- If the SPO2 is 90%, the PO2 is probably around 60
- If the SPO2 is 80%, the PO2 is probably around 50%
- If the SPO2 is 70%, the PO2 is probably around 40%
If the patient is not in respiratory distress, you will not need to obtain an ABG in most cases. Yet, as you and I both know, an ABG is quite often ordered regardless. Yet all of the above have been developed cheats for you and I to use, and to share with nurses and physicians, based on the oxyhemoglobin dissociation curve.
Basically, as you can see by this post at Ventworld.com, "the curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called 'hemoglobin's affinity for oxygen,' that is, how readily hemoglobin acquires and releases oxygen molecules from its surrounding tissue." The curve is based on oxygen's affinity for hemoglobin.
Each hemoglobin molecule has a limited capacity for holding oxygen molecules. How much of that capacity is filled up is expressed as the oxygen saturation, or SpO2. Thus, under normal conditions, a normal SpO2 should be 98%, which would result in a PO2 of greater than 90, which is generally about 102.
In the curve above, the blue curve represents that of a normal person. Under normal conditions, where the pH is normal, the above 4-5-6, 7-8-9 rule holds true. According to the curve, as the PO2 decreases, so does the PO2. And the PO2 is just about 30 below the SPO2. Thus, the 4-5-6,7-8-9 rule basically purports that the PO2 is generally 30 below SPO2.
Likewise, by observing the curve, the higher the SpO2 the less change in PO2 will occur. Thus, if the SpO2 is 90%, you will have minimal changes in PO2. However, as the SpO2 drops below 90%, you start to have a more significant drops in PO2 for each SpO2 recorded.
Or, stated another way, the lower the SpO2 the greater the corresponding drop in PO2 will be. So you can see that the carrying capacity of hemoglobin (expressed as your SpO2) is directly related to the PO2 of which the hemoglobin is exposed to in the lungs.
These events, in effect, will cause the curve to shift to the right or left. A curve shifts to the right when the hemoglobin has a decreased affinity for oxygen, and has a "harder" time making the bond with oxygen.
This increases the hemoglobin's ability to release oxygen to tissues in the body's attempt to keep tissues well oxygenated despite whatever irregular conditions that are occurring. Thus, when a curve shifts to the right, you will have the following conditions:
- Lower SpO2 for a given PO2
- Requires a higher PO2 to achieve the desired SpO2
- Hemoglobin more likely to dump oxygen into tissues (active muscles need more oxygen)
- Think Heat. Anything that creates heat will move curve to right. Acidosis or low pH (heat)
- High CO2 (causes Acidosis)
- Exercise (heats up body)
- Increased 2.3 DPG (we'll describe this below)
When the curve shifts to the left, you will have the following conditions:
- Higher SpO2 for a given PO2
- Hemoglobin is more likely to cling to O2 and not let go (activity is minimal)
- Think Cold. The colder your body, the slower activity will be.
- Hypothermia (cold tissues)
- Rest (minimal exertion)
- CO2 poisoning
- Cold tissues Alkalotic
- Decreased 2.3 DPG
- Fetal Hemoglobin (fetus needs less oxygen and can live off lower PO2s)
Increasing 2.3 DPG: This is your bodies way of responding to lack of oxygen, and lowers hemoglobin's affinity for oxygen, and thus releases more oxygen from RBCs to the blood stream for tissues to use. This moves the curve to the right. The following conditions cause the body to increase production of 2.3 DPG:
- Anemia (it may take 24 hours after transfusion to replenish supply, and return curve to normal)
- Cystic Fibrosis
- Congenital heart diseases
- Anything that increases metabolism (HEAT), such as acidosis, exercise, fever, etc.
- Large blood transfusion