The effects of nitrous oxide on the auditory evoked potential index during sevoflurane anaesthesia

We have studied the effects of nitrous oxide on the auditory evoked response index (AAI-index) derived from the A-line monitoring device during sevoflurane anaesthesia in 21 patients undergoing minor ambulatory surgery. During sevoflurane anaesthesia with an AAI-index < 30, the addition or withdrawal of nitrous oxide in a concentration of 66% end tidal did not show any linear dose dependent change in AAI-index . However, comparing nitrous oxide > 40% to nitrous oxide < 10% end tidal concentration the AAI-index did decrease, p < 0.05. The AAI-index is either non-linear at deeper anaesthetic levels or is insensitive to the anaesthetic effects of nitrous oxide in terms of MAC-multiples.     

Nitrous Oxide Exposure during Routine Anaesthetic Work

Nitrous oxide exposure in a modern hospital during routine anaesthetic work was measured using a technical exposure measurement technique and compared to measurement of biologic exposure from urine samples. The study included different anaesthetic situations and also a study of the efficiency of close scavenging and general air-conditioning in reducing nitrous oxide exposure. Exposure to nitrous oxide varied greatly. The mean nitrous oxide exposure in the total material was 53 ppm corresponding to approximately half the Swedish control limit (100 ppm) for 8 h time-weighted average (TWA). The only anaesthetic situation regularly resulting in 8 h TWA exposure exceeding the control limit was paediatric anaesthesia (92 +/- 67 ppm, mean +/- s.d.). The use of close scavenging significantly reduced the 8 h TWA nitrous oxide exposure in paediatric anaesthesia. The reduction of exposure was not significant during other forms of anaesthesia where low levels were found when anaesthetic equipment with excess gas scavenging was used in theatres with non-recirculating air-conditioning. The correlation between conventional technical exposure measurement and urine headspace nitrous oxide measurement was good. Both theoretical arguments and practical experience indicate that this method can be used for assessing nitrous oxide exposure during routine anaesthetic work.     

Bronchoscopy and nitrous oxide pollution

The purpose of the investigation was to assess the extent of nitrous oxide pollution during bronchoscopy (rigid ventilation bronchoscope) and further to elucidate the possible reduction in air pollution during local scavenging of leaking gas by a suction catheter placed in the pharynx. The study included 14 adult patients. The leakage of anaesthetic gas was 11.4 +/- 3.2 l min-1 and the median value of the nitrous oxide concentration in the breathing zone of the endoscopist was greater than 300 ppm. Air pollution could not be reduced significantly by scavenging with a suction catheter. The use of inhalation anaesthetics is therefore inadvisable during bronchoscopy in adults unless sufficient anaesthetic scavenging can be established. Other ways of reducing air pollution are discussed and the use of total intravenous anaesthesia or local anaesthesia is advocated.     

Experimental and Preliminary Clinical Studies of a Method of Reducing Anaesthetic Gas Contamination

Long-term exposure to low concentrations of anaesthetic gases is potentially hazardous. In spite of efforts to reduce the exposure of personnel by increased air conditioning, by scavenging of excess circuit gases and by improvements in anaesthetic equipment, high concentrations of gases have still been measured. Problems arise during mask induction, in paediatric anaesthesia, and in certain situations with unavoidable leaks, especially in small rooms but also in recovery rooms. Experimental and preliminary clinical studies have shown that nitrous oxide pollution can be considerably reduced by using a separate close scavenging system in the area where high gas concentrations occur. The geometrical and flow dimensions of a close scavenging device used in paediatric. anaesthesia are described.     

A study of waste gas scavenging in operating theatres.

Measurements of atmospheric nitrous oxide concentrations were made in eight hospitals, using 30 anaesthetic machines and locations, during 41 anaesthetic administrations or simulations. All areas were air conditioned, two by laminar flow devices. All anaesthetic machines and ventilators were fitted with commercially available gas collector (scavenger) valves. In all areas except one, venturi suction was used to exhaust gases. Pollution levels during endotracheal anaesthesia did not exceed the recommended 30 ppm. except where leaking anaesthetic machines or nitrous oxide supplies were used. In two instances where paediatric anaesthesia was administered through uncuffed endotracheal tubes nitrous oxide levels were also excessive. Of nine anaesthetics administered through face masks, only in one was the ambient nitrous oxide concentration acceptable. During induction of anaesthesia using nitrous oxide, unacceptable peaks of concentration were encountered. In two air conditioned recovery rooms tested, nitrous oxide concentration was acceptable. The collector valves performed their function satisfactorily, but laminar flow air conditioning was insufficient in itself to maintain acceptably low nitrous oxide concentration without the use of scavenging.     

