Ignition and burning of biological tissue under simulated CO2-laser surgery conditions

Background. Laser surgery in endoscopy has greatly enhanced the surgical armamentarium for treating various laryngeal and hypopharyngeal disorders, but harbours a risk of tissue ignition and inflammation of surgical and anaesthetic instrumentation. However, even if non-inflammable material is used, there is still a residual fire hazard from endogenous tissue that may develop an ignitable aerosol (so-called laser smog) as an effect of laser irradiation. The aim of this study was to investigate how tissue carbonisation and vaporisation contributes to the risk of airway fire. Methods. For the simulation of gas accumulation in the hypopharynx and larynx following the European standard ISO-11990, a cylindrical steel chamber with an open and a closed end has been used to simulate the operative setting. Pork meat chunks with a tissue composition similar to the larynx and popharynx such as fat, muscle, cartilage and bone were introduced into this chamber. Ventilation was achieved through jet ventilation with disposable, non-inflammable laser jet catheters. The tissue was then repeatedly exposed to a laser beam in super-pulse mode (pulse rate 250 Hz) with various intensities and exposure lengths at an impact angle of 75 degrees. The laser intensity was varied from 2 to 15 W. The type, duration, intensity and incidence of tissue ignition were recorded and analysed. Results. The degree of tissue ignition correlated with laser intensity. Low laser intensity caused spark formation whereas high intensity resulted in sustained tissue fire. The type of tissue had an impact on ignition intensity thereby showing lower ignition thresholds and higher ignition susceptibility in fat-containing tissue compared to muscle. The most important factor for occurrence of tissue ignition was the chamber oxygen concentration which displayed an inverse correlation with the time until tissue ignition. Oxygen concentrations of 35% led to tissue ignition in 42 s, 40% oxygen in 20 s. Oxygen concentrations higher than 60% resulted in immediate tissue ignition. Conclusions. Despite the use of non-inflammable materials in endoscopic laser surgery of the upper airway and hypopharynx, the risk of tissue ignition remains due to the inflammable laser smog which is easily ignited in an oxygen-rich environment. Hence to minimise this risk, we recommend using oxygen concentrations lower than 40%, low laser intensities (< 6 W) and limiting continuous laser activation to periods shorter than 10 s.