A SIMPLE IDEA HAS MADE A big difference in a new stimulator for peripheral nerve blocks. By varying the duration of the stimulator's electrical pulses, we can locate nerves easier and faster, and “lock on” to the nerve plexus as we advance the needle without losing the muscle response. After performing close to 500 peripheral nerve blocks with this technology, I now use it exclusively, and I'll never go back.
To understand how it works, consider how most professionals do blocks now. We attach the needle to the stimulator, aim the needle at the nerve, insert it into the skin and turn it on. The stimulator directs electrical pulses of consistent duration ahead of the needle.
Initially, when we are far away from the targeted nerve, we turn the amplitude up. When we reach the proximity of the nerve, the current causes the muscle that is innervated by the targeted nerve to twitch. We then slowly advance the needle, reducing the amplitude as we go, until we can get the muscle to twitch at very low amperage. That's a sign that the needle tip is very near the nerve or within the neural plexus—right where the anesthetic needs to go.
But the process is far from foolproof. We often have to hunt and peck to get the needle well positioned so it elicits a muscle response. And every practitioner has experienced the frustration of finding—then losing—the response due to inadvertent needle movement. Even tiny changes in direction can cause you to lose the targeted nerve.
It works like a homing signal by emitting long and short pulse durations over and over. The long pulse can elicit a muscular response even when the needle is far from and/or not directly aimed at the nerve. Even though this pulse is the same amperage as the shorter pulses, it delivers more charge due to its duration. To begin you only see one twitch every second. As you get nearer to the nerve the shorter pulses start to elicit a response, and as a result you see three twitches every second.
Clinically, this seemingly small change makes a big difference. After achieving the motor response, I can push the needle to one side and the other, and evaluate the strength and/or speed of the muscle twitches. I can then redirect the needle toward the stronger or faster twitches, without the twitches disappearing completely. It also gives me feedback about surrounding nerve structures so I can avoid unwanted nerves, or even target two nerves at once. During a sciatic nerve block, for example, I can position the needle to obtain both a tibial and perineal nerve response, and intentionally involve one nerve more than the other.
With block failure rates already at only 1 percent, this device may not improve block success. But it's bound to make experienced practitioners faster and make less experienced ones more facile, and that will surely benefit our patients.
Dr. Urmey is with the Department of Anesthesiology, Hospital for Special Surgery, Weill Medical College of Cornell University.