Co-oximetry refers to the measurement of various forms of hemoglobin by dedicated multiwavelength spectrophotometry. It is a measure of the potential oxygen-carrying capacity of the blood.
Use
It usually measures concentrations of oxygenated hemoglobin (oxyHb), deoxygenated hemoglobin (deoxyHb or reduced Hb), COHB, and methemoglobin (MetHb) as a percentage of the total hemoglobin concentration in the blood sample.
Indications
History consistent with toxin exposure
Hypoxia fails to improve with the administration of oxygen
Existence of a discrepancy between the PaO2 on a blood gas determination and the oxygen saturation on pulse oximetry (SpO2)
Suspected other dyshemoglobinemias such as methemoglobinemia or carboxyhemoglobinemia
Oxygen Saturation (SO2)
Calculated as O2 Hb/O2 Hb + HHB 100%.
The availability of oxygen to tissues is dependent not only on SO2 but also on the affinity of O2 to Hb. It is clinically useful in cyanosis and erythrocytosis. It may differentiate between diminished oxygenation of blood, as in pulmonary diseases and admixture of venous blood, as in an AV shunt.
Percent saturation in newborns is 40 " 90% and thereafter 94 " 98%; values decrease with age.
Oxyhemoglobin
This represents the fraction of oxygenated Hb in relation to the total Hb present, including non " oxygen-binding Hb. In healthy individuals, oxyhemoglobin and oxygen saturation are approximately equal. In the presence of dyshemoglobins, oxyhemoglobin can be considerably lower than oxygen saturation. Although the oxygen saturation often remains within the reference limits, the oxygen-carrying capacity of the blood may be severely decreased.
Normal range: 94 " 100%.
Carboxyhemoglobin
This is Hb that has carbon monoxide instead of the normal oxygen bound to it. Carbon monoxide (CO) has a much great affinity than oxygen for hemoglobin.
Carboxyhemoglobin is formed in carbon monoxide poisoning. The source of the carbon monoxide may be exhaust (such as from a car, truck, boat, or generator), smoke from a fire, or tobacco smoke. The carboxyhemoglobin level is useful in judging the extent of CO toxicity and in considering the effect of smoking on the patient. A direct correlation has been claimed between CO level and symptoms of atherosclerotic diseases, angina, and MI.
Nonsmokers: 0.5 " 1.5% saturation of Hb
Smokers: 1 " 2 packs/day: 4 " 5%
Heavy smokers: >2 packs/day: 8 " 9%
Methemoglobin
This is produced by the oxidation of the normal ferrous iron of Hb to ferric iron, making it chemically useless for respiration.
A small amount of methemoglobin is normally present in blood, but the conversion of a larger fraction of hemoglobin into methemoglobin results in perceptible cyanosis. Methemoglobinemia may be acquired anytime in life by exposure to a number of different chemical agents, such as nitrites, or it may be congenital due a genetic condition.