Evaluation of intercostal pacing to provide artificial ventilation in quadriplegics

Epidural electrical spinal cord stimulation of the thoracic segments (T2-T5) facilitates respiratory function in patients with complete spinal cord injury

INTRODUCTION

Patients with high cervical Spinal Cord Injury (SCI) usually require mechanical ventilation support. Phrenic Nerve Stimulation (PNS) both direct and indirect is the main alternative for these patients to wean off ventilator although PNS has several limitations and phrenic nerve could be also damaged after cervical spinal cord injury.

OBJECTIVE

In this study, we assessed if the spinal cord Epidural Electrical Stimulation (EES) at the segments T2-T5, related to intercostal muscles, can facilitate respiratory function and particularly inspired tidal volume during mechanic ventilation.

METHODS

Two patients with a high cervical injury were selected for this study with ethical committee permission and under review board supervision. A phrenic nerve conduction study with diaphragm electromyography (DEMG) was performed before and after trial of EES.

RESULTS

Results demonstrate that EES at T2-T5 substantially increase the inspired volume. The results of this study also demonstrate that EES at spinal segments T2-T5 can bring patients dependent from mechanical ventilation to pressure support (on CPAP), preventing Baro-trauma and other complications related to mechanical ventilation.

CONCLUSION

These findings suggest that tested approach applied alone or in combination with the phrenic nerve stimulation could help to reduce time on mechanical ventilation and related complications.

Pharmacological disinhibition enhances paced breathing following complete spinal cord injury in rats

Respiratory dysfunction is one of the most devastating and life-threatening deficits that occurs following cervical spinal cord injury (SCI). Assisted breathing with mechanical ventilators is a necessary part of care for many cervical injured individuals, but it is also associated with increased risk of secondary complications such as infection, muscle atrophy and maladaptive plasticity. Pre-clinical studies with epidural stimulation (EDS) have identified it as an alternative/additional method to support adequate lung ventilation without mechanical assistance. The full potential of EDS, however, may be limited by spinal inhibitory mechanisms within the injured spinal cord. The goal of the present work is to assess the potential improvement for EDS in combination with pharmacological disinhibition of spinal circuits following complete high cervical SCI. All experiments were performed in decerebrate, unanesthetized, non-paralyzed (n = 13) and paralyzed (n = 8) adult Sprague-Dawley rats 6 h following a complete C1 transection. The combination of high-frequency EDS (HF-EDS) at the C4 spinal segment with intrathecal delivery of GABA and glycine receptors antagonists (GABAzine and strychnine, respectively) resulted in significantly increased phrenic motor output, tidal volume and amplitude of diaphragm electrical activity compared to HF-EDS alone. Thus, it appears that spinal fast inhibitory mechanisms limit phrenic motor output and present a new neuropharmacological target to improve paced breathing in individuals with cervical SCI.

Mechanisms of compensatory plasticity for respiratory motor neuron death

Respiratory motor neuron death arises from multiple neurodegenerative and traumatic neuromuscular disorders. Despite motor neuron death, compensatory mechanisms minimize its functional impact by harnessing intrinsic mechanisms of compensatory respiratory plasticity. However, the capacity for compensation eventually reaches limits and pathology ensues. Initially, challenges to the system such as increased metabolic demand reveal sub-clinical pathology. With greater motor neuron loss, the eventual result is de-compensation, ventilatory failure, ventilator dependence and then death. In this brief review, we discuss recent advances in our understanding of mechanisms giving rise to compensatory respiratory plasticity in response to respiratory motor neuron death including: 1) increased central respiratory drive, 2) plasticity in synapses on spared phrenic motor neurons, 3) enhanced neuromuscular transmission and 4) shifts in respiratory muscle utilization from more affected to less affected motor pools. Some of these compensatory mechanisms may prolong breathing function, but hasten the demise of surviving motor neurons. Improved understanding of these mechanisms and their impact on survival of spared motor neurons will guide future efforts to develop therapeutic interventions that preserve respiratory function with neuromuscular injury/disease.

