Clinical presentation of methaemoglobinaemia

Clinical effects of methaemoglobinaemia are listed in Clinical effects of methaemoglobinaemia related to the fraction of methaemoglobin in the blood. These effects relate to the fraction of methaemoglobin in the blood and the effectiveness of the compensatory cardiovascular response to reduced oxygen-carrying capacity. The methaemoglobin fraction in whole blood is normally less than 1%.

Haemolytic anaemia may accompany methaemoglobinaemia, but onset may be delayed by 12 to 24 hours.

Table 1. Clinical effects of methaemoglobinaemia related to the fraction of methaemoglobin in the blood
Methaemoglobin fraction in the blood	Clinical effects
less than 10%	none
10 to 20%	slate grey skin discolouration, cyanosis
20 to 30%	headache, anxiety, tachycardia
30 to 50%	drowsiness, fatigue, confusion, tachypnoea
50 to 70%	increasing central nervous system depression, coma, seizures, arrhythmias, metabolic acidosis
more than 70%	lethal