Key points

Introduction

There are more than 11,000 new cases of spinal cord injury (SCI) each year. Approximately 50% of these patients have tetraplegia, and of those, approximately 4% require mechanical ventilation long term. The patient with tetraplegia faces significant challenges beyond the mobility and sensory impairments imposed by the injury. Chief among these issues is that of respiratory impairment. Patients with tetraplegia, depending on whether the injury is complete or incomplete, will have varying degrees of respiratory dysfunction based on the amount of residually intact innervation to the muscles of inspiration and expiration. The muscles of expiration include the abdominal musculature and intercostals, which are innervated by nerves coming from the thoracic cord, and are less likely to be functionally intact in a patient with a cervical injury. The chief muscle of inspiration is the diaphragm, which is innervated by motoneurons from the cervical spinal cord, and thus susceptible to dysfunction in tetraplegia.

The standard of care for patients who cannot adequately mobilize their diaphragm (in addition to a host of other comorbidities that could compromise respiratory function) is to perform a tracheostomy and mechanically ventilate the patient. This article outlines the negative implications of mechanical ventilation, and discusses the possibility and benefits of allowing the patient to breathe with the assistance of exogenous diaphragm pacing.

Introduction

There are more than 11,000 new cases of spinal cord injury (SCI) each year. Approximately 50% of these patients have tetraplegia, and of those, approximately 4% require mechanical ventilation long term. The patient with tetraplegia faces significant challenges beyond the mobility and sensory impairments imposed by the injury. Chief among these issues is that of respiratory impairment. Patients with tetraplegia, depending on whether the injury is complete or incomplete, will have varying degrees of respiratory dysfunction based on the amount of residually intact innervation to the muscles of inspiration and expiration. The muscles of expiration include the abdominal musculature and intercostals, which are innervated by nerves coming from the thoracic cord, and are less likely to be functionally intact in a patient with a cervical injury. The chief muscle of inspiration is the diaphragm, which is innervated by motoneurons from the cervical spinal cord, and thus susceptible to dysfunction in tetraplegia.

The standard of care for patients who cannot adequately mobilize their diaphragm (in addition to a host of other comorbidities that could compromise respiratory function) is to perform a tracheostomy and mechanically ventilate the patient. This article outlines the negative implications of mechanical ventilation, and discusses the possibility and benefits of allowing the patient to breathe with the assistance of exogenous diaphragm pacing.

Physiology of breathing

As previously mentioned, the primary muscle of inspiration is the diaphragm, a thin, dome-shaped sheet of skeletal muscle, with muscular tissue converging on a central tendon that forms the crest of the dome. The muscle fibers originate from various structures, including the lumbar vertebrae and abdominal wall posteriorly, the ribs laterally, and the xiphoid process and floating ribs anteriorly. The central tendon is closer to the anterior of the thorax and, thus, the posterior muscle fibers are longer, traveling a farther course to converge on the tendon. The diaphragm is pierced by 3 apertures, which allow passage of the vena cava, the esophagus, and the aorta.

During inspiration, the diaphragm contracts, which creates a negative-pressure vacuum, and draws air into the thoracic cavity through the respiratory system. The diaphragm contracts volitionally during the daytime hours and automatically during sleep, based on CO2 levels monitored in the brain’s respiratory centers. When the diaphragm relaxes, air is exhaled by the elastic recoil of the lung and the pleural cavity. In forced exhalation, such as a cough, the internal intercostal muscles and abdominal muscles work antagonistically to the diaphragm. Additionally, the diaphragm can be used in nonrespiratory capacities by suddenly increasing intra-abdominal pressure, as in the processes of vomiting, defecating, and urination.

The diaphragm is innervated by the phrenic motoneurons, which are supplied by cervical spinal nerves C3, C4, and C5. These spinal nerves combine peripherally to form the paired phrenic nerves, which progress caudally through the thorax and insert into the diaphragm. In the event of a cervical SCI, the interruption of respiratory bulbospinal pathways can lead to respiratory paresis or paralysis.

In higher-level cervical injuries, the spinal roots, which directly contribute to the phrenic nerves and innervate the diaphragm, are spared, but the roots from the respiratory centers in the medulla to the cord are still interrupted. These patients will definitely require exogenous ventilation. However, there are many disadvantages to mechanical ventilation, and these higher-level injuries may be candidates for diaphragm pacing.

Mechanical ventilation

In such cases of acute or chronic respiratory failure, the use of positive-pressure mechanical ventilation can serve as a life-sustaining measure. Some patients may tolerate less-invasive means of mechanical ventilation, but at least initially, many patients are managed via the traditional measure of positive-pressure ventilation via a tracheostomy. Up to 20% of newly injured patients with SCI may require mechanical ventilation initially, but many patients improve in the following weeks. Regardless, between 200 and 400 patients per year become dependent on lifelong ventilator support.

