AbstractBackground:This study was designed to examine the spread of local anesthetic (LA) via magnetic resonance imaging after a standardized ultrasound-guided thoracic paravertebral blockade.Methods:Ten volunteers were enrolled in the study. We performed ultrasound-guided single-shot paravertebral blocks with 20 ml mepivacaine 1% at the thoracic six level at both sides on two consecutive days. After each paravertebral blockade, a magnetic resonance imaging investigation was performed to investigate the three-dimensional spread of the LA. In addition, sensory spread of blockade was evaluated via pinprick testing.Results:The median (interquartile range) cranial and caudal distribution of the LA relative to the thoracic six puncture level was 1.0 (2.5) and 3.0 (0.75) [=4.0 vertebral levels] for the left and 0.5 (1.0) and 3.0 (0.75) [=3.5 vertebral levels] for the right side. Accordingly, the LA distributed more caudally than cranially. The median (interquartile range) number of sensory dermatomes which were affected by the thoracic paravertebral blockade was 9.8 (6.5) for the left and 10.7 (8.8) for the right side. The sensory distribution of thoracic paravertebral blockade was significantly larger compared with the spread of LA.Conclusions:Although the spread of LA was reproducible, the anesthetic effect was unpredictable, even with a standardized ultrasound-guided technique in volunteers. While it can be assumed that approximately 4 vertebral levels are covered by 20 ml LA, the somatic distribution of the thoracic paravertebral blockade remains unpredictable. In a significant percentage, the LA distributes into the epidural space, prevertebral, or to the contralateral side.