Scott K. Berman and Cynthia S. Chin
Introduction
The diaphragm is the predominant muscle of inspiration and accounts for approximately 65% of the vital capacity in a fully functional individual. Without its proper functioning, a patient can experience mild-to-severe respiratory symptoms. The diaphragm is innervated by the phrenic nerve, which is composed of nerve fibers from cervical 3, 4, and 5 nerve roots.
The majority of patients referred for diaphragmatic surgery has unilateral diaphragm paralysis and are treated with diaphragm plication. A smaller percentage of patients have bilateral diaphragm paralysis, which can lead to respiratory failure requiring ventilator dependence. Diaphragmatic pacing is utilized in patients with intact phrenic nerves but interruption of the nerve signal conduction from the respiratory center in the brainstem to the phrenic nerve. Diaphragm pacers are most commonly placed for patients with a high cervical spinal cord injury (SCI) or central alveolar hypoventilation. The initial reports on diaphragm pacing were published in the late 1700s. Sarnoff was the first to show that phrenic nerve stimulation could produce adequate ventilation. His techniques utilized transcutaneous electrodes, which were not suitable for long-term use. It was Glenn’s pioneering work in the 1960s, which produced successful results by employing an implantable diaphragm pacing device in patients with high cervical injuries or central alveolar hypoventilation. Intrinsic motor neuron disease was considered a contraindication to placement of a diaphragm pacer; however, recent studies have not supported this thought.
INDICATIONS/CONTRAINDICATIONS
Injury to the cervical spine above the level of C1 to C2 results in quadriplegia and prevents stimulation of the diaphragm. It is important to realize that these patients have intact phrenic nerves but are simply unable to transmit a nerve impulse to the diaphragm. Every year, there are approximately 12,000 new patients affected with SCI. More than 50% of this group will develop quadriplegia with >4% requiring mechanical ventilation. The medical cost of a mechanically ventilated patient can approach $200,000 a year. In addition to this astronomical cost, patients with mechanical ventilation have a much poorer quality of life and are riddled with complications. Ventilated patients can experience difficulty with speech, inability to eat properly leading to frequent aspirations, increased production of secretions requiring frequent suctioning, and an increase rate of ventilator-associated pneumonias. It has been reported that the estimated life expectancy for a 20 year old with SCI requiring mechanical ventilation, is more than 41 years shorter than a person of the same age who has a SCI that does not require mechanical ventilation. Diaphragm pacers are placed in this cohort of patient with the goal of decreasing ventilator dependence.
Patients with central alveolar hypoventilation do not have the normal increased respiratory response when hypoxic or hypercapneic. The diminished response is present night and day; however, the patient is able to make a conscious effort to breathe during the day, which is not the case at night. This form of hypoventilation can be congenital or acquired. The congenital form affects 1 in 50,000 live births. The diagnosis of congenital central hypoventilation syndrome requires documentation of hypoventilation during sleep in the absence of primary respiratory, cardiac, or neuromuscular disease or a brainstem lesion. Once, it is diagnosed, children require nighttime positive pressure ventilation for the rest of their lives. The acquired form can be secondary to brainstem stroke, surgical trauma, tumor, hemorrhage, or meningoencephalitis. Implantation of a diaphragmatic pacer in these patients, whether a child or adult, can drastically improve the patient’s quality of life by releasing them from a lifelong requirement of nightly positive pressure ventilation. It is important to differentiate between central alveolar hypoventilation and obstructive sleep apnea as the latter does not benefit from implantation of a diaphragmatic pacer.
Contrary to initial thoughts, some recent studies have reported utility of diaphragmatic pacing in the amyotropic lateral sclerosis (ALS or Lou Gehrig disease). ALS patients have an idiopathic motor neuron degeneration in the cerebral cortex, brainstem, and spinal cord. This is a progressive and ultimately fatal disease. All of the muscles utilized for respiration are adversely affected in ALS patients resulting in progressive respiratory failure requiring mechanical ventilation. More than 80% of deaths in ALS are attributed to pulmonary failure and complications. It was conceptualized that in these patients, the utilization of diaphragm pacing before the onset of respiratory failure may help maintain diaphragm strength and provide trophic effects allowing the phrenic nerve neurons to remain viable much longer. The goal with diaphragm pacing would be to increase the time from diagnosis to the onset of respiratory failure requiring mechanical ventilation in ALS patients.