However, mechanical ventilation is susceptible to increased morbidity from pneumonia, as well as earlier mortality. When compared with an able-bodied 20-year-old, the life expectancy for a 20-year-old patient with SCI on long-term mechanical ventilation decreases markedly from 58.6 to only 17.1 years. According to the National Spinal Cord Injury 2002 Database, survival rates decreased from 84% in the nonventilated patient to only 33% in the ventilated population.

Unfortunately, mechanical ventilation presents several obstacles for the patient, especially in light of the compounded impact on independence and mobility beyond that imposed by the injury’s impact on motor function in the extremities. Additionally, mechanical ventilation imposes varying degrees of physical discomfort, and impairments in speech and olfaction. The cost and care burden of total mechanical ventilation can make living in the home environment impossible, thus shifting more of the care responsibilities to long-term facilities. Care of a ventilated patient may cost up to $200,000 per year, and lifetime costs are outlined in Table 1 . The care of the ventilated patient requires 24-hour supervision by a trained caregiver. The caregiver must feel comfortable in manipulating ventilator settings to optimize the respiratory function and adapt to periodic changes in oxygenation. Also, the caregiver must be able to provide adequate pulmonary toilet, whether it is via chest percussion to loosen secretions or provide frequent suctioning.

Phrenic nerve pacing

In patients with respiratory failure secondary to a cervical SCI, mechanical ventilation is a lifesaving intervention. Under certain circumstances, some patients may be able to wean to noninvasive ventilation or use noninvasive ventilation for short periods of time. More typically, patients are subjected to positive-pressure ventilation with a tracheostomy. However, the use of mechanical ventilation impairs speech and olfaction, and leads to atrophy of the diaphragm muscle fibers.

The advantages of avoiding ventilation include reduction in airway pressure, increased posterior lobe ventilation, and maintenance of negative chest pressures. Phrenic nerve pacing is a more realistic approximation of the patient’s native respiratory drive in that it proceeds via negative-pressure ventilation by contraction of the patient’s own muscle fibers as opposed to exogenously induced inflation. Speech quality is improved with the lessened noise, better enabling the voice to be heard. Olfaction is improved, which in turn improves the patient’s sense of well-being. Phrenic nerve pacing also has more cosmesis when compared with the extensive tubing, machinery, and noise associated with a mechanical ventilator. Shedding the tethering to this ancillary machinery also obviously liberates the patient for increased household and community mobility and hence may lead to increased reintegration into society.

Direct diaphragm pacing

Exogenous electrical stimulation of the phrenic nerve has been a viable treatment option, given appropriate physiologic circumstances, for more than 30 years. Until recently, direct electrical stimulation of the phrenic nerves was the standard procedure for this kind of intervention, but required a thoracotomy or neck surgery for electrode placement and the surgical manipulation posed a risk of damaging the nerves themselves. A less-invasive procedure allowing percutaneous placement of electrodes over strategically located areas of the diaphragm has emerged as a viable alternative with reduced risk. As far back as 1980, Mortimer demonstrated that the diaphragm could be directly stimulated at its motor points. The motor points are determined using laparoscopy via electrical stimulation on the abdominal surface of the diaphragm.

Selection criteria

The placement of a diaphragm pacemaker comes with very specific criteria for selecting potential surgical candidates. The grade and level of injury is of paramount importance. An injury at any level can theoretically tolerate mechanical ventilation, but patients with C3-level to C5-level injuries cannot be paced because of the Wallerian degeneration that affects the phrenic nerves and, thus, would not respond to pacing. In such patients, diaphragmatic pacing is not an option because the motor pools and phrenic nerves are not intact. One criterion for placement is that the lesion must typically be above the third cervical level. The cell bodies of the nerves that supply the phrenic nerve are C3 to C5 and must be intact for the diaphragm or phrenic nerve to be paced.

Patients who are candidates for phrenic nerve or diaphragmatic pacing are almost universally on mechanical ventilation at the time of that determination. The assessment of the patient’s ventilatory status and the viability of the phrenic nerves are the primary determinants of candidacy for pacing. A patient cannot sustain spontaneous breathing with a vital capacity less than 10 mL/kg and a maximum inspiratory pressure of less than 20 cm H20.

There also remain certain physiologic limitations when compared with normal physiologic breathing. First and most obviously, as the breathing mechanism is now being exogenously paced, there is no spontaneous control of breathing. Important to consider, as well, is that only the inspiratory aspect of the breathing mechanism is facilitated by pacing the diaphragm. The muscles of expiration are not stimulated, which means the patient’s cough mechanism will continue to be greatly impaired. This will leave the patient susceptible to having difficulty clearing secretions, and thus prone to atelectasis and pneumonia.