Diaphragm Pacing Technique

Updated: Aug 14, 2017
  • Author: Shabir Bhimji, MD, PhD; Chief Editor: Zab Mosenifar, MD, FACP, FCCP  more...
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Technique

Insertion of Pacemaker

The surgical procedure for inserting the diaphragmatic pacemaker is not complicated. To stimulate the phrenic nerve, one may place the electrode in the neck or in the chest.

Neck approach

With the neck approach, a small supraclavicular incision is made just above the clavicle. The sternocleidomastoid muscle is retracted and the phrenic nerve is identified as it courses between the middle and anterior scalene muscles. The identification of the nerve can be confirmed with a nerve simulator and use of intraoperative fluoroscopy. The fascia around the phrenic nerve is dissected and a U-shaped electrode is placed around the nerve and secured. During dissection, damage to the phrenic nerve must be avoided. The electrode wire is then tunneled subcutaneously to a subcutaneous pocket made just below the clavicle. The electrode is hooked to the receiver and the skin incisions are closed.

Chest placement

Stimulation of the phrenic nerve in the chest was once done using a thoracotomy, but today, it is done via a thoracoscopic approach. In the right chest cavity, the phrenic nerve can be found coursing just posterior to the esophagus; in the left chest, it is located lateral to the pericardium. The mediastinal pleura is incised parallel to the phrenic nerve, but a few millimeters of tissue are left on either side of the nerve. This incision is made both anterior and posterior to the nerve. The electrode is then gently passed behind the mobilized nerve, with care taken to ensure that the nerve only lies on the surface of the electrode. Minor oozing during dissection is common, but it stops in most cases. It is important not to use the cautery during this part of the dissection, because it may injure the nerve. The Silastic portion of the electrode is then secured firmly to the pleura overlying the nerve. The electrode must lie on the mediastinal surface, without distortion or kinks; any twisting or kinking may result in long-term dysfunction of the nerve. The end of the electrode is then passed through the chest wall in an atraumatic fashion and into a small pocket developed via a separate incision just below the costal surface of the chest. Any redundant wire is then looped gently on the surface of the lung within the chest cavity.

Other options

There are some patients who may not have an intact phrenic nerve or may have only one unilateral diaphragmatic function. In these patients, the diaphragm may be stimulated directly with electrodes. Before implanting electrodes in the diaphragm, motor mapping is done and a grid map algorithm is created. Once the points of maximal contraction are identified, the electrodes are implanted into these sites and again hooked to an external stimulator. Today, laparoscopy is often used to implant bilateral electrodes on the diaphragm. [1, 2, 3, 4]

The pacing wires must be handled with care because they are much thinner (and more fragile) than regular pacing wires. It is important to make sure that the copper coil on the receiver and the metal plate of the indifferent electrode are facing outward and away from the body. This ensures optimal communication between the receiver and the external antenna and also avoids unwanted stimulation of other chest wall muscles. All of the subpockets are closed firmly to keep the components from moving. [1, 2]

As with any subcutaneous implantation of a foreign body, the incision is made a few centimeters away from the implanted hardware to prevent tension on the incision, as well as to prevent extrusion of the device and allow proper healing. The device is always checked in the operating room to confirm functioning of the overall system before the patient is undraped.

Excellent contractions of the diaphragm with pacing should be confirmed with imaging, and the threshold should be in the range of 1-2 mA. Failure to pace or the presence of a high threshold should lead to reassessment to detect potential causes, such as improper connections, wire injury, lead dislodgment, or interposition of excess tissues between the electrode and the nerve.

The current is set on the basis of fluoroscopic testing in the operating room. In most cases, a standardized method is developed to assess the optimum diaphragmatic excursion, and this system is followed each time. Maximal voluntary descent of the diaphragm during inspiration is determined for each patient. In most adults, the diaphragm should move 6-10 cm. The amount of current required to generate this descent is assessed, and the current is kept at the minimum setting that produces the desired result. [1, 2, 3]

Prophylactic antibiotics should be given preoperatively to prevent infection. It is recommended that the right and left implantations be done in 2 separate procedures, usually performed 2-3 weeks apart.

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Conduct of Pacing

Unlike cardiac pacemakers, diaphragmatic pacemakers cannot be used immediately. Pacing is usually not started for 2-3 weeks after implantation is complete. Early pacing usually leads to development of pleural effusions, most likely from the breakage of lung and chest wall adhesions that bleed.

Pacing is done either unilaterally or bilaterally on alternate sides at 12-hour intervals; sometimes, the 2 sides are even paced simultaneously. In most patients with central alveolar hypoventilation, unilateral pacing is more than adequate.

A program of gradual conditioning is started to allow the atrophied diaphragm to regain its muscle strength and bulk. The adaptive process should be slow and gradual. The initial pacing period is 10-15 minutes per waking hour. This is gradually increased to 30 minutes per waking hour over several days. The increase is continued until the pacing period reaches 12 hours. The rest period is also gradually and progressively shortened until full-time pacing is achieved.

