Block Evaluation
TAKE TIME TO LOOK BEFORE YOU LEAP
Beyond visualizing a successful block on ultrasound, I usually will want to see an immediate effect of my local by asking the patient to raise their arm toward the ceiling (deltoids, C5 for an ISB) or extend their knee (for a femoral block). There are reasons why they still may be able to perform these tasks despite a successful block, but that topic is for another day. Today, I want to talk about the other end of the spectrum of false positive and false negative physical exams. [nonmember]…
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The point that I want to stress -especially as you begin doing lots of blocks (so you will develop good practice patterns, or good ‘memorized habits’) is to take a moment to ask about current symptoms and evaluate the body part to be blocked before you perform your block. Ultimately, you may have only moments to evaluate your results before being rushed off to other responsibilities. You can be reassured on ‘questionable’ blocks and hold off on worrying about seemingly unacceptable immediate block results by just a few quick things that you do beforehand or clarifications of patients’ responses. As well, you might keep from being fooled by signs that you would otherwise believe are confirmatory to the success of your block.
Let’s start with the first example I gave above. You want to know the patient’s ability to raise their arm before you do an interscalene nerve block, so ask them to raise it before you get started. If they have a significant rotator cuff injury, they may not be able to move their arm up at all. You assume you are ‘the man’ because your block set up so quickly only to discover rather too late that it was a total failure. They might not raise it due to significant sedation medication as well. Conversely, you see them raise it afterward just fine, and you are puzzled because it seemed perfect. It may be, especially with a posterior approach, that your block has already knocked out sensation significantly which allowed them to raise it at all. Some patients will raise their arm with significant pain, but they can do it. If you checked this beforehand, you would see the significant difference was the ease in which they raised their arm.
I will also ask the patient to tell me if they are in pain at rest ‘right now’ prior to a block. Sometimes this is obvious, but if you ask afterward if their ‘X’ hurts, and they say ‘no’, it may be that it didn’t hurt before the block either. Also, your asking about their post-block pain may be the only thing that they now come to recognize is different from just beforehand. Further, asking them if their ‘X’ tingles is not helpful information if their ‘X’ always tingles. It has happened to me more than once that a patient has answered in the affirmative that they do, in fact, have a ringing in their ears. It now occurs to me to ask them if that is a new sensation or not.
Another point to make here is about ‘the absence of sensation of the sensation of absence.’ Asking a patient if they ‘feel’ numbness sometimes will lead you astray. If they are not touching their thigh after you do a femoral nerve block, for example, unless they had just been hurting, they will likely say, ‘no’ even if they are numb. They are not receiving any sensory input from their thigh because nothing is really stimulating it at that moment. That is not the sensation of ‘numbness’; it is just the absence of stimulation. They don’t know they are numb until you actively stimulate them and recognize the absence of stimulation. Don’t ask, ‘Are you numb?’ Check and see if they are numb.
Further, when you stimulate them, distinguish clearly the sensation of touch from the sensation of sharp. If you ask while you scratch them with fingernails, ‘Do you feel me touching you here…and now over here?’, they are likely to say ‘yes’ even if the sensations are different. You need to ask if the two sensations within and outside the block area feel different or not. Sometimes that still needs more clarification, or they will just not be able to perceive the difference in sensation that you are asking them to differentiate. Using cold objects helps, but you have to ask if this is cold here, too. Don’t ask,’Does this feel the same as that?’ because they might answer ‘yes’ meaning that they recognized the pressure of each touch, not the difference in temperature.
Finally, make sure you are actively stimulating an area covered by the particular nerve block that you performed. I don’t mean to say that you are unaware of what area is supposed to be covered. What I mean is that you want to be sure you don’t overlook this due to haste. For example, don’t reach under the blanket to test sensation to the sciatic nerve distribution below the knee and not pay attention to the fact that you are stimulating the medial aspect of the calf. That is a saphenous distribution. The other point to this is to try to stay away from ‘border zones’. Don’t test the lateral side of the thigh for a femoral block which might actually be the distribution of the lateral femoral cutaneous nerve.
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