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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. 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.

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.

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.

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.

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.

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.

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 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.

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.

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.