Thick or thin, it matters not. Just don't use too much

A fashionable tenet among obstetric anaesthetists in recent years has been to use large volume – low concentration epidural solutions to achieve labour analgesia. The theory has been that dilute concentration do not penetrate the myelin of motor nerves, but do penetrate the unmyelinated axons of the C fibres that carry the afferent pain stimuli from the contracting uterus. Larger volumes also seemed to help the physical spread of the epidural solution from the mid thoracic nerve roots to the low sacral ones. And all using a dose of local anaesthetic that was not dangerous either in the CSF or the circulation.

However, things may not be what they seem. Ngan Kee (in IJOA July 2006) compared doses of 30 mg ropivacaine (without opioid) in 5, 10 and 20 ml boluses at induction of epidural analgesia. There were no statistical differences in analgesia, motor block, satisfaction or delivery outcomes between the groups of Hong Kong women. Similarly, Bernard (Anes Analg 2003) found no differences between different concentrations of the same dose in PCEA boluses. And this month in IJOA Jan 2007 Buyse et al conclude that the ED50 of 10 ml of bupivacaine is 0.149%. In 20 ml the ED50 is 0.07%; the dose is the same.

Buyse et al also explore the effect of sufentanil in epidural solutions and conclude that 7.5 ug reduces the MLAC of bupivacaine by 90%! However the up-down graph for bupivacaine in this paper looks a bit funny to me; it’s not the usual saw-tooth appearance and I wonder if it suffers from chance. Nonetheless it supports the notion that for many women labour analgesia can be achieve with very low doses of local anaesthetic with opioid. If 10 ml is used, 0.1 to 0.15% bupivacaine is usually adequate.

I still use a higher volume technique because of the technical advantage: priming the epidural space with fluid (and I usually use 8-10 ml of 0.08% bupivacaine with fentanyl) facilitates its catheterisation; it may also hasten analgesia if difficulties do occur with catheterisation. But I cannot continue to believe the large volume-low concentration regimen makes that much difference to low dose alone; but 10 – 15 mg of epidural bupivacaine with opioid is enough to cover most women, and is a dose that is unlikely to harm if it goes into the wrong place.