Histotoxic hypoxia

Histotoxic hypoxia, also known as histoxic hypoxia, occurs when cells cannot absorb or utilize oxygen from the blood, even though oxygen delivery to those cells and tissues is normal. This condition stems from tissue poisoning, for instance by cyanide or hydrogen sulfide.

Cyanide poisoning is a classic example. It blocks the final step in the electron transport chain by binding to the ferric ion on cytochrome oxidase a3, halting oxidative phosphorylation and ATP production in mitochondria. Other chemicals like rotenone and antimycin A disrupt the mitochondrial chain similarly. As oxygen consumption drops, oxygen extraction decreases, raising venous oxygen content and partial pressure. Although cyanide stimulates peripheral respiratory chemoreceptors, increasing the fraction of inspired oxygen offers no benefit because the cells already have sufficient oxygen they cannot use.

Treatment for cyanide-induced histotoxic hypoxia typically involves a three-part antidote kit: amyl nitrite, sodium nitrite, and sodium thiosulfate. The nitrites convert hemoglobin into methemoglobin, which binds cyanide. Since cyanide has a stronger affinity for the ferric ion on methemoglobin than for that on cytochrome oxidase a3, it is drawn out of the mitochondria, allowing ATP production to resume and halting the hypoxia.

Histotoxic hypoxia can also follow ischemia, as in stroke or inflammation. In stroke, interrupted blood supply followed by reperfusion leads to an accumulation of reactive oxygen species (ROS), causing the condition. In neuro-inflammatory diseases such as Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis, ROS from mitochondrial damage in active lesions can trigger histotoxic hypoxia. Additionally, excessive release of inflammatory mediators like heme oxygenase-1 (HO-1) can cause iron sequestration, contributing to histotoxic hypoxia in these same diseases.