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Introduction

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The technique of applying electrical stimulation to the phrenic nerve to induce diaphragm pacing was first described by Cavallo in 1777.1 Early proponents used this technique for a variety of conditions associated with impaired respiration, including asphyxia (Hufeland, 1783), cholera (Duchenne, 1849), apnea (Israel, 1927), and polio (Sarnoff, 1950).2 Diaphragm pacing was introduced into contemporary thoracic surgical practice in the 1970s by Glenn, who pioneered its application in patients with central apnea (Glenn, 1966) and quadriplegia (Glenn, 1972).24 With the standard pacing devices, the electrical stimulus is applied at the phrenic nerve, as originally described. However, a newer approach applies the stimulus directly into the muscle at the phrenic nerve motor point for more direct control.5 The primary conditions amenable to pacing are high cervical spinal cord injuries (i.e., C3–C5) and congenital or acquired central hypoventilation syndrome. Onders et al. have extended pacing via the motor point technique to other populations, including patients with the progressive neuromuscular degenerative disorder, amyotrophic lateral sclerosis (ALS), and other more transient problems, such as intensive care unit patients who demonstrate difficulty weaning from mechanical ventilation.6,7

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General Principles

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The successful application of diaphragm pacing requires a fundamental knowledge of the anatomy and physiology of the respiratory system. A full discussion of the neural control of breathing is beyond the scope of this chapter. Briefly, the major components of the respiratory system include respiratory centers in the brainstem that control voluntary and involuntary breathing through connections to the diaphragm and muscles of respiration via the phrenic and intercostal nerves. Respiratory sensors deliver feedback to the brain via mechanoreceptors and chemoreceptors, which help regulate ventilation based on oxygen and carbon dioxide levels, as well as neural reflexes from smooth muscles in the airways and chest wall. The three respiratory centers in the brainstem include the medullary, which controls rhythmic inspiration and expiration, the apneustic center in the lower pons, which controls the prolongation of inspiration, and the pneumotaxic center in the upper portion of the pons. The latter inhibits inspiration to prevent overexpansion of the lungs.

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The diaphragm is innervated by the phrenic nerve bundle, which exits the spinal cord from the upper motor nerve centers in the brainstem at C3 to C5 and passes from the neck between the heart and respective lung to its most distal extent on the hemidiaphragm. The phrenic nerve can be accessed in the cervical region or in the chest. The nerve descends obliquely with the internal jugular vein across the anterior scalene deep to the prevertebral layer of the deep cervical fascia and the transverse cervical and suprascapular arteries. On the left, the phrenic nerve crosses anterior to the first part of the subclavian artery. On the right it lies on the anterior scalene muscle and crosses anterior to the second part of the subclavian artery. On both sides, the phrenic nerve runs ...

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