Oxygen Dissociation Curve

The oxyhemoglobin dissociation curve is a map of how oxygen tension relates to haemoglobin saturation. It is not, by itself, a measure of total oxygen transport or tissue oxygenation. Use the curve to answer two linked questions: how tightly is haemoglobin holding oxygen, and is enough oxygen actually reaching cells? A patient may have a reassuring SpO2 but poor oxygen delivery because haemoglobin is severely reduced, oxygen-binding sites are occupied by carbon monoxide, or perfusion is failing. Conversely, a right shift can improve unloading without correcting inadequate cardiac output or anaemia. The curve is especially useful when a saturation value does not fit the patient. In Canadian practice, follow local oxygen protocols and remember that Health Canada cautions that pulse-oximeter accuracy may be reduced in people with darker skin pigmentation. The January 2025 FDA pulse-oximeter proposal is a United States draft regulatory document, not a Canadian requirement. The physiology that follows explains when the displayed number can be trusted, when it needs confirmation, and how affinity changes alter treatment decisions.