17. Poisons inducing methemoglobinemia Flashcards
meth
reduced or ferrous form (Fe2+) can combine with oxygen by sharing an electron, thus forming oxyhemoglobin. - incapable of oxygen transport
methemoglobin - When hemoglobin loses an electron and becomes oxidized, Fe is converted to the ferric state (Fe3+)
Spontaneously formed methemoglobin is normally reduced (regenerating normal hemoglobin) by protective enzyme systems NADH methemoglobin reductase (cytochrome-b5)
transfers electrons from NADH to methemoglobin - removal of 95-99% of the methemoglobin that is produced under normal circumstances
NADPH methemoglobin reductase (minor pathway) and to a lesser extent the ascorbic acid and glutathione enzyme systems - utilizes glutathione production and glucose-6-phosphate dehydrogenase (G6PD) to reduce methemoglobin to hemoglobin.
The NADPH-dependent methemoglobin reduction pathway can be accelerated by exogenous cofactors such as methylene blue to as much as five times its normal level of activity.
Mechanism - reversible/irreversible changes in the structure of Hb → Hb unable for the normal O2 transport
Methemoglobin - reversible changes, Hb Fe++ (ferro) is oxidized to Fe+++ (ferri)
Verdoglobin
in case of irreversible changes (substances that can lead to methemoglibinaemia in higher conc) - oxidation and splitting of the porphyrin ring
porphyrin production without Fe
They may damage also the RBCs (membrane, strome, globin) may appear the Heinz bodies → hemolysis → anuria
Symptoms cyanosis, hypoxia, collapse
Therapy:
1% methylene blue 1–2 mg/ttkg in slow iv infusion
