- Email: [email protected]
Sevoflurane analgesia in labour (Sevo’n’ox)
‘‘I would have everie man write what he knowes and no more.’’—Montaigne
BRITISH JOURNAL OF ANAESTHESIA Volume 98, Number 1, January 2007 British Journal of Anaesthesia 98 (1): 1–2 (2007)
doi:10.1093/bja/ael328
Editorial I
This issue of the British Journal of Anaesthesia sees the publication of two studies of the analgesic effects of sevoflurane in oxygen-enriched air during labour.1 2 The first study determines the optimum concentration of sevoflurane and the second compares this concentration with the current standard inhalation analgesic used during labour, 50% nitrous oxide in 50% oxygen, Entonox . The use of volatile agents to provide analgesia during labour dates back to James Young Simpson in 1847. A variety of volatile agents, ether, chloroform, trilene, methoxyflurane, isoflurane and desflurane have all been tried and gradually discarded or replaced. Nitrous oxide has stood the test of time and has been in use in labour since 1881 and in the form of Entonox from 1961.3 Air, oxygen and Entonox have each been used as carrier gases. Sevoflurane is a relatively new volatile agent and its low blood gas partition coefficient of 0.65 ensures rapid uptake into the brain and a fast washout. The first study in this month’s journal describes the technique of administration of sevoflurane, which utilized an Oxford Miniature Vaporiser (OMV) to which oxygen was added to a reservoir tube on the inspiratory limb at 4 litre min 1. The oxygen enriched air/sevoflurane mix was then delivered to the mother via a non-return valve. No antipollution safeguards were used. This compact arrangement, in contrast to previous apparatus used to administer sevoflurane may have practical application in the setting of a busy labour ward. A visual analogue score (VAS) of ‘pain relief’ was used in addition to the standard ‘pain intensity’ score and proved a more sensitive measure of the efficacy of analgesia. This may reflect the altered pain perception caused by the inhalation agent and the relief of pain experienced by the mother. Inspired sevoflurane concentrations were measured rather than end-tidal because of practical issues during intermittent administration and reflect the manner in which inhalation agents are used in labour. Mothers received sevoflurane in the first stage of labour at >3 cm cervical dilatation and had not received any form of pain relief before recruitment. Inspired gas concentrations
were measured by an AS/3 anaesthetic monitor (Datex Ohmeda) via a continuous sampling port on the inspiratory reservoir tube. Inspired oxygen concentrations varied between 37% and 51% depending on inspiratory effort. The authors also found it difficult to achieve a fixed inspiratory concentration of sevoflurane, which was also dependent on inspiratory effort. Each labouring mother was aided to establish a regular breathing pattern during each labour contraction, which is standard clinical practice by midwives. A regular breathing pattern helped establish constant inspired oxygen and sevoflurane concentrations with the OMV. Each mother started breathing oxygen-enriched air alone for the first contraction and then the concentration of sevoflurane was increased by 0.2% for each subsequent contraction up to a maximum of 1.4%. Excessive sedation was defined when the mother had difficulty using the VAS slide ruler or had difficulty with comprehension. If this occurred the sevoflurane concentration was reduced by 0.2% for the next contraction. An inspired concentration of 0.8% was found to be both an acceptable and effective analgesic in labour. This concentration balanced the benefit of pain relief against the risk of sedation. The second study compared this optimum concentration of 0.8% inspired sevoflurane in oxygen-enriched air using the same OMV apparatus with Entonox. This was a small open-label, three part sequence, random-order study. The study was clearly impossible to double-blind and also impossible to avoid some novelty bias comparing a new technique to the established use of Entonox. In this second study an impressive 29 of the 31 mothers preferred sevoflurane to Entonox. Specifically, more mothers had an increased level of sedation compared with Entonox, but they still preferred it. However, none of the mothers experienced excessive drowsiness and all were able to comply and co-operate throughout the study period. The mothers were again aided to establish a regular breathing pattern during each contraction but were allowed to use the agents and apparatus without interference, which was greatly appreciated.
