A sigmoid curve with P50 of about 3.5 kPa or 27 mmHg
Mnemonic
“CADET, face Right!” for a right shift, meaning reduced affinity so oxygen is released more readily to the tissues:
- C - CO2 raised
- A - Acid, so a low pH
- D - 2,3-DPG raised
- E - Exercise
- T - Temperature raised
Everything opposite shifts it left, increasing affinity so oxygen is held onto: alkalosis, hypothermia, low 2,3-DPG (as in stored blood), fetal haemoglobin, carboxyhaemoglobin and methaemoglobin.
Fetal haemoglobin shifts left because it binds 2,3-DPG poorly, which lets it strip oxygen from the maternal circulation.
Key points on the curve: P50 is 3.5 kPa (26 mmHg), saturation is 90 per cent at 8 kPa, and the curve is sigmoid because of cooperative binding, which is why saturation falls slowly at first and then precipitously below 8 kPa.
Expansion
The curve is sigmoid because haemoglobin binds oxygen cooperatively: each bound molecule increases the affinity of the remaining sites.
Key points:
- P50 (50 per cent saturation) at about 3.5 kPa (27 mmHg)
- Arterial point: 13.3 kPa (100 mmHg), 97 per cent saturated
- Mixed venous point: 5.3 kPa (40 mmHg), 75 per cent saturated
The shape is functionally elegant:
- The flat upper portion means arterial saturation is well maintained even if alveolar oxygen falls substantially, and it also means supplemental oxygen adds little once saturation is high
- The steep middle portion means large amounts of oxygen are released for a small fall in tension at tissue level
A useful clinical landmark is that a saturation of 90 per cent corresponds to an oxygen tension of about 8 kPa (60 mmHg), below which the curve falls away steeply and small further falls cause large desaturation.