PREOPERATIVE PLANNING
Diaphragmatic pacing has been proven successful in removing ventilator dependency in a highly specific subset of patients. It is critical to select the appropriate patients for this procedure. Only patients who have an intact phrenic nerve and functional diaphragm should be selected for implantation. Patients with SCI have had partial and sometimes full recovery of the phrenic nerve up to 12 months after the initial injury. Therefore, it is important to wait 12 months before assessing these patients for a diaphragm pacer. Once phrenic nerve and diaphragmatic function is confirmed the patient can be considered for surgical placement of a diaphragm pacer.
Standard preoperative workup includes:
History and physical examination
Particular attention needs to be paid to signs and symptoms of respiratory and neurologic deficits. Patients with underlying intrinsic lung disease may not benefit from diaphragmatic pacing because severe lung pathology may be the major contributor to poor oxygenation and ventilation. In which case, a pacer may not have a dramatic impact on the patient’s respiratory status. CT scans of the brain, cervical spine, neck, and chest are important to rule out symptoms secondary to a mass lesion.
Pulmonary function tests (PFTs)
In patients with a paralyzed diaphragm, a restrictive process is seen on PFTs. There is a loss of vital capacity, which is worse when measured in the supine position.
It is important to look for evidence of intrinsic lung disease to better select patients who will benefit from pacing.
Chest x-ray (CXR)
A CXR is more useful in identifying patients with unilateral diaphragmatic paralysis as it will show an elevated hemidiaphragm on the affected side compared to the normal diaphragm on CXR. Patients with bilateral diaphragmatic paresis may not have an obvious finding on CXR as both hemidiaphragms may elevate and thus appear in “normal” position.
Fluoroscopic “sniff test”
When a patient takes a deep breath, intercostal and accessory muscles are the main contributors to the respiratory excursion. Breathing through one’s nose, “sniffing,” ensures diaphragmatic involvement. A patient is asked to sniff while in the supine position. Radiopaque markers are used to measure maximal diaphragmatic movement. In a patient with normal phrenic nerves, the sniff will result in a quick downward deflection of the diaphragm. The test is positive for diaphragmatic paralysis if there is paradoxical upward movement of the diaphragm during inspiration.
Percutaneous cervical electrical stimulation
This is the gold standard for testing phrenic nerve function. Electrodes are placed in the neck and electrical stimulation is performed. And intact phrenic nerve results in hemidiaphragm stimulation and contraction.
Prolonged latency or failure to conduct indicates poor phrenic nerve conduction.
Social assessment
Patients undergoing diaphragmatic pacemakers need to be highly motivated with a solid social and economic foundation. It is vital that the caregivers are enthusiastic as well since postoperative manipulations of the pacer may require frequent visits with the medical team.
Of special note, patients with congenital central hypoventilation syndrome as well as patients with high cervical SCI, have been reported to have bradyarrhythmias requiring cardiac pacing systems. Diaphragmatic pacers do not have sensing capabilities so there is no risk of interference from a cardiac pacemaker. However, there is the potential for a cardiac pacemaker to be influenced by the diaphragmatic pacemaker. In patients with cardiac pacemakers, it is important to discuss the case with the cardiac electrophysiology service as they may need to adjust the cardiac pacemaker settings prior to implantation of a diaphragm pacer. Although this is a theoretical concern, Onder et al. published a report on 20 patients with cardiac pacemakers in whom a diaphragmatic pacer was placed. None of the patients experienced immediate or long-term device-to-device interactions.
Amyotropic Lateral Sclerosis
The preoperative forced vital capacity (FVC) must be greater than 40% in ALS patients otherwise it is been found that there is a high risk of failure to extubate the patient at the conclusion of the implantation procedure.
The American Academy of Neurology recommends that ALS patients with respiratory symptoms and a FVC of less than 50% should be offered noninvasive positive pressure ventilation (NIPPV). Patients considered for diaphragm pacing should have these masks fitted and utilized prior to surgery so they are accustomed to them if they are needed in the immediate postoperative period.
SURGERY
Currently there are four devices that are available worldwide: Vienna Phrenic Pacemaker (Medimplant, Vienna, Austria), Astrostim (Atrotech Ltd., Tampere, Finland), the Avery Mark IV Phrenic Pacemaker (Avery biomedical, Commack, NY, USA), and the NeuRx Diaphragm Pacing System (DPS: Synapse Biomedical Inc., Oberlin, OH, USA). The first three systems utilize direct phrenic nerve stimulation by directly implanting the electrodes on the phrenic nerve. The NeuRx system places the stimulating electrode directly onto the under surface of the diaphragm. All systems then require connection of the stimulating electrode to a receiver usually placed in a subcutaneous pocket.
