An apparatus for the delivery of 50% premixed nitrous oxide and oxygen on demand or by controlled ventilation

A small, portable and robust apparatus for the administration of 50% premixed nitrous oxide and oxygen mixture for analgesia for hospital and emergency use is described. The equipment can deliver the mixture on demand or by intermittent positive pressure ventilation. It has important advantages over earlier devices particularly in the accident and emergency field in which it has already proved successful.     

The loss of sevoflurane from a closed breathing system

When volatile anaesthetics are used in a closed breathing system it is usually assumed that inflow of anaesthetic to the system matches uptake by the patient. Early laboratory reports on the interactions between sevoflurane and soda lime cast doubt on that assumption. We have measured the loss of sevoflurane, desflurane and isoflurane from a closed breathing system and found no differences of consequences.     

The effect of nitrous oxide on cerebral electrical activity

Recordings of cerebral electrical activity produced by the cerebral function analysing monitor ( CFAM ) were obtained from nine healthy volunteers breathing increasing concentrations of N2O in O2. Concentrations of 30 and 50% N2O produced significant reductions in CFAM trace amplitude. Withdrawal of N2O resulted in an increase in amplitude with an initial increase above the control level. There were no consistent changes in the frequency distribution of the weighted electroencephalographic signal with increasing concentrations of N2O.     

Inhalation sedation with 25% nitrous oxide

A field trial is reported of the use of a fixed concentration of 25% nitrous oxide to produce sedation in anxious dental patients. Three hundred and ninety-four patients were treated on 1005 occasions. Ninety-nine per cent of patients were treated successfully without any incidence of loss of consciousness, including 47 children who were treated on 88 occasions.     

An assessment of a method of inflating cuffs with a nitrous oxide gas mixture to prevent an increase in intracuff pressure in five different tracheal tube designs apparatus

We assessed the efficacy of inflating cuffs with a nitrous oxide gas mixture to minimise changes in intracuff pressure during anaesthesia. Patients were randomly assigned to one of five groups of 15 subjects each, and the trachea was intubated with the Profile Soft-Seal Cuff, Hi-Contour, Reinforced, Sheridan or Trachelon tracheal tubes. Cuffs were inflated with 40% nitrous oxide and cuff pressure was measured during anaesthesia with 67% nitrous oxide. Concentration of nitrous oxide in the cuff was measured at the end of anaesthesia. Cuff pressure increased slightly but significantly in the Reinforced and Trachelon groups. Nitrous oxide concentration in the Reinforced, Sheridan, or Trachelon groups was slightly but significantly higher than that in the Profile or Hi-Contour groups. Cuff pressure never exceeded 22 mmHg and there were no air leaks. Therefore, inflating cuffs with 40% N2O preserves stable cuff pressure in all five tracheal tubes, despite differences in cuff and pilot balloon design.     

Choice of cuff inflation medium during nitrous oxide anaesthesia

Air, saline and a mixture of nitrous oxide and oxygen were used to inflate the cuffs of tracheal tubes inserted into three groups of patients, during nitrous oxide anaesthesia. Large increases in intracuff pressure were seen in the air group, but no morbidity resulted. The intracuff pressure decreased in the gas mix group and leaks occurred in some patients. Initial adjustment of cuff pressure was more difficult in the saline group but the pressure remained stable thereafter.     

The estimation of inspired desflurane concentration in a low-flow system

We have examined the predictability of inspired desflurane concentration during low-flow anaesthesia using a to-and-fro breathing system. Twenty-two adult patients requiring mechanical ventilation of the lungs during surgery took part in the study. Using a fresh gas flow of 1 l.min¹, the ratio of inspired desflurane concentration to desflurane vaporizer setting was found to be approximately 0.75 after 9 min of anaesthesia and at 2 l.min¹ fresh gas flow the ratio was approximately 0.9 after 2 min of anaesthesia. These ratios were maintained throughout the procedure, except for a few minutes following each change in vaporizer setting.     

New agents, the circle system and short procedures

Sevoflurane, desflurane and isoflurane were compared using a circle system in 97 patients undergoing short surgical procedures. Using initial high flows, the time intervals to equilibration between inspired and end-expired agent concentrations were measured; equilibration was defined as F _E/ F ₁ =0.8. The mean (SD) times obtained for sevoflurane, desflurane and isoflurane were 8.2 (2.1) min, 3.8 (0.7) min and 19.7 (6.5) min, respectively. These times were significantly different from each other (p <0.0001). After equilibration total flows were reduced to 500 ml.min⁻¹; at these flows the initial decline in end-expired agent concentration was minimal with desflurane, intermediate with sevoflurane and greatest with isoflurane. Both desflurane and sevoflurane are appropriate for efficient use of the circle system during short anaesthetics.     

