Influence of volume on the spread of local anesthetic-methylene blue solution after injection for intercostal block

The purpose of this study was to evaluate the influence of the volume of methylene blue-local anesthetic on the spread of the injectate along the costal pleura. Twenty patients undergoing elective thoracotomy were studied. Twelve patients received intercostal nerve injection with 10 mL of 0.5% bupivacaine with methylene blue (10-mL group), and eight patients received 5 mL of 0.5% bupivacaine with methylene blue (5-mL group). The area of spread of the methylene blue was measured after the pleural cavity was incised. The 10-mL group had a mean area of spread of 51.1 cm2 as opposed to 17.6 cm2 for the 5-mL group (P less than 0.05). In the 10-mL group, eight patients had bupivacaine-methylene blue spread to two intercostal spaces, three patients to three intercostal spaces, and one patient to four intercostal spaces. In the 5-mL group, seven patients had bupivacaine methylene blue spread confined to one intercostal space and one patient to two intercostal spaces. We conclude that a potential anatomic space exists between the costal pleura and the internal intercostal muscle and that the spread of local anesthetic after intercostal nerve block injection is volume dependent.