In the 1980s it was shown that direct diaphragmatic stimulation can be achieved. Mortimer et al. were able to produce diaphragmatic contractions when they stimulated areas where the phrenic nerve enters the diaphragm. Electrodes placed in this area, known as motor points, were able to produce diaphragmatic contractions similar to those obtained with direct phrenic nerve stimulation. Onders has since published extensively on the effectiveness of laparoscopic electrode placement at motor points in patient with respiratory insufficiency.
The remainder of this chapter will discuss the Avery Mark IV and NeuRx systems.
Components
Avery Mark IV
Electrodes surgically placed on bilateral phrenic nerves
Lead wires that connect electrodes to subcutaneous receivers
Antennae that are taped over the receivers
External transmitter
NeuRx
Electrodes surgically placed at motor points on bilateral diaphragms
Grounding electrode
Control unit
Cable and external battery-powered pulse generator
Anesthetic Considerations
Overall strategy should be to avoid paralytics since motor points need to be identified during the surgery.
Rapid reversible, short-acting anesthesia is preferred.
In ALS patients, succinylcholine is contraindicated because it can trigger hyperkalemia in these patients who have denervated muscles with increased acetylcholine receptors.
Local anesthesia should be used in all incisions to decrease pain response and minimize the amount of general anesthesia required.
Surgical Procedure
The implantation of phrenic nerve electrodes used for diaphragmatic pacing can be performed through a cervical, thoracic, abdominal approach.
All of these approaches first require an understanding of the path and course taken by the right and left phrenic nerves as they exit the spinal cord and travel to the diaphragm.
While the phrenic nerve contains motor and sensory fibers, the right and left phrenic nerves provide the only motor innervation of the diaphragm. In addition, they supply sensation to the central tendon of the diaphragm. They originate in the neck from C3 to C5 with most of the fibers of the phrenic nerve primarily originating from the fourth cervical nerve. See Figure 20.1.
Figure 20.1 Course of phrenic nerve in neck: Note the phrenic nerve traveling on the anterior surface of the anterior scalene muscle as it travels from lateral to medial. The nerve is deep to the transverse cervical artery and the suprascapular artery which can be injured in the dissection. Note the nerve entering the thoracic inlet just lateral to the junction of the internal jugular vein and the subclavian vein.
Right Phrenic Nerve
After leaving the vertebral foramen from C3 to C5, the right phrenic nerve is identified on the posterolateral aspect of the internal jugular vein.
The nerve exits between the middle scalene muscle posteriorly and the anterior scalene muscle anteriorly to travel obliquely across the anterior surface of the anterior scalene muscle.
At this level on the anterior scalene muscle, the phrenic nerve is deep to the prevertebral layer of the deep cervical fascia as well as the transverse cervical artery and the suprascapular artery. As it descends and reaches the inferior and medial aspect of the anterior scalene muscle, the phrenic nerve is superficial to the second portion of the right subclavian artery and as the nerve passes medially it is deep to the right subclavian vein.
The nerve then passes deep to the under surface of the first rib and at the level of the first costochrondral junction it will cross the innominate artery.
As it enters the right hemithorax, the phrenic nerve is found anterior to the superior vena cava and pulmonary hilum running along the pericardium lateral to the right atrium.
The right phrenic nerve leaves the right hemithorax by passing through the vena caval hiatus in the diaphragm at the level of T8.
The right phrenic nerve then enters the diaphragm through the tendinous portion of the diaphragm just lateral to the inferior vena caval foramen.
Upon entering the diaphragm the right phrenic nerve will break into three branches on the inferior undersurface of the diaphragm forming an anterior branch, a lateral branch, and a posterior branch.
These branches will then spread out in a radial pattern to supply motor function to the right hemidiaphragm.
Left Phrenic Nerve
The left phrenic nerve course through the neck is a mirror image of its right-sided counterpart. As the left phrenic nerve enters the left hemithorax, it passes superficial from lateral to medial on the arch of the aorta.
The left phrenic nerve then passes along the pericardium anterior to the left pulmonary hilum.
The left phrenic nerve then curves anteriorly.
It will enter the diaphragm anterior to the central tendon and just lateral to the pericardium.