Waste gas exposure to sevoflurane and nitrous oxide during anaesthesia using the oesophageal-tracheal Combitube small adult

Exposure to sevoflurane (SEV) and nitrous oxide during ventilationusing a Combitube (37Fr) small adult (SA)^TM was compared withwaste gas exposure using conventional endotracheal tubes. Traceconcentrations of SEV and nitrous oxide were assessed usinga direct reading spectrometer during 40 gynaecological laparoscopicprocedures under general anaesthesia. Measurements were madeat the patients’ mouth and in the anaesthetists’breathing zone. Mean (SD) concentrations of SEV and nitrousoxide measured at the patients’ mouth were comparablein the Combitube SA^TM (SEV 0.6 (0.2) p.p.m.; nitrous oxide 9.7(8.5) p.p.m.) and endotracheal tube group (SEV 1.2 (0.8) p.p.m.;nitrous oxide 17.2 (10.6) p.p.m.). These values caused comparablecontamination of the anaesthetists’ breathing zone (SEV0.6 (0.2) p.p.m. and nitrous oxide 4.3 (3.7) p.p.m. for theCombitube SA^TM group, compared with SEV 0.5 (0.2) p.p.m. andnitrous oxide 4.1 (1.8) p.p.m. for the endotracheal tube group).We conclude that the use of the Combitube SA^TM during positivepressure ventilation is not necessarily associated with increasedwaste gas exposure, especially when air conditioning and scavengingdevices are available. Br J Anaesth 2001; 86: 124–6     

RAPID INHALATION INDUCTION WITH ISOFLURANE IN HUMIDIFIED CARRIER GAS

This study was designed to determine if rapid inhalation inductionof anaesthesia (Rll) with 5% isoflurane (4.5 MAC equivalent)in oxygen offers any advantage in terms of complication rate,speed of induction and patient acceptance in comparison witha previously described technique of Rll using 2 % isofluranein nitrous oxide and oxygen. Forty ASA I and II unpremedicatedpatients were allocated randomly to receive Rll with either5% isoflurane in oxygen or 2% isoflurane and nitrous oxide inoxygen. The carrier gas was humidified in order to limit airwayirritation associated with the use of isoflurane for inhalationinduction. In this study the observed difference in mean inductiontime (7 s) between the two groups was neither clinically norstatistically significant. The groups were similar also withregard to complication rate and patient acceptance.     

Concentration and Elimination of Anaesthetic Gases in Operating Theatres Influence of Anaesthesia Apparatus Leakages

Halothane and nitrous oxide (N2O) concentrations were measured in operating theatres, in the areas corresponding to theinhalation zones of the anaesthetists and operating nurses. The measurements were performed in an operating theatre with a non-recirculating air exchange rate of 20/h. This was performed partly in model experiments and partly during the administration of anaesthesia by intubation. In the model experiments. the measurements were taken both with and without a specially constructed scavenging system. During anaesthesia, the measurements were taken exclusively with the scavenging system, although well-defined leakages were fitted into the otherwise gas-tight anaesthetic system. The results were supplemented by smoke experiments which showed the air distribution patterns. The investigation showed that the gases were concentrated over and around the operating table. Activities during surgery diluted this concentration. Furthermore, it was shown that leakage in the anaesthetic system significantly influences the achieving of a low gas-air mixture. Halothane concentrations in the inhalation zone of the anaesthetist and operating nurse can be reduced to 0.02 and 0.01 p.p.m. respectively, if the anaesthetic system is completely gas-tight.     

Efficacy of the Ejector Flow-meter. A Scavenging Device for Anaesthetic Gases

Measurements of air concentrations of nitrous oxide and halothane in the breathing zone of the anaesthetist and the operating-room nurse were carried out during inhalation anaesthesia with a Mapleson D system. Gas removal was performed from inside the breathing system at the same rate as that of the fresh gas inflow by means of an ejector flow-meter. The concentrations of nitrous oxide and halothane were maintained below the Danish Threshold Limit Values of 100 and 5 parts per million, respectively, by using this type of scavenging. When these anaesthetics were used simultaneously, the reduced Threshold Limit Values were not exceeded during endotracheal anaesthesia.     

Reduction of nitrous oxide contamination in a paediatric hospital.

Ambient nitrous oxide concentrations were recorded in an operating room during delivery of the gas at low and medium flow-rates, with and without the application of simple scavenging devices. Residual background levels of nitrous oxide were still present more than one hour after disconnection of the flowmeters and use of the room. Scavenging reduced concentrations ten-fold. Adjacent corridors and the post-anaesthetic recovery room were contaminated with nitrous oxide from the operating rooms and from patients. A vigorous programme of checking for leaks and repairing and maintaining equipment, coupled with the use of suction scavenging, can reduce atmospheric contamination with nitrous oxide below 30 p.p.m., which is well within the limit suggested by the U.S. National Institute for Occupational Safety and Health.     

