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Rapid Responses: Rapid Responses published:
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Concern about measurement of the primary outcome
- Geraldine Barrett, Sally Spencer (20 July 2006)
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Effectiveness and acceptability of lidocaine spray in reducing perineal pain during spontaneous vaginal delivery - Misleading conclusions
- Rachel E Farmer, Dr Geraldine O'Sullivan (Consultant Obstetric Anaesthetist), Professor Andrew Shennan (Professor of Obstetrics and Gynaecology). (20 July 2006)
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Geraldine Barrett, Senior Lecturer Brunel University, School of Health Sciences and Social Care, Uxbridge UB8 3PH, Sally Spencer
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Sanders et al address an important but neglected topic in their paper (1), however we have concerns about the measurement of the trial’s primary outcome. From the information given it appears that perineal pain was measured using a direct estimation method based on one item, probably a visual analogue scale. Our first concern is that a one item measure is prone to measurement error as the reliability of a scale is related to the number of items it contains (2). Secondly, direct estimation methods have a response bias problem known as end-of-scale aversion, where respondents are reluctant to put responses at the end of the scale (2). These two problems are of concern in the context of a clinical trial: the effect of a reduced scale makes it harder to detect differences in scores between groups and the inherent unreliability of one item compounds the problem. Thirdly, it is not stated whether any procedures to establish the validity of the measure were carried out prior to the trial. The most obvious danger is that the scale is measuring the pain of second stage labour generally as opposed to perineal pain specifically. Our final concern is with the estimate of a clinically significant reduction in pain as 13 scale points or 0.5 standard deviations (1). A difference of 0.5 standard deviations has previously been suggested as an estimate of minimally clinically important difference in relation to health-related quality of life (3), however this is not a universally accepted rule of thumb (4). Applied to a measure of undocumented validity and reliability, it may be spurious. Given the problems outlined above, it is entirely possible a true difference in perineal pain existed between the two groups but was not detected. 1) Sanders J, Peters TJ, Campbell R. Effectiveness and acceptability of lidocaine spray in reducing perineal pain during spontaneous vaginal delivery: randomised controlled trial BMJ doi:10.1136/bmj.38878.833241.7C 2) Streiner and Normal Streiner DL, Norman GR. Health measurement scales: A practical guide to their development and use. 2nd Ed. Oxford: Oxford University Press, 1995. 3) Norman GR, et al. Interpretation of changes in health-related quality of life: the remarkable universality of half a standard deviation. Med Care 2003, 41:582-92. 4) Beaton DE. Simple as possible? Or too simple? Possible limits to the universality of the one half standard deviation. Med Care 2003, 41:593 -96. Competing interests: None declared |
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Rachel E Farmer, Anaesthetic SHO St Thomas' Hospital, Lambeth Palace Road, London, SE1 7EH, Dr Geraldine O'Sullivan (Consultant Obstetric Anaesthetist), Professor Andrew Shennan (Professor of Obstetrics and Gynaecology).
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Dear Editor, Sanders et al conclude that there is no evidence of a significant reduction in second stage labour pain when lidocaine is sprayed on the intact perineum prior to a spontaneous vaginal delivery. This is hardly surprising since the rational for using topical lidocaine (Xylocaine) spray on intact squamous epithelium, in order to achieve analgesia, is fundamentally flawed. High concentrations of local anaesthetic, i.e. 40% lidocaine, are required for absorption through intact skin, and even then absorption is unreliable and slow 1. Similarly the dose of lidocaine spray used in this study was minimal, (0.5ml of 10% Xylocaine spray i.e. 50mg lidocaine). The recommended dose for adequate infiltration analgesia during delivery is up to four times this amount of lidocaine 2. The perineal area to which this 0.5ml was applied is not small and this volume of local anaesthetic spray could never have been expected to reduce the pain of the second stage of labour. In addition more than half of the women received pethidine, and although not specifically targeted at perineal pain, its use could influence the pain scores and also explain the lack of effect. Since the lidocaine spray did not reduce the pain in the second stage, there is no plausible reason why it should influence trauma, by allowing ‘the more controlled delivery of the fetal head’ as suggested by the authors. It is thus more likely this is a chance finding, (type I statistical error), given the multiple endpoints investigated. It is a concern that this study highlights how ‘acceptable’ this ineffective technique is to women and midwives when it fails to show a reduction, and may even have shown an increase in discomfort and pain. More alarmingly the discussion and accompanying editorial also hint at how the technique may reduce perineal trauma. The effect of this publication could result in the technique being adopted by midwives, with the laudable aim of reducing perineal trauma. The BMJ would have better served it readers by highlighting that this was a negative study and that the technique currently has no place in modern obstetric practice. 1 Update in Anaesthesia. Issue 4 (1994) Article 7:p2. The Pharmacology of Local Anaesthetic Agents. 2 BNF No.51 March 2006. 15.2 Local Anaesthetics. Xylocaine (AstraZeneca) Competing interests: None declared |
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