The oxyhemoglobin dissociation curve is basic science, but it is not trivia. It explains why a patient desaturates the way they do, why stored blood behaves strangely, and why carbon monoxide is so dangerous. It also shows up in hotspot and graphic items, where you have to point at the curve itself.
Three anchor points
Memorize these and you can sketch the curve on the note board in ten seconds.
| PaO2 | SaO2 | Where you see it |
|---|---|---|
| 27 mmHg | 50% | The P50 |
| 40 mmHg | 75% | Normal mixed venous blood |
| 60 mmHg | 90% | The edge of the cliff |
| 100 mmHg | about 98% | Normal arterial blood |
The shape is sigmoid because of cooperative binding: each oxygen that binds makes the next one bind more easily. The flat top means arterial saturation barely changes until PaO2 drops below about 60. The steep middle means small drops in PaO2 below 60 cause large drops in saturation.
Clinically: a saturation of 90 percent already means a PaO2 of about 60. There is very little reserve left before the steep part.
Right shift and left shift
The P50 is the PaO2 at which hemoglobin is 50 percent saturated. It is the single number that tells you which way the curve moved.
- Right shift: P50 goes up. Hemoglobin holds oxygen less tightly and unloads more to the tissues.
- Left shift: P50 goes down. Hemoglobin holds oxygen more tightly and unloads less.
What shifts it right
The mnemonic is CADET, face Right:
- CO2 increased
- Acid (decreased pH, more hydrogen ion)
- DPG (2,3-diphosphoglycerate) increased
- Exercise
- Temperature increased
Every one of these describes a working, hot, acidic tissue that needs oxygen. The curve shifts right exactly where unloading helps.
What shifts it left
- Alkalosis and hypocapnia
- Hypothermia
- Decreased 2,3-DPG, as in stored bank blood
- Fetal hemoglobin
- Carbon monoxide
- Methemoglobin
Fetal hemoglobin shifts left on purpose. The fetus needs to pull oxygen away from maternal blood, so its hemoglobin has to grab more tightly.
Bohr versus Haldane
These two get swapped constantly. Keep them straight by what each one is about.
- Bohr effect: CO2 and acid change how hemoglobin holds oxygen. More CO2 in the tissues pushes the curve right and releases oxygen.
- Haldane effect: oxygen changes how blood carries CO2. Deoxygenated hemoglobin carries more CO2, so in the lungs, as hemoglobin picks up oxygen, it dumps CO2.
A simple tag: Bohr is about oxygen delivery, Haldane is about CO2 transport.
Carbon monoxide, the dangerous one
Carbon monoxide binds hemoglobin with an affinity about 200 to 250 times that of oxygen. It does two harmful things at once:
- It takes up binding sites, so oxygen content falls.
- It shifts the curve left, so the oxygen that is bound is released less readily.
A standard two wavelength pulse oximeter reads carboxyhemoglobin as oxyhemoglobin, so the SpO2 can look normal in a poisoned patient. Co-oximetry is required, and treatment is 100 percent oxygen, with hyperbaric oxygen in severe cases.
Oxygen content, the calculation
The NCE likes this one as a calculation item.
CaO2 = (1.34 x Hgb x SaO2) + (0.003 x PaO2)
For a hemoglobin of 15, saturation of 98 percent and PaO2 of 100:
- Bound: 1.34 x 15 x 0.98 = 19.7 mL/dL
- Dissolved: 0.003 x 100 = 0.3 mL/dL
- Total: about 20.0 mL/dL
Notice how little is dissolved. That is why hemoglobin, not PaO2, drives oxygen content.
Quick hits for test day
- P50 is about 27 mmHg. Right shift raises it.
- 40 mmHg is 75 percent, 60 mmHg is 90 percent.
- CADET, face right.
- Stored blood, fetal hemoglobin, CO and methemoglobin shift left.
- Bohr is oxygen, Haldane is CO2.
- CO fools the pulse oximeter. Use co-oximetry.
The respiratory physiology lessons on CRNA Dojo walk the curve step by step, and the mnemonics library has printable cards for shortcuts like this one.
Sources
- Barash, Clinical Anesthesia
- Nagelhout, Nurse Anesthesia
- West, Respiratory Physiology: The Essentials