The Board of Management and Trustees of the British Journal of Anaesthesia 2007. All rights reserved. For Permissions, please e-mail: [email protected]
Downloaded from http://bja.oxfordjournals.org/ at University of Nebraka-Lincoln Libraries on March 12, 2015
Sevoflurane analgesia in labour (Sevo’n’ox)
Editorial I
analgesia using short-acting analgesics17 are more efficacious in labour and more environmentally friendly.
Nausea and vomiting were also more common with Entonox compared with sevoflurane with a relative risk ratio of 2.7. Two mothers withdrew because of the unpleasant odour of sevoflurane. No mother experienced an oxygen saturation less than 98%. In this small study sevoflurane was found to provide statistically superior ‘pain relief’ scores compared with Entonox (P<0.0001) and its sedative effects were found to be helpful. So will this new inhalation technique replace the wellestablished use of Entonox in labour? When one assesses the efficacy and undesirable effects of Entonox it is certainly worth considering. Studies of the efficacy of Entonox as an analgesic in labour have shown that 30–40% of mothers find no benefit.4–7 Entonox has some undesirable effects including nausea and vomiting, depression of consciousness, confusion and respiratory depression.4–7 These side-effects are presumably exacerbated by combination with parenteral opioids but there are few data on this, other than an increased risk of arterial desaturation.8 It has been postulated that because of pain during a contraction and an attempt to increase analgesia, mothers may hyperventilate when breathing Entonox resulting in hypocapnia.9 This may lead to hypoventilation and hypoxaemia between contractions.10–12 Severe hypoxaemia during intermittent selfadministration of Entonox in the absence of opioids has been reported.13 Local control of air pollution in the labour rooms should be more of a priority when one considers the repeated exposure of midwives to inhalation agents. Better ventilation of rooms may help but this will not help global pollution. Exposure to low background levels of nitrous oxide has been implicated in affecting early fetal development and causing miscarriage in healthcare workers.14 Background concentrations of nitrous oxide have been shown to exceed safe environmental limits when Entonox is used in labour wards without specialized ventilation systems.15 16 The OMV used to administer sevoflurane in these studies is not only small and portable but also lacks pollution control, as does most apparatus used for administering Entonox in labour. Nitrous oxide use appears to be declining rapidly in anaesthetic practice and it will be interesting to observe whether market forces may influence the continued production of nitrous oxide and Entonox. It would be useful to have an alternative inhalation agent available for analgesia in labour, although it is likely that the i.v. route of administration of analgesic or anaesthetic drugs may become a more popular option, as appears to behappeningduringanaesthesiaandforpainreliefaftersurgery. Sevoflurane may be a useful alternative as an inhalation analgesic but perhaps epidural and patient controlled i.v.
J. H. McClure Edinburgh, UK E-mail: [email protected]
References
2
Downloaded from http://bja.oxfordjournals.org/ at University of Nebraka-Lincoln Libraries on March 12, 2015
1 Yeo ST, Holdcroft A, Yentis SM, Stewart A. Analgesia with sevoflurane during labour: I. Determination of the optimum concentration. Br J Anaesth 2007; 98: 105–9 2 Yeo ST, Holdcroft A, Yentis SM, Stewart A. Analgesia with sevoflurane during labour: II. Sevoflurane compared with Entonox for labour analgesia. Br J Anaesth 2007; 98: 110–15 3 Tunstall ME. Obstetric analgesia. The use of a fixed nitrous oxide and oxygen mixture from one cylinder. Lancet 1961; 2: 964 4 McAneny TM, Doughty AG. Self-administration of nitrous oxide/ oxygen in obstetrics. Anaesthesia 1963; 18: 488–97 5 Rosen M, Mushin WW, Jones PL, Jones EV. Field trial of methoxyflurane, nitrous oxide, and trichloroethylene as obstetric analgesics. Br Med J 1969; 3: 263–7 6 Report to the Medical Research Council of the Committee on Nitrous Oxide and Oxygen Analgesia in Midwifery. Clinical trials of different concentrations of oxygen and nitrous oxide for obstetric analgesia. Br Med J 1970; 1: 709–13 7 Carstoniu J, Levytam S, Norman P, Daley D, Katz J, Sandler AN. Nitrous-oxide in early labor. Safety and analgesic efficacy assessed by a double-blind, placebo-controlled study. Anesthesiology 1994; 80: 30–5 