Ultrasound used with nerve stimulation offers great advantages over nerve stimulation alone. It can enable us to see the interaction of the needle and nerve, the location of important structures like the lung, and the location of the local anesthetic medicine injection. In fact, our research shows that simply using ultrasound to mark landmarks can improve block success among beginners, and ultrasound can be especially helpful for practiced anesthesiologists during more difficult blocks. Additionally, it can be of great value when nerve stimulation is not feasible. For example, inducing a muscular twitch in a patient with an unstable fracture can cause severe, emanating pain with no identifiable endpoint. Nerve stimulation may also be difficult or impossible in patients who are uncooperative or who have insensate limbs.
However, current ultrasound technology has its own limirations. The exact location of the needle tip can be ambiguous, even when the ultrasound "beam" is well aligned with the needle. Nerves can also be difficult to identify on ultrasound. Their appearance changes when they are surrounded by fluid, and they change in appearance with anatomic location. Nerves appear blackened, or hypoechoic, above the clavicle but brighter, or hyperechoic, in the lower extremity. For these reasons, we confirm needle placement with stimulation whenever possible, even when we are using ultrasound.
Sometimes, ultrasound is just not clinically viable. Just recently, I performed a lateral popliteal block on an ICU patient with uncontrolled, severe foot pain. I could nto use ultrasound because her unstable medical condition prevented me from moving her at all and made it impossible to place the ultrasound probe properly.
So, in our program, we ask residents to master anatomy, block indications, block options and nerve stimulation skills before we ever introduce ultrasound techniques. When we do, we first teach residents to identify landmarks with ultrasound. WHen they are facile with this task, they graduate to performing "real-time" ultrasound simultaneously with nerve stimulation.
We have found ultrasound to be a tremendous technology for education and problem-solving, and for generally helping us guide the needle to its destination. However, we never teach our residents to rely on ultrasound alone. It is an excellent tool in our arsenal that works with - and not to the exclusion of - nerve stimulation.
