ENDOTRACHEAL TUBE FIRES

A feared complication of laser use during airway surgery is endotracheal tube fire. The estimated incidence of this complication during such operations is 0.5% to 1.5%.^{[49] [50]} Although a survey of otolaryngologists did not provide an incidence for fires, it did demonstrate that laser-induced ignition of endotracheal tube, cuff, or cottonoids was responsible for most (41%) perioperative complications, followed by postoperative laryngeal web (19%) and laser-related or laser-induced facial burns (11%).^[51] The largest published (retrospective) series of CO₂ laser airway procedures documented six airway fires in 4416 cases (0.14% incidence),^[52] confirming it as the most frequent laser-related complication in patients undergoing this type of procedure. Given the proximity of the endotracheal tube to surgical sites around the larynx, the potential for fire or other airway complications is clear. Electrosurgical devices have also caused airway fires when leaks from the breathing circuit raise the oxygen concentration in proximity to the cautery. Many of these fires, when appropriately handled, result in minimal or no harm to the patient, ^{[49] [53] [54]} but catastrophic consequences are possible.^{[55] [56]}

With the energy delivery rates described earlier, any hydrocarbon material, including tissue, plastic, or rubber, can ignite and burn, particularly in an oxygen-enriched atmosphere. Fires can result from direct laser illumination, reflected laser light, or incandescent particles of tissue blown from the surgical site.^[57] Initially, most fires are located solely on the external surface of the endotracheal tube, where they can cause local thermal destruction. If a fire is unrecognized and burns through to the interior of the tube, the oxygen-enriched gas combined with the to-and-fro gas flow due to ventilation will produce a blowtorch-like flame, blowing heat and toxic products of combustion down to the pulmonary parenchyma ( Fig. 67-7 ). Puncture and unrecognized deflation of the tube cuff may also permit oxygen-enriched gas to flood the operative site and increase the chance of a devastating fire after a laser burst.

Three strategies are used to reduce the incidence of airway fire: reduction of the flammability of the endotracheal tube, removal of flammable materials from the airway by using a metallic Venturi jet ventilation cannula or intermittent extubation with or without apnea, and reduction of the available oxygen content to the minimum

Figure 67-7 Blowtorch ignition of an endotracheal tube. (From Emergency Care Research Institute: Airway fires: Reducing the risk during laser surgery. Health Devices 19:109, 1990.)

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Relative Flammability: Effect of Tube Composition

Given that all common endotracheal tubes are potentially flammable, the relative risks of the various types of construction material have been well studied. For many years, reusable red rubber tubes were commonplace, but these were suplanted by clear polyvinylchloride (PVC) plastic tubes. Modern PVC strongly absorbs far-infrared light and is very sensitive to CO₂ laser energy. PVC tubes appear to be much more easily ignited by CO₂ lasers than red rubber tubes^{[52] [58] [59]} and to produce more toxic combustion products. In vitro, PVC is transparent and therefore immune to Nd:YAG and visible laser light; however, a thin coating of mucus or blood in vivo can absorb energy and restore the hazard.^[60] Commercially available tubes fabricated of PVC without opaque lettering or a barium stripe make claims of laser resistance based on the in vitro tests—however, caveat emptor (i.e., let the buyer beware).

Two studies of the effects of Nd:YAG laser energy on common types of tubes, including those designed to be resistant to laser irradiation, revealed that endotracheal tubes of all materials are quite vulnerable.^{[61] [62]} Ossoff and colleagues^[58] compared the extent of acute damage to the trachea from blowtorch-type ignitions in dogs ventilated with 1% halothane and 70% nitrous oxide (balance oxygen) through PVC, rubber, or silicone endotracheal tubes. PVC ignited and developed intense flame the soonest, resulting in widespread deposit of carbonaceous debris and significant ulceration and inflammation of the trachea on postmortem analysis. Red rubber tubes were more resistant to ignition, and produced less debris and inflammation. Silicone tubes were the most resistant to ignition, but they produced copious white silica ash, suggesting the potential for late development of silicosis. Ossoff^[63] subsequently measured the time to intraluminal ignition during exposure CO₂ laser energy to determine resistance to ignition and reversed the ranking of silicone and red rubber, with PVC remaining the most vulnerable. He also found that addition of 2% halothane vapor retarded ignition. After ignition occurs, the index of flammability is the minimum inspired oxygen fraction (FIO₂ ) necessary to maintain combustion. Wolf and Simpson^[64] report that PVC is less flammable than silicone or red rubber, having a flammability index of 0.26 versus 0.19 for silicone and 0.19 for red rubber. When nitrous oxide was used as the oxidant, PVC retained the highest index (0.46), followed by silicone (0.41) and red rubber (0.37).

Despite the conflicting data, many physicians recommend the use of red rubber endotracheal tubes during laser surgery of the areodigestive tract, and base this suggestion on criteria of resistance to ignition and least toxic combustion products. However, at the University of California, San Francisco, one surgeon has performed more than 4000 microdirect laryngoscopies using a CO₂ laser, incurring only two fires (0.005%) and no significant fire-related morbidity (H. H. Dedo, personal communication, 1992) with aluminum-taped PVC endotracheal tubes and moistened pledgets (discussed later). Regardless of the tube substrate, there are additional safety considerations and maneuvers to consider, as outlined later.