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The pressure in the internal jugular vein was measured with the head in three different positions using, first, a short cannula and then a catheter with a round occluded end and two side-holes. These pressures were compared with the central venous pressure. In the horizontal position using the catheter, the two venous pressure measurements were similar. With the open-ended cannula, results could be unreliable with the head turned towards the side of cannulation.

Difficulties were experienced in weaning a patient from a ventilator by means of intermittent mandatory ventilation (IMV). The difficulty was overcome by installing an alternative IMV system (Hudson “disposable IMV valve”) through which the patient drew her spontaneous breaths. Laboratory measurements showed that the resistance of the ventilator breathing system was much higher than that of the alternative system, mainly as a result of the resistance of the humidifier. It was calculated from measurements with a preset pattern of simulated breathing that the extra, external, work of breathing through the ventilator breathing system was approximately 1.5 times the normal internal mechanical work of breathing for a normal patient; with the alternative system, the extra work was only 0.5 times the normal. It is stressed that the breathing systems of IMV ventilators should be judged with the same rigour as other systems through which the patient is required to breathespontaneously. It is recommended that manufacturers should pro vide the necessary information.

Three cases of accidental intra-arterial injection of drugs into indwelling arterial cannulae are described. The factors contributing to, and the dangers of, such accidents are outlined and a number of preventative measures are suggested.

A 16-gauge suction catheter (i.d. 4.0 mm) was used to intubate the trachea in two patients in whom the larynx was narrowed as a result of pathological lesions, and normal, small diameter tracheal tubes were too short.

In 15 patients anaesthesia for elective Caesarean section was maintained with 50% nitrous oxide in oxygen and a 0.65% halothane supplement. In a further 15 mothers a 0.2% halothane supplement was used. In those mothers receiving 0.65% halothane intermittent measurements were made of maternal arterial halothane concentrations during the uptake and the excretion of the agent. At delivery the foetal umbilical venous concentrations were measured also. The mean time between administering halothane and delivery was 10.5 min (SD 3.5). The mean maternal arterial halothane concentration at delivery was 6.03 mg/100 ml (SD 0.75) and the mean umbilical vein concentration was 2.13 mg/100 ml (SD 0.69). The regression of Apgar scores at 1 min after delivery on umbilical vein halothane concentration at delivery was not significant. In the mothers receiving 0.2% halothane measurements of halothane concentration were made in five patients only. The mean maternal arterial halothane concentration at delivery was 1.56 mg/100 ml (SD 0.52) and the mean umbilical vein and artery concentrations were 0.8 and 0.38 mg/100 ml respectively. The use of 0.2% and 0.65% halothane supplements prevented awareness in all the mothers. However, dreaming occurred in two patients given a 0.2% halothane supplement. Studies are required to establish the minimum halothane supplement required to prevent awareness in a larger series of patients.

Mean venous pressures were measured in 20 patients in whom a catheter, inserted via the external jugular vein, failed to pass centrally. The catheter used had a rounded end and two side holes. Pressures were measured with the patient's head in three different positions. The pressures were compared with the central venous pressures measured from a catheter inserted through an antecubital vein (and confirmed to be placed centrally). In all patients the differences between the mean pressures from the two sites were all within 3 mm Hg and were not influenced by alteration in head position or the side of jugular. catheterization.

Measurements were made in 15 mothers of the arterial concentrations of nitrous oxide at the end of periods of inhalation of 50% nitrous oxide. The mean arterial concentrations, from all the patients, would be equivalent to breathing 26.4% nitrous oxide until equilibrium had been reached between the inspired concentration and the blood. The low measured blood levels are related to such factors as the short duration of inhalation, ventilation-perfusion defects in labour and the almost complete elimination of nitrous oxide between each contraction.

The blood concentration of methoxyflurane and the ventilation during intermittent self-administered inhalation analgesia were measured in 16 mothers (12 multigravidae) during labour. The blood methoxyflurane concentrations did not show accumulation over a mean period of inhalation of 77.6 min. Yet from theoretical deductions some accumulation would be anticipated. In a longer case it was shown that the mother, in spite of constant overall ventilation, breathed less methoxyflurane per contraction as labour progressed, so maintaining a reasonably steady blood level. This indicates that the conscious mother, unlike the anaesthetized subject, controls the uptake of methoxyflurane.

In fifteen mothers undergoing elective Caesarean section methoxyflurane 0.1 per cent vapour was added before the birth of the infant to a mixture of 40 per cent oxygen and 60 per cent nitrous oxide. Analysis of uptake and excretion of methoxyflurane was performed. Prompt recovery of consciousness occurred at the end of the procedure. In none of the mothers was there evidence of awareness during the procedure. There was no relationship between the Apgar score of the infants at one minute after delivery and the umbilical vein methoxyflurane concentration at delivery, and no evidence that methoxyflurane supplementation was responsible for foetal depression in this series.