Thermocamera, a macroscopic method for the study of pollution with nitrous oxide in operating theatres

Nitrous oxide (N2O) is used in high concentrations in inhalation anaesthesia and can serve as a tracer of other, more potent anaesthetic agents polluting the air of operating theatres. It has the quality of absorbing infra-red light with a characteristic peak of 4.5 micrometer in the absorption spectrum. N2O in the operating-room atmosphere will absorb infra-red light emitted from a heat screen, and can therefore be registered by an infra-red-camera equipped with a filter eliminating waves outside the 4.5 micrometer waveband. The method was tested during paediatric inhalation anaesthesia. The infra-red-camera measurements are semi-quantitative and sensitive to an extinction of about 1000 ppmcm, comparable to a N2O concentration of 100 ppm measured by an infra-red N2O monitor. It was demonstrated that major pollution occurs during mask anaesthesia and after extubation. The polluting gas is insufficiently evacuated by the operating theatre ventilation. The method makes it possible to visualize the dispersion of spilled or leaking N2O, and is therefore of value when constructing and evaluating new scavenging equipment and in producing educational material.