Current trends in obstetric epidural practice.

Abstract

The article by Howell et al. (Anaesthesia 1998; 53: 238–43) highlights the present trends of current obstetric epidural practice. The paper suggests that epidural insertion with the loss of resistance to saline is the favoured technique, for practical and teaching purposes. The utilisation of loss of resistance to air remains popular, especially with consultants. Three of the anaesthetists surveyed practice loss of resistance to saline with an air cushion. Their choice is neither explained nor discussed. Are they simply sitting on the ‘epidural fence’, unable to decide whether to opt for air or saline, or does their technique have certain advantages? Location of the epidural space depends on accurate determination of ligamentum flavum penetration with an epidural needle. Whilst the sensation of a needle passing through the ligament flavum is reassuring, most techniques involve detection of a pressure change as the potential epidural space is entered. The loss of resistance technique involves the generation of positive pressure at the needle tip by exerting a force on the syringe plunger. The operator must detect the change in the pressure via the plunger, as the needle tip crosses the ligamentum flavum. The syringe acts as effector and sensor, its integrity vital for accurate detection of the epidural space. If the plunger of the syringe sticks, any force applied is exerted on the walls of the syringe chamber, without an increase in pressure of the syringe contents. Failure to generate pressure at the needle–ligamentum interface, when the syringe plunger sticks in the barrel, will prevent detection of a pressure change as the ligamentum flavum is breached. Such syringe failure, although rare, should be established if inadvertent dural puncture is to be avoided. Air, unlike saline, is compressible, its volume decreasing as syringe pressure increases. This volume change may be seen in a ‘loss of resistance syringe’, if an air bubble is included within the saline. This confirms that plunger pressure is transmitted to the syringe contents and that the syringe is functioning as an effective unit. With this knowledge, loss of resistance with saline at the needle–ligamentum interface can then accurately be achieved. The three respondents of this survey using saline with air have perhaps taken the saline versus air discussion one step further. Are saline and air together a better choice than either one alone?