Phrenic nerve pacing was developed in the early 1960s by Glenn and colleagues (Glenn and Phelps, 1985). In appropriate candidates (see Volume II, Chapter 37 on Rehabilitation in Spinal Cord Injury), phrenic nerve pacing has the potential to improve mobility, speech, and overall health, as well as to reduce anxiety and the volume of respiratory secretions, improve the level of comfort, and reduce required nursing care in ventilator dependent tetraplegic individuals (DiMarco, 2001). Confirmation of phrenic nerve function via nerve conduction studies or observation of diaphragm movement under fluoroscopy is necessary before considering phrenic nerve stimulation and diaphragmatic pacing (MacLean and Mattioni, 1981).
The implanted components of neuroprosthetic systems for phrenic nerve pacing consist of nerve electrodes placed around or adjacent to the phrenic nerves, radio-frequency receivers, and cabling. The external components are the radio-frequency transmitter and the antenna. There are currently three manufactures of these devices: Avery, Atrotech, and MedImplant. The Avery and Atrotech devices are available in the USA. The Avery device has received pre-market approval by the food and drug administration (FDA), and is the one most widely used. The Atrotech system is available under an FDA Investigational Device Exemption. The Atrotech has a more sophisticated four-pole electrode system, which is thought to reduce the chance for fatigue. All of these systems allow for changes in stimulus frequency, amplitude, and train rate.
Surgically, the electrodes are placed on or around the phrenic nerve in the neck or thorax. The receiver is placed in a subcutaneous pocket on the anterior chest wall, and the leads are tunneled through the third or fourth inter-costal space. Stimulation is initiated approximately 2 weeks post-operatively. Reconditioning is then required to improve the fatigue resistance of the diaphragm. Low-frequency stimulation (7-12 Hz) is required to help convert the diaphragm to primarily oxidative, slow-twitch Type I fibers (Oda et al., 1981). The respiratory rate usually lies within 8-14 breaths/min during initiation of therapy, and then is reduced to 6-12 breaths/min later on.
The possible complications of phrenic nerve pacing include infection, mechanical injury to the phrenic nerve, upper airway obstruction, reduction in ventilation due to altered respiratory system mechanics, and technical malfunctions. Most patients using electrophrenic respiration maintain their tra-cheostomy stoma to be used at night and for suctioning, although many of them can plug the sites during the day.
Recent advances for respiratory pacing have been contributed by the work of DiMarco and colleagues, who have demonstrated the use of a minimally invasive approach for introducing electrodes into the diaphragm with a laparoscope. This technique significantly reduces the surgical procedure required. Electrodes are placed bilaterally near the motor points to activate the muscles simultaneously for inspiration; expiration is due to passive relaxation. Of five subjects who have been entered into a clinical protocol and are through the exercise period, four have sufficient ventilation for breathing without other aids (e.g., respirator) for many hours per day. Subjects also report the return of smell as a result of the procedure (DiMarco, 2001).
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