Pressure profile in the caudal extradural space of standing horses before and after extradural drug administration

The objectives of this study were to measure the pressure in the caudal extradural space of standing horses and to evaluate the usefulness of pressure waves to identify correct needle placement. Caudal extradural pressure was measured in 12 healthy horses. The pressure and any extradural pressure waves were recorded for 3 min after puncture, for 1 min after testing for lack of resistance (LOR), and for 10 min after lidocaine injection. Successful extradural drug administration was confirmed in all horses. The median extradural pressure findings after puncture, after LOR, immediately after injection and 10 min after needle placement were −1.60 kPa (range −2.27 to 1.33 kPa), −0.67 kPa (−2.27 to 5.73 kPa), 5.00 kPa (0.93 to 9.87 kPa) and 0.13 kPa (−0.67 to 4.53 kPa), respectively. Extradural pressure waves were not always present. Extradural space pressure was sub-atmospheric in most horses and extradural injection significantly increased this pressure for up to 10 min. Extradural pressure waves had limited usefulness in the confirmation of the correct placement of the needle.