What level of lactic acidemia is concerning for concurrent cyanide poisoning in carbon monoxide poisoning?

From the Guidelines

A lactic acid level greater than 10 mmol/L in a patient with carbon monoxide poisoning should raise concern for concurrent cyanide poisoning, particularly in victims of smoke inhalation from enclosed-space fires, and empiric treatment with hydroxocobalamin should be considered. This is because the high lactic acid level results from cellular hypoxia caused by cyanide's inhibition of cytochrome c oxidase in the mitochondrial electron transport chain, which blocks aerobic metabolism and forces cells to rely on anaerobic glycolysis 1. The most recent guidelines recommend hydroxocobalamin as the primary treatment for patients with suspected cyanide poisoning, especially when carbon monoxide poisoning is also a concern, due to its effectiveness and lack of hypotensive effects 1.

Some key points to consider in the management of patients with suspected concurrent cyanide and carbon monoxide poisoning include:

• The importance of early recognition and treatment, as cyanide poisoning can progress rapidly and be fatal
• The use of 100% oxygen as therapy, even with a normal partial pressure of oxygen, as it may enhance the effectiveness of cyanide-specific antidotes 1
• The consideration of other clinical findings that may support concurrent cyanide toxicity, such as persistent metabolic acidosis, altered mental status disproportionate to the carbon monoxide level, and cardiovascular instability
• The potential benefits of combining cyanide-specific antidotes, such as hydroxocobalamin and sodium thiosulfate, to enhance cyanide elimination 1

In terms of specific treatment recommendations, the guidelines suggest that hydroxocobalamin (Cyanokit, initial adult dose 5g IV over 15 minutes, may repeat once if needed) is a suitable option for empiric treatment of suspected cyanide poisoning in patients with carbon monoxide poisoning 1. Sodium nitrite and sodium thiosulfate may also be considered as alternative or adjunctive treatments, although their use requires careful consideration of the patient's individual circumstances and the potential risks and benefits of each treatment option 1.

From the FDA Drug Label

Although hypotension is highly suggestive of cyanide poisoning, it is only present in a small percentage of cyanide-poisoned smoke inhalation victims Also indicative of cyanide poisoning is a plasma lactate concentration greater than or equal to 10 mmol/L (a value higher than that typically listed in the table of signs and symptoms of isolated cyanide poisoning because carbon monoxide associated with smoke inhalation also contributes to lactic acidemia)

• A plasma lactate concentration of ≥ 10 mmol/L is concerning for concurrent cyanide poisoning in carbon monoxide poisoning 2

From the Research

Lactic Acidemia and Cyanide Poisoning in Carbon Monoxide Poisoning

• A plasma lactate concentration ≥ 10 mmol/L in fire victims without severe burns and ≥ 8 mmol/L in pure cyanide poisoned patients is a sensitive and specific indicator of cyanide intoxication 3.
• In the context of carbon monoxide poisoning, lactic acidosis can persist even after the cessation of toxic exposure, and treatment with oxygen and sodium thiosulfate may not hasten the recovery of this lactic acidosis 4.
• The level of lactic acid can be a useful prognostic factor in cases of carbon monoxide poisoning, with higher initial lactate levels associated with more severe clinical presentation and worse outcomes 5.
• A study on canine models of combined carbon monoxide and cyanide poisoning found that lactic acidosis persisted for at least 10 minutes after toxic exposure, despite the administration of oxygen and sodium thiosulfate 4.
• In patients with carbon monoxide poisoning, initial lactate levels > 2.1 mmol/L were associated with altered mental status, higher white blood cell count, glucose, and carboxyhemoglobin levels, and lower base deficit on admission 5.

Diagnostic Considerations

• The examination of lactic acid level is significant in carbon monoxide poisoning, and routine examination of acid-base balance and lactate level can be important for considering the severity of the poisoning 6.
• Lactate level can correlate well with the condition severity and may be even more significant for assuming oxygen deficit and total prognosis of the patient 6.
• In cases of suspected cyanide poisoning, a plasma lactate concentration ≥ 10 mmol/L in fire victims without severe burns and ≥ 8 mmol/L in pure cyanide poisoned patients can be used as a diagnostic indicator 3.