Intraoperative PNS

One of the more frequently used tools in the OR is the peripheral nerve stimulator (PNS). The battery powered device delivers depolarizing current via two electrodes. When properly placed, it can be used to evaluate the level of neuromuscular blockade in a patient and tell us when it is appropriate to use neuromuscular blockade reversal agents. One of the more commonly used devices is pictured below. The device is pretty intuitive to use, so I won't go into too many details. This tutorial is more focused on proper lead placement to optimize your results. Just know that the black represents the negative lead and the red represents the positive lead.

Early on in my education, I felt like it was one tool I was not using properly. In particular, my lead placement was kind of all over the place. There were often times when I would be getting no PNS response only to have the surgeon yelling about the patient moving moments later. Briefly, I am going to go over how to properly place the leads using the more common nerves. These are not the only nerves you can use, but they are utilized often because they exhibit a visible motor reaction when stimulated, are close to the surface of the skin, and are often easy to access during a variety of surgeries.

Ulnar Nerve: In general, this is your best option for evaluating a patient for extubation and reversal of neuromuscular blockade. When giving muscle relaxants, this nerve response will be the first to go and last to return. That means it is not a great indicator for intubation but gives a larger margin of safety when determining status for reversal and extubation.

For proper PNS, extend the arm in a relaxed state with the palm up. The two electrodes will be placed over the path of the ulnar nerve. The more distal and negative electrode is placed at the level of the wrist on the ulnar surface at the flexor crease. The positive lead is then place a few cm proximal to the first. See the image below.

Facial Nerve: When giving muscle relaxants, this nerve is one of the last responses to disappear and one of the first to reappear. For this reason, it can be reliably used for intubation (not something you will often see) but isn't the standard of care for extubation and reversal, even though the onset, duration, and sensitivity of the muscles stimulated are the same as most of the respiratory muscles.

The positive electrode is placed on face at the outer canthus of the eye. The negative lead is then attached a few cm beloe, at the same level of the tragus of the ear. See picture below.

Posterior Tibial Nerve: This nerve is often used in surgeries when the head of the bed is turned 180 degrees (craniotomy, ENT, etc). Sensitivity should be very similar to that of the ulnar nerve.

The negative electrode is placed more distally, just posterior to the medial malleolus. The positive electrode should be placed a few cm above the first, more proximal. See image below.