A trigonometric method to confirm needle tip position during out-of-plane ultrasound-guided regional blockade

Abstract

To the Editor, A common challenge using the out-of-plane approach during ultrasound-guided regional blocks is ensuring that the hyperechoic dot visualized on the screen is the needle tip and not the cross-section of the needle shaft. For superficial targets (1-2 cm), this can often be overcome by employing the ‘‘walkdown’’ approach. This technique involves inserting the needle at incrementally steeper angles, allowing identification of the tip until it is observed adjacent to the target nerve. With this method, the needle is inserted at a final angle of 45 , and the distance between the needle and probe equals the depth of the nerve. At depths greater than 2 cm, the ‘‘walkdown’’ approach becomes impractical. Targeting any nerve deeper than 2 cm requires a long needle because the needle insertion depth increases in proportion to the target depth and insertion distance. For example, using the Pythagorean theorem (a ? b = c), where a is a target depth of 3 cm, b is an insertion distance of 3 cm, and c is the needle insertion depth required to reach the target (see Figure), we find that a 5 cm needle would be barely long enough to reach the target (c = H(3 ? 3) = 4.24). Moreover, the further distance the needle is from the transducer, the further distance the needle must traverse tissue without real-time ultrasound guidance, in effect ‘‘blinding’’ it to the structures in its trajectory. Alternatively, we can circumvent these problems by reducing the distance between the insertion site and the ultrasound beam. Ideally, the needle would be inserted perpendicularly along the ultrasound scanning plane; however, in reality, the minimum distance is about 0.5 cm due to the space occupied by the footprint of the transducer. Again, applying the principles of trigonometry, a distance B 1 cm between the needle and the ultrasound beam becomes negligible as the target depth increases, and the needle insertion depth becomes comparable with the target depth. For instance, the difference between the depth of needle insertion and the actual depth of the target is minimal when the needle is directly adjacent to the transducer with a maximum distance of 1 cm away from the ultrasound beam: Needle length required 1⁄4 p 2 cm target depth ð Þ þ 1 cm insertion distance ð Þ 1⁄4 2:2 cm