Paced breathing and phrenic nerve responses evoked by epidural stimulation following complete high cervical spinal cord injury in rats

Spinal cord injury (SCI) at the level of cervical segments often results in life-threatening respiratory complications and requires long-term mechanical ventilator assistance. Thus restoring diaphragm activity and regaining voluntary control of breathing are the primary clinical goals for patients with respiratory dysfunction following cervical SCI. Epidural stimulation (EDS) is a promising strategy that has been explored extensively for nonrespiratory functions and to a limited extent within the respiratory system. The goal of the present study is to assess the potential for EDS at the location of the phrenic nucleus (C₃-C₅) innervating the diaphragm: the main inspiratory muscle following complete C₁ cervical transection. To avoid the suppressive effect of anesthesia, all experiments were performed in decerebrate, C₁ cervical transection, unanesthetized, nonparalyzed ( n = 13) and paralyzed ( n = 7) animals. Our results show that C₄ segment was the most responsive to EDS and required the lowest threshold of current intensity, affecting tracheal pressure and phrenic nerve responses. High-frequency (200-300 Hz) EDS applied over C₄ segment (C₄-EDS) was able to maintain breathing with normal end-tidal CO₂ level and raise blood pressure. In addition, 100-300 Hz of C₄-EDS showed time- and frequency-dependent changes (short-term facilitation) of evoked phrenic nerve responses that may serve as a target mechanism for pacing of phrenic motor circuits. The present work provides the first report of successful EDS at the level of phrenic nucleus in a complete SCI animal model and offers insight into the potential therapeutic application in patients with high cervical SCI. NEW & NOTEWORTHY The present work offers the first demonstration of successful life-supporting breathing paced by epidural stimulation (EDS) at the level of the phrenic nucleus, following a complete spinal cord injury in unanesthetized, decerebrate rats. Moreover, our experiments showed time- and frequency-dependent changes of evoked phrenic nerve activity during EDS that may serve as a target mechanism for pacing spinal phrenic motor networks.

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.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

Functional Magnetic Stimulation of Inspiratory and Expiratory Muscles in Subjects With Tetraplegia

BACKGROUND

Respiratory complications are major causes of morbidity and mortality in persons with a spinal cord injury, partly because of respiratory muscle paralysis. Earlier investigation has demonstrated that functional magnetic stimulation (FMS) can be used as a noninvasive technology for activating expiratory muscles, thus producing useful expiratory functions (simulated cough) in subjects with spinal cord injury.

OBJECTIVE

To evaluate the effectiveness of FMS for conditioning inspiratory and expiratory muscles in persons with tetraplegia.

DESIGN

A prospective before and after trial.

SETTING

FMS Laboratory, Louis Stokes Cleveland VA Medical Center, Cleveland, OH.

PARTICIPANTS

Six persons with tetraplegia.

METHOD

Each subject participated in a 6-week FMS protocol for conditioning the inspiratory and expiratory muscles. A magnetic stimulator was used with the center of a magnetic coil placed at the C7-T1 and T9-T10 spinous processes, respectively. Pulmonary function tests were performed before, during, and after the protocol.

MAIN OUTCOME MEASUREMENTS

Respiratory variables included maximal inspiratory pressure (MIP), inspiratory reserve volume (IRV), peak inspiratory flow (PIF), maximal expiratory pressure (MEP), expiratory reserve volume (ERV), and peak expiratory flow (PEF).

RESULTS

After 6 weeks of conditioning, the main outcome measurements (mean ± standard error) were as follows: MIP, 89.6 ± 7.3 cm H2O; IRV, 1.90 ± 0.34 L; PIF, 302.4 ± 36.3 L/min; MEP, 67.4 ± 11.1 cm H2O; ERV, 0.40 ± 0.06 L; and PEF, 372.4 ± 31.9 L/min. These values corresponded to 117%, 107%, 136%, 109%, 130%, and 124% of pre-FMS conditioning values, respectively. Significant improvements were observed in MIP (P = .022), PIF (P = .0001), and PEF (P = .0006), respectively. When FMS was discontinued for 4 weeks, these values showed decreases from their values at the end of the conditioning protocol, which suggests that continual FMS may be necessary to maintain improved respiratory functions.

