Epidural volume extension at Caesarean section
London, UK
E-mail: oxygen{at}freshgasflow.com
Editor—I read with interest the article on epidural volume extension (EVE).1 The proposed mechanism of EVE is that it fills the epidural space with liquid, which results in compression of the subarachnoid space leading to the local anaesthetic in it to be pushed upwards. On the few occasions I have used EVE, I have often wondered why the injected fluid does not run back out of the Tuohy needle. With fluid under pressure, one would expect some of it to track back out the Tuohy needle. During the study, did the authors find a significant backflow out the Tuohy after they injected 7 ml of saline? If they did not observe backflow, is it possible that more volume is required to cause compartmental compression? Perhaps, a study of the effects of EVE with enough saline to cause a backflow out of the Tuohy needle (a ‘fill to spill’ technique), thus guaranteeing compartmental compression, is required. However, this would have to be done cautiously as fluid boluses into the epidural space are known to have caused retinal venous haemorrhage.2
London, UK
* E-mail: gstocks{at}hhnt.org
Editor—Thank you for giving us the opportunity to respond to Dr Tilakaratna's query about the occurrence of fluid spill back from the Tuohy needle when saline is injected during epidural volume extension (EVE). In all previous studies of EVE, the fluid was injected into the epidural space via the epidural catheter and the problem of fluid leakage was not documented. When we began using this technique, we decided to inject directly through the Tuohy needle for two reasons. First, the epidural sets we use contain relatively soft catheters that occasionally cannot be inserted and a second attempt with a stiffer catheter is required. Previous studies have shown the effect of EVE to be time dependant and, by injecting via the Tuohy, we avoided this potential delay. Second, a study by Verniquet3 showed that flushing the Tuohy in this way reduced the incidence of a bloody tap with the catheter. Leakage of fluid back through the Tuohy needle does occasionally occur, but when it does we simply occlude the end with a thumb for no more than a couple of seconds, after which the flow ceases. However, in our experience, this happens infrequently and we speculate why this might be.
Hoffmann and colleagues4 demonstrated that the degree of protrusion of a spinal needle beyond the tip of a Tuohy needle required to successfully puncture the dura ranges from 2.5 to 15 mm, suggesting that there is wide variation in the size of the lumbar epidural space. We speculate that this will also cause variations in epidural space compliance. Just as this might explain why the same volume of EVE can have a variable effect on eventual block height achieved, variations in compliance might also explain why, in some, fluid may leak out of the Tuohy, but in others it does not. In this context, patient positioning may also be important in affecting epidural compliance. We find fluid leak to be more common when patients are placed in the sitting position during the combined spinal-epidural procedure. However, in our study, all patients were placed in the lateral position and leakage was not a significant problem. The infrequency of fluid leakage might also be related to the position of the tip of the Tuohy needle within the epidural space. Fat, nerves, and blood vessels are all present and it is quite possible that any of these may occlude the end of the needle and prevent leakage of fluid.
Although Dr Tilakaratna speculates that to guarantee intrathecal compression there is a need for ‘fill to spill’, the implication from this is that unless fluid does leak from the Tuohy needle, no compression of the dural sac can occur. As most EVE studies have injected volumes via the epidural catheter, it is impossible to say whether this is correct; however, clinical and radiological evidence cited in our paper demonstrates that an EVE effect can be obtained with volumes of 5–10 ml and in our experience this does not result in spill.
References
1 Beale N, Evans B, Plaat F, Columb MO, Lyons G, Stocks GM. (2005) Effect of epidural volume extension on dose requirement of intrathecal hyperbaric bupivacaine at Caesarean section. Br J Anaesth 95:500–3.
2 Clifford G. (2006) Complications of epidural corticosteroid injections. Topics Pain Manag 21:1–6.
3 Verniquet AJW. (1980) Vessell puncture with epidural catheters. Anaesthesia 35:660–2.[Web of Science][Medline]
4 Hoffmann VL, Vercauteren MP, Buczkowski PW, Vanspringel GL. (1997) A new combined spinal-epidural apparatus: measurement of the distance to the epidural and subarachnoid spaces. Anaesthesia 52:350–5.[CrossRef][Web of Science][Medline]
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