Anesthetic management in a case of Friedreich's ataxia

We describe the anaesthetic management of a 20 years old female with a Friedreich's ataxia for curettage of the uterus. After the premedication with thalamonal, diazepam and atropine the anaesthesia was induced with thiopental. Isoflurane and nitrous oxide were used for maintenance. We review the literature about this disease and its implications in anaesthesia. Anesthetic hazards to the patient with Friedreich's ataxia include potential risk of cardiac dysrhythmias and heart failure and also marked sensitivity to muscle relaxants. Respiratory complications and diabetes mellitus are other main problems in postoperative period. We conclude that this patients should be careful monitored specially cardiovascular function and neuromuscular transmission during and after anaesthesia.     

Exposure of operating room personnel to anaesthetic agents: An examination of the effectiveness of scavenging systems and the importance of maintenance programs

Ten Alberta hospitals were monitored to assess the exposure of operating room personnel to nitrous oxide, halothane and enflurane during the average work day. Exposure to these agents in operating rooms without scavenging systems was determined to be well above the recommended guidelines. Contaminants were significantly lower in operating rooms with scavenging systems of either the wall suction type or the air-conditioning exhaust type. In the majority of studies, leakage points of anaesthetic gases were found throughout the anaesthesia systems. Where maintenance programs for the anaesthesia equipment were designed for the detection and elimination of leakage, contaminant levels were significantly lower than in those hospitals where the maintenance programs were concerned solely with proper equipment function as it relates to patient care. A regular, frequent maintenance program for all anaesthesia and scavenging systems designed to identify and correct leakage points is therefore recommended. We consider a nitrous oxide monitor to be the ideal tool for detection of leakage.     

Effect of humidification on inhalation induction with isoflurane in children

A well practised technique is important in achieving quiet inductionof anaesthesia in children, but the volatile agent used alsoinfluences the process. Isoflurane results in more airway problemsthan halothane but recent studies have described several waysof improving the acceptability of isoflurane for inhalationinduction of anaesthesia. We have studied the effect of humidificationof inspired gases on respiratory complications and hypoxic episodesduring induction. Forty-one children undergoing inhalation inductionof anaesthesia with isoflurane and nitrous oxide in oxygen wereallocated randomly to receive either humidified or non-humidifiedgas. Humidification did not result in a lesser incidence ofproblems. These findings are contrary to the results in a similarstudy in a predominantly adult sample which demonstrated a significantlylower incidence of complications using humidified gases. Present address: Department of Anaesthesia, Royal Perth Hospital,Wellington Street, Perth, Western Australia 6000 Present address: Department of Anaesthesia, Frenchay Hospital,Frenchay Park Road, Bristol BS16 1LE Present address: Department of Anaesthesia, The Ipswich Hospital,Ipswich IP4 5PD     

The effect of nitrous oxide on cerebral blood flow velocity in children anaesthetised with sevoflurane

To determine the effects of nitrous oxide on middle cerebral artery blood flow velocity (CBFV) during sevoflurane anaesthesia in children, CBFV was measured using transcranial Doppler sonography in 16 ASA I or II children. Anaesthesia consisted of 1.0 MAC sevoflurane in 30% oxygen with intermittent positive pressure ventilation maintaining FEco2 at 38 mmHg (5.0 kPa) and a caudal epidural block using 0.25% bupivacaine 1.0 ml.kg-1. The remainder of the inspired gas was varied in one of two sequences either air/nitrous oxide/air or nitrous oxide/air/nitrous oxide. The results showed that CBFV decreased when nitrous oxide was replaced by air (p = 0.03) and returned to its initial value when nitrous oxide was reintroduced. CBFV increased when air was replaced by nitrous oxide (p = 0.04) and returned to its initial value when air was reintroduced. Mean heart rate and blood pressure remained constant. We conclude that nitrous oxide increases cerebral blood flow velocity in healthy children anaesthetised with 1.0 MAC sevoflurane.     

The Effect of Local Scavenging on Occupational Exposure to Nitrous Oxide

Despite evacuation of excess anaesthetic gases at the expiratory valve of the anaesthetic circuit and a general ventilation system producing 17–20 air changes per hour, mask anaesthesia often causes occupational exposure to anaesthetic gases exceeding the threshold limit values. The effect of a local air exhaust system, a local scavenger, on occupational exposure to nitrous oxide during paediatric mask anaesthesia was studied. The scavenger evacuated 140 m³ of air per hour and was placed at a distance of 20 cm from the face mask. In a very poorly ventilated operating theatre the exposure to nitrous oxide was reduced by 75% during the anaesthetic sessions and exposure to concentrations above 500 ppm was almost eliminated. The experiences from the installation and clinical use are discussed. Local scavenging is an excellent complement to the scavenging of excess gases at the expiratory valve, and it may be considered an alternative to expensive, high-capacity ventilation systems.     

