With the DPS system, the leads are inserted at the phrenic nerve motor point on the underside of the diaphragm (Fig. 152-3). Inadvertent nerve injury caused by the necessity of implanting the lead directly on the phrenic nerve is thus avoided. The procedure is currently performed laparoscopically. Four ports are used including two 10-mm ports in the supraumbilical and epigastric regions in addition to two lateral 5-mm ports. A portion of the falciform ligament is taken down to improve exposure of both hemidiaphragms. The motor point is then mapped to determine the location of the optimal diaphragm contraction both visually and via pressure measurements using a transducer connected to one of the ports. Once two points of maximal contraction have been mapped on each side for redundancy, a laparoscopic insertion device is used to implant the electrodes (Fig. 152-4A,B).
NeuRx Diaphragm Pacing System. This device is implanted laparoscopically with two leads inserted into the musculature of each hemidiaphragm. The leads are then tunneled to exit the skin level where they connect with the stimulator. Device settings for each lead are programmed into the stimulator via a clinical work station (not shown).
A. Diaphragm mapping. The laparoscopic view of the right hemidiaphragm during mapping is shown. The mapping electrode is positioned in different areas (left) and then stimulated (right) until the ideal phrenic nerve motor point is found. The point of maximal contraction is determined by measurement of pneumoperitoneum pressure displacement, tidal volumes generated, and visual inspection. B. Electrode insertion. Two leads are inserted into each hemidiaphragm to optimize diaphragm contraction. The implantation device is carefully inserted into the muscle at the optimal mapping point and then pulled out as the barbs at the tip of each lead hold the wire electrode in place. Care is taken to avoid inadvertent capnothorax.
Each lead is loaded into the shaft of the inserter such that the lead is released into the muscle at the mapped motor point when the device needle tip is pulled back and out. If the needle crosses into the pleural space, it is important to assess for capnothorax. If proper hemodynamics are maintained, treatment may be unnecessary. Otherwise, aspiration and/or temporary chest tube insertion may be required. The leads are each tested with train stimulations to confirm proper capture and then tunneled out the epigastric port to the right upper quadrant with a fifth subcutaneous lead serving as the ground electrode. The leads are then connected to the stimulator via a small switchboard adapter. Stimulus parameters are programmed into the primary and backup stimulator units using the clinical workstation console. Although the DPS is not completely implantable, as in the case of the phrenic nerve pacemakers, the tunneling serves as a good barrier for infection and the device can be implanted in the outpatient setting with laparoscopic technique. Currently, the NeuRx device is FDA approved only for patients 18 years and older, although clinical trials are being conducted in children aged 5 to 17 years, with encouraging results.
Setup and Perioperative Planning
When embarking on placement of a diaphragm pacer, it is wise for the surgeon to establish a multidisciplinary team to help in the perioperative assessment and planning. This assembly of clinicians can include a pulmonologist, rehabilitation physician, respiratory therapist, neurologist, electromyelography technician, physician assistant/nurse practitioner and nursing aides, as well as the patient and family members. There is an important educational component for the various team members in the operating room, ward, and rehab units that can be taken for granted if not considered ahead of time. Communication between these team members after implantation will help the weaning process significantly. There is also a lot of behind the scenes work that goes into the scheduling process from an administrative standpoint between the hospital, purchasing unit, biomedical engineering, insurance company, and institutional review board.
Ample support from the device manufacturers has always seemed available throughout the entire perioperative period. Additionally, in the surgical community, an experienced person is usually approachable for questions and personal support. Irrespective of device, the initial settings are made in the operating room with a specialist from the company present to assist with programming. Pacing thresholds and tidal volumes are determined for each patient to optimize performance. The conduct of pacing initiation and ventilator weaning is patient-dependent and can vary with pacer device and indication. Long-term results seem comparable between devices, but the NeuRx patients are able to pace sooner and wean faster. If there is substantial atrophy of the diaphragm, a common occurrence with spinal cord injury patients, a longer period of conditioning may be needed before full-time pacing can be instituted. The necessity for conditioning results from the conversion of type I fatigue-resistant slow twitch muscle fibers to the less efficient type II fast twitch fibers during prolonged periods of ventilator dependence. For these patients, progress with conditioning by electrical stimulation is monitored by clinical observation of the respiratory work of breathing along with measurements of pulse oximetry, end-tidal CO2 values, or even arterial blood gases, before weaning is attempted.
