There are two basic types of commercially available DP pacing systems. In the first, electrodes are implanted directly on the phrenic nerves (direct phrenic nerve stimulation system). In another, more recently developed system, electrodes are implanted within the diaphragm (intramuscular DP system).
Direct Phrenic Nerve Stimulation Systems
Each of these systems is very similar in design, consisting of both implanted materials and external components (Fig. 62-1A). The electrodes and radiofrequency receivers comprise the surgically implanted components. An external power supply, radiofrequency transmitter, and antenna wires comprise components outside of the body.
Basic design of commercially available diaphragmatic pacing systems. Direct phrenic nerve stimulation system (A) and intramuscular diaphragmatic pacing system (B). With the direct phrenic nerve stimulation system, the internal components consist of a single electrode implanted on each phrenic nerve in the thorax with a wire connected to each implanted radiofrequency receiver. The external components consist of a stimulus transmitter and attached rubberized antennae, which must be positioned over the radiofrequency receivers. The receiver converts radiofrequency signals from the transmitter into electrical signals, which stimulate the phrenic nerves to activate the diaphragm. With the intramuscular diaphragmatic pacing system, two wires are implanted into the body of each hemidiaphragm near the phrenic nerve motor points. These wires are tunneled subcutaneously to a site over the chest wall where they exit the skin and are attached to an external electrode connector.
With each system, a single stimulating electrode is surgically positioned on each phrenic nerve. With unipolar systems, an indifferent electrode is also implanted subcutaneously.10 Wires are tunneled subcutaneously to connect each electrode to a radiofrequency receiver that is positioned over the anterior chest wall, usually over the lower rib cage.10 The external battery-powered transmitter generates radiofrequency signals, which are inductively coupled to the receivers via circular rubberized antennas. Each antenna must be positioned directly over each receiver and secured in place to ensure proper transmission of the signal. The transmitter signal is demodulated by the receivers, converting the radiofrequency signals to electrical signals, which are transmitted to the electrodes to stimulate the phrenic nerves and activate the diaphragm.10,20
The transmitter allows adjustment of stimulus amplitude (milliamperes [mA]) and stimulus frequency (Hertz [Hz]) to modulate the magnitude of tidal volume. Respiratory rate can be adjusted by altering the train rate. Inspiratory time and inspiratory flow rate can be adjusted, in tandem, by alterations of stimulus on-time. Sigh breaths can also be provided.
Surgical techniques have been developed for both cervical and thoracic placement of phrenic nerve electrodes.8–12 Although less invasive from a surgical standpoint, the cervical approach has significant disadvantages. First, an accessory branch from a lower segment of the cervical cord may join the main trunk of the phrenic nerve low in the neck or in the upper thorax.9 Activation of the cervical portion of the phrenic nerve may therefore lead to incomplete activation of the phrenic nerve and reduce the chance of successful pacing. Second, other nerves in close proximity to the phrenic nerve may also be activated, resulting in pain and/or unwanted movement.41 Finally, neck movement may displace the electrode, resulting in incomplete diaphragm activation and/or place significant mechanical stress on the nerve increasing the risk of injury. Consequently, the thoracic approach is the preferred method of implantation.26,32,48
Phrenic nerve electrodes have been successfully placed thorascopically.30,48 This procedure involves placement of trocars in several intercostal spaces and is technically quite demanding. Successful pacing was achieved for 12 to 14 hours/day while awake in a small group of children, primarily for management of congenital CHS. Moreover, although less invasive than thoracotomy, these patients also developed complications of pneumonia, atelectasis, and pneumothorax postoperatively. Additional studies are necessary to determine the long-term success of this procedure and its applicability to patients with cervical spinal cord injury.
Of the three commercially available DP systems, only the Avery system is available worldwide. Table 62-2 lists the technical characteristics of each device.
