Nitrous oxide exposure during routine anaesthetic work. Measurement of biologic exposure from urine samples and technical exposure by bag sampling

Nitrous oxide abuse direct measurement for diagnosis and follow-up: update on kinetics and impact on metabolic pathways

Recreational use of nitrous oxide (N2O) has become a major health issue worldwide, with a high number of clinical events, especially in neurology and cardiology. It is essential to be able to detect and monitor N2O abuse to provide effective care and follow-up to these patients. Current recommendations for detecting N2O in cases of recreational misuse and consumption markers are lacking. We aimed to update current knowledge through a review of the literature on N2O measurement and kinetics. We reviewed the outcomes of experiments, whether in preclinical models (in vitro or in vivo), or in humans, with the aim to identify biomarkers of intoxication as well as biomarkers of clinical severity, for laboratory use. Because N2O is eliminated 5 min after inhalation, measuring it in exhaled air is of no value. Many studies have found that urine and blood matrices concentrations are connected to ambient concentrations, but there is no similar data for direct exposure. There have been no studies on N2O measurement in direct consumers. Currently, patients actively abusing N2O are monitored using effect biomarkers (biomarkers related to the effects of N2O on metabolism), such as vitamin B12, homocysteine and methylmalonic acid.

Exposure to anaesthetic trace gases during general anaesthesia: CobraPLA vs. LMA classic

BACKGROUND

To prospectively investigate the performance, sealing capacity and operating room (OR) staff exposure to waste anaesthetic gases during the use of the Cobra perilaryngeal airway (CobraPLA) compared with the laryngeal mask airway classic (LMA).

METHODS

Sixty patients were randomly assigned to the CobraPLA or the LMA group. Insertion time, number of insertion attempts and airway leak pressures were assessed after induction of anaesthesia. Occupational exposure to nitrous oxide (N(2)O) and Sevoflurane (SEV) was measured at the anaesthetists' breathing zone and the patients' mouth using a photoacoustic infrared spectrometer.

RESULTS

N(2)O waste gas concentrations differed significantly in the anaesthetist's breathing zone (11.7+/-7.2 p.p.m. in CobraPLA vs. 4.1+/-4.3 p.p.m. in LMA, P=0.03), whereas no difference could be shown in SEV concentrations. Correct CobraPLA positioning was possible in 28 out of 30 patients (more than one attempt necessary in five patients). Correct positioning of the LMA classic was possible in all 30 patients (more than one attempt in three patients). Peak airway pressure was higher in the CobraPLA group (16+/-3 vs. 14+/-2 cmH(2)O, P=0.01). The average leak pressure of the CobraPLA was 24+/-4 cmH(2)O, compared with 20+/-4 cmH(2)O of the LMA classic (P<0.001; all values means+/-SD).

CONCLUSION

Despite higher airway seal pressures, the CobraPLA caused higher intraoperative N(2)O trace concentrations in the anaesthetists' breathing zone.

Effects of gas flow management on postintubation end-tidal anesthetic concentration and operating room pollution

STUDY OBJECTIVE

To study how different anesthetic practices during the transition from anesthetic delivery by mask to endotracheal intubation affect end-tidal postintubation anesthetic concentration and operating room (OR) pollution.

DESIGN

Prospective study.

SETTING

Anesthesia research laboratory.

MEASUREMENTS AND MAIN RESULTS

We studied four gas flow management practices: practice vaporizer off, only the anesthetic vaporizer was turned off; all off, oxygen (O2), nitrous oxide (N2O), and the vaporizer were turned off; gas off: O2 and N2O were turned off; and all on: neither the gas flows nor the vaporizer were turned off. A model of inhalational anesthetic induction was simulated by using an adult circle system attached to a reservoir bag ("artificial lung"). By using a fixed gas flow, we achieved an end-tidal N2O (ETN2O) concentration of 70% and end-tidal halothane (ETHal) concentration of 3%, then stopped mechanical ventilation and performed the four practices for a 30-second "intubation" period. During this time, the reservoir bag was disconnected from the circuit, and the gas volume exiting the circuit (pollution volume) was measured. After this 30-second disconnect period, the bag was reconnected to the anesthetic circuit, and the original ventilation, gas flows, and vaporizer setting were resumed. The anesthetic concentrations were measured at 10, 20, and 30 seconds after reconnection. For the vaporizer off practice, ETHal was low and did not return to equilibrium within 30 seconds (p < 0.05); ETN2O clinically was unaltered. In the all off practice, anesthetic concentrations were below equilibrium at 10, 20, and 30 seconds (p < 0.05). For the gas off practice, ETHal was slightly below equilibrium at all times; ETN2O was below equilibrium at 10, 20, and 30 seconds (p < 0.05). In the all on practice, end-tidal anesthetic concentrations were unchanged when compared with equilibrium (p > 0.05). Pollution volumes in the vaporizer off and all on practices were ten-fold higher than in the all off and gas off practices (p < 0.05).

CONCLUSION

In a mechanical model of anesthetic induction, turning the gas flows off before "intubation" and leaving the vaporizer on (the gas off practice) maintained "postintubation" end-tidal drug concentrations close to "preintubation" equilibrium and minimized OR pollution.

Interferences of urinary tract infection in the measurement of urinary nitrous oxide

OBJECTIVES

To investigate the effective role of micro-organisms in producing N2O.

METHODS

The N2O in either urine samples inoculated with 24 microbial strains or urine samples from patients with urinary tract infections were measured.

RESULTS

Gram negative bacilli generally produced high amounts of nitrous oxide (N2O), whereas Gram positive cocci and yeasts did not. The production of N2O depends on the incubation time and follows exponential kinetics, reaching a plateau at 48 hours. Furthermore, the results of urinocultures agreed well with N2O concentrations found in urine samples: samples negative for bacteria were found to contain very low concentrations of N2O whereas those positive--for example, for Enterobacteriaceae--gave highest N2O values.

CONCLUSION

The urinary tract infections caused by Gram negative bacilli are important confounding factors in biological monitoring practices of exposure to inhalation anaesthetics. The current methods adopted to avoid these factors (urine acidification, storage of samples at 4 degrees C) are not good enough because of the relative acid tolerance of some strains and the production of N2O directly into the bladder.

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.

Evaluation of exposure to isoflurane (Forane): environmental and biological measurements in operating room personnel

The concentration of isoflurane (Forane) in the ambient atmosphere was determined in 11 operating theaters of 5 hospitals in Italy. The concentration of isoflurane in the ambient air exceeds the recommended time-weighted average exposure levels (median value: 113 mumol/m3). Isoflurane was detected in the urine of 45 exposed subjects (anesthetists, surgeons, and nurses). A significant correlation was found between the isoflurane concentration in urine produced during the shift (Cu' nmol/l) and isoflurane environmental concentration (Cl' mumol/m3) (Cu = 0.243 X Cl + 3.712) (r = .90). The results show that the urinary isoflurane concentration can be used as an appropriate biological exposure index. The authors suggest a biological exposure index of 18 nmol/l (3.4 micrograms/l). This is the biological value obtained after 4 h of an average environmental exposure to 81 mumol/m3 (2 ppm).