All of these efforts are worthwhile in getting a return on investment that can be seen within 3 years of pacing when taking into account the costs of the devices against the costs of mechanical ventilation, its disposables and monitored care personnel wages. From a clinical standpoint, the patients benefit from natural breathing with successful pacing by simplified nursing care, reduced ventilator-associated pneumonia, improved speech patterns and olfactory sensations, physical freedom from mechanical ventilation and eventual conversion to a button or outright decannulation.
An early report of results using the Avery device showed that in 165 patients out of 477 implanted for spinal cord injury who had detailed follow-up, 47% were pacing completely with 35% pacing part-time and 17% not pacing.18 The unsuccessful patients were not pacing largely due to socioeconomic reasons as opposed to device malfunction. In a more recent study, Elefteriades found that 6 out of 12 tetraplegic patients were pacing full time 10 years out from implantation and that thresholds had not increased over time.19 No patient lost the ability to pace, and pathology specimens showed that there was no evidence of nerve injury on microscopic analysis, demonstrating that the electrode does not cause damage to the nerve despite direct implantation.
In a study of 50 spinal cord patients implanted with the NeuRx system followed for 2 years, 50% of those implanted over 6 months were completely off mechanical ventilation while 66% were pacing over 12 hours a day without the need of an attendant.20 No patient was on the ventilator full time. The longest patient with the device was pacing out a 8 years and another was weaned from mechanical ventilation 28 years after injury. The latter patient took longer to wean from the ventilator, but the case demonstrates that it was still possible effectively pace despite this long period due to the intact phrenic nerves.
In its application to ALS patients, the DPS has been shown by Onders to be an effective means of delaying the inevitable need for mechanical ventilation in this patient population by up to 2 years.7 In the first 16 ALS patients, the decline in FVC with pacing was 0.9% per month compared to controls of 2.4% per month. Ultrasound studies in several patients showed thickening with improved excursion on fluoroscopy.
By virtue of the differences in implantation schemes, the Avery unit puts the phrenic nerve at greater risk for devascularization or mechanical injury during the dissection and insertion of the electrode. This risk is largely avoided with NeuRx system, which implants at the neuromuscular motor point. While each system is susceptible to device-related infection, the risks are lower with the Avery system, which is completely implantable making percutaneous infection theoretically less likely. Capnothorax is a known complication for the NeuRx system since the electrode is inserted from the underside of the diaphragm and the needle may inadvertently puncture the pleura during insertion. The condition may be symptomatic requiring treatment or occult requiring additional close observation.
Each device is quite durable from a mechanical standpoint, but both may require electrode or electrode component replacement due to wear and tear over time. The Avery electrode on the nerve cannot be removed easily or safely, but the end attached to the receiver can be replaced should the receiver malfunction. The NeuRx electrodes can fray or be damaged outside the body, but are easily spliced back together at the bedside with experience. Additionally, these electrodes implanted in the diaphragm can be simply cut similar to epicardial pacing wires or perhaps carefully pulled out if necessary.
Diaphragm pacing is an uncommon procedure in thoracic surgery. Many patients with unilateral diaphragm paralysis are referred for diagnostic evaluation, but few will be suitable for pacing. Others will have idiopathic, viral, or traumatic etiology that can be managed surgically with diaphragm plication, if clinically warranted. For patients with high cervical spinal cord injury or central hypoventilation (congenital or acquired), the integrity of the phrenic nerve must be carefully evaluated, including full review of the neurologic and pulmonary systems, and careful assessments conducted via diagnostic procedures. During implantation, the ability of the phrenic nerve or motor point to conduct a signal is retested on-the-spot before proceeding to full implantation of the device. All four devices mentioned in this chapter are suitable for pacing. However, the author's experience is limited to the Avery Mark IV and the NeuRx DPS, as these are the only two approved for implantation in the United States. Nevertheless, the surgical access techniques and technical principles are all very similar. Recently, it has been shown that diaphragm pacing may play a valuable role in patients with acute respiratory insufficiency and with the neuromuscular disease, ALS. This expanded indication may benefit patients by promoting weaning long-term result. Future applications of this technique in patients with marginal pulmonary function following high-risk resection, after lung transplantation, or for temporary pacing in the intensive care unit for difficult to wean patients may prove viable but will require further exploration. Thoracic surgeons should keep this enabling technology in their armamentarium.