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Background: Isoflurane (0.25%) in premixed nitrous oxide and oxygen 50/50, v/v, (IN(2)O), has been suggested for pain relief in labour., Methods: Possible phase separation of this mixture was studied by analysis of samples from pre-filled cylinders as they were cooled., Results: Condensation of isoflurane was found at 3.1 degrees C in a cylinder, which held 8.7 MPa at 15 degrees C. In a cylinder holding 13.8 MPa, which is the standard filling pressure stipulated by the National Health Service, the condensation temperature was -2.3 degrees C. At the highest cylinder filling pressure investigated (14.15 MPa) the separation temperature was even less, -3.0 degrees C. After exposure of cylinders to -40 degrees C and complete phase separation of the mixture, complete mixing was achieved by 24 h storage in the horizontal position at room temperature and, either three complete inversions of the cylinder or mechanical rolling at 30 r.p.m., Conclusions: These findings should assist the use and storage of IN(2)O.

Background: Isoflurane (0.25%) in premixed nitrous oxide and oxygen, 50/50, v/v (IN(2)O), has been suggested for pain relief in labour. Possible phase separation of the mixture was studied during simulated administration., Methods: A sinusoidal pump set at stroke volume of 2 litres and a rate of 20-22 bpm and cycling for 1 min in three was used to simulate breathing during the painful contractions of labour., Results: The temperature inside a 10-litre capacity cylinder did not drecrease sufficiently to cause separation of the gas mixture. Temperature in the demand valve decreased to -15.5 degrees C and this caused a small amount of liquid formation within the valve. Accordingly, the inspired concentration during the first breath of mixture in a cycle could be transiently as high as 0.55%. The concentration observed at the patient connection after the first breath varied between 0.17 and 0.28%., Conclusions: The system delivered a clinically acceptable performance although further development to avoid liquid condensation is needed.

To compare the efficacy and acceptability of conventional intravenous sedation with patient-controlled inhalational isodesox, 57 women undergoing outpatient oocyte recovery were randomly allocated to receive isodesox by face mask, while 55 women were given intravenous fentanyl and midazolam. Women's satisfaction with pain relief, peroperative pain, clouding of memory and the surgeons' assessment of operating conditions were evaluated. Thirty-eight women in the inhalation group (67%) and 41 (75%) women in the intravenous group were 'very satisfied' with their analgesia (p = 0. 41). The mean (SD) pain score in women given isodesox was 46.8 (34. 7), while in the intravenous group it was 34.1 (21.3) (p = 0.02). Oxygen saturation levels < 94% were recorded in one woman using isodesox and in 16 (29%) women given intravenous analgesia. Despite higher pain scores, in comparison with the conventional analgesia, patient-controlled isodesox offers a safer method of pain relief with comparable satisfaction rates.

The addition of 0.25% isoflurane to 50% nitrous oxide in oxygen provides more effective pain relief in labour than 50% nitrous oxide alone. This study was carried out to determine whether self-administration by demand valve of 0.25% isoflurane in 50% nitrous oxide in oxygen premixed in cylinders at 13.7 MPa (IN2O) was practical and safe during labour. Two hundred and twenty-one mothers used IN2O in labour after 50% nitrous oxide had become inadequate for pain relief. Data on IN2O use was recorded during labour and details of the course of labour and opioid usage were taken from the clinical notes. The duration of IN2O use was 0.1-12.35 h (median 2.3). Thirty-two mothers (14.5%) required an epidural and intolerance to IN2O was seen in a maximum of 17 cases (7.7%). One hundred and twenty-six cases were primiparous and 93 parous with 151 deliveries being spontaneous and 70 interventional, of which 12 were by Caesarean section. Maternal blood loss was 20-1500 ml (median 200 ml). Apgar scores at 1 and 5 min were unaffected by IN2O use although a positive correlation was found between the use of opioids and the number of neonates with a 1-min score below 8 and the number requiring resuscitation. Six neonates had an Apgar score below 8 at 5 min, but their condition was adequately explained by factors other than the sedative technique used. Self-administered IN2O was found to be a safe and practical technique for sedation in labour when 50% nitrous oxide alone had become inadequate.

Thirty-five patients who had undergone uncomplicated cardiac surgery were randomly allocated to receive either Entonox or isoflurane 0.25% in Entonox as inhalational analgesia for the removal of their two chest drains. The gases were presented premixed in high-pressure cylinders and were self-administered by means of a demand valve. The removal of the second drain was more painful than the first but that pain was better controlled by isoflurane 0.25% in Entonox than by Entonox alone.

A prospective study of complications associated with epidural and spinal blockade in obstetrics was carried out during the years 1990 and 1991. 79 obstetric units in the United Kingdom took part. These units had a total of 467 491 deliveries during the two years. 108 133 mothers received epidural blockade and 14856 received spinal blockade. 22% of all mothers received epidural analgesia in labour, and 50% of caesarean sections were performed under either epidural or spinal anaesthesia. 128 complications (not including post dural puncture headache) were reported. Of these, 46 were neuropathies involving a single spinal or peripheral nerve. 26 unexpectedly high blockades were encountered. Backache was reported in 21 mothers and urinary retention in 8. Cardiac arrest occurred twice. Although resuscitation restored normal cardiac function in both cases, one patient was decerebrate and died some days later. Postmortem examination revealed evidence of amniotic fluid embolus. 20 reports were classified as 'miscellaneous' and presented individual clinical diagnoses. The overall complication rate (excluding post dural puncture headache) was approximately 1 per thousand.