CONCLUSION

FMS conditioning of the inspiratory and expiratory muscles improved voluntary inspiratory and expiratory functions. FMS may be a noninvasive technology for respiratory muscle training in persons with tetraplegia.

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.

Activation of inspiratory muscles via spinal cord stimulation

Diaphragm pacing is a clinically useful modality providing artificial ventilatory support in patients with ventilator dependent spinal cord injury. Since this technique is successful in providing full-time ventilatory support in only ~50% of patients, better methods are needed. In this paper, we review a novel method of inspiratory muscle activation involving the application of electrical stimulation applied to the ventral surface of the upper thoracic spinal cord at high stimulus frequencies (300 Hz). In an animal model, high frequency spinal cord stimulation (HF-SCS) results in synchronous activation of both the diaphragm and inspiratory intercostal muscles. Since this method results in an asynchronous pattern of EMG activity and mean peak firing frequencies similar to those observed during spontaneous breathing, HF-SCS is a more physiologic form of inspiratory muscle activation. Further, ventilation can be maintained on a long-term basis with repetitive stimulation at low stimulus amplitudes (<1 mA). These preliminary results suggest that HF-SCS holds promise as a more successful method of inspiratory muscle pacing.

Diaphragm activation via high frequency spinal cord stimulation in a rodent model of spinal cord injury

As demonstrated in a canine model, high frequency spinal cord stimulation (HF-SCS) is a novel and more physiologic method of electrical activation of the inspiratory muscles compared to current techniques. The dog model, however, has significant limitations due to cost and societal concerns. Since the rodent respiratory system is also a relevant model for the study of neuronal circuitry function, the aims of the present study were to a) assess the effects of HF-SCS and b) determine the methodology of application of this technique in rats. In 9 Sprague Dawley rats, diaphragm multiunit and single motor unit EMG activity were assessed during spontaneous breathing and HF-SCS applied on the ventral epidural surface of the spinal cord at the T2 level following C1 spinal section. As in dogs, HF-SCS results in the activation of the diaphragm at physiological firing frequencies and the generation of large inspired volumes. Mean maximum firing frequencies of the diaphragm during spontaneous breathing and HF-SCS were 23.3 ± 1.4 Hz (range: 9.8-51.6 Hz) and 26.6 ± 1.3 Hz; range: 12.0-72.9 Hz, respectively, at comparable inspired volumes. Moreover, HF-SCS was successful in pacing these animals over a 60-min period without evidence of system fatigue. Our results suggest that, similar to the dog model, HF-SCS in the rat results in the activation of spinal cord tracts which synapse with the phrenic motoneuron pool, allowing the processing of the stimulus and consequent physiologic activation of the inspiratory muscles. The rat may be a useful model for further studies evaluating phrenic motoneuron physiology.

Distribution of electrical activation to the external intercostal muscles during high frequency spinal cord stimulation in dogs

In contrast to previous methods of electrical stimulation of the inspiratory muscles, high frequency spinal cord stimulation (HF-SCS) results in more physiological activation of these muscles. The spatial distribution of activation to the external intercostal muscles by this method is unknown. In anaesthetized dogs, multiunit and single motor unit (SMU) EMG activity was monitored in the dorsal portion of the 3rd, 5th and 7th interspaces and ventral portion of the 3rd interspace during spontaneous breathing and HF-SCS following C2 spinal section. Stimulus amplitude during HF-SCS was adjusted such that inspired volumes matched spontaneous breathing (Protocol 1). During HF-SCS, mean peak SMU firing frequency was highest in the 3rd interspace (dorsal) (18.8 ± 0.3 Hz) and significantly lower in the 3rd interspace (ventral) (12.2 ± 0.2 Hz) and 5th interspace (dorsal) (15.3 ± 0.3 Hz) (P <0.05 for each comparison). Similar rostrocaudal and dorsoventral gradients of activity were observed during spontaneous breathing prior to C2 section. No significant activity was observed in the 7th interspace during either spontaneous breathing or HF-SCS. Since peak discharge frequencies of the SMUs were higher and rib cage movement greater during HF-SCS compared to spontaneous breathing, stimulus amplitude during HF-SCS was adjusted such that rib cage movement matched (Protocol 2). Under these conditions, mean peak SMU frequencies and rostrocaudal and dorsoventral gradients of activity during HF-SCS were not significantly different compared to spontaneous breathing. These results indicate that (a) the topographic pattern of electrical activation of the external intercostal muscles during HF-SCS is similar to that occurring during spontaneous breathing and (b) differential descending synaptic input from supraspinal centres is not a required component of the differential spatial distribution of external intercostal muscle activation. HF-SCS may provide a more physiological method of inspiratory muscle pacing.

