Phrenic Nerve Stimulation PNS

Phrenic nerve stimulation (PNS) or diaphragm pacing for treatment of respiratory failure is possible only with normal phrenic nerves and muscles. Diseases of voluntary muscles (myopathy) or nerves (neuropathy) generally are a contraindication to PNS. Respiratory failure with normal phrenic nerves and diaphragm muscles is caused by either
- malfunction of the respiratory center in the brainstem or
- loss of the connection between the respiratory center and the nerve cells of the phrenic nerve in the cervical spinal cord.

In the latter case, also cells of the phrenic nerve may be destroyed, which decreases the amount of fibers (axons) in the nerve; this, in turn, decreases the amount of working muscle fibers and thus, muscle strength.

The phrenic nerves arise at both sides from the cervical segments 3 through 6 of the spinal cord . An electrode fixed to the nerve above the clavicle in the neck cannot stimulate the fibers coming from segment 6. The shortest way from the surface of the skin to the whole nerve is through the second intercostal space (between the second and the third rib) just outside of the borders of the sternum on both sides. The incisions needed to access the nerve are about 5 to 8 cm long. On the right side the nerve almost exactly drops vertically down from the clavicular groove to the center of the right diaphragm. On the left side, the nerve has to follow laterally the outer line of the heart but also ends in the center of the diaphragm. The center of the diaphragm is a tendon plate, which is fixed to the lower rib cage by the diaphragm muscles. The nerve separates into smaller units that innervate the different parts of the left and right diaphragm muscles. The fibers (axons) from C3 to C6 intermingle in the phrenic nerve. One axon serves muscle fibers in about 70% of the diaphragm muscle. Therefore, when electrically stimulating one quarter of the nerve a contraction results of one quarter of the muscles fibers, but this quarter is almost evenly distributed over the whole muscle. Naturally, we never contract more than one sixth of our muscle fibers at the same time.