Phrenic Nerve Pacing: Current Concepts

Anatomy: The diaphragm is the main inspiratory muscle in mammals, it is a highly active muscle with a duty cycle of 30-40%, and therefore, the diaphragm may be particularly susceptible to inactivity or disuse (1). It is innervated by the phrenic nerves that arise from the nerve roots C3 through C5, and is primarily composed of fatigue-resistant slow-twitch type I and fast-twitch IIa myofibers. As the diaphragm contracts, the abdominal contents are displaced caudally, abdominal pressure increases and the lower ribcage expands (2). The phrenic nerve contains sensory, motor and sympathetic nerve fibers, and it supplies motor supply to the diaphragm and sensory supply to the central tendon (3). Both phrenic nerves run along the anterior scalene muscle deep to the carotid sheath. The right phrenic nerve passes over the brachiocephalic artery and posterior to the subclavian vein and over the right atrium. It enters the diaphragm at the level of T8 vertebrae. The left phrenic nerve also runs posterior to the left subclavian vein and passes over the pericardium of the left ventricle and penetrates the left hemidiaphragm separately (3). See Figures 1 and 2.