Upon entering the diaphragm the left phrenic nerve will break into three branches on the inferior undersurface of the diaphragm forming an anterior branch, a lateral branch, and a posterior branch.
These branches will then spread out in a radial pattern to supply motor function to the left hemidiaphragm.
Cervical Approach Technique
The cervical approach is usually considered a minimally invasive procedure since it does not require a formal thoracotomy, thoracoscopy, or laparoscopy and is frequently performed as an outpatient procedure. This procedure can be performed under general anesthesia or using local anesthesia with intravenous sedation.
The patient is placed on the operating room table with a small roll under the patient’s shoulders to aid in the cervical visualization.
The patient is prepped and draped in the usual sterile fashion.
A 3- to 5-cm incision is made approximately 2 cm above and parallel to the midportion of the clavicle.
The platysma can then be divided and the sternocleidomastoid muscle dissected and reflected medially providing exposure of the prescalene fat pad laterally.
The prevertebral layer of the deep cervical fascia can be incised exposing the anterior surface of the anterior scalene muscle and internal jugular vein.
The phrenic nerve is identified running superficial on the anterior surface of the anterior scalene muscle.
If there is any question as to the identity of the phrenic nerve, a nerve test probe can be used to test and identify the nerve.
Meticulous dissection is used to free a portion of the phrenic nerve taking care to avoid electrical injury to the nerve.
Depending on the particular anatomy, the nerve can be exposed between the transverse cervical artery and the suprascapular artery or it can be exposed inferior to the suprascapular artery before the nerve dips behind the subclavian vein.
A tunnel is then created beneath the nerve lifting it off the superficial surface of the anterior scalene muscle.
The tunnel should measure approximately 10 to 12 mm and can be created with a right angle clamp or tonsil clamp.
The electrode is then passed under the phrenic nerve and secured to itself and sutured in place.
At this point remove any retractors to ensure that the nerve is not compromised or kinked.
Make an incision approximately 5 cm below the clavicle and create a pocket to create the receiver.
Using a tonsil clamp make a subcutaneous tunnel connecting the two incisions traveling over the clavicle.
Making sure to leave slack on the electrode wire where it connects to the nerve, pull the electrode wire through the subcutaneous tunnel so that it may be connected to the receiver in the infraclavicular pocket.
Secure the electrode to the receiver and place the receiver in the pocket making sure that the anode disc side is down and making good contact with the anterior chest wall.
Test the receiver function to demonstrate diaphragmatic stimulation.
Once appropriate function is demonstrated, place a nonabsorbable tie around the electrode connector to the receiver to prevent any fluid from entering the receiver, which could disrupt function.
Excess wire may be coiled in the subcutaneous pocket but should not be placed on top of or beneath the receiver itself.
The incisions can then be closed in a subcuticular fashion.
Test again after skin closure for proper functioning.
Note that when both hemidiaphragms are to be paced, two receivers are implanted.
Thoracic Approaches and Technique
Thoracic approaches to the implantation of a phrenic nerve pacemaker can be divided into a traditional or limited thoracotomy, or some form of a minimally invasive procedure. Minimally invasive procedures can include a traditional VATS (video-assisted thoracic surgery) approach or a robotic VATS approach.
Thoracotomy Method and Technique
Although a formal standard posterolateral thoracotomy was previously employed during the infancy of phrenic nerve pacemaker insertions, this is rarely performed today.
More commonly, a limited anterior thoracic thoracotomy approach is now employed.
Before placing the patient on the operating room table, an I roll should be made which will facilitate in the positioning of the patient and exposure during the surgery. This is created by rolling a sheet that will be placed vertically under the patient’s spine. Two additional sheets are rolled, which will be placed horizontally under the patient’s shoulders and the small of the patient’s back. The three rolls are then taped together forming an I.
The upper horizontal portion of the I should be no wider than the patient’s shoulders.
Place the patient on the operating room table.
The I roll is placed under the patient with the vertical portion under the patient’s spine, the upper horizontal portion under the patient’s shoulders, and the lower horizontal portion in the small of the patient’s back. The patient lies supine on the operating room table. The patient’s arms can either be placed over their head or at their sides with the patient’s shoulders slightly externally rotated and allowed to fall slightly posterior.
Although a bronchial blocker has great utility in thoracic surgery, it is most efficient to use a double-lumen endotracheal tube (ETT) for this procedure so lung isolation can be achieved by adjusting clamps, which obviates the need for the intraoperative manipulation that would be required if a blocker was used.
