Laterality of injectate spread in bilateral ultrasound-guided thoracic paravertebral block: a pilot cadaveric study

Although thoracic paravertebral block (TPVB) is an established regional analgesia technique in thoracic surgery, the extent of injectate spread is variable. Furthermore, while it is known that a larger capacity exists in the left thoracic paravertebral space (TPVS) compared with the right due to mediastinal asymmetry, it remains unclear whether this characteristic translates to functional differences in injectate spread. We performed bilateral TPVB in twelve Thiel-embalmed cadavers using an intercostal approach—six were placed in the left lateral decubitus position and six in the right lateral decubitus position. To control for positional bias, 20 mL of dye was administered to both the upper and lower sides while maintaining the same position. The numbers of stained vertebral segmental TPVSs and intercostal spaces (ICSs) were compared between the left and right sides. TPVB performed on the left side demonstrated significantly more stained segments (median [interquartile range]) than that performed on the right side (TPVS: 4.0 [3.0–5.0] vs. 2.0 [1.0–3.0], p = 0.01; ICS: 4.0 [3.0–5.0] vs. 3.0 [2.0–4.0], p = 0.04). This tendency remained consistent regardless of the injection side (upper or lower). In addition, we consistently observed extensive dye distribution when the parietal pleura medial to the transverse process was markedly displaced, irrespective of laterality. This study demonstrates a significant laterality in injectate spread during TPVB, with more extensive longitudinal distribution on the left side. Our findings may provide important insights for understanding the variability of injectate distribution in TPVB.