In most cases, the pacing changes are made at intervals of 10-14 days, depending on patient tolerance. Careful assessment of lung function and arterial blood gases to look for fatigue may be advisable. At the same time, the frequency of stimuli in the pulse train is decreased progressively to minimize fatigue of the diaphragm and allow longer pacing periods.

Concurrently, the respiratory rate is gradually decreased to the lowest number that provides adequate ventilation—typically about 8-10 breaths/min. To ensure that the adequacy of ventilation is maintained in the sitting position, all quadriplegics must have a firm abdominal binder. In addition, the respiratory rate is increased by 1 breath/min until the patient appears comfortable and not hypoxic.

In quadriplegic patients, this conditioning phase may take anywhere from 3 to 9 months. Patients with central alveolar hypoventilation tend to do much better because their diaphragm is not atrophied from disuse at the start of the program. [1, 2]

Other monitoring measures that are frequently employed include pulse oximetry, pulmonary function testing, and periodic arterial blood gas measurement. In addition, there is no substitute for a thorough physical examination. These patients need hourly monitoring during the initial phase of conditioning until pacing is advanced and tolerated. Noninvasive oxygen monitoring is routinely done, and carbon dioxide is also monitored to ensure adequate ventilation.

If, at any time during the conditioning phase, tidal volume falls or carbon dioxide tension starts to rise, diaphragmatic fatigue should be suspected. In such cases, the patient should be allowed to rest on a mechanical ventilator for a minimum of 12 hours or, if that is not feasible, at least overnight. The next day, pacing should resume at a shorter duration.

Patients with central alveolar hypoventilation usually need to be paced at night. The daytime break often gives the diaphragm enough time to recover from fatigue. [3, 4]

Close monitoring is required during the initial conditioning period. Even after discharge, monitoring continues to be necessary because the system is still prone to errors and fatigue. Patient factors that influence pacing include hormonal changes, infection, fever, stress, and any other factors that place additional stress on the diaphragm and thereby lead to extra ventilatory requirements. Often, when mechanical ventilation is started in these patients, it again leads to disuse of the diaphragm. [1]

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Tracheostomy

A permanent tracheostomy is highly recommended as a safety measure for all patients who undergo diaphragm pacing. Diaphragm pacing is known to be associated with intermittent upper airway obstruction resulting from strong diaphragmatic contractions that are not synchronized with the muscles of the upper airway. At night, all patients are told to leave the tracheostomy stoma open. Thus, if periodic dysfunction occurs at night, the tracheostomy offers secure access to the airways for the institution of positive-pressure ventilation.

Once full-time ventilation is achieved, the stoma can be closed with a button instead of a tracheostomy tube. The stoma button not only is acceptable cosmetically but also allows normal speech and reduces tracheal irritation and injury. [1, 2]

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Approach Considerations

Which procedure?

Some surgeons prefer the cervical approach because it avoids the morbidity associated with a thoracic procedure. Many of these patients have marginal pulmonary function and have other comorbidities. The disadvantage of the cervical approach is that in a small number of patients, the current amplitude required to stimulate the phrenic nerve may also stimulate adjacent nerves in the area. For example, it is reported that some people with cervical phrenci nerve packers frequently develop stimulation of the brachial plexus, which may cause involuntary movements of the upper arm.

The benefit of the thoracic approach is that one avoids stimulation of the brachial plexus. Further, there is evidence that an accessory branch of the phrenic nerve does reconnect to the main trunk in the chest and that this results in good diaphragmatic stimulation. For patients who are critically ill or not able to tolerate a thoracotomy, a thoracoscopic approach is recommended.

Diaphragmatic conditioning

In patients who have a paralyzed diaphragm or those who have not utilized the diaphragm for 6 or more months, conditioning of the diaphragm is required. This training period may last 9-12 months to help achieve adequate function of the diaphragm. There is no universal approach for diaphragmatic conditioning that is applicable in all patients, and the duration does vary from patient to patient.

In general, the following steps are undertaken to condition the diaphragm:

  • Gradually increase the duration of diaphragm pacing each day or week. Allow for rest at night, when the patient is on a mechanical ventilator.
  • During pacing, monitor the patient's arterial blood gas and pulse oximetry to assess for oxygenation and ventilation.
  • Avoid hypercapnia; if it occurs, increase the ventilatory rate.
  • The manufacturer of each device does provide guidelines on the conditioning process, and their recommendations should be followed.

Complications

Complications of diaphragmatic pacing may occur from the surgical procedure or may be pacing related. They include the following:

  • Skin infection after surgery
  • Lung complications after a thoracic procedure (eg, atelectasis, pneumonia, pain)
  • Dislodgement of electrode
  • Transmission of electrical impulses to the brachial plexus, with involuntary arm movements
  • Hardware malfunction

 

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