A new method of identifying the epidural space

I should like to describe what I believe to be a novel method of identifying the epidural space. The most commonly used techniques for identification are loss of resistance to either air or saline. Both techniques require equipment, usually a purpose-built low-friction syringe, and the introduction of a fluid into the epidural space. The injection of air into the epidural space has been suggested to be associated with both inadequate analgesia and life-threatening complications [1]. The new technique involves using loss of resistance to the passage of the catheter itself as the means of identifying, and subsequently catheterising, the epidural space. A standard Tuohy needle is inserted into the interspinous ligament in the usual fashion. The trochar is removed, the feeding guide is placed in the hub of the needle and the catheter is passed through it down the needle. The catheter stops against the tissue at the bevel of the needle. The catheter is now grasped with the thumb and index fingers about 5–10 mm from the feeding guide, i.e. at approximately the 12-cm mark on the catheter. The needle is advanced in increments of 1–2 mm and, at each stop, an attempt is made to advance the catheter. The pressure applied is limited to the first sign of buckling of the catheter. When the catheter passes through the ligamentum flavum, a distinct loss of resistance is felt and the catheter can then be advanced into the epidural space. Bench tests have shown that a 16G Portex epidural catheter is able to exert a pressure of up to 100 mmHg at the end of the needle before buckling when used in this fashion, while an 18G catheter can exert a pressure of up to 70 mmHg. Studies with pig dissections have confirmed that these pressures are sufficient to allow the catheter to pierce the ligamentum flavum that has been scored by the needle, but not the dura mater (1, 2). Microscope inspection of used catheters has revealed no damage to their tips. Tenting of the ligamentum flavum by an epidural catheter seen in a dissected pig cadaver. Penetration of the ligamentum flavum in the same specimen as Fig. 1. I have successfully used this technique in over 400 obstetric and surgical patients without a single incident of accidental dural puncture. I am currently embarking on a study to determine the success and complication rates when the technique is used by anaesthetic trainees. I would welcome the comments of readers on this technique.