Diaphragmatic Pacing

Anatomy and Physiology

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 the floating ribs. It is 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. The 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.[3]

Indications

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 the neck, the thorax, or the diaphragm. In comparison, patients with mid-cervical SCI cannot be stimulated at the 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, cerebrovascular accidents, Pompe disease, syringomyelia, and meningomyelocele.[9] 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.[6][10]

Contraindications

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.[11] The authors of this trial did a long-term follow up of these patients and confirmed their original findings.[12] However, there are two trials that have shown decreased survival in these patients.[13][14] 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.

Equipment

There are two common varieties of diaphragmatic pacers:

• Conventional DP - An internal electrode is attached to the phrenic nerve at the cervical, thoracic, or diaphragmatic level. Pacing wires connect this electrode to a receiver under the skin. An external transmitting box is placed above the receiver on the surface of the skin. This box is the main control unit and emits radiofrequency signals.
• Diaphragmatic Pacing System (DPS) - DPS consists of four electrodes implanted in the diaphragm to provide direct muscle stimulation and a fifth electrode implanted under the skin, which acts as a grounder. An electrode connector groups the five electrodes exiting the skin into a socket called an external pulse generator (EPG). A removable cable connects the electrode socket to the EPG.

Personnel

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 a physical medicine and rehabilitation clinician, physical therapist, occupational therapist, and speech therapist.

Preparation

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.[15]

Technique or Treatment

There are three approaches to placing the pacing electrodes depending upon the position of placement, i.e., cervical, thoracic, and diaphragmatic.

• Cervical approach - The nerve is identified in the mid-portion of the neck under the scalene fat pad by retraction of sternocleidomastoid (SCM) muscle.[16] The nerve is tested again by a stimulator and identifying the muscle contraction by fluoroscopy. Once tested, the fascial coverings are carefully dissected off. Two electrodes are hooked onto the nerve and secured to underlying connective tissue. A wire is tunneled subcutaneously that connects the electrodes to a pulse generator, which is also placed subcutaneously on the ipsilateral side of the chest.
• The thoracic approach is usually undertaken via video-assisted thoracoscopic surgery (VATS).[17] The right phrenic nerve is found just posterior to the esophagus, and the left phrenic nerve is located lateral to the pericardium. Once the nerve is identified, it is freed from its fibrous sheath, and electrodes are hooked onto the nerve. The nerve is tested in a similar fashion as in the cervical approach. Pacemakers are usually placed one at a time two weeks apart.
• Pacing through the diaphragmatic approach is done laparoscopically.[17] The electrodes are attached to the insertion points of the phrenic nerve to the diaphragm. These points are mapped out intraoperatively by phrenic nerve stimulation. Once all the electrodes are connected, the connecting wire is brought out through the epigastric port and connected to the EPG (placed in a subcutaneous pocket in the chest).

Complications

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.

Clinical Significance

Diaphragmatic 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.

Enhancing Healthcare Team Outcomes

Diaphragm pacing is a perfect example of the need for an interprofessional team for improving outcomes in patients. This procedure not only involves clinicians 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 clinician is the key to the successful rehabilitation of these patients. [Level 4]