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Loss of Resistance to Saline Reduces Responses Accompanying Spinal Needle Insertion during Institution of ‘Needle-Through-Needle’ Combined Spinal-Epidural Analgesia

Normal saline or air is used to identify loss of resistance during identification of the epidural space for combined spinal-epidural analgesia. Following epidural needle placement using air for loss of resistance, up to 80% of parturients move, grimace, vocalise or experience paraesthesia or dysaesthesia during subsequent dural puncture by a spinal needle. We compared the effects of saline versus air for loss of resistance on the occurrence of these subjective and objective responses during thecal penetration. With institutional approval, 55 parturients presenting for labour analgesia were studied. After infiltration of lignocaine at an L2-L5 vertebral interspace, a 17 gauge Tuohy epidural needle attached to a 5 ml loss of resistance syringe containing either saline or air was inserted and advanced until loss of resistance was identified by injection of 3 to 5 ml of content. During subsequent ‘needle-through-needle’ insertion of a 27 gauge pencil-point spinal needle through the meninges, all subjective and objective patient responses were recorded, as well as each patient's reply to the question “Did you feel that?”. The two groups (n=28, n=27) were comparable. In those given saline and air respectively, 5 (18%) and 12 (44%) parturients responded to and/or acknowledged having perceived dural puncture (P <0.005). Overall, 7 and 31 (P <0.0005) subjective and objective responses occurred during dural puncture in those given saline and air, respectively. The study found that use of saline to determine loss of resistance is associated with fewer patient responses at the moment of thecal penetration during ‘needle-through-needle’ placement of the spinal needle at combined spinal-epidural analgesia.