Table 62-2: Technical Features of Diaphragmatic Pacing Systems |Favorite Table|Download (.pdf)
Table 62-2: Technical Features of Diaphragmatic Pacing Systems
|Manufacturer||Avery Laboratories||Atrotech OY||Medimplant|
|Transmitter (stimulus generator)||Mark IVa||PX 244||Medimplant 8-channel stimulator|
|Size (mm)||146 × 140 × 25||185 × 88 × 28||170 × 130 × 51|
|Transmitter/battery weight (kg)||0.54|
0.45 + 0.6 (12V)
0.45 + 0.046 (9V)
|Rate (breaths/min)||6 to 24||8 to 35||5 to 60|
|Amplitude (mA)||0 to 10||0 to 6||0 to 4|
|Pulse width (μs)||150||200||100 to 1000|
|Battery life (hours)||400||160–320 (12V) 8 (9V)||24|
|Antenna||902A||TC 27-250/80||RF transmission coil|
|Receiver||Model I-110A||RX 44-27-2||Implantable receiver|
|Size (mm)||30 (diam) × 8||49 (diam) × 8.5||56 × 53 × 14|
|No. of receivers to stimulate both hemidiaphragm||2||2||1|
Avery Laboratories Mark IV
Glenn et al performed the basic science and clinical studies that led to the commercial application of the Avery (Commack, NY) system in the 1960s.49,50 The electrode consists of a semicircular platinum-iridium ribbon embedded in molded silicone rubber. The electrode contains a shallow trough for placement of the phrenic nerve. Monopolar and bipolar electrodes (recommended for patients with cardiac pacemakers) are available.9 The recent Avery Mark IV transmitter is lighter and provides a wider range of stimulus amplitudes than previous models.33 An optional interface also allows biofeedback control from pulse oximetry and CO2 monitoring.33 Transtelephonic monitoring is available, which allows the electronic output and neurophysiologic response of the pacing system to be monitored by telephone.51
The unique feature of the Atrotech (Tampere, Finland) system lies in its electrode technology, which consists of a four-pole electrode design.25,52 The electrode consists of two identical strips of Teflon fabric with two platinum buttons mounted onto each strip. Theoretically, when appropriately placed, the phrenic nerve is divided equally into four stimulation compartments. Each quadrant of the nerve, which supplies a specific set of diaphragm motor units, is stimulated sequentially. During one stimulus sequence, which consists of four current combinations, one pole in turn acts as a cathode and one pole on the opposite side as an anode. The result is four excitation compartments around the nerve. Combined stimulation of all quadrants of the nerve (each at 5 to 6 Hz) results in activation of the diaphragm near its optimum fusion frequency of 20 to 25 Hz, resulting in smooth contraction of the diaphragm.25,52
By this method, the stimulation frequency of individual axons is less than with monopolar stimulation. The slower stimulus frequency should provide more time for recovery, improve the endurance characteristics of the diaphragm, and shorten the conditioning process, compared to conventional unipolar stimulation.25,52,53 This technique had been approved by the FDA through an investigational device exemption. As of October 2005, however, the investigational device exemption study has been terminated. Therefore, this device is no longer available in the United States.
Medimplant Biotechnisches Labor
The Medimplant Biotechnisches Labor (Vienna, Austria) system is also differentiated by a unique electrode design.26 A complex microsurgical technique involving placement of four electrode leads around each phrenic nerve is required. The nerve tissue between each electrode lead comprises different stimulating compartments, only one of which is stimulated during any single inspiration. The various compartments are stimulated in sequence during subsequent inspirations (carousel stimulation).54 Sixteen different electrode combinations can be adjusted individually for each nerve. Similar to the Atrotech device, only a portion of the nerve is stimulated at any given time, allowing more recovery time and therefore less chance for the development of fatigue compared to the unipolar design. Availability of this system is limited predominantly to Austria and Germany.
