Diaphragm dysfunction an underdiagnosed condition and causes unexplained dyspnea. The dysfunction can range from partial weakness to complete paralysis of either one hemidiaphragm or both hemidiaphragm. Spinal cord injuries (SCI) and critical care polyneuropathies encompass a big chunk of the cases of diaphragmatic dysfunction. According to the National Spinal Cord Injury Statistics Center, the incidence of traumatic SCI in the United States was approximately 17,000 in 2016.
Diaphragmatic paralysis usually results from a high spinal cord injury, whereas mid-cervical lesions lead to partial weakness. SCI leads to chronic impairment and disability. Half of these patients develop tetraplegia, with 4% of these patients requiring long term mechanical ventilation. Critical illness polyneuropathy (CIP) is a common complication of critical illness affecting the motor and sensory neurons. Muscle involvement causing loss of muscle mass and eventual weakness has been referred to as critical illness myopathy. The involvement of the phrenic nerve and diaphragm leads to its weakness and, at times, complete paralysis. These patients have worse outcomes with prolonged weaning, higher hospital length of stays, and dependency on mechanical ventilation.
Traditional approaches to management have been mainly focused on waiting for recovery through innervation while supporting the patient on mechanical ventilation. However, this is fraught with complications. In the past few years, diaphragm pacing (DP) has been a proven therapy to wean SCI patients from mechanical ventilation (MV). A small feasibility study evaluating DP in critically ill mechanically ventilated patients demonstrated that the diaphragm could significantly help with the work of breathing when activated by a catheter-based, transvenous DP. A large multi-center randomized clinical trial (RESCUE 2) is underway to compare temporary transvenous diaphragm pacing vs. standard of care for weaning from mechanical ventilation.
The diaphragm is a dome-shaped structure that separates the abdominal and thoracic cavity. It's composed of slow-twitch type I and fast-twitch type IIa myofibers. These muscle fibers originate from the lumbar vertebrae and abdominal wall, the ribs, and the xiphoid process and floating ribs. Its innervated by phrenic nerves that arise from the nerve roots from C3 to C5. The phrenic nerve trifurcates at the dome of the diaphragm just prior to enervation. Diaphragm opposes the lower rib cages from the sides called a zone of apposition. As the diaphragm contracts, it pushes down into the abdominal cavity, generating a negative pressure in the lungs, which drives the air into the lungs. As the diaphragm relaxes, the elastic recoil of the lungs pushes the air out. In a cervical SCI, there is direct damage to the respiratory bulbospinal pathways. In higher SCI, the spinal roots to phrenic nerves are spared; however, there is an interruption in the roots from the respiratory center in the brain to the medulla.
Patients with SCI above C3 are the most obvious candidates as their phrenic nerve is intact and can be easily stimulated. Their phrenic nerve can be stimulated at either neck, thorax, or diaphragm. In comparison, patients with mid-cervical SCI cannot be stimulated at neck or thorax levels as their phrenic nerve is not functional. An advanced technique of stimulating the nerve endings at the insertion of the diaphragm has shown promising results (direct diaphragmatic pacing stimulation).
Other indications include disorders like congenital central alveolar hypoventilation, Arnold-Chiari malformations, basilar meningitis, brainstem tumors, strokes, Pompe disease, syringomyelia, and meningomyelocele. There have been published case series and reports of DP being successfully used in patients with accidental unilateral phrenic nerve injuries and lower motor neuron diseases like Charcot Marie Tooth disease, spinal muscular atrophy, polio, diaphragmatic flutter, and acute flaccid myelitis.
Controversy surrounds the use of DP in patients with amyotrophic lateral sclerosis (ALS). One study showed improved survival by delaying the need for mechanical ventilation. The authors of this trial did a long term follow up of these patients and confirmed their original findings. However, there are two trials that have shown decreased survival in these patients. Until more conclusive evidence is available showing clear benefit, we suggest weighing the risks and benefits of the procedure with a team in a specialized center. DP should not be done in patients who do have phrenic nerve function as determined by nerve conduction studies.
There are two common varieties of diaphragmatic pacers:
Diaphragm pacing requires an interprofessional team approach before and after the surgery. Alongside the surgeon who performs the procedure, neurology consultation should be sought for careful performance and interpretation of the phrenic nerve conduction velocities prior to surgery. These patients are usually severely deconditioned at the time of surgery and need a skilled rehabilitation team consisting of physical medicine and rehabilitation physician, physical therapist, occupational therapist, and speech therapist.
Patient selection is the key step in preparation, as discussed above. Phrenic nerve conduction velocities should be done to assess the viability of the phrenic nerve. Preoperative workup should be done in accordance with the institute’s protocol. These patients should have a tracheostomy prior to the surgery. DP can cause sudden upper airway closure due to dyssynchronous muscle contractions of the diaphragm and upper airway. Tracheostomy acts as emergency access to the airway in case of such an event. A careful anesthesia plan needs to be in place prior to surgery. The use of paralytic agents must be avoided intraoperatively, as the phrenic nerve is stimulated to assess for diaphragmatic muscle contraction.
There are three approaches to placing the pacing electrodes depending upon the position of placement, i.e., cervical, thoracic, and diaphragmatic.
Potential complications include but are not limited to intraoperative damage to the phrenic nerve, vascular injuries, acute lung injury, pericardial and esophageal injuries, diaphragmatic perforation, viscous perforation, and peritonitis.
Diaphragm pacing is a therapy aimed at patients with ventilatory failure due to bilateral diaphragm paresis or paralysis. This is aimed at giving these patients an opportunity to become free or delay the need for invasive mechanical ventilation, improving the quality of life. DP is a relatively rare procedure, so patients should be evaluated at centers of expertise.
Diaphragm pacing is a perfect example of the need for an interprofessional team for improving outcomes in patients. This procedure not only involves physicians across specialties, but there is also consistent involvement of ancillary services like nursing care, physical therapy, occupational therapy, and speech therapy. The rehabilitative process involves diaphragmatic conditioning after pacer implantation, which can take a long time. Education of all ancillary staff to identify clinical signs and symptoms of pacemaker malfunction and timely communication to the treating physician is the key to the successful rehabilitation of these patients. [Level 4]
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