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Diaphragmatic pacing, a medical intervention to aid patients with spinal cord injuries to breathe, has gradually expanded through the last few centuries. The first credible reference dates back to 1777, when Cavallo suggested in his treatise that electricity may be of use to assist the respiration. After the discovery of the animal electricity by Galvani in 1787, Ure in 1818 applied electricity to the phrenic nerve of a recently hanged criminal and reported the resulting contractions. He noted that “The chest heaved and fell; the belly was protruded and again collapsed, with the relaxing and retiring diaphragm.” In 1948, Sarnoff and colleagues devised the term “electrophrenic respiration” to refer to the investigations on the role of diaphragmatic pacing as a possible treatment to aid respiration in victims of bulbar poliomyelitis (Sarnoff, Whittenberger, & Hardenbergh, 1948). Although Sarnoff’s early work remains critical to the understanding of diaphragmatic pacing, Farmer and colleagues are credited for developing the modern systems and employing chronic diaphragmatic stimulation using radiofrequency signals to stimulate phrenic nerves through intact skin (Farmer et al., 1978). Moreover, this group initiated investigations regarding muscle fatigue and conditioning, and they are responsible for the implementation of the safety guidelines for the effective use of diaphragmatic pacing.
The main objective of diaphragmatic pacing is to provide ways to improve the ventilation and to eliminate the requirements for the positive pressure ventilatory support. Although the standard way has been the using of phrenic nerve pacing, there are some clinical evidence that the direct pacing of the diaphragm muscle can be beneficial for some people. Diaphragmatic pacing is able to lead to the great improvement in the life quality for quadriplegic ventilator-dependent individuals. It is able to improve the pulmonary function and decrease the possibility of the pulmonary infections. Most of the patients are able to speak when they are being paced and it substantially contributes to its clinical value. While restoring the negative pressure ventilation, diaphragmatic pacing also improves or restores olfaction, which is usually lost during the positive pressure ventilation (Adler et al., 2009).
Diaphragmatic pacing is a complex procedure involving several steps. Here, I will be describing few of these topics.
The selection of patients for Diaphragmatic pacing is elaborate, because it is only beneficial to some specific groups of patients. Historically, the two groups most likely to benefit from electrical stimulation of the diaphragm have been patients with high cervical spinal injuries and patients with central alveolar hypoventilation. However, with recent advances in microsurgery, there has been a recent attempt to expand the useful of diaphragmatic pacing to other patients. The patients with cervical spine lesions above the level of the phrenic roots (C3–5) usually die or are faced with a lifetime of mechanical ventilation. With the advent of diaphragmatic pacing, some patients with intact phrenic nerves and injury at the proper cervical level have a chance to become independent of mechanical ventilation. These patients are able to synchronize their upper airway movements and plug their tracheostomies.
In neonates, several case series involving children paced from infancy to as old as 10 years, involving bilateral pacing and noncontinuous stimulation have been reported. Even in the best cases, these children usually do not tolerate the 24-hour support, but instead respond better with pacing during the day and ventilation during sleep. In the patients with anatomical defects causing respiratory insufficiency, most are able to maintain normal respiratory physiology while awake, but develop significant apnea and hypoventilation during sleep. This subset of patients is unique in that they have a somewhat functional respiratory system while awake and some refuse surgery fearing that medical and surgical complications related to diaphragmatic pacing could leave them worse off than before. Diaphragmatic pacing has been feasible only in patients with intact lower motoneurons, a prerequisite which excluded a significant group of patients in the past, including C3–5 quadriplegics, owing to axonal loss of the phrenic nerve.
Currently, there are numerous commercially available diaphragmatic pacing devices. Although each has unique characteristics, four basic components are common: receiver, electrode, antennae and transmitter. The receiver and electrode assembly require permanent surgical implantation, whereas the transmitter and antennae are external devices. The subcutaneously implanted receiver transforms radiofrequency signals from the transmitter into electrical impulses carried to electrodes placed in the proximity of the phrenic nerve. The radiofrequency signal generated by the external transmitter produces a train of pulses lasting between 1.2 and 1.45 seconds, which corresponds to the length of inspiration. The respiratory rate is determined by the number of pulse trains per minute. An externally secured antenna transfers the radiofrequency signal from the transmitter across the skin to the subcutaneous receiver. The antenna must be within 2.5 cm of the receiver and should be secured in place to prevent migration and possible signal disruption. The receiver uses inductive electromagnetic coupling to obtain energy and stimulus information from the external transmitter. The signal is then demodulated and sent in the form of a unidirectional current to the electrodes. The implanted electrodes are composed with highly flexible stainless steel fibers, insulated by silicone rubber, with a platinum nerve contact. Recently, a device called NeuRx Diaphragm Pacing System™ - H100006, for amyotrophic lateral sclerosis (ALS) patients with breathing problems has been approved by the Center for Devices and Radiological Health of the Food and Drug Administration (FDA, 2011).
Long-term success with diaphragmatic pacing is dependent on a well-educated family and careful monitoring. Pulse oximetry is mandatory, as patients with central alveolar hypoventilation may not sense a decrease in oxygenation and quadriplegics must have an alarm they are able to access (Ali & Flageole, 2008). Pacing system malfunction can occur at the external and internal level. Internal failure, usually requiring the surgical intervention to correct, occurred at a rate of approximately 25% per patient-year in one study of children (Flageole et al. 1995). Most commonly, this internal failure is receiver malfunction, which requires a minimally invasive exchange.
Malfunction of other components, including the wires and electrodes, has been reported as well (DiMarco, 2009). Electrode malfunction can occur with growth, vigorous activity, or fibrous nerve entrapment, but the incidence has decreased, since bipolar cuff-shaped electrodes have declined in use (Hunt, Brouillette, Weese-Mayer, Morrow, & Ilbawi, 1988). Rarely, the etiology of failure is at the neuromuscular junction or phrenic nerve level. These have been linked to diabetes, toxins, nutritional deficiencies, anticholinergic drugs, hypermagnesemia and hypocalcemia. Infections have been reported over the receiver site, sometimes long after surgery. Physicians should inspect all pacing equipment annually. In addition to this physical exam, polysomnography should be performed in a well-equipped sleep laboratory. During this exam, arterial blood gas sampling should be included.
Diaphragmatic dysfunction is the result of a variety of disease processes and is degenerative. Though the intervention with diaphragmatic pacing greatly improves the patients’ condition, only a select few are eligible for the treatment. An intact peripheral (phrenic) nerve and muscle (diaphragm) are prerequisites for potential pacing candidates. Some patients with intrinsic paralysis of the accessory respiratory muscles (high cervical spine injuries) and those with central alveolar hypoventilation may be candidates for diaphragmatic pacing. Patients with respiratory insufficiency secondary to lower the motor neuron (phrenic nerve) dysfunction, amyotrophic lateral sclerosis, muscular dystrophy or extensive pulmonary parenchymal disease have traditionally not been candidates. Innovative approach and sound clinical research is required to improve the life of the patient. In the case of the implantation of diaphragm pacing in the actor Christopher Reeve, an unprecedented negative publicity and media awareness about this procedure has been accrued. For now, though the potential patient pool is limited, diaphragmatic pacing offers a clear advantage in the quality of life, compared to traditional ventilatory systems, for appropriately selected patients (Onders, Elmo & Khansarinia, 2009).
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