Bicarbonate Levels in Cyanide Poisoning
Bicarbonate levels will be LOW in cyanide poisoning due to severe metabolic acidosis, which is a hallmark finding of this toxicity. 1
Pathophysiology of Acid-Base Disturbance
Cyanide causes profound lactic acidosis by blocking aerobic cellular metabolism at the mitochondrial level, specifically by reversibly binding to ferric ion cytochrome oxidase, which halts adenosine triphosphate production and forces cells into anaerobic metabolism. 2
The resulting accumulation of lactate consumes bicarbonate, leading to a severe anion gap metabolic acidosis with markedly decreased serum bicarbonate levels. 3, 4
Lactic acidosis is both a sensitive and specific finding for cyanide poisoning, making it one of the most reliable laboratory indicators when cyanide toxicity is suspected. 2, 1
Diagnostic Thresholds
A plasma lactate level ≥10 mmol/L in fire victims without severe burns or ≥8 mmol/L in pure cyanide poisoning cases is a sensitive and specific indicator of cyanide intoxication. 3
Severe metabolic acidosis with pH <7.20 correlates with high short-term mortality (30-50%) in cyanide-poisoned patients, regardless of the specific cyanide level. 2
Deep metabolic acidosis is the rule in severe cyanide intoxications, and cyanide poisoning should always be suspected in cases of coma with severe acidosis. 5
Clinical Implications for Treatment
Bicarbonate administration is part of supportive care to correct the severe metabolic acidosis while definitive antidote therapy is being administered. 6, 5
Treatment decisions should be based on clinical presentation (including the metabolic acidosis) rather than waiting for laboratory confirmation of cyanide levels, as immediate intervention is critical. 1
The combination of hydroxocobalamin (5-10 grams IV for adults) plus 100% oxygen, with or without sodium thiosulfate, is the recommended antidote regimen for cyanide poisoning causing metabolic acidosis. 2, 1
Common Pitfalls
Do not delay antidote therapy while waiting for cyanide level confirmation—the presence of severe metabolic acidosis with elevated lactate in the appropriate clinical context (smoke inhalation, industrial exposure) warrants immediate treatment. 1, 3
In house fire victims with severe metabolic acidosis (pH <7.20) or lactate ≥10 mmol/L, empiric treatment for cyanide poisoning with hydroxocobalamin should be strongly considered even if carbon monoxide is also present. 2, 7
The low bicarbonate is a consequence of the poisoning, not the primary problem—while bicarbonate replacement helps stabilize the patient, definitive treatment requires cyanide-specific antidotes to restore cellular respiration. 6, 5