Physiotherapy secretion removal techniques in people with spinal cord injury: a systematic review

OBJECTIVE

To address whether secretion removal techniques increase airway clearance in people with chronic spinal cord injury (SCI).

DATA SOURCES AND STUDY SELECTION

MEDLINE/PubMed, CINAHL, EMBASE, and PsycINFO were searched from inception to May 2009 for population keywords (spinal cord injury, paraplegia, tetraplegia, quadriplegia) paired with secretion removal-related interventions and outcomes. Inclusion criteria for articles were a research study, irrespective of design, that examined secretion removal in people with chronic SCI published in English.

REVIEW METHODS

Two reviewers determined whether articles met the inclusion criteria, abstracted information, and performed a quality assessment using PEDro or Downs and Black criteria. Studies were then given a level of evidence based on a modified Sackett scale.

RESULTS

Of 2416 abstracts and titles retrieved, 24 met the inclusion criteria. Subjects were young (mean, 31 years) and 84% were male. Most evidence was level 4 or 5 and only 2 studies were randomized controlled trials. Three reports described outcomes for secretion removal techniques in addition to cough, whereas most articles examined the immediate effects of various components of cough. Studies examining insufflation combined with manual assisted cough provided the most consistent, high-level evidence. Compelling recent evidence supports the use of respiratory muscle training or electrical stimulation of the expiratory muscles to facilitate airway clearance in people with SCI.

CONCLUSION

Evidence supporting the use of secretion removal techniques in SCI, while positive, is limited and mostly of low level. Treatments that increase respiratory muscle force show promise as effective airway clearance techniques.

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.

Intercostal muscle pacing with high frequency spinal cord stimulation in dogs

High frequency spinal cord stimulation (HF-SCS) is a novel and more physiologic method of inspiratory muscle activation which involves stimulation of spinal cord pathways. In the present study, we determined if activation of the inspiratory intercostal muscles alone by this technique could be utilized to maintain artificial ventilation. In 7 anesthetized dogs, following C2 spinal cord section and bilateral phrenicotomy, trains of electrical stimulation (12 times/min) were applied at the T2 level. Eucapnea was maintained during an initial 5.5h period of continuous stimulation. During a subsequent 0.5h period, stimulus parameters were increased to induce hyperventilation resulting in a sustained fall in end-tidal P(CO(2)) to 29.3 + or - 0.4 mmHg. Single motor unit peak firing frequencies of the intercostal muscles during HF-SCS were similar to those occurring during spontaneous breathing. This technique holds promise as a method to restore ventilation in ventilator-dependent tetraplegics who do not have adequate phrenic nerve function for diaphragm pacing.

High-frequency spinal cord stimulation of inspiratory muscles in dogs: a new method of inspiratory muscle pacing

Despite clinically available methods of diaphragm pacing, most patients with ventilator-dependent tetraplegia are still dependent on mechanical ventilation. Given the significant disadvantages of these devices, additional pacing options are needed. The objective of this study was to evaluate a novel and potentially more physiological method of inspiratory muscle activation, which involves the application of high-frequency (>200 Hz) stimulation to the ventral surface of the spinal cord in the high thoracic region. Studies were performed in 13 anesthetized dogs. High-frequency spinal cord stimulation (HF-SCS) results in the activation of both the diaphragm and inspiratory intercostal muscles, in concert, at physiological firing frequencies and the generation of large inspired volumes. Mean maximum firing frequencies of motor units in the parasternal (2nd interspace), the external intercostal (3rd interspace), and the diaphragm muscles were 10.6 +/- 0.4, 11.7 +/- 0.4, and 10.4 +/- 0.3 Hz, respectively. These values were not significantly different from those occurring during spontaneous breathing at comparable inspired volumes. Maximum inspired volume was 0.93 +/- 0.01 liter, which approximates the inspiratory capacity of these animals. Moreover, ventilation can be maintained on a chronic basis by this method (6 h) without evidence of system fatigue. Our results suggest that HF-SCS results in activation of spinal cord tracts that synapse with the inspiratory motoneuron pools, allowing processing of the stimulus and consequent physiological activation of the inspiratory muscles. HF-SCS has the potential to provide an effective method of inspiratory muscle pacing.

