Toxicology

Methemoglobinemia

TRMetemoglobinemi
Also known as:Methaemoglobinaemia

A condition in which an increased fraction of hemoglobin contains oxidized ferric (Fe³⁺) iron; this methemoglobin cannot carry oxygen and shifts the dissociation curve to the left, impairing tissue oxygen delivery.

Formula

MetHb (%) = MetHb / total Hb × 100

In depth

Methemoglobin (MetHb) normally stays at ≤1–2% of total hemoglobin (Hb), mainly through NADH-cytochrome b5 reductase; a minor NADPH-dependent pathway becomes important only when methylene blue is given. Acquired forms follow oxidant drugs, often via metabolites: topical benzocaine, prilocaine, dapsone, primaquine, nitrates and nitrites, inhaled nitric oxide and rasburicase. Infants and people with cytochrome b5 reductase deficiency are more susceptible.

Cyanosis persists despite oxygen, blood looks chocolate-brown, and pulse oximetry reads low while PaO2 is normal; co-oximetry confirms the diagnosis. Treatment is withdrawal of the cause, oxygen and methylene blue (methylthioninium chloride); in G6PD deficiency methylene blue is ineffective and may cause hemolysis, so ascorbic acid or exchange transfusion are alternatives.

Clinical pearl

Suspect it when cyanosis and a low SpO2 fail to respond to oxygen despite a normal PaO2; before giving methylene blue, ask about G6PD deficiency and serotonergic drugs, since methylene blue is a potent MAO inhibitor.

Example drugs

Dapsone Benzocaine Prilocaine Primaquine Rasburicase
TRIn Turkish

Metemoglobinemi

Hemoglobinin artmış bir bölümünün oksitlenmiş ferrik (Fe³⁺) demir içerdiği durumdur; oksijen taşıyamayan bu metemoglobin, oksijen-hemoglobin ayrışma eğrisini sola kaydırarak dokulara oksijen sunumunu bozar.

Metemoglobin (MetHb) normalde total hemoglobinin (Hb) %1–2’sini aşmaz; bu düzey esas olarak NADH-sitokrom b5 redüktaz ile korunur, NADPH’ye bağımlı küçük yol ise ancak metilen mavisi verildiğinde önem kazanır. Edinsel formlar sıklıkla metabolitleri aracılığıyla etki eden oksidan ilaçlarla ortaya çıkar: topikal benzokain, prilokain, dapson, primakin, nitrat ve nitritler, inhale nitrik oksit ve rasburikaz. Bebekler ve sitokrom b5 redüktaz eksikliği olanlar daha duyarlıdır.

Oksijene yanıt vermeyen siyanoz, çikolata kahverengisi kan ve PaO2 normalken düşük SpO2 tipiktir; tanı ko-oksimetriyle doğrulanır. Tedavi nedenin kesilmesi, oksijen ve metilen mavisidir (metiltiyoninyum klorür); G6PD eksikliğinde metilen mavisi etkisizdir ve hemolize yol açabilir, bu durumda askorbik asit ya da kan değişimi seçenektir.

Normal PaO2’ye karşın oksijene yanıt vermeyen siyanoz ve düşük SpO2 varsa akla gelmelidir; metilen mavisi güçlü bir MAO inhibitörü olduğundan, uygulamadan önce G6PD eksikliği ve serotonerjik ilaç kullanımı sorgulanmalıdır.

Diğer adları:Methemoglobinemi
Updated 24.09.2026 1 views Educational reference — verify against current sources before clinical use.

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