8 Zelcer J, Owers H, Paull JD. A controlled oximetric evaluation of inhalational, opioid and epidural analgesia in labour. Anaesth Intensive Care 1989; 17: 418–21 9 Fadl ET, Utting JE. A study of maternal acid–base state during labour. Br J Anaesth 1969; 41: 327–37 10 Northwood D, Sapsford DJ, Jones JG, Griffiths D, Wilkins C. Nitrous oxide sedation causes post-hyperventilation apnoea. Br J Anaesth 1991; 67: 7–12 11 Wilkins CJ, Reed PN, Aitkenhead AR. Hypoxaemia after inhalation of 50% nitrous oxide and oxygen. Br J Anaesth 1989; 63: 346–7 12 Yacoub O, Doell D, Kryger MH, Anthonisen NR. Depression of hypoxic ventilatory response by nitrous oxide. Anesthesiology 1976; 45: 385–9 13 Lucas DN, Siemaszko O, Yentis SM. Maternal hypoxaemia associated with the use of Entonox in labour. Int J Obstet Anesth 2000; 9: 270–2 14 Dale O, Husum B. Nitrous oxide: at threat to personnel and global environment? Acta Anaesthesiol Scand 1994; 38: 777–9 15 Henderson KA, Matthews IP. An environmental survey of compliance with Occupational Exposure Standards (OES) for anaesthetic gases. Anaesthesia 1999; 54: 941–8 16 Mills GH, Singh D, Longan M, O’Sullivan J, Caunt JA. Nitrous oxide exposure on the labour ward. Int J Obstet Anesth 1996; 5: 160–4 17 Volmanen P, Akural E, Raudaskoski T, Ohtonen P, Alahuhta S. Comparison of remifentanil and nitrous oxide in labour analgesia. Acta Anaesthesiol Scand 2005; 49: 453–8
BRITISH JOURNAL OF ANAESTHESIA Volume 98, Number 1, January 2007 British Journal of Anaesthesia 98 (1): 1–2 (2007)
doi:10.1093/bja/ael328
Editorial I
This issue of the British Journal of Anaesthesia sees the publication of two studies of the analgesic effects of sevoflurane in oxygen-enriched air during labour.1 2 The first study determines the optimum concentration of sevoflurane and the second compares this concentration with the current standard inhalation analgesic used during labour, 50% nitrous oxide in 50% oxygen, Entonox . The use of volatile agents to provide analgesia during labour dates back to James Young Simpson in 1847. A variety of volatile agents, ether, chloroform, trilene, methoxyflurane, isoflurane and desflurane have all been tried and gradually discarded or replaced. Nitrous oxide has stood the test of time and has been in use in labour since 1881 and in the form of Entonox from 1961.3 Air, oxygen and Entonox have each been used as carrier gases. Sevoflurane is a relatively new volatile agent and its low blood gas partition coefficient of 0.65 ensures rapid uptake into the brain and a fast washout. The first study in this month’s journal describes the technique of administration of sevoflurane, which utilized an Oxford Miniature Vaporiser (OMV) to which oxygen was added to a reservoir tube on the inspiratory limb at 4 litre min 1. The oxygen enriched air/sevoflurane mix was then delivered to the mother via a non-return valve. No antipollution safeguards were used. This compact arrangement, in contrast to previous apparatus used to administer sevoflurane may have practical application in the setting of a busy labour ward. A visual analogue score (VAS) of ‘pain relief’ was used in addition to the standard ‘pain intensity’ score and proved a more sensitive measure of the efficacy of analgesia. This may reflect the altered pain perception caused by the inhalation agent and the relief of pain experienced by the mother. Inspired sevoflurane concentrations were measured rather than end-tidal because of practical issues during intermittent administration and reflect the manner in which inhalation agents are used in labour. Mothers received sevoflurane in the first stage of labour at >3 cm cervical dilatation and had not received any form of pain relief before recruitment. Inspired gas concentrations
were measured by an AS/3 anaesthetic monitor (Datex Ohmeda) via a continuous sampling port on the inspiratory reservoir tube. Inspired oxygen concentrations varied between 37% and 51% depending on inspiratory effort. The authors also found it difficult to achieve a fixed inspiratory concentration of sevoflurane, which was also dependent on inspiratory effort. Each labouring mother was aided to establish a regular breathing pattern during each labour contraction, which is standard clinical practice by midwives. A regular breathing pattern helped establish constant inspired oxygen and sevoflurane concentrations with the OMV. Each mother started breathing oxygen-enriched air alone for the first contraction and then the concentration of sevoflurane was increased by 0.2% for each subsequent