Epidural test dose

There is no agreement on the selection of an appropriate epidural test dose. In their survey of thoracic epidural analgesia, Pennefather et al. report that in 2004, 60% of respondents used a test dose of 3–5 ml of either 0.25% or 0.5% bupivacaine [1]. There is also variation in selection for the obstetric population [2]. The aim of a test dose is to detect misplacement of needle or catheter in the intravascular, intrathecal or subdural space [3]. It should be safe, effective and reliable, allowing practical interpretation within a reasonable timescale, traditionally at 5 min. The test doses using bupivacaine (between 7.5 mg and 25 mg) reported by Pennefather cannot reliably exclude epidural misplacement. Intravenous injection of similar doses of bupivacaine elicited no observable signs in 70% of subjects [4]. Intrathecal injection of bupivacaine may not produce observable signs of subarachnoid block within the review period [5]. Additionally, subdural misplacement is ‘the third space to go astray’[6]. Moore and Batra [4] proposed using a test dose of 45 mg of lidocaine with 15 μg epinephrine. The utility of the epidural test dose has been systematically reviewed, with a focus on testing for accidental intravascular epidural placement [3]. Achievement of a sensitivity and a positive predictive value both equal to or greater than 80% in at least two randomised controlled trials from two different groups was considered effective. For non-pregnant adult patients, a heart rate change following a test dose containing 10 or 15 μg of epinephrine was considered to be the most useful. In contrast, the ability of a test dose to discern accidental intrathecal or subdural placement has not been systematically reviewed. Given the lack of systematic evidence, it is not really surprising that test dose selection is so variable. We believe that there is a good case for a new approach to test dose selection. National co-ordination of this approach would be helpful. Epidural analgesia is a routine part of our practice, and is generally considered to be ‘safe’, yet there is still scope to improve our practice with a rational choice of a test dose.