume (by 41% ± 16%). In every subject, superimposed electrical stimulation improved peak expiratory flow during cough efforts (by 0.99 ± 0.12 L/s; range, 0.41-1.80 L/s). Wearing an abdominal binder did not improve stimulated cough flows or pressures. Conclusions. The increases in Pga and PEF with electrical stimulation using the novel posterolateral electrode placement are 2 to 3 times greater than improvements reported in other studies. This suggests that posterolateral electrical stimulation of abdominal muscles is a simple noninvasive way to enhance cough in individuals with SCI.

Stimulating multiple respiratory muscles with intramuscular Permaloc electrodes

OBJECTIVE

To test the feasibility of implanting intramuscular electrodes (Permaloc, Synapse Biomedical Inc, Oberlin OH) with self-securing polypropylene anchors to stimulate upper-intercostal and abdominal muscles plus the diaphragm.

METHODS/RESULTS

In 6 anesthetized dogs, 12 Permaloc electrodes were implanted in the 3 respiratory muscles (4 in each muscle group). Tidal volume with diaphragmatic stimulation was 310 +/- 38 mL (mean +/- SE); with upper intercostal stimulation, it was 68 +/- 18 mL; and with combined diaphragm intercostal stimulation, it was 438 +/- 78 mL. By study design, stimulation in the upper intercostal muscles was limited to not more than slight/moderate contraction of the serratus and latissimus muscles overlying the ribs. Abdominal muscle stimulation produced exhaled volumes of 38 +/- 20 mL (this stimulation was limited by the maximal output of the stimulator of 25 milliamperes). Combined diaphragm intercostal stimulation followed by abdominal muscle stimulation increased exhaled volumes from 312 +/- 31 mL to 486 +/- 58 mL (P = 0.024).

CONCLUSIONS

Permaloc electrodes can be successfully implanted in upper intercostal and abdominal muscles in addition to the diaphragm. Combined diaphragm intercostal stimulation followed by abdominal muscle stimulation increased the exhaled volumes recorded with diaphragmatic stimulation alone.

Cough following low thoracic hemisection in the cat

A function of the abdominal expiratory muscles is the generation of cough, a critical respiratory defense mechanism that is often disrupted following spinal cord injury. We assessed the effects of a lateral T9/10 hemisection on cough production at 4, 13 and 21 weeks post-injury in cats receiving extensive locomotor training. The magnitudes of esophageal pressure as well as of bilateral rectus abdominis electromyogram activity during cough were not significantly different from pre-injury values at all time points evaluated. The results show that despite considerable interruption of the descending pre-motor drive from the brainstem to the expiratory motoneuron pools, the cough motor system shows a significant function by 4 weeks following incomplete thoracic injury.

Lower thoracic spinal cord stimulation to restore cough in patients with spinal cord injury: results of a National Institutes of Health-sponsored clinical trial. Part I: methodology and effectiveness of expiratory muscle activation

OBJECTIVE

Evaluation of the capacity of lower thoracic spinal cord stimulation (SCS) to activate the expiratory muscles and generate large airway pressures and high peak airflows characteristic of cough, in subjects with tetraplegia.

DESIGN

Clinical trial.

SETTING

Inpatient hospital setting for electrode insertion; outpatient setting for measurement of respiratory pressures; home setting for application of SCS.

PARTICIPANTS

Subjects (N=9; 8 men, 1 woman) with cervical spinal cord injury and weak cough.

INTERVENTIONS

A fully implantable electrical stimulation system was surgically placed in each subject. Partial hemilaminectomies were made to place single-disk electrodes in the epidural space at the T9, T11, and L1 spinal levels. A radiofrequency receiver was placed in a subcutaneous pocket over the anterior portion of the chest wall. Electrode wires were tunneled subcutaneously and connected to the receiver. Stimulation was applied by activating a small portable external stimulus controller box powered by a rechargeable battery to each electrode lead alone and in combination.

MAIN OUTCOME MEASURES

Peak airflow and airway pressure generation achieved with SCS.

RESULTS

Supramaximal SCS resulted in high peak airflow rates and large airway pressures during stimulation at each electrode lead. Maximum peak airflow rates and airway pressures were achieved with combined stimulation of any 2 leads. At total lung capacity, mean maximum peak airflow rates and airway pressure generation were 8.6+/-1.8 (mean +/- SE) L/s and 137+/-30 cmH2O (mean +/- SE), respectively.

CONCLUSIONS

Lower thoracic SCS results in near maximum activation of the expiratory muscles and the generation of high peak airflow rates and positive airway pressures in the range of those observed with maximum cough efforts in healthy persons.

Surface functional electrical stimulation of the abdominal muscles to enhance cough and assist tracheostomy decannulation after high-level spinal cord injury

OBJECTIVE

Evaluation of noninvasive stimulation modalities to augment cough and assist tracheostomy decannulation in high-level tetraplegia.

STUDY DESIGN

Single case study.

METHODS

A 65-year-old man with C4 ASIA C tetraplegia had delayed rehabilitation due to a tracheostomy and recurrent pneumonia primarily resulting from ineffective cough. Anterior surface electrical stimulation (SES) of the abdominal musculature was conducted to train an effective cough and enable decannulation. Training occurred daily for 4 weeks. The patient was tested 1 year later with posterolateral SES to determine the relative clinical effect of this delivery method.

RESULTS

At baseline, the addition of anterior SES increased maximal expiratory pressure (80%), maximal expiratory cough pressure (67%), and peak expiratory flow rate (11%). Three weeks after training began, the patient was decannulated following a program of SES and assisted and voluntary coughing. Upon testing 1 year later, SES with posterolaterally placed electrodes also produced an enhancement of voluntary cough attempts.

CONCLUSIONS

Noninvasive SES can potentially assist decannulation of tracheostomies.