Inhalational and parenteral analgesia

Abstract

Systemic or parenteral medication can be classed in two broad groups; inhalational analgesics and parenteral opioids. These are the most commonly used forms of pain relief in labour both in the USA and Europe, despite being less efficacious than regional analgesia. This may be due to their availability and ease of use. Of the inhalational analgesics, only Entonox has stood the test of time. Newer inhalational agents in the form of isoflurane, desflurane and sevoflurane have been used for labour analgesia sporadically and seem to be at least as efficacious as Entonox. Environmental concerns may limit their use in labour rooms unless there is active scavenging of the gases. Of the parenteral opioids, pethidine is the most widely used but also the least efficacious. It has detrimental effects on both the woman and her fetus. In the UK, midwives have been able to prescribe pethidine independently since 1950, which might explain its continued use. Morphine has been compared to pethidine and has similarly poor analgesic efficacy and side effects. Diamorphine has been reported to be more efficacious but there are few randomized controlled studies to support its use. Other partial opioid agonists/antagonists have been compared to pethidine and there are no significant differences in efficacy. Fentanyl patient-controlled analgesia (PCA) is an alternative when better control of analgesia is required or when regional analgesia is contraindicated. A promising parenteral opioid is PCA remifentanil. It is extremely potent and is rapidly metabolized to inactive metabolites. In contrast to pethidine, it causes minimal detrimental effects to the neonate.