Site of action of continuous extrapleural intercostal nerve block. Continuous extrapleural intercostal nerve block has been shown in a randomized, controlled study to be effective in reducing postoperative pain after thoracotomy and in restoring pulmonary mechanics. To assess the extent of spread of bupivacaine infused through an extrapleurally placed cannula inserted at thoracotomy, iohexol (Omnipaque) was infused at 5 days postoperatively in 5 patients and computed tomography performed. The contrast medium was confined to the paravertebral space covering on average six intercostal spaces. This study demonstrated that anatomically, the site of action of the bupivacaine infused through an extrapleural cannula was primarily in the paravertebral space. 