Intramuscular Diaphragmatic Pacing System
The phrenic nerves can also be activated via placement of electrodes directly into the diaphragm (Fig. 62-1B).27,28,55,56 The major advantage of this method compared to direct phrenic nerve stimulation systems is that electrode placement does not require a thoracotomy. A thoracotomy is associated with significant perioperative morbidity, requiring an inpatient hospital stay and high cost. These disadvantages have limited the number of patients undergoing this procedure and are significant obstacles for patients undertaking phrenic nerve pacing. Intramuscular diaphragm electrodes can be positioned using minimally invasive laparoscopic techniques. This procedure can be performed on an outpatient basis or with an overnight observational stay, significantly reducing costs.27,28,56,57 The risk of nerve injury is virtually eliminated because this procedure does not require manipulation of the phrenic nerve. Postoperative pain is less for patients with CHS and recovery times are reduced significantly.
Conventional laparoscopy is employed for electrode placement of the Synapse Biomedical (Oberlin, OH) system.58 Four laparoscopic ports are required to provide access to the abdominal cavity for visualization, insufflation of the abdominal cavity, diaphragm mapping, and insertion of the implant tool (Fig. 62-2). Specially designed surgical tools and intramuscular electrodes are required for implantation.27–29,59–61 Two intramuscular electrodes are implanted into each hemidiaphragm near the phrenic nerve motor points (Fig. 62-3).62,63 A mapping procedure is required for appropriate electrode placement.27,28 By this method, full-time respiratory support can also be maintained in ventilator-dependent tetraplegic patients (both children and adults) with success rates similar to those of direct phrenic nerve stimulation.19,27,28,34,64 It is important to note that despite the fact that electrodes are positioned within the diaphragm muscle, the mechanism of diaphragmatic activation by this method is stimulation of the phrenic nerve motor points. This technique, therefore, represents a form of phrenic nerve stimulation, rather than direct diaphragm muscle stimulation.
Schematic of laparoscopic implant materials required for implantation of intramuscular diaphragmatic electrodes. Four laparoscopic ports are necessary to provide access to the abdominal cavity. Ports are necessary for visualization, insufflation of the abdominal cavity, diaphragmatic mapping, and insertion of the electrode implant tool. (Reprinted with permission of the American Thoracic Society. Copyright © 2012 American Thoracic Society. DiMarco AF, Onders RP, Kowalski KE, et al. Phrenic nerve pacing in a tetraplegic patient via intramuscular diaphragm electrodes. Am J Respir Crit Care Med. 2002;166:1604–1606. Official Journal of the American Thoracic Society.)
Schematic of the anatomy of the diaphragm from the abdominal surface and entrance points of each phrenic nerve into each hemidiaphragm (phrenic nerve motor points). (Reprinted with permission of the American Thoracic Society. Copyright © 2012 American Thoracic Society. DiMarco AF, Onders RP, Kowalski KE, et al. Phrenic nerve pacing in a tetraplegic patient via intramuscular diaphragm electrodes. Am J Respir Crit Care Med. 2002;166:1604–1606. Official Journal of the American Thoracic Society.)
Combined Intercostal and Diaphragmatic Pacing
Many patients with cervical spinal cord injury have damage to one or both phrenic motor neuron pools in the spinal cord and/or phrenic rootlets, and therefore cannot be offered DP.31,65,66 By placing electrodes epidurally on the ventral surface of the upper thoracic spinal cord, however, the inspiratory intercostal muscles of the upper rib cage can also be stimulated to produce large inspired volumes.67–69 Moreover, gas exchange during intercostal breathing alone is comparable to diaphragmatic breathing.70
In initial clinical trials, in patients with absent diaphragmatic function, stimulation of the intercostal muscles alone produced inspired volumes of similar magnitude to those resulting from activation of a single hemidiaphragm.69 Inspired volume, however, was not sufficient to support ventilation for prolonged periods. In subsequent trials in tetraplegic patients with unilateral diaphragm function, however, stimulation of the intercostal muscles in combination with unilateral phrenic nerve stimulation was successful in providing long-term ventilatory support.71
Side effects of intercostal muscle stimulation included mild flexion of both hands and contraction of the muscles of the upper torso, which was well tolerated.69,71 Intercostal pacing may be a useful adjunct to enhance tidal volume in patients whose inspired volume with phrenic nerve pacing is suboptimal.
This technique is not yet commercially available but has received approval by the FDA through an investigational device exemption.