Use a Longer Needle
I discussed in a previous Tip of the Week about some of the advantages of using a larger gauge needle, and today I would like to discuss advantages of using a longer needle to improve your success. I encounter commonly the view, not put into words exactly, but expressed in behavior and choice, that a superficial block is best approached by a short path with a short needle. I believe that [nonmember]…
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the thought process is that a shorter path will be less trespass (so less pain for the patient) and a more simple approach (technically more simple for the operator). The operator then lines up the target under the probe and enters the skin just lateral (in-plane at about a 45 degree angle or out of plane) to the probe on the shortest path to the target. The probe never needs to move from the original position. I get it, and this may be a reasonable approach as you are beginning your ‘conversion’ to using ultrasound in your practice, but I believe that you are making it more difficult for yourself and even jeopardizing analgesic effect for the patient in the end by this approach.
Beyond the points that I want to make today, my general opinion is that two inch needles are worthless. I favor a three and a half or four inch needle for superficial nerve blocks. Having two inch needles around only leads to the accidental opening of the wrong tray (which always seems to happen on a very busy block day) and having to search for the correct tray, wasting valuable time. Maybe we should lobby manufacturers to label trays with different colors depending on the needle within.
Anyway, back to the topic at hand. The difference between what you see at about a 45 degree angle and a near 90 degree angle of the needle to the probe is significant and allows us to take advantage of what ultrasound technology really affords us. That is, it allows for greater precision in guidance and evaluation of the position of our needle and local anesthetic spread relative to our target and to adjacent structures. At a 45 degree angle, much of the sound is reflected away from the probe (I know that there are echogenic needles and ultrasound features that can make up for this to some degree), so you are left with watching tissue distraction and a shadow below the shaft of the needle (if you remain in-plane, or you will just be using tissue distraction and possibly nerve-stimulation to arrive at your goal). At closer to 90 degrees, the (even un-echogenic 25 gauge) needle will brightly light up, and you can differentiate the tip of a curved (Tuohy) needle easily. Focusing on a bright needle takes much less mental focus to track than the movement of a shadow. For this reason, I will almost always choose an in-plane versus an out-of-plane approach to nerve blocks (certainly for superficial approaches). Not only can I locate the needle tip easier, I can also more quickly recognize correct and incorrect local anesthetic spread in and around the target.
The objection to this, I measure by the change in the size of people’s eyes when I show them where I recommend entering the skin to achieve this angle. It is usually an inch or two away from their intended entry site. I hope that you can take from my comments in the last paragraph that I do not intend for this path to be taken blindly. It will necessitate some probe movements and localizing the underlying tissue a little more. I will save more on that for another day, but it is really not a dramatic technique.
Further, entering from a distance allows more of the catheter to be under the skin, improving its securement in place (both at the skin and at the target location) and minimizing the negative effects of ‘track back’ (see last week’s comments for more on this). It is certainly important to choose the particular path appropriately for each type of nerve block that you perform such that you do not mitigate these advantages or even cause a higher likelihood of these problems to occur. That sounds like a great introduction to another Tip of the Week!
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