contraction up to a maximum of 1.4%. Excessive sedation was defined when the mother had difficulty using the VAS slide ruler or had difficulty with comprehension. If this occurred the sevoflurane concentration was reduced by 0.2% for the next contraction. An inspired concentration of 0.8% was found to be both an acceptable and effective analgesic in labour. This concentration balanced the benefit of pain relief against the risk of sedation. The second study compared this optimum concentration of 0.8% inspired sevoflurane in oxygen-enriched air using the same OMV apparatus with Entonox. This was a small open-label, three part sequence, random-order study. The study was clearly impossible to double-blind and also impossible to avoid some novelty bias comparing a new technique to the established use of Entonox. In this second study an impressive 29 of the 31 mothers preferred sevoflurane to Entonox. Specifically, more mothers had an increased level of sedation compared with Entonox, but they still preferred it. However, none of the mothers experienced excessive drowsiness and all were able to comply and co-operate throughout the study period. The mothers were again aided to establish a regular breathing pattern during each contraction but were allowed to use the agents and apparatus without interference, which was greatly appreciated.
The Board of Management and Trustees of the British Journal of Anaesthesia 2007. All rights reserved. For Permissions, please e-mail: [email protected]
Downloaded from http://bja.oxfordjournals.org/ at University of Nebraka-Lincoln Libraries on March 12, 2015
Sevoflurane analgesia in labour (Sevo’n’ox)
Editorial I
analgesia using short-acting analgesics17 are more efficacious in labour and more environmentally friendly.
Nausea and vomiting were also more common with Entonox compared with sevoflurane with a relative risk ratio of 2.7. Two mothers withdrew because of the unpleasant odour of sevoflurane. No mother experienced an oxygen saturation less than 98%. In this small study sevoflurane was found to provide statistically superior ‘pain relief’ scores compared with Entonox (P<0.0001) and its sedative effects were found to be helpful. So will this new inhalation technique replace the wellestablished use of Entonox in labour? When one assesses the efficacy and undesirable effects of Entonox it is certainly worth considering. Studies of the efficacy of Entonox as an analgesic in labour have shown that 30–40% of mothers find no benefit.4–7 Entonox has some undesirable effects including nausea and vomiting, depression of consciousness, confusion and respiratory depression.4–7 These side-effects are presumably exacerbated by combination with parenteral opioids but there are few data on this, other than an increased risk of arterial desaturation.8 It has been postulated that because of pain during a contraction and an attempt to increase analgesia, mothers may hyperventilate when breathing Entonox resulting in hypocapnia.9 This may lead to hypoventilation and hypoxaemia between contractions.10–12 Severe hypoxaemia during intermittent selfadministration of Entonox in the absence of opioids has been reported.13 Local control of air pollution in the labour rooms should be more of a priority when one considers the repeated exposure of midwives to inhalation agents. Better ventilation of rooms may help but this will not help global pollution. Exposure to low background levels of nitrous oxide has been implicated in affecting early fetal development and causing miscarriage in healthcare workers.14 Background concentrations of nitrous oxide have been shown to exceed safe environmental limits when Entonox is used in labour wards without specialized ventilation systems.15 16 The OMV used to administer sevoflurane in these studies is not only small and portable but also lacks pollution control, as does most apparatus used for administering Entonox in labour. Nitrous oxide use appears to be declining rapidly in anaesthetic practice and it will be interesting to observe whether market forces may influence the continued production of nitrous oxide and Entonox. It would be useful to have an alternative inhalation agent available for analgesia in labour, although it is likely that the i.v. route of administration of analgesic or anaesthetic drugs may become a more popular option, as appears to behappeningduringanaesthesiaandforpainreliefaftersurgery. Sevoflurane may be a useful alternative as an inhalation analgesic but perhaps epidural and patient controlled i.v.
