Ultrasound guidance has advantages over traditional methods for localizing peripheral nerves. There are studies demonstrating more rapid block placement, faster block onset times, and reduction in the dose of local anesthetic required.1–3 Needle visualization is a specific advantage of ultrasound guidance because it enables precise needle placement in proximity of a target nerve. However, visualization of local anesthetic spread is also important. Failure to observe local anesthetic spread may be an indication of intravascular injection. We present a case in which intravascular injection of local anesthetic occurred and was quickly detected despite negative aspiration for blood during an ultrasound-guided axillary block.
A 54-yr-old 78 kg ASA physical class II patient was scheduled for left small finger proximal interphalangeal joint arthrodesis. The patient's medical history was significant for hypertension that was well controlled. The anesthetic plan included an ultrasound-guided axillary brachial plexus block in combination with IV sedation. After attaching electrocardiogram, noninvasive arterial blood pressure monitors, and Spo2 and the administration of supplemental oxygen, the patient was sedated with IV midazolam 2 mg and fentanyl 100 mcg. The axillary artery and surrounding nerves were easily visualized (Fig. 1) using ultrasound guidance (Sonosite MicroMaxx, 13–6 MHz 38-mm broadband linear array transducer). A 22 gauge 2 in. (50 mm) Stimuplex (B Braun, Bethlehem, PA) needle was inserted using an in-plane approach from the anatomic lateral side of the ultrasound probe. In an attempt to localize the ulnar nerve, the needle was advanced past the axillary artery (Fig. 2). No motor response was obtained despite a stimulating current of 0.5 mA, and an apparent intimate relationship between needle tip and ulnar nerve. After negative aspiration, 1 mL of ropivacaine 0.5% was injected without visualization on the ultrasound monitor. After repeat negative aspiration, another 2 mL of ropivacaine was injected without visualization. Injection was halted, and the amount of pressure applied to the skin with the ultrasound transducer was reduced. The basilic vein, which had previously been collapsed from transducer pressure, was visualized (Fig. 3) with the stimuplex needle in its lumen. Aspiration at that time revealed venous blood, and the needle was repositioned. The nerve block was completed and 30 mL of ropivacaine 0.5% was injected with good visualization of local anesthetic spread. The block proved to be adequate for surgical anesthesia, and the patient did not exhibit any signs of local anesthetic toxicity from the small amount of presumed intravascular injection.
Local anesthetic toxicity has been reported during axillary nerve block despite negative aspiration of blood.4,5 The mechanism of local anesthetic toxicity from peripheral nerve block can be due to either a direct intravascular injection or from systemic absorption of a toxic dose of local anesthetic. Toxicity from direct intravascular injection typically occurs at the time of injection, whereas toxicity from absorption necessarily has a delayed onset. Two commonly used measures to avoid direct intravascular injection include the addition of epinephrine to local anesthetics and frequent aspiration for the presence of blood. Although epinephrine may help identify an intravascular injection, it has the potential to increase nerve injury if injected intraneurally,6 and we do not typically add it to our local anesthetics. The Raj test is commonly used to confirm appropriate local anesthetic spread,7 however, because there was no motor response elicited at the time of injection, it was not possible. The fact that we did not persist in attempts to elicit a motor response was reasonable because we had visual confirmation of needle to nerve proximity by ultrasound.8
A purported benefit of ultrasound guidance is that its use will decrease complications. In this case, its use may have increased the likelihood of an intravascular injection but reduced the amount of intravascular local anesthetic injected. With a nerve stimulator-guided technique for an axillary block, the needle insertion site is usually proximal to the palpating fingers which either rest over or straddle the arterial pulse. Because of that, the tissues underlying the needle tip are not compressed, and aspiration of blood would be more likely to occur if a vascular structure was punctured. Using an in-line ultrasound-guided approach, this is not the case. The needle tip is beneath the probe at the time of injection. Aspiration may only be possible when the pressure from the probe is reduced, as in this case.
The fact that local anesthetic spread was not visualized on ultrasound imaging alerted us to a possible intravascular injection even though the needle appeared to be correctly positioned near the ulnar nerve. Although ultrasound guidance in regional anesthesia clearly has its advantages (visualization of nerve, needle, surrounding structures, and local anesthetic spread), there are certain pitfalls that must be anticipated and avoided when incorporating its use into practice.
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