The sevoflurane-sparing effect of nitrous oxide: a clinical study

BACKGROUND: We studied the sevoflurane-sparing effect of nitrous oxide in a prospective randomised study. METHODS: Forty-two ASA I-II patients scheduled for elective knee arthroscopy under general anaesthesia were randomly assigned to a fresh gas flow consisting of oxygen in air or oxygen in nitrous oxide 1:2. All patients received a standardised anaesthesia consisting of induction with fentanyl and propofol and maintenance with sevoflurane adjusted according to clinical signs. The sevoflurane consumption was studied by means of weighing the vaporiser before and after every anaesthesia. RESULTS: The mean sevoflurane consumption was reduced from 0.62 to 0.25 g/min, a 60% reduction, by the use of oxygen in nitrous oxide 1:2 in the fresh gas flow. The emergence was faster for the patients receiving nitrous oxide. No major differences were observed during recovery. CONCLUSION: Nitrous oxide was found to be cost-effective for use during short ambulatory knee arthroscopy.     

Sedation by the use of inhalation agents in dental care

Nitrous oxide/oxygen has long been the mixture of gases used in dental practice to produce light sedation. The main indication for use of nitrous oxide/oxygen sedation is fear-anxiety. The equipment used is a continuos flow machine with a fail safe system set at a minimum of 30 or 40 per cent oxygen. A standardized sedation technique starting with administration of 100 per cent oxygen, is recommended. The concentration of nitrous oxide is then slowly increased and individually set, mostly between 30 and 50 per cent. In most cases the analgesia produced by nitrous oxide is not sufficient to ensure pain-free dental treatment. The sedation must therefore be supplemented by local anaesthesia. Side effects, e.g. restlessness, vomiting, and nausea are infrequent. About 90 per cent patients, who have difficulty in co-operating during dental treatment, mainly because of anxiety, show excellent or fair co-operation during nitrous oxide/oxygen sedation. Occupational exposure to nitrous oxide can be minimized by the use of scavenging systems, local exhaust systems, careful sedation technique, and equipment management.     

Exposure of operating room personnel to nitrous oxide during paediatric anaesthesia

This study was undertaken to quantify the exposure of operating room staff to nitrous oxide during routine paediatric otolaryngeal surgery and to determine the influence of the method of induction of anaesthesia on this exposure. The nitrous oxide exposure of the anaesthetist, the surgeon and the circulating nurse were measured, using body-worn passive atmospheric samplers, during twelve routine paediatric otolaryngeal surgical lists. During six of the lists an inhalational technique, with nitrous oxide, oxygen and halothane, was used for the induction of anaesthesia. During the other six lists anaesthesia was induced using intravenous thiopentone. In all cases, anaesthesia was maintained using nitrous oxide, oxygen and halothane. Regardless of the induction technique used, the mean nitrous oxide exposures of the anaesthetist, the surgeon and the nurse all exceeded the maximum level of 25 ppm.hr-1 recommended by the United States National Institute for Occupational Safety and Health (NIOSH). The use of an intravenous technique for the induction of anaesthesia reduced the nitrous oxide exposure of the anaesthetist and the circulating nurse. This suggests that, although the use of an intravenous induction may reduce exposure to nitrous oxide, the NIOSH recommendations for maximum exposure of operating room personnel to nitrous oxide are currently unattainable in practice.     

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.     

Occupational exposure to inhaled anaesthetics: a follow-up study on anaesthetists of an eastern European university hospital

BACKGROUND: Although no dose-response relationship for the health risks associated with the occupational exposure to inhaled anaesthetics exists, public health authorities recommend threshold values. The aim of the present study was to assess if and to what extent these threshold values are exceeded in an eastern European university hospital before and after measures had been taken to reduce occupational exposure. METHODS: At nine workplaces occupational exposure of anaesthetists to nitrous oxide and halothane or isoflurane was measured by means of photoacoustic infrared spectrometry. The measurements were carried out in 1996 and were repeated in 1997 after the installation of active scavenging devices at five workplaces and an air-conditioning system at one workplace. RESULTS: Occupational exposure to nitrous oxide and halothane or isoflurane was lower in 1997 compared to 1996. In 1997 most of the nitrous oxide values still exceeded the threshold value of 100 ppm, whereas most of the halothane and isoflurane values were already below the threshold values of 5 ppm and 10 ppm in 1996. CONCLUSION: The measures taken were effective in reducing waste gas exposure. Nevertheless, further efforts are necessary, especially for nitrous oxide, to reach western European standards. These efforts comprise structural measures such as active scavenging devices and air-conditioning systems at all workplaces, the use of total intravenous anaesthesia, low-flow anaesthesia and an appropriate working technique.