Inspiratory muscle pacing in spinal cord injury: case report and clinical commentary

BACKGROUND/OBJECTIVE

A significant fraction of patients with cervical spinal cord injury suffer from respiratory muscle paralysis and dependence on chronic mechanical ventilation. In selected patients, diaphragm pacing (DP) through electrical stimulation of the phrenic nerves provides an alternative to mechanical ventilation with significant advantages in life quality.

METHODS

A case report of an individual who successfully underwent DP using intramuscular diaphragm electrodes. A brief review of the state of the art of DP including the clinical benefits of DP, patient selection and evaluation, description of equipment, methods of transition from mechanical ventilation to DP, potential complications and side effects, long-term outcome, and potential future developments in this field is included.

RESULTS

Several available DP systems are available, including conventional ones in which electrodes are positioned directly on the phrenic nerves through thoracotomy and less invasive systems in which electrodes are placed within the diaphragm through laparoscopy. For patients with only unilateral phrenic nerve function, a combined intercostal and unilateral diaphragm pacing system is under development.

CONCLUSIONS

In patients with ventilator-dependent tetraplegia, there are alternative methods of ventilatory support, which offer substantial benefits compared to mechanical ventilation.

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3 +/- 4.5 cm H2O (Mean +/- SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0 +/- 3.9 cm H2O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p < 0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0 +/- 9.4 L/ min) and 77.5% of the volume (626 +/- 556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.

Functional Electrical Stimulation for Neuromuscular Applications

Paralyzed or paretic muscles can be made to contract by applying electrical currents to the intact peripheral motor nerves innervating them. When electrically elicited muscle contractions are coordinated in a manner that provides function, the technique is termed functional electrical stimulation (FES). In more than 40 years of FES research, principles for safe stimulation of neuromuscular tissue have been established, and methods for modulating the strength of electrically induced muscle contractions have been discovered. FES systems have been developed for restoring function in the upper extremity, lower extremity, bladder and bowel, and respiratory system. Some of these neuroprostheses have become commercialized products, and others are available in clinical research settings. Technological developments are expected to produce new systems that have no external components, are expandable to multiple applications, are upgradable to new advances, and are controlled by a combination of signals, including biopotential signals from nerve, muscle, and the brain.

Combined intercostal and diaphragm pacing to provide artificial ventilation in patients with tetraplegia

OBJECTIVE

To evaluate the usefulness of combined intercostal and diaphragm pacing to maintain independence from mechanical ventilation.

DESIGN

A prospective trial.

SETTING

Clinical research center at a large tertiary hospital.

PARTICIPANTS

Four ventilator-dependent subjects with spinal cord injury with only unilateral phrenic nerve function.

INTERVENTION

During an initial surgical procedure, a multipolar epidural disk electrode was positioned on the ventral surface of the upper-thoracic spinal cord via a hemilaminectomy to activate the inspiratory intercostal muscles. A phrenic nerve electrode was implanted unilaterally via the thoracic approach.

MAIN OUTCOME MEASURES

Inspired volume production and duration that subjects could be comfortably maintained when off mechanical ventilatory support.

RESULTS

Initial maximum inspired volumes from combined intercostal and diaphragm stimulation ranged between .23 and .93L and significantly increased over the course of reconditioning period to between 0.55 and 1.31L; subjects could be maintained off mechanical ventilation between 16 and 24 hours a day.

CONCLUSIONS

Combined intercostal and unilateral diaphragm pacing may be a useful therapeutic modality capable of maintaining long-term ventilatory support in patients with only unilateral phrenic nerve function.

