Interpleural local anaesthesia: anatomical basis for mechanism of action.

India ink was injected through an extradural catheter placed in the right interpleural space in cadavers at autopsy. The lung was removed and the spread of the ink noted and photographed. In six cadavers, block sections were cut at the anterior axillary line through three ribs, intercostal muscles and parietal pleura. The blocks were fixed in formalin and then decalcified, sectioned, stained and mounted on slides. In all cases the ink covered the chest wall, lung and diaphragm, and was seen to pool in the paravertebral area. Examination of the histological slides revealed black pigment on the pleural surface and in the subpleural space. The mechanism of transport across the parietal pleura is discussed.