J. H. McClure Edinburgh, UK E-mail: [email protected]
References
2
Downloaded from http://bja.oxfordjournals.org/ at University of Nebraka-Lincoln Libraries on March 12, 2015
1 Yeo ST, Holdcroft A, Yentis SM, Stewart A. Analgesia with sevoflurane during labour: I. Determination of the optimum concentration. Br J Anaesth 2007; 98: 105–9 2 Yeo ST, Holdcroft A, Yentis SM, Stewart A. Analgesia with sevoflurane during labour: II. Sevoflurane compared with Entonox for labour analgesia. Br J Anaesth 2007; 98: 110–15 3 Tunstall ME. Obstetric analgesia. The use of a fixed nitrous oxide and oxygen mixture from one cylinder. Lancet 1961; 2: 964 4 McAneny TM, Doughty AG. Self-administration of nitrous oxide/ oxygen in obstetrics. Anaesthesia 1963; 18: 488–97 5 Rosen M, Mushin WW, Jones PL, Jones EV. Field trial of methoxyflurane, nitrous oxide, and trichloroethylene as obstetric analgesics. Br Med J 1969; 3: 263–7 6 Report to the Medical Research Council of the Committee on Nitrous Oxide and Oxygen Analgesia in Midwifery. Clinical trials of different concentrations of oxygen and nitrous oxide for obstetric analgesia. Br Med J 1970; 1: 709–13 7 Carstoniu J, Levytam S, Norman P, Daley D, Katz J, Sandler AN. Nitrous-oxide in early labor. Safety and analgesic efficacy assessed by a double-blind, placebo-controlled study. Anesthesiology 1994; 80: 30–5 8 Zelcer J, Owers H, Paull JD. A controlled oximetric evaluation of inhalational, opioid and epidural analgesia in labour. Anaesth Intensive Care 1989; 17: 418–21 9 Fadl ET, Utting JE. A study of maternal acid–base state during labour. Br J Anaesth 1969; 41: 327–37 10 Northwood D, Sapsford DJ, Jones JG, Griffiths D, Wilkins C. Nitrous oxide sedation causes post-hyperventilation apnoea. Br J Anaesth 1991; 67: 7–12 11 Wilkins CJ, Reed PN, Aitkenhead AR. Hypoxaemia after inhalation of 50% nitrous oxide and oxygen. Br J Anaesth 1989; 63: 346–7 12 Yacoub O, Doell D, Kryger MH, Anthonisen NR. Depression of hypoxic ventilatory response by nitrous oxide. Anesthesiology 1976; 45: 385–9 13 Lucas DN, Siemaszko O, Yentis SM. Maternal hypoxaemia associated with the use of Entonox in labour. Int J Obstet Anesth 2000; 9: 270–2 14 Dale O, Husum B. Nitrous oxide: at threat to personnel and global environment? Acta Anaesthesiol Scand 1994; 38: 777–9 15 Henderson KA, Matthews IP. An environmental survey of compliance with Occupational Exposure Standards (OES) for anaesthetic gases. Anaesthesia 1999; 54: 941–8 16 Mills GH, Singh D, Longan M, O’Sullivan J, Caunt JA. Nitrous oxide exposure on the labour ward. Int J Obstet Anesth 1996; 5: 160–4 17 Volmanen P, Akural E, Raudaskoski T, Ohtonen P, Alahuhta S. Comparison of remifentanil and nitrous oxide in labour analgesia. Acta Anaesthesiol Scand 2005; 49: 453–8