The effect of nitrous oxide on laryngeal mask cuff pressure

We have studied the effect of nitrous oxide on the cuff pressure of a laryngeal mask both in vitro and in vivo. In laboratory tests, we showed that nitrous oxide and carbon dioxide diffuse across the cuff wall much more rapidly than nitrogen and oxygen. Differing partial pressures of these gases across the cuff wall therefore give rise to changes in volume and pressure within the cuff. We then studied 18 patients undergoing general anaesthesia with nitrous oxide, and found a consistent and linear increase in cuff pressure in all patients. After 30 min, the mean pressure had increased by 30 mmHg, and there was approximately 10% nitrous oxide in the cuff. It is difficult to relate these findings to pressure on pharyngeal structures, but methods of limiting the rise in intracuff pressure are discussed.     

Occupational exposure to nitrous oxide in four hospitals

Diffusive samplers were used to measure the exposure of the anaesthetist, anaesthetic assistant and circulating nurse to nitrous oxide in every operating theatre of four Glasgow hospitals. Each theatre was surveyed over 1 or 2 weeks; samples were taken during every operating session. The exposures were in general surprisingly high. The anaesthetist's exposure was greater than ppm on about half the occasions. The study underlines the need for theatre ventilation systems and scavenging equipment to be properly maintained and for scavenging facilities to be used effectively. It is concluded that regular monitoring of staff exposures is required to check the effectiveness of antipollution measures.     

Predictable nitrous oxide uptake enables simple oxygen uptake monitoring during low flow anaesthesia

The uptake rate of oxygen and nitrous oxide were studied during low flow anaesthesia with enflurane or isoflurane in nitrous oxide with either spontaneous or controlled ventilation. The excess gas flow and composition were analysed. The nitrous oxide uptake rate was in agreement with Severinghaus'formula _N20 1000.t^-0.5. The composition of excess gas was predictable and the following formula for oxygen uptake could be derived: O₂=fgO₂ -0.45 (fgN²O -(kg: 70.1000.t^-0.5)) where oxygen uptake rate (O₂, ml.min^-1) equals oxygen fresh gas flow (fgO₂) minus 0.45 times the difference between the fresh gas flow of nitrous oxide (fgN₂O), ml.min^-1 and estimated uptake of nitrous oxide. The equation assumes constant inspired gas concentrations of 30% oxygen and 65–70% nitrous oxide. The oxygen uptake rates calculated from this formula were in good agreement with measured uptake rates. Thus, continuous monitoring of oxygen uptake rates is possible by using only reliable flowmeters and analysis of inspried oxygen concentration.     

The use of nitrous oxide in anaesthetic practice: a questionnaire survey

Where practicable, exposure to a hazardous substance should be eliminated or adequately controlled. A postal questionnaire survey was sent to 10% of consultants from the Association of Anaesthetists of Great Britain and Ireland, to identify the level of use of nitrous oxide in current anaesthetic practice and identify any change of practice over the last 5 years. Details of anaesthetic practice were requested in three areas: nitrous oxide usage, availability of medical air on anaesthetic machines and the use of total intravenous anaesthesia. Replies were received from 320 consultants (75%). Of these, 49% felt that, over the last 5 years, their use of nitrous oxide had decreased. Twenty per cent of anaesthetists felt that there should be some restriction in availability of nitrous oxide. Where medical air was available, 32% felt that they would use it frequently. A total of 263 (83%) stated that they use total intravenous anaesthesia to some extent. The results showed that, although 49% of consultant anaesthetists had reduced their use of nitrous oxide, this was due to medical considerations rather than concerns over health and pollution issues arising from the use of nitrous oxide.     

The moisture-conserving performance of breathing system filters in use with simulated circle anaesthesia breathing systems

Breathing system filters can be used to humidify gases delivered to patients. Performance can be determined by measuring the net moisture loss (the difference between expired and inspired levels of humidity) from a patient model. The net moisture loss should be decreased by increasing the level of humidity in the breathing system by, for example, using a circle breathing system. The effect of four different filters, three different levels of humidity in the breathing system (7, 13 and 19 g.m-3) and two tidal volumes (0.5 l and 1.0 l) on the net moisture loss from a patient model was measured. The net moisture loss decreased as the humidity in the breathing system increased and was less for the lower tidal volume. Adequate levels of humidity (>/= 20 g.m-3) will be delivered to patients by most filters provided they are used in conjunction with circle breathing systems and low fresh gas flows.