The patient should receive intravenous antibiotics within 60 minutes prior to incision.
After prepping and draping in the usual sterile fashion, a 5- to 7-cm transverse incision is made over the second or third intercostal space just lateral to the sternum. The incision is extended down to the costal cartilage and rib surface.
The intercostal space is entered. If necessary, the costal cartilage can be removed as in a Chamberlain procedure to help facilitate exposure.
A small rib spreader is then inserted.
The lung is deflated, the pleural space entered, and the lung and can be packed off superiorly and inferiorly.
The pericardium is visualized and the phrenic nerve is identified running anterior to the pulmonary hilum on the anterolateral surface of the pericardium.
On the right side, the phrenic nerve location is usually chosen at the junction of the superior vena cava and right atrium.
On the left side, the phrenic nerve is chosen at the level of the main pulmonary artery as it leaves the pericardial reflection.
The phrenic nerve bundle is then dissected off the pericardium, taking care to avoid either mechanical or electrical injury to the nerve.
A tunnel is made under the phrenic nerve allowing it to be elevated off the pericardium. The tunnel should measure between 10 to 12 mm in width.
The electrode is passed under the phrenic nerve and secured to itself and the electrode is then fixed with sutures to the pericardium.
A pocket can then be made at the inferior and lateral portion of the thoracic incision for implantation of the receiver on the anterior chest wall.
The electrodes are brought out through the incision and connected to the receiver and the receiver is placed in the pocket with the anode side in direct contact with the chest wall.
The receiver is then tested and the electrode function confirmed.
Nonabsorbable ties are then placed around the insertion point of the electrode into the receiver to prevent leakage of fluid at the level of the connection, which could interfere with proper functioning.
Excess electrode wire may be coiled and placed elsewhere in the pocket.
Is important to make sure that the anode surface of the receiver is in good contact with the chest wall.
The incision can then be closed in a layered fashion and the skin closed in a subcuticular fashion.
The procedure is then repeated on the opposite side.
A chest tube can be placed through a separate incision.
If chest tubes are to be avoided, a red rubber catheter attached to suction, can be placed into the chest through the incision. The incision can be closed around the catheter. Once the incision is closed and suction is applied, ask the anesthesiologist to give a breath and hold. The red rubber catheter can be quickly removed at this point. A CXR can then be obtained in the operating room to determine if a chest tube is required.
Minimally Invasive Thorascopic Approach and Technique
The minimally invasive thorascopic approach can be performed in either a traditional VATS fashion with the patient placed sequentially in the right lateral decubitus and then the left lateral decubitus position, or the patient can be placed in a supine position similar to the approach used for VATS mediastinal surgery. In addition, surgery can be performed as a traditional VATS procedure or with the da Vinci robotic assistance.
The advantage of the supine position is that repositioning is not required.
If the supine position is used, then an I roll (previously described) is placed under the patient with the patient’s arms either over their head with care being taken to avoid injury to the brachial plexus, or with the arms at the patient’s side in a supine position with the shoulders slightly externally rotated and posteriorly deviated.
Lung isolation is obtained using a double-lumen ETT.
CO2 insufflation may be used at the preference and discretion of the individual surgeons.
Once general anesthesia has been induced and the chest or chests prepped and draped, thorascopic or robotic incisions can be placed which are designed to facilitate exposure of the upper pulmonary hilum.
If the patient is in the supine position, port incisions can be made in the fourth intercostal space, 2 cm anterior to the anterior axillary line, and in the second and sixth intercostal spaces at the level of the anterior axillary line.
If the patient is in the lateral decubitus position, port incisions can be made at the seventh intercostal space in the posterior axillary line, in the fifth intercostal space in the midaxillary line, and in the ninth intercostal space in the posterior axillary line.
The phrenic nerve is usually identified at the superior aspect of the pericardium.
On the right side this is where the superior vena cava enters the right atrium.
On the left side this is just inferior to the aortic arch where the pulmonary artery exits the pericardium to enter the left pulmonary hilum.
Care is taken to avoid directly grasping the phrenic nerve.
Incisions are made in the mediastinal pleura anterior and posterior to the phrenic nerve and a tunnel is created beneath the phrenic nerve freeing it from the pericardium and measuring 10 to 12 mm in width.
A small subcostal incision is made and a subcutaneous pocket created, which will house the receiver.
