Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (838 page)

   Hypoxia fails to improve with the administration of oxygen

   Existence of a discrepancy between the PaO
₂
on a blood gas determination and the oxygen saturation on pulse oximetry (SpO
₂
)

   Suspected other dyshemoglobinemias such as methemoglobinemia or carboxyhemoglobinemia

Oxygen Saturation (SO
₂
)

   Calculated as O
₂
Hb/O
₂
Hb + HHB × 100%.

   The availability of oxygen to tissues is dependent not only on SO
₂
but also on the affinity of O
₂
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%