02b. Carbon Monoxide Poisoning Complicated By Cyanide Poisoning

Carbon monoxide and cyanide poisoning frequently occur simultaneously in victims of smoke inhalation.(70-76) In combination, these two agents exhibit synergistic toxicity.(77,78) HBO2 should be strongly considered in such cases. In addition to its effect on CO, HBO2 may have a direct effect in reducing the toxicity of cyanide(79-83) and in augmenting the benefit of antidote treatment.(84‑86) Clinical reports involving the use of HBO2 in pure cyanide poisoning are infrequent; however, some reports suggest a benefit.(87-89) Since the condition carries a high mortality risk, HBO2 treatment is justified if standard therapy is unsuccessful. The traditional antidote for cyanide poisoning involves formation of methemoglobin through the infusion of sodium nitrite.(90,91) This treatment has the potential to impair the oxygen carrying capacity of hemoglobin. In the smoke inhalation victim, with concomitant COHb and possible pulmonary injury, there is an obvious added risk associated with methemoglobin formation. The HBO2‑mediated increase in plasma-dissolved oxygen content offers a direct benefit. However, one must be cautious in this setting because the methemoglobin level may be directly lowered by hyperoxia (at least at 4 atm abs), possibly reducing the efficacy of antidotal therapy.(92) 

Antidotal therapies other than nitrite‑methemoglobin formation exist, although their use is still investigational. Hydroxocobalamin and dicobalt EDTA directly bind cyanide, obviating the need for methemoglobin formation,(93,96) however, since these agents possess their own toxicities, their use is currently limited. Until direct antidotes become available, HBO2 is recommended as an adjunct to the treatment of combined CO poisoning complicated by cyanide poisoning.