Restoration of respiratory muscle function following spinal cord injury

Respiratory complications are a leading cause of morbidity and mortality in patients with spinal cord injury. Several techniques, currently available or in development, have the capacity to restore respiratory muscle function allowing these patients to live more normal lives and hopefully reduce the incidence of respiratory complications. Bilateral phrenic nerve pacing, a clinically accepted technique to restore inspiratory muscle function, allows patients with ventilator dependent tetraplegia complete freedom from mechanical ventilation. Compared to mechanical ventilation, phrenic nerve pacing provides patients with increased mobility, improved speech, improved comfort level and reduction in health care costs. The results of clinical trials of laparoscopically placed intramuscular diaphragm electrodes suggest that diaphragm pacing can also be achieved without the need for a thoracotomy and associated long hospital stay, and without manipulation of the phrenic nerve which carries a risk of phrenic nerve injury. Other clinical trials are being performed to restore inspiratory intercostal function. In patients with only unilateral phrenic nerve function who are not candidates for phrenic nerve pacing, combined intercostal and unilateral diaphragm pacing appears to provide benefits similar to that of bilateral diaphragm pacing. Clinical trials are also underway to restore expiratory muscle function. Magnetic stimulation, surface stimulation and spinal cord stimulation of the expiratory muscles are promising techniques to restore an effective cough mechanism in this patient population. These techniques hold promise to reduce the incidence of respiratory tract infections, atelectasis and respiratory failure in patients with spinal cord injury and reduce the morbidity and mortality associated with these complications.

Functional magnetic ventilation in dogs

OBJECTIVE

To investigate the efficacy of the magnetic stimulation of inspiratory muscles as an alternative to mechanical ventilation and functional electric stimulation.

DESIGN

A prospective before-after trial.

SETTING

Functional magnetic stimulation laboratory in a Veterans Administration health care system.

ANIMALS

Six male mongrel dogs, each weighing between 25 and 35 kg.

INTERVENTIONS

Commercially available magnetic stimulators with a round magnetic coil were used. The center of the magnetic coil was placed posteriorly over the C5-7 vertebrae of the spinal cord transected dogs. Magnetic stimulation parameters were set at 80% intensity, 20 Hz, and a 1.2-second on and 3.8-second off pulse train.

MAIN OUTCOME MEASURES

The major outcomes were changes in tidal volume (VT), tracheal pressure (Ptr), and arterial partial pressure of carbon dioxide (PaCO2) and oxygen sustained by magnetic stimulation over time.

RESULTS

The average Vt and Ptr produced during functional magnetic ventilation (FMV) were.47+/-.07 L and -4.7+/-.51 cmH2O, respectively. Blood gas data showed that PaCO2 increased from a baseline of 33 to 75 mmHg, whereas pH decreased from 7.33 to 6.99 at the end of the 1-hour FMV period.

CONCLUSIONS

FMV was achieved for 2 hours in dogs with C2 spinal cord transection. Additional refinements in magnetic stimulation are needed to improve ventilation in animals.

Gas exchange during separate diaphragm and intercostal muscle breathing

In patients with diaphragm paralysis, ventilation to the basal lung zones is reduced, whereas in patients with paralysis of the rib cage muscles, ventilation to the upper lung zones in reduced. Inspiration produced by either rib cage muscle or diaphragm contraction alone, therefore, may result in mismatching of ventilation and perfusion and in gas-exchange impairment. To test this hypothesis, we assessed gas exchange in 11 anesthetized dogs during ventilation produced by either diaphragm or intercostal muscle contraction alone. Diaphragm activation was achieved by phrenic nerve stimulation. Intercostal muscle activation was accomplished by electrical stimulation by using electrodes positioned epidurally at the T2 spinal cord level. Stimulation parameters were adjusted to provide a constant tidal volume and inspiratory flow rate. During diaphragm (D) and intercostal muscle breathing (IC), mean arterial Po2 was 97.1 ± 2.1 and 88.1 ± 2.7 Torr, respectively ( P < 0.01). Arterial Pco2 was lower during D than during IC (32.6 ± 1.4 and 36.6 ± 1.8 Torr, respectively; P < 0.05). During IC, oxygen consumption was also higher than that during D (0.13 ± 0.01 and 0.09 ± 0.01 l/min, respectively; P < 0.05). The alveolar-arterial oxygen difference was 11.3 ± 1.9 and 7.7 ± 1.0 Torr ( P < 0.01) during IC and D, respectively. These results indicate that diaphragm breathing is significantly more efficient than intercostal muscle breathing. However, despite marked differences in the pattern of inspiratory muscle contraction, the distribution of ventilation remains well matched to pulmonary perfusion resulting in preservation of normal gas exchange.