A 3- to 4-in length of Penrose drain is placed over the electrode connector and tied in place with a suture.
The electrode is then fed into the chest cavity through the lowest trocar port space.
A tonsil clamp is then passed from the subcutaneous pocket, under the ribs and through the anterolateral peripheral aspect of the diaphragm into the chest cavity under direct visualization.
The free end of the Penrose drain is grasped and the connector and excess wire is delivered into the subcutaneous pocket.
The phrenic nerve electrode is then brought under the phrenic nerve through the tunnel previously created below the nerve.
The electrode is sutured in position on either side of the nerve using 4-0 or 5-0 nonabsorbable sutures. See Figure 20.2.
The electrode is then connected to the receiver and the receiver is inserted into the subcutaneous pocket with the anode disc side down making contact with the abdominal wall musculature.
After the connection is made, the receiver and phrenic nerve electrode are tested to ensure diaphragmatic stimulation.
Figure 20.2 Electrode placement on phrenic nerve. The Avery Mark IV system requires the attachment of an electrode to the phrenic nerve, either in the neck or in the chest. Note the electrode is tunneled under the phrenic nerve and sutured to itself. It is then secure here to the pericardium.
Once diaphragmatic stimulation is confirmed, a nonabsorbable tie is passed around the connector where it enters the receiver to prevent any fluid from disrupting the connection.
It is important to have sufficient excess wire in the chest cavity to prevent any traction on the phrenic nerve and any remaining wire can be coiled and left in the subcutaneous pocket.
If a chest tube is to be used, it can then be placed through the inferior most trocar port and secured in place.
If a chest tube is not to be used, place a red rubber catheter, as previously described, to remove the pneumothorax.
Port incisions are closed in layers using 2-0 absorbable sutures for the deep layer and either 3-0 or 4-0 absorbable sutures for the skin.
Laparoscopic Approach and Technique
This laparoscopic approach utilizes a standard four-port technique, which allows visualization of the entire diaphragm.
The patient is placed in the supine position under general endotracheal anesthesia. The patient cannot be paralyzed to evaluate the diaphragm.
In addition to the usual grounding pads, which are used with electrocoagulating instruments, an additional grounding pad will be placed on the patient and attached to the clinical station which will be used intraoperatively to map diaphragmatic stimulation and record changes in intra-abdominal pressure during stimulation.
The initial port is usually placed several centimeters above the umbilicus in the midline using either a 5- or 10-mm port depending on the laparoscope, which is employed. CO2 insufflation is then performed.
Either a 0-degree or a 30-degree laparoscope may be used.
A right and left lateral 5-mm subcostal port are placed.
These ports are frequently used to aid in the mapping of the right and left hemidiaphragms.
The lateral ports are also used to completely divide the falciform ligament, which can be accomplished either using a Bovie or harmonic scalpel.
Complete division of the falciform ligament aids in visualization of the right hemidiaphragm.
A 12-mm epigastric port is then placed. This port will primarily be used for the implantation instrument. See Figure 20.3.
After laparoscopic access is obtained, mapping of the hemidiaphragm proceeds.
Mapping identifies the area on the diaphragm where electrical stimulation causes the greatest amount of diaphragmatic excursion.
The mapping instrument is passed through one of the lateral ports.
The mapping instrument consists of a hollow flexible tubing, which sits inside of a rigid outer metal cannula. This is then connected to the typical operating room suction canister.
Once the mapping instrument has been placed in the abdomen, a grasper is used through the other lateral port and the tip of the probe is pulled out of the outer metal cannula.
The tip of the electrode is grasped with the dissector and then touched along the undersurface of the diaphragm. It is attached to the diaphragm with suction.
Once attached, a stimulating current is then applied. See Figure 20.4.
Changes in intra-abdominal pressure are measured as well as direct observations of diaphragmatic contractions.
The closer the testing electrode is to a diaphragmatic motor point, the stronger the observed contraction will be and the greater the magnitude of change in measured intra-abdominal pressure.
Figure 20.3 Typical laparoscopic port placement utilizing an upper epigastric port, two lateral ports, and one supraumbilical port for visualization and mapping of both hemidiaphragms a placement of electrodes.
When the area of maximal stimulation is identified, this area can be marked using the Bovie as the primary electrode placement site.
More recently, some surgeons have elected to avoid formal mapping using the mapping instrument. Instead, an alligator clip can be connected to a Maryland dissector which is then touched to the diaphragm to evaluate diaphragmatic stimulation.
