Intrathecal diamorphine or intrathecal fentanyl to supplement spinal anaesthesia for Caesarean section?

D. W. Cooper1, C. M. Cowan2, K. Smith2, J. B. Kendall3 and R. G. Wilkes3

1 Middlesbrough, UK 2 Wirral, UK 3 Liverpool, UK

Editor—I was interested to read the paper by Cowan and colleagues.1 The study compared post-Caesarean section analgesia when i.v. morphine patient-controlled analgesia (PCA) was used, after supplementation of spinal anaesthesia from bupivacaine with intrathecal diamorphine, fentanyl or saline. There were only marginal benefits from using intrathecal diamorphine compared with intrathecal fentanyl. Intrathecal diamorphine was associated with lower i.v. morphine PCA use, and with a lower incidence of mild drowsiness at one of the assessment times, but postoperative pain and nausea scores were not significantly different. When choosing between intrathecal diamorphine and intrathecal fentanyl, the relative risk of a dosing error, or contamination, occurring during preparation of the local anaesthetic/opioid combination, should be considered. The risk is less with fentanyl because reconstitution and dilution are not required. Another advantage of fentanyl is that it is stable in solution with bupivacaine. If sterile-wrapped syringes containing pre-mixed solutions of fentanyl and bupivacaine were to become commercially available, this would increase the convenience, speed and safety, with which intrathecal opioid could be given at Caesarean section. I believe that this would shift the overall risk/benefit ratio in favour of supplementing spinal anaesthesia with intrathecal fentanyl, rather than with intrathecal diamorphine, especially for emergency Caesarean section.

D. W. Cooper

Middlesbrough, UK

Editor—We thank Dr Cooper for his comments on our paper,1 but find ourselves unable to agree with many of them. The use of intrathecal diamorphine resulted in a 50% reduction in postoperative morphine requirements compared with both fentanyl and saline. In many centres, this reduction has obviated the need for PCA devices, with associated cost savings.

Dr Cooper notes that postoperative pain scores were not significantly different, a valid but not unexpected observation, for two reasons. First, the use of a PCA device allowed all patients to self-medicate until they felt comfortable, a feature which tended to mask any difference in pain scores. Second, the study was not primarily designed to detect differences in pain scores and is under-powered to so do. Using the 6-h data as an example, the mean (SD) scores for fentanyl and diamorphine are 26 (8) and 22 (9) mm, respectively. In order to avoid Type II error, we would have needed to study 119 patients per group.

No one can dispute that because fentanyl is presented as a solution, the dilution aspect of drug preparation is removed. But, as with any potent drug, the clinician still must exercise vigilance and caution in ensuring only 0.4 ml of the 2 ml ampoule is injected intrathecally. Hospital pharmacy sterile services are capable of providing pre-diluted diamorphine in saline, which is stable for up to 15 days when stored at 4°C as a 1 mg ml–1 solution.2 Such a service makes differences in presentation negligible.

The potential dangers associated with the use of fentanyl must not be overlooked. Although not specifically powered to detect a difference in sedation scores, the study showed that fentanyl recipients tended to be more sedated, both acutely and after 12 h. Since our paper has been published, another case report of respiratory arrest associated with intrathecal fentanyl has appeared,3 reinforcing our findings. Any tendency toward somnolence, and ultimately respiratory depression, is a cause for concern, and as such, should not be disregarded.

C. M. Cowan1

K. Smith1

J. B. Kendall2

R. G. Wilkes2

1Wirral, UK

2Liverpool, UK

References

1 Cowan CM, Kendall JB, Barclay PM, Wilkes RG. Comparison of intrathecal fentanyl and diamorphine in addition to bupivacaine for Caesarean section under spinal anaesthesia. Br J Anaesth 2002; 89: 452–8[Abstract/Free Full Text]

2 Kleinberg ML, Duafala ME, Nacov C, et al. Stability of heroin hydrochloride in infusion devices and containers for intravenous administration. Am J Hosp Pharm 1990; 47: 377–81[ISI][Medline]

3 Kuczkowski KM. Respiratory arrest in a parturient following intrathecal administration of fentanyl and bupivacaine as part of a combined spinal–epidural analgesia for labour. Anaesthesia 2002; 57: 939–40[ISI][Medline]





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