Mechanism of expiratory muscle activation during lower thoracic spinal cord stimulation

Lower thoracic spinal cord stimulation (SCS) may be a useful method to restore an effective cough mechanism. In dogs, two groups of studies were performed to evaluate the mechanism of the expiratory muscle activation during stimulation at the T(9)-T(10) level, which results in the greatest changes in airway pressure. In one group, expiratory muscle activation was monitored by evoked muscle compound action potentials (CAPs) from the internal intercostal muscles in the 10th, 11th, and 12th interspaces and from portions of the external oblique innervated by the L(1) and L(2) motor roots. SCS, applied with single shocks, resulted in short-latency CAPs at T(10) but not at more caudal levels. SCS resulted in long-latency CAPs at each of the more caudal caudal recording sites. Bilateral dorsal column sectioning, just below the T(11) spinal cord level, did not affect the short-latency CAPs but abolished the long-latency CAPs and also resulted in a fall in airway pressure generation. In the second group, sequential spinal root sectioning was performed to assess their individual mechanical contribution to pressure generation. Section of the ventral roots from T(8) through T(10) resulted in negligible changes, whereas section of more caudal roots resulted in a progressive reduction in pressure generation. We conclude that 1) SCS at the T(9)-T(10) level results in direct activation of spinal cord roots within two to three segments of the stimulating electrode and activation of more distal roots via spinal cord pathways, and 2) pathway activation of motor roots makes a substantial contribution to pressure generation.

The intercostal to phrenic nerve transfer: an effective means of reanimating the diaphragm in patients with high cervical spine injury

Nerve transfers have been well described for the treatment of congenital and traumatic injuries in the brachial plexus and extremities. This series is the first to describe nerve transfers to reanimate the diaphragm in patients confined to long-term positive pressure ventilation because of high cervical spine injury. Patients who have sustained injury to the spinal cord at the C3 to C5 level suffer axonal loss in the phrenic nerve. They can neither propagate a nerve stimulus nor respond to implanted diaphragmatic pacing devices (electrophrenic respiration). Ten nerve transfers were performed in six patients who met these conditions. The procedures used end-to-end anastomoses from the fourth intercostal to the phrenic nerve approximately 5 cm above the diaphragm. A phrenic nerve pacemaker was implanted as part of the procedure and was placed distal to the anastomosis. Each week, the pacemaker was activated to test for diaphragmatic response. Once diaphragm movement was documented, diaphragmatic pacing was instituted. Eight of the 10 transfers have had more than 3 months to allow for axonal regeneration. Of these, all eight achieved successful diaphragmatic pacing (100 percent). The average interval from surgery to diaphragm response to electrical stimulation was 9 months. All patients were able to tolerate diaphragmatic pacing as an alternative to positive pressure ventilation, as judged by end tidal CO2 values, tidal volumes, and patient comfort. Intercostal to phrenic nerve transfer with diaphragmatic pacing is a viable means of liberating patients with high cervical spine injury from long-term mechanical ventilation.

Pattern of expiratory muscle activation during lower thoracic spinal cord stimulation

