Sevoflurane and analgesia
London, UK
* E-mail: davidwilkinson1{at}compuserve.com
Editor—We were interested to read the papers by Yeo and colleagues1 2 on the use of sevoflurane as an analgesic in labour. The anaesthetic properties of sevoflurane have been well documented since its introduction into clinical practice and, like all other modern inhalation agents, it has not demonstrated any significant analgesia. Its use, therefore, in general anaesthesia is usually associated with the use of analgesic adjuvant.
Yeo and colleagues have suggested that sevoflurane acts as a significant analgesic in obstetric patients; the observation, originally made by Toscano and colleagues,3 being supported by patient-completed visual analogue scores. We believe that it is still unclear that what has been measured is a direct analgesic effect of sevoflurane and may represent a small change in conscious level or even an alteration in uterine contractility due to the known tocolytic effect of the drug.
In the mid-1980s, there was significant correspondence related to whether unconscious patients feel pain and if analgesics were really required. It has been argued that analgesia is a matter of self-report. Thus, the immobile patient with no memory of intraoperative events cannot reveal the presence or absence of analgesia. However, modern teaching has supplanted these views reinforced by understanding of the neuroendocrine response to painful stimulus, and thus analgesia is normally routinely used as a part of balanced anaesthesia technique.
A paper published by Tomi and colleagues4 in 1993 showed that sevoflurane or isoflurane at 0.2 MAC has no effect on the perception of pain in volunteers. The BNF 52 states that ‘Early post operative pain relief may be required with sevoflurane because emergence and recovery are particularly rapid’, a suggestion that is not in keeping with the hypothesis that a sub-anaesthetic concentration of sevoflurane may have an analgesic effect.
Although we recognize the importance of these interesting findings in obstetrics, we are concerned that the repercussions of suggesting significant analgesic effects of sevoflurane may lead to practice that avoids the use of concomitant analgesia with sevoflurane anaesthesia. We believe that this could lead to a detrimental effect on patient care.
London, UK
* E-mail: seng.yeo{at}hhtr.nhs.uk
Editor—We wish to thank Drs Grover and Wilkinson for the comments regarding our study,1 2 but fear that they may be extrapolating more from our paper than is reported. Our study of the use of sevoflurane as an inhalation analgesic was specifically for painful contractions during the first stage of labour. We have not presumed nor stated that there would be concomitant analgesia with the use of sevoflurane during anaesthesia. We would therefore agree that any practitioner using solely sevoflurane for both anaesthesia and analgesia during a surgical procedure would have a detrimental effect on patient care.
In response to Grover and Wilkinson's comments regarding analgesia or rather lack of analgesia with sevoflurane, we would like to explain that pain and its associated analgesia is a highly complex entity. Although acute pain such as labour pain is considered to have at least two separate dimensions, a sensory and an affective component, in both research and clinical practice, the primary focus is often isolated to the physical transmission of pain stimuli. Melzack and Casey5 added to this complexity by describing the importance of the sensory, motivational, and cognitive components. Chapman6 formulated a model that noxious or nociceptive stimuli are centrally received and the interpretation of the stimuli is augmented by a wide variety of emotional, motivational, social, cultural, environmental, and cognitive variables unique to the individual. Therefore, the personal interpretation of the painful stimuli is individually private and unique to person and time. Chapman's model is consistent with a definition by Bonica7 of ‘acute pain is a complex constellation of unpleasant sensory, perceptual, and emotional experiences and certain associated autonomic, psychological, emotional, and behavioural responses’. Therefore with logical consequence, the definition of analgesia for labour pain is equally as complex.
In our study, we attempted to evaluate the complexity of pain and analgesia focusing on a prospective assessment of several sensory modalities. We discovered that ‘pain relief’ scores demonstrated statistically significant change with the use of sevoflurane compared with Entonox®. Grover and Wilkinson have suggested that either an alteration in conscious level or a tocolytic effect may have resulted in the analgesic effect. However, all the participants were fully conscious throughout the study with no amnesic effects observed. Also, a tocolytic effect would have resulted in reduced pain intensity scores as supported by a paper by Corli,8 which was not observed in our study.
In conclusion, labour pain and its associated analgesic modalities are highly complex. We have been careful not to conclude in our paper significant analgesic effects. However, we have demonstrated and reported that the use self-administered, sub-anaesthetic sevoflurane can provide useful pain relief during the first stage of labour, and to a greater extent than Entonox®.
References
1 Yeo ST, Holdcroft A, Yentis SM, Stewart A. Analgesia with sevoflurane during labour: 1. Determination of the optimum concentration. Br J Anaesth (2007) 98:105–9.
2 Yeo ST, Holdcroft A, Yentis SM, Stewart A, Bassett P. Analgesia with sevoflurane during labour: 11. Sevoflurane compared with Entonox for labour analgesia. Br J Anaesth (2007) 98:110–5.
3 Toscano A, Pancaro S, Giovannoni G, et al. Sevoflurane in obstetrics; a pilot study. Int J Obst Anesth (2003) 12:79–82.[CrossRef]
4 Tomi K, Mashimo T, Tashiro C, et al. Alterations in pain threshold and psychomotor response associated with subanaesthetic concentrations of inhalation anaesthetics in humans. Br J Anaesth (1993) 70:684–6.
5 Melzack R, Casey KL. Sensory, motivational and central control determinants of pain: a new conceptual model. In: The Skin Senses—Kenshalo D, ed. (1968) Springfield, Illinios: Thomas. 423–43.
6 Chapman CR. Sensory decision theory methods in pain research: a reply to Rollman. Pain (1977) 3:295–305.[CrossRef][Web of Science][Medline]
7 Bonica JJ. Definitions and taxonomy of pain. In: The Management of Pain—Bonica JJ, ed. (1990) 2nd Edn. Philadelphia: Lea & Febiger. 18–27.
8 Corli O, Grossi E, Roma G, Battagliarin G. Correlation between subjective labour pain and uterine contractions: a clinical study. Pain (1986) 26:53–60.[CrossRef][Web of Science][Medline]
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