Transversus Thoracic Muscle Plane Block on a Cadaver With History of Coronary Artery Bypass Grafting

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To the Editor:
In February 2015, Ueshima and Kitamura1 published an article describing the ultrasound-guided transversus thoracic muscle plane (TTP) block for anesthesia and analgesia of breast cancer resection. In a cadaveric study using 3 embalmed cadavers, they showed that 15 mL of blue dye injected into the plane between the transversus thoracic and internal intercostal muscle closer to the lateral border of the sternum covered the intercostal nerves from T2–T6 anteriorly, which supply innervation to the sternum.2,3 In a subsequent report, the same authors described 2 cases where the TTP block provided effective analgesia for patients undergoing median sternotomy.4
Cardiac surgeries are most commonly performed by median sternotomy, with coronary artery bypass grafting (CABG) making up the majority. A number of studies have shown that cardiac surgical patients have significant sternal pain after surgery, and the highest pain scores are reported in the immediate postoperative period.5 Patients who undergo internal mammary artery (IMA) harvesting for CABG also experienced more intense pain.6 According to these studies, patients complain about the most severe pain on postoperative days 1 and 2 (mean of 3.7 and 3.9, respectively, on a pain scale of 0–10).
Theoretically, the TTP block should provide significant analgesia after cardiac surgery. In order to confirm the spread of local anesthetics in this patient population, we performed bilateral TTP blocks on a fresh cadaver with a surgical history of CABG with left IMA harvest. The block was performed in the supine position. Under dynamic ultrasound guidance, 20 mL of blue dye (15 mL of 5% dextrose, 2 mL methylene blue, and 3 mL methyl cellulose) was injected bilaterally between the fourth and fifth ribs adjacent to the sternal border using a 17-gauge Tuohy needle. On the right chest wall, the muscle plane between the internal intercostal muscle and the transversus thoracic muscle was clearly visualized under ultrasound, whereas the border was vague on the left.
Twenty minutes after the block, we dissected the cadaver. We confirmed the spread of blue dye in the appropriate plane between the transversus thoracic muscle and the internal intercostal muscle. On the right side, the spread was between the first and sixth intercostal spaces; however, on the left side, the dye was confined to the left fourth and fifth intercostal space (Fig. 1). On further dissection, we noted significant postsurgical scarring, as well as excessive fat tissue on the left compared with the right side, presumably because of previous left IMA takedown. This scar tissue prevented the dye from spreading.
Our study differs from Ueshima and colleagues' previously published reports in 2 major aspects. First, our TTP injections were performed on fresh cadavers, which simulates injections in living patients. Second, our probe position was parallel to the sternum, and injection was performed in-plane caudad to cephalad unlike the technique described by Ueshima et al (transverse probe position, in-plane approach).
The TTP block is a novel regional technique that has the potential to provide significant analgesia for patients undergoing median sternotomy; however, in patients who undergo IMA harvest, the TTP block may have decreased efficacy because of tissue disruption and scar tissue formation in the TTP. We are the first to show that preexisting parasternal scarring can limit the extent of local anesthetic spread. In addition, the transversus thoracic muscle is a thin muscle with digitations in its attachments to the ribs; hence, care should be taken to correctly identify the intermuscular plane.

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