Continuous or intermittent loss of resistance for identifying the epidural space.

Abstract

Dr Menon (Anasthesia 2000; 53: 92) describes a technique using pressure on the epidural catheter to identify the epidural space while intermittently advancing the Tuohy needle and suggests that this strategy might be taught to anasthetic trainees. We disagree with this advice. Saline is preferred to air for identification of the epidural space not only because it avoids specific risks associated with injection of air into the body as stated by Dr Menon, but also because there is a growing body of evidence that the incidence of dural puncture is reduced [1, 2] Two mechanisms have been suggested to explain this. First, using the ‘loss of resistance to saline’ technique as described by Doughty [3] and Lewis [4], constant pressure is exerted on the plunger of the syringe as the Tuohy needle is carefully but continuously advanced. The nondominant hand is continuously used as a brace against the parturient's back, and needle advancement ceases the instant the epidural space is reached. With ‘loss of resistance to air’, as with Dr Menon's ‘loss of resistance to epidural catheter’, intermittent advancement and testing results in intermittent loss of control of the Tuohy needle and increases the possibility of overshooting into the subarachnoid space. The second mechanism that may protect against accidental dural puncture is that the pressurised saline pushes the dura away from the tip of the Touhy needle immediately it enters the epidural space. Dr Menon suggests that pressure on the epidural catheter might breach the last few fibres of the ligamentum flavum, conferring a slight advantage of his technique over the use of air. However, it is likely that, even with the 1–2 mm increments recommended, the Tuohy needle will frequently enter the epidural space directly. We would suggest that any technique where the Tuohy needle is advanced and tested intermittently (using epidural catheter, air or indeed saline) is inferior to loss of resistance to saline under continuous pressure [3, 4] and should not be taught to or used by novice anasthetists.