Ultrasound-guided transversus abdominis plane block in obese patients

Sir, Ultrasound-guided (USG) transversus abdominis plane (TAP) block is routinely performed in adults for abdominal surgeries like caesarean section and open appendectomy.[12] This block is usually performed in the supine position under ultrasound guidance (in-plane approach). The ideal position of the needle during TAP block is along the anterior axillary line, as the lateral cutaneous branch of the intercostal nerves, which supply the antero-lateral aspect of the abdominal wall, leave the neurovascular plane (TAP) along the mid-or anterior axillary line.[3] In obese patients, the performance of this block can be challenging due to excessive subcutaneous fat and increased depth of TAP. It is recommended that USG-TAP block is performed under real-time ultrasound guidance in obese patients. In the supine position, the subcutaneous fat hangs over the flank and leads to difficulty in probe handling and poor visualization of the needle during the procedure [Figure 1]. Hence, we perform USG-TAP block with the patient in a semilateral position by placing a wedge under their flank on the ipsilateral side of the block, which pushes the subcutaneous fat medially and flattens the procedure field. This makes handling of the ultrasound probe easier and visualization of the needle is also improved because the depth of the TAP is decreased and we are able to introduce the needle at a more obtuse angle in relation to the ultrasound probe [Figure 2].Figure 1: Line diagram depicting the ultrasound-guided transversus abdominis plane block in the supine position in obese patients and α=needle-beam angle. The needle-beam angle in the supine position is acute and, therefore, needle visualization is difficult. Also, the handling of the probe is difficult in view of the overhanging subcutaneous fat tissue in the flank regionFigure 2: Line diagram depicting the ultrasound-guided transversus abdominis plane block in the semilateral position in obese patients and α=needle-beam angle. The subcutaneous fat is pushed medially and, hence, probe handling is easier. In the semilateral position, weare able to achieve better alignment between needle and ultrasound probe and the needle-beam angle is obtuse. Hence, we have better needle visualization in the semilateral positionNeedle-beam angle is one of the most important factors influencing the visualization of the needle during the in-plane approach of an USG procedure.[4] It is difficult to attain a desired (>55 degrees) needle-beam angle with the increasing depth of the target during USG procedures. Hence, we suggest performing USG-TAP block in a semilateral position rather than in a supine position in obese patients, as the handling of the ultrasound probe is easier and needle visualization improves due to better needle-to-probe alignment.

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