Previous attempts to introduce non-invasive monitoring of ventilation of spontaneously breathing patients into routine practice have been unsuccessful. The Aberdeen University Respiratory Alarm (AURA) allows such monitoring by utilizing the pyroelectric property of polarized polyvinylidine fluoride sensors to detect temperature changes that occur during breathing into an oxygen delivery face mask. A quartz crystal oscillator generates pulses that allow measurement of interexpiratory time and ventilatory frequency. The system incorporates LED digital displays, a bargraph and audiovisual alarms. An analog output permits display and analysis of the sensed signals. AURA performed satisfactorily in both volunteer studies and six patients in the clinical setting. AURA may be an appropriate respiratory transducer in those patients requiring oxygen therapy.

The Aberdeen University Respiratory Alarm (AURA) is designed to monitor the respiratory rate and the inter- expiratory period (respiratory gap) in the spontaneously breathing patient using polarised polyvinylidene fluoride sensors incorporated in the Oxygen delivery mask. The monitor is composed of three modules--the sensor module includes a pair of polyvinylidene film strips mounted on the inner surface of the Oxygen mask. The difference in temperature between the ambient Oxygen enriched air and the warmer expired air allows the pyroelectric property of the sensors to be harnessed. The sensor arrangement allows a differential signal to develop between them which is amplified, filtered and rectified prior to triggering a counter. The timing module is based on a 32,768 Hz crystal oscillator that generates pulses allowing the period between expirations to be measured and compared to the maximum allowable period set by the user. The respiratory rate is also measured and compared to the minimum set by the user using EPROMS and comparators. The display module consists of a bar-graph display of the level of the sensed signal with each breath with a soft audible click. Eight segment LEDs are used to provide a digital display of the respiratory rate and the respiratory gap. Audiovisual alarms are triggered when the rate falls short of the set rate and/or when the gap exceeds the set gap. The need for continuous respiratory monitoring of the spontaneously breathing adult in hospital practice led us to develop this prototype with sensors whose design and configuration can be considerably improved.

Entonox (50% nitrous oxide in oxygen) and isoflurane (0.75% in oxygen) were compared as analgesics in the first stage of labour in 32 consenting women. The drugs were self-administered and given in random sequence, each during five consecutive uterine contractions. Each patient acted as her own control. Linear analogue pain scores were significantly lower (p less than 0.001) with isoflurane than with Entonox, but scores for drowsiness were higher for isoflurane. Further study is needed to assess the effects of more prolonged use of isoflurane in labour.

An electrical alarm which can be operated by the tongue of a quadraplegic patient by contact with an electrode on the chin is described.

The effect on halothane uptake of changing the nitrous oxide concentration during the first few minutes of a general anaesthetic for Caesarean section was investigated. In 10 mothers anaesthesia was maintained with halothane 0.4%, nitrous oxide 33% and oxygen 66%. In 10 others the sole difference in anaesthetic technique was that the ratio of nitrous oxide to oxygen was reversed for the first 3 minutes only. Serial maternal arterial blood samples showed a significant increase in halothane levels in the group where the nitrous oxide was increased, with p less than 0.02 at 1 and 2 minutes and p less than 0.05 at 3.4 and 5 minutes. After 5 minutes there was no significant difference between the two groups. Cord blood concentrations between the two groups were comparable. The difference in halothane levels is a demonstration of the influence of the concentration effect of nitrous oxide on the uptake of halothane, the second gas effect. The relevance of anaesthetic uptake to obstetric anaesthesia and awareness is discussed.

The effects of 0.08% (Group A) and 0.25% (Group B) solutions of bupivacaine were compared in a random manner, to assess continuous pump infusion epidural analgesia in labour. Both solutions were infused at a dose rate of 20 mg bupivacaine/hour. The results in all the mothers who had received infusions lasting more than 4 hours were studied. There were 25 in Group A and 28 in Group B. Any treatment during the infusion epidural for inadequate analgesia, hypotension, etc., was recorded as an intervention. The mean of the intervention-free intervals was significantly greater in Group A than in Group B, and significantly fewer top-up injections were required in Group A. The results show that the administration of a 0.08% solution of bupivacaine into the epidural space by continuous pump infusion is more labour saving than the infusion of a 0.25% solution. The concept that a greater volume infusion rate maintains a more extended liquid sleeve of local anaesthetic in the epidural space is supported.

