An estimated 50,000 patients seek medical attention in the United States each year due to carbon monoxide (CO) poisoning (Hampson & Weaver, 2007). Although CO poisoning presents infrequently, it is a consequential and serious component of burn-related injuries, especially those injured via structure fire. Approximately half of fire-related deaths are secondary to CO poisoning (Heimbach & Waeckerle, 1988). A multitude of retrospective reviews and prospective trials have attempted to establish evidence demonstrating the ideal modality for oxygen administration in CO-poisoned patients; however, a consensus recommendation has not been reached. In addition, the majority of these studies either excluded burn patients (Annane et al., 2010; Weaver et al., 2002) or only included small numbers of this subpopulation (Huang et al., 2017; Rose et al., 2018). This leaves a dearth of information for the treatment of trauma-related CO poisoning.

Toxicity occurs secondarily to inspiration of CO, a colorless and odorless gas created by incomplete combustion of organic materials. When inspired, CO competes with oxygen for binding to hemoglobin. Hemoglobin's affinity for CO is greater than 200-fold higher than its affinity for oxygen. When CO binds to hemoglobin, carboxyhemoglobin (COHb) is formed and effectively reduces the amount of hemoglobin available to deliver oxygen to peripheral tissues (Hoyte, 2017). Carbon monoxide toxicity is most commonly diagnosed via co-oximetry analysis from an arterial or venous blood gas sampling. Normal COHb levels are less than 3% in nonsmokers and less than 10% in smokers (Hoyte, 2017). Treatment of CO toxicity requires the administration of 100% oxygen, which decreases binding of CO to hemoglobin, thus reducing its half-life. Ultimately, hemoglobin is once again available to carry oxygen (Hoyte, 2017).

Administration of 100% oxygen is accomplished with either normobaric oxygen or hyperbaric oxygen (HBO) delivery. Normobaric oxygen is delivered via nonrebreather mask or endotracheal tube depending on clinical status of the patient. Hyperbaric oxygen is the delivery of 100% oxygen in a chamber that is pressurized to two to three times the atmospheric pressure at sea level (2-3 ATA) (Hoyte, 2017). Such chambers are constructed to hold either a single patient or rooms that can hold several patients. Pressurizing the inspired oxygen hastens the dissociation of CO from hemoglobin and it also enables oxygen to dissolve freely in the bloodstream (Hoyte, 2017). This allows for a second mechanism to increase oxygen delivery to peripheral tissues by bypassing hemoglobin as a carrier. For decades, there has been substantial controversy regarding which method is optimal, normobaric oxygen versus HBO.

Several recently published trials have suggested benefit of HBO for CO poisoning (Huang et al., 2017; Rose et al., 2018; Weaver et al., 2002). In perhaps the most well-designed prospective, randomized trial to date, Weaver et al. (2002) concluded that three sessions of HBO within 24 hr of CO poisoning led to a lower incidence of long-term neurologic sequelae when compared with normobaric oxygen. The patients in this study were transferred from three states to a single institution upon diagnosis of CO toxicity, limiting the logistical capability of transferring traumatically injured patients. In addition, no burn patients were included in this study, further limiting generalizability. In a retrospective analysis from the registry data of the National Poisoning Database from Taiwan (1999-2012), researchers found a lower mortality rate among HBO-treated patients (Huang et al., 2017). Furthermore, those who received two or more sessions of HBO had a lower mortality compared with one session. Unfortunately, no subgroup analysis was performed for CO-poisoned patients with concomitant burns or traumatic injuries from fires.

Most recently, Rose et al. (2018) conducted a retrospective cohort study of CO-poisoned patients over a 14-year period derived from a Pittsburgh area health system repository. This analysis determined that the administration of HBO reduced inpatient mortality as well as 1-year mortality. Interestingly, a subgroup analysis of patients with fire exposure (n = 195) was performed and deduced that HBO treatment was associated with reduced inpatient mortality, but not 1-year mortality. No information regarding number of HBO sessions or neurologic sequelae was described.

Conversely, some prospective trials have shown negative results when comparing HBO to normobaric oxygen for CO poisoning (Annane e al., 2010; Scheinkestel et al., 1999). Annane et al. (2010) described two prospective, randomized trials that were performed concurrently in France. One study evaluated HBO in patients with transient loss of consciousness, whereas the other assessed those presenting comatose. Neither study demonstrated benefit of HBO, and further analysis correlated two sessions of HBO with worse outcomes than one session in the population of CO-poisoned patients who presented comatose. However, the authors' conclusions should be interpreted with careful attention because administration of HBO in this study was on the lower end of ATA than the majority of published research, which may not have produced equivalent clinical outcomes. Also, extended caution should be exercised with trauma patients because patients with fire-related CO poisoning were excluded.

The ideal treatment modality for CO poisoning requires a well-organized multi-institutional prospective, randomized trial and must include all pertinent patients (e.g., fire-related CO poisoning) to truly determine what treatment modality is superior. This echoes the request by other researchers and authors of authority on this subject matter. For the reasons presented, the lack of data in this subpopulation of CO-poisoned patients leaves trauma providers without clear guidance. Given that half of the fire-related patients succumb to CO poisoning, this is an imperative area of research.

KEY POINTS

* Carboxyhemoglobin levels greater than 3% in nonsmokers or greater than 10% in smokers indicate CO exposure.

* Administration of 100% oxygen decreases the half-life of CO in the bloodstream; the ideal method of delivery of oxygen is debatable.

* Available literature does not commonly include subgroup analysis of fire/burn patients when assessing the difference between oxygen delivery via HBO or via nonrebreather/endotracheal tube.

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