Large positive airway pressures (Paws) can be generated by lower thoracic spinal cord stimulation (SCS), which may be a useful method of restoring cough in spinal cord-injured patients. Optimal electrode placement, however, requires an assessment of the pattern of current spread during SCS. Studies were performed in anesthetized dogs to assess the pattern of expiratory muscle recruitment during SCS applied at different spinal cord levels. A multicontact stimulating electrode was positioned over the surface of the lower thoracic and upper lumbar spinal cord. Recording electromyographic electrodes were placed at several locations in the abdominal and internal intercostal muscles. SCS was applied at each lead, in separate trials, with single shocks of 0.2-ms duration. The intensity of stimulation was adjusted to determine the threshold for development of the compound action potential at each electrode lead. The values of current threshold for activation of each muscle formed parabolas with minimum values at specific spinal root levels. The slopes of the parabolas were relatively steep, indicating that the threshold for muscle activation increases rapidly at more cephalad and caudal sites. These results were compared with the effectiveness of SCS (50 Hz; train duration, 1-2 s) at different spinal cord levels to produce changes in Paw. Stimulation at the T9 and T10 spinal cord level resulted in the largest positive Paws with a single lead. At these sites, threshold values for activation of the internal intercostal (7-11th interspaces) upper portions of external oblique, rectus abdominis, and transversus abdominis were near their minimum. Threshold values for activation of the caudal portions of the abdominal muscles were high (>50 mA). Our results indicate that 1) activation of the more cephalad portions of the abdominal muscles is more important than activation of caudal regions in the generation of positive Paws and 2) it is not possible to achieve complete activation of the expiratory muscles with a single electrode lead by using modest current levels. In support of this latter conclusion, a two-electrode lead system results in more uniform expiratory muscle activation and significantly greater changes in Paw.

Functional magnetic stimulation of the respiratory muscles in dogs

This study assessed the ability of functional magnetic stimulation (FMS) to activate the respiratory muscles in dogs. With the animal supine, FMS of the phrenic nerves using a high-speed magnetic stimulator was performed by placing a round magnetic coil (MC) at the carotid triangle. Following hyperventilation-induced apnea, changes in volume (deltaV) and airway pressure (deltaP) against an occluded airway were determined. FMS of the phrenic nerves produced substantial inspired function (deltaV = 373 +/- 20.5 mL and deltaP = -20 +/- 2.0 cm H2O). After bilateral phrenectomies, maximal inspired deltaV (219 +/- 12.2 mL) and deltaP (-10 +/- 1.0 cm H2O) were produced when the MC was placed near the C6-C7 spinous processes, while maximal expired deltaV (-199 +/- 22.5 mL) and deltaP (11 +/- 2.3 cm H2O) were produced following stimulation near the T9-T10 spinous processes. We conclude: (1) FMS of either the phrenic or upper intercostal nerves results in inspired volume production; (2) FMS of the lower intercostal nerves generates expired volume production; and (3) FMS of the respiratory muscles may be a useful noninvasive tool for artificial ventilation and assisted cough in patients with spinal cord injuries or other neurological disorders.

Efficacy of combined inspiratory intercostal and expiratory muscle pacing to maintain artificial ventilation

Many patients with ventilator-dependent quadriplegia have coincident phrenic nerve injury and therefore cannot be offered phrenic nerve pacing. The purpose of this study was to assess the utility of combined inspiratory intercostal and expiratory muscle pacing to provide complete ventilatory support. Studies were performed in 15 anesthetized dogs. An electrode was positioned on the epidural surface of the upper thoracic spinal cord to activate the inspiratory intercostal muscles; a separate electrode was positioned on the epidural surface of the lower thoracic spinal cord to activate the expiratory muscles. In an attempt to replicate the effects of inspiratory intercostal pacing alone in humans, stimulus parameters during upper thoracic spinal cord stimulation were adjusted to provide suboptimal levels of ventilation (end-tidal PCO2 of 55 to 60 mm Hg). Expiratory muscle activation was triggered electrically by the inspiratory signal with a 4.2-s delay resulting in alternate inspiratory and expiratory muscle pacing at a combined rate of 14 breaths/min. Combined pacing was maintained for an arbitrary period of 3 h. Initial intercostal muscle pacing alone resulted in an end-tidal PCO2 of 57.1 +/- 1.1 mm Hg. After the addition of expiratory muscle pacing, end-tidal PCO2 fell to 36.3 +/- 1.2 mm Hg. Tidal volume during both inspiratory and expiratory muscle pacing and end-tidal PCO2 remained stable throughout the study period. Our results suggest that combined alternate inspiratory and expiratory muscle pacing may be a viable alternative method of artificial ventilation in ventilator-dependent quadriplegic patients.