Measurements of maternal blood-gas tensions, (PAO2-PaO2), calculated pulmonary venous admixture (physiological shunt) and deadspace/tidal volume ratio (VD/VT) were made in 14 patients undergoing Caesarean section. Measurements were carried out after induction of general anaesthesia, and before delivery, with the patient tilted to the left. With a mean inspired oxygen concentration of 67.5% mean PaO2 was 44.3 kPa, mean (PAO2-PaO2) 18.4 kPa, mean Vd/Vt 32% and mean physiological shunt 9.5%. The findings are compared with published values for non-anesthetized pregnant patients.

The outcome of 175 pregnancies during which nitrous oxide was administered has been evaluated. All anaesthetics were of short duration (20-30 min) and 97% were administered during the second trimester. No adverse effects were identified. The mechanism of potential toxicity and the implications of the use of nitrous oxide during the first and second trimesters of pregnancy are discussed.

Four mothers receiving chlormethiazole for pre-eclampsia and their babies were the subjects of the investigation. Blood samples at delivery and blood and breast milk samples in the postpartum period were analysed for chlormethiazole. Concentrations ranged from 1.340 to 1.640 micrograms/g of sample of umbilical artery blood at birth, and fell to 0.010 to 0.153 micrograms/g in capillary blood 20 to 26 hours later. After the start of breast feeding, chlormethiazole in the babies was detectable in only 3 out of 27 serial blood samples and was 0.018, 0.009 and 0.006 micrograms/g. The highest calculated amount of chlormethiazole ingested at a breast feed was 37.2 micrograms. It is suggested that breast feeding should not be delayed solely on account of chlormethiazole therapy.

The need is identified of certain women in labour for a more potent analgesic than entonox. We describe a modification of the Entonox demand system incorporating an Oxford Miniature Vaporizer and there is a discussion directed towards its supervision.

We examined the feasibility of administering nearly 100% oxygen throughout the induction-delivery period of general anaesthesia for 113 Caesarean sections. Isoflurane 1.25% was compared with 1.5% enflurane for maintenance of anaesthesia. The level of anaesthesia was monitored by use of the isolated forearm technique. There was a greater amount of isolated forearm movement when enflurane was used. The three main criteria for a satisfactory general anaesthetic technique for Caesarean section were fulfilled, namely no maternal awareness, no undue depression of the fetus and no adverse effect on uterine contractility. Isoflurane and enflurane appear to be suitable anaesthetic agents for facilitating hyperoxygenation during Caesarean section.

A double-blind comparison of pethidine and meptazinol in the relief of pain during labour was undertaken in 205 healthy women. The protocol allowed 100 mg of the test drug to be repeated at intervals of 2 h to a maximum of three doses. It was noteworthy that only 29 mothers were given a second dose of narcotic. Every woman receiving one injection of meptazinol complained of moderate to severe pain after 2 h; 97% of those receiving one injection of pethidine were complaining of moderate to severe pain after 2 h. There was no difference between the two drugs with regard to pain relief or in side-effects both in mother and baby.

The effects of 0.08% and 0.5% bupivacaine, given in equal volumes as the loading dose prior to continuous infusion epidural analgesia, were compared. The incidence of hypotension, as well as the height of sensory blockade, was markedly lowered by reducing the loading dose of bupivacaine. However, the efficacy of the epidural analgesia was maintained by using the same volume for the loading dose. The mean onset of analgesia was delayed from 9.1 to 13.6 minutes. There were fewer calls for intervention consequent upon the reduction in adverse effects. The use of 8 ml 0.08% bupivacaine for the loading dose or for subsequent top-ups is recommended. The concept of a liquid sleeve of local anaesthetic in the epidural space is further supported.

The management of failure to intubate the trachea during emergency Caesarean section in a 116.7-kg woman is described. General anaesthesia was continued with the aid of a Gordon and Don Michael Esophageal Gastric Tube Airway. The forward displacement of the larynx caused by the tube in the oesophagus improved the patency of the airway.

Wakefulness during general anaesthesia was assessed by the isolated forearm technique in two groups of patients for Caesarean section. The anaesthetic was based on the use of oxygen 66%, nitrous oxide 33% and halothane 0.4% during the induction delivery interval. There was a highly significant reduction in wakefulness in the Group who received nitrous oxide at 66% for the first 3 min only. The wakefulness was amnesic in all cases.

A patient became wakeful during a general anaesthetic for Caesarean section. The isolated forearm technique for assessing wakefulness was in use. In response to questioning, the patient indicated by hand signals her wish to remain awake. She felt no pain. After the operation she was able to remember the experience, being told about the delivery, and hearing her baby cry. The words from a tape-recording of the anaesthetist's postoperative visit on the next day are presented.

A study was undertaken to investigate the changes in pressure inside the epidural space in parturients in the first stage of labour. The purpose was to see whether acute local pressure changes in the epidural space were consistent with the hypothesis that there is physical movement of cerebrospinal fluid with certain changes in maternal posture. Pressure changes were measured via the epidural catheter through which continuous infusion epidural analgesia was administered, using a pressure transducer kept at the level of the lumbar spine. The mothers moved through a series of postures that reflected those expected during the course of labour and delivery. The pressure within the catheter was recorded during these manoeuvres. The pressure changes which were found, taken in conjunction with the associated investigations of others, are consistent with the hypothesis.