What is the site of injection that shows the highest rate of systemic absorption of local anesthetic?

The intercostal injection area demonstrates the highest rate of systemic absorption of local anesthetic due to its anatomical positioning and the rich vascularization in that region. When local anesthetics are injected into the intercostal space, they easily diffuse into the surrounding tissues and are quickly absorbed into the blood vessels present, leading to faster systemic absorption compared to other injection sites.

This is particularly relevant because the intercostal space contains many small blood vessels and is situated in proximity to the highly vascular pleura, which further enhances the likelihood of anesthetic entering the systemic circulation quickly. In contrast, other injection sites like epidural, caudal, and brachial plexus injections typically have lower systemic absorption rates due to factors such as deeper positioning (in the case of epidural and caudal injections) or their location being more distal from major vascular structures (as is often the case with brachial plexus injections). Hence, the combination of anatomical features and vascularization makes intercostal injections notably more efficient for rapid systemic uptake of local anesthetic.