The area with the next highest level of contraction is marked as the secondary electrode sites.
On the right hemidiaphragm, the motor point is usually noted just lateral to the central tendon while on the left hemidiaphragm, the motor point tends to be more lateral since the phrenic nerve enters the diaphragm from a more lateral aspect on the left.
Also note that the phrenic nerve bundles tend not to run with the vascular bundles unlike other areas of the body.
Once the primary and secondary electrode locations have been identified in the right and left hemidiaphragms, an empty implant instrument can be used to decide, which port provides the best approach angle.
Figure 20.4 The NeuRx system does not attach directly to the phrenic nerve, but operates by stimulation of the diaphragmatic motor point to directly cause diaphragm contraction. The mapping instrument demonstrated here shows the flexible inner plastic hollow tubing protruding from the rigid metal outer cannula. Suction applied through the hollow tubing allows for contact to the diaphragm.
Figure 20.5 In the laparoscopic NeuRx system, the electrode is delivered through the rigid delivery device and pulled through the diaphragm, securing it in place.
The electrode is loaded into the lumen of the implant instrument so that only a small portion of the hooked tip of the electrode and a small portion of the blue polypropylene barb extends beyond the needle.
The most posterior diaphragmatic electrode site should be inserted first to prevent interference with the excess lead from the placement of the other electrodes.
With the needle closed, the insertion device is passed into the abdomen.
The needle is then advanced into the diaphragm perpendicular to the muscle fibers to increase the likelihood of capture of the barb on the muscle fibers.
After insertion of the electrode the needle is withdrawn by applying some backward traction on the needle and counter pressure on the hemidiaphragm using a separate dissector. The barbed electrode lodged in the diaphragm, remains behind. See Figure 20.5.
The electrode should then be tested to ensure that proper stimulation and contraction of the hemidiaphragm is obtained.
The remaining additional electrodes are then implanted in a similar fashion until all electrodes have been placed.
It is important to keep the right and left hemidiaphragm electrodes separate.
The electrodes wires are brought out of the epigastric port.
Excess wire can remain intra-abdominally and is frequently placed over the dome of the liver.
An area of the chest is identified, which is easily accessible to either the patient or their health care provider.
The electrodes are tunneled into this area of the chest by making a subcutaneous tunnel for each electrode separately keeping the right electrodes inferior to the left electrodes.
The electrodes are again checked to ensure proper functioning.
It is also important to check that there is no capture of the patient’s cardiac rhythm.
The port incisions can then be closed in the usual manner.
The electrode wires are then connected to a connection block, which is inserted into the diaphragmatic stimulator.
Diaphragmatic Pacemaker Receiver Replacement
It is occasionally necessary to replace the phrenic nerve stimulator receiver.
A CXR should be obtained before replacing the phrenic nerve receiver to identify the location and direction of the connectors and the anode disc. This will allow for planning of the incision and decrease the likelihood of transection of the wires.
If the new receiver is smaller than the present one, a smaller pocket must be made to ensure good electrical contact between the bottom anode plate of the receiver and the chest wall.
Receiver replacement can frequently be performed under local anesthesia on an outpatient basis.
Injection of the local anesthetic into the fibrous sheath that frequently forms around the receiver may also facilitate its removal.
An incision is made directly over the receiver and carries down onto the surface of the receiver.
The fibrous sheath over the receiver can be incised and then cut in a cruciate fashion to facilitate delivery of the receiver into the surgical wound.
The anode disc is removed.
Any previously placed sutures to secure the connectors are cut.
Disconnect the connectors by gently pulling and rolling the connector out of the receiver.
Examine the plating material on the connectors of the phrenic nerve electrodes.
If there is evidence of discoloration or oxidation then you can wipe the connectors with a dry sponge or scrape them lightly with a scalpel.
The replacement receiver is then connected to the electrodes and tested using a sterile antenna.
Ties are then placed around the connectors where they insert into the receiver to prevent fluid from entering and disrupting the connection.
If necessary, a new subcutaneous pocket can be made.
The receiver is then placed in the pocket making sure that the anode disc is in direct contact with the chest wall.
The incision can then be closed and layers closing the subcutaneous tissues with absorbable sutures and the skin with a subcuticular suture.
The pacemaker may be used immediately upon completion of surgery if necessary.
