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Carbon Monoxide & Hydrogen Cyanide

CO and HCN are both chemical asphyxiates commonly found in fire smoke, and exposure should be considered in every case of smoke inhalation. CO is produced when a carbon-containing material, such as wood, is burned and there isn’t enough oxygen present to completely transform into carbon dioxide (CO2). It’s also commonly found near burning hydrocarbon fuels (e.g., gasoline and diesel). Although the fire smoke or fuel fumes are easy to identify, CO itself is colorless, odorless, tasteless and initially nonirritating. Hemoglobin has a high affinity for CO and once bound, the new molecule (carboxyhemoglobin [COHb]) is no longer capable of binding oxygen. Since oxygen dissolves poorly in blood plasma, high COHb levels result in poor oxygen-carrying capacity and hypoxemia.

HCN is a by-product of the combustion of materials such as green wood, tobacco, cotton, paper, wool and silk. When burned, these materials release nitrogen gas into the air. You should assume that both CO and HCN are present in fire smoke and that fire victims have been poisoned by both, resulting in both oxygen transport and utilization issues. The immediate danger to life and health toxicity for cyanide is much lower (50 ppm) when compared to CO (1200 ppm), so firefighters should be extremely cautious about working in a smoky environment unless wearing self-contained breathing apparatus (SCBA).




Immediate Care

The first priority of managing smoke inhalation victims is assuring the airway remains patent. Apneic and near-apneic patients should be ventilated with 100% oxygen by bag-valve mask and the appropriate airway adjuncts. Smoke inhalation victims with a patent airway should be treated with high-flow oxygen administered with a tight-fitting mask. For most EMS systems, the best mask available is the standard non-rebreather mask, but a double-sealing silicone mask will deliver higher fraction of inspired oxygen.

The best initial treatment for CO poisoning is to provide the highest fraction of inspired oxygen possible. The half-life of CO bound to hemoglobin while breathing air is approximately 5.5 hours, but breathing 100% oxygen reduces the half-life to 80 min. Most cases of mild CO poisoning respond well to this therapy alone. Severe cases of CO poisoning may be treated with hyperbaric oxygen. Patients are placed in a hyperbaric chamber and exposed to oxygen at pressures higher than those experienced at sea level. Hyperbaric oxygen delivered at 2 atmosphere’s absolute pressure decreases the half-life of CO to 23 minutes.4 The high pressures cause oxygen to dissolve into the blood plasma, allowing the tissues to oxygenate while the CO is being removed from hemoglobin. Not every hospital will have a hyperbaric facility and not all physicians support hyperbaric oxygen therapy. It’s important to know your local protocols for hyperbaric oxygen indications and the centers that are capable of delivering that therapy.

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