Diaphragmatic Conditioning
In individuals with diaphragm paralysis for 6 months or longer and in particular those individuals who had a paralyzed diaphragm for 2 years or longer, a period of diaphragmatic conditioning is required. This may last anywhere from 3 to 9 months. This is necessary to achieve optimization of diaphragmatic functioning using a diaphragmatic pacemaker.
Throughout the day, the amount of time that the diaphragm is paced per hour is gradually increased, followed by a 12-hour period of rest when the patient is placed back on positive pressure ventilation.
Long-term mechanical ventilation frequently results in chronic hyperventilation and decreased levels of CO2. Since the diaphragmatic pacing systems are designed to return the patient to a physiologic state, patients may frequently experience the sensation of shortness of breath or dyspnea during initiation of pacing. This can be minimized by adjusting the CO2 levels while the patient is on the ventilator during conditioning.
Diaphragm conditioning usually proceeds in a step-by-step fashion by gradually increasing the amount of time the patient’s diaphragm is able to be paced.
When the patient is sitting, higher levels of electrical stimulation may be required because of the effects of gravity on the diaphragm.
The ultimate goal is to achieve full-time pacing without developing diaphragmatic fatigue and loss of contractility.
COMPLICATIONS
Diaphragmatic pacing complications are related both to the function of pacing and to the surgical procedure employed.
Infection, as with any surgical procedure and especially one where a foreign body is introduced, can develop following surgery. Wound infections have been reported with a rate of approximately 3%.
When a thoracic approach is employed, pulmonary complications can develop such as pneumonia, pneumothorax, hemorrhage, and empyema.
When a cervical approach is employed, the patient can develop stimulation and movement of the upper extremity due to transmission of the pacemaker impulses to the brachial plexus.
Phrenic nerve electrode malfunction or fracture can result in the loss of the ability to pace the diaphragm.
When the laparoscopic approach is performed, a capnothorax can occur from the electrodes being placed in a thin diaphragm with escape of carbon dioxide into the hemithorax. This occurs in up to 50% of patients and usually resolves spontaneously over a short period of time or can be aspirated.
Most common complication reported has been the iatrogenic injury to the phrenic nerve itself. The incidence of this has decreased as the techniques for implantation have evolved.
RESULTS
Initial studies done in SCI multicenter trials showed 98% of patient gained independence for mechanical ventilation. In addition to reporting short-term positive results with diaphragm pacers, Glenn reported long-term follow-up in 12 patients who underwent bilateral phrenic nerve pacemaker placement 15 to 20 years prior. The report confirms that patients with phrenic nerve pacers were able to achieve long-term respiratory independence from mechanical ventilation. In highly selected ventilated patients, there was been multiple reports of improved quality of life after placement of a diaphragmatic pacer. In SCI and central alveolar hypoventilation patients, such life-changing improvements include an improved olfactory sense, which has been associated with increased pleasure, increased independence from noisy machinery, which allows for better reinsertion into the community, improved posterior lobe ventilation resulting in decreased respiratory infections, increased ability to clear secretions leading to decreased need for suctioning, and reduced time on the ventilator and possible complete freedom from the ventilator and decannulation of the tracheosotomy.
There have been reports in which diaphragmatic pacers were placed safely in ALS patients under general anesthesia with no 30-day mortality. It has also been reported in a study of 38 ALS patients who had diaphragmatic pacer placed, that there was a 2-year delay for the need of ventilatory support when compared to historical controls. Treatment of ALS patients with diaphragm pacers is an area of active investigation.
There has not been any direct comparison of the different devices or the surgical techniques required for their placement. The cervical placement of the Avery Mark IV had the advantage of being able to be done under local anesthesia and, therefore, there is a decreased risk of exposure to general anesthesia. There, however, have not been any reports of anesthesia complications associated with either thoracic or abdominal placement of the devices. There is a risk of injury of the phrenic nerve when placing an Avery Mark IV which is not present when placing the NeuRx pacer. However, the incidence of this has decreased as the surgical procedure for implantation has evolved. Diaphragm pacers placed in each of the three locations have had excellent results with relatively minimal-associated morbidity and mortality. The choice of surgical technique and which pacer to use should be based on surgeon comfort.
CONCLUSION
Compared with mechanical ventilation, diaphragmatic pacing, either via phrenic nerve pacing or direct pacing of motor end plates on the diaphragm, can have a dramatic improvement on a patient’s quality of life and sense of well being. In addition to the great physical and emotional benefits, there is a significant economical advantage to those patients who have successful placement of a diaphragm pacer.
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