Wednesday, July 7, 2010

Oxyhemoglobin Dissociation Curve

If you are like me, you have explained to one or more co-worker time and again that the older a patient gets the lower that patient's normal SpO2 is going to be. So if you have a 100 year old man with a resting SpO2 of 90% there is no need for supplemental oxygen. You may also have explained the following 4-5-6, 7-8-9 rule. This is where you can estimate the patients PO2 based on your knowledge of the oxyhemoglobini dissociation curve in the following manner:
  • 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%
Now this is assuming the patient is less than 60 and is not a chronic CO2 retainer. However, more often than not, I find the 4-5-6, 7-8-9 rule works pretty well. By using the above chart most ABGs can be avoided, especially if all you want to determine is oxygenation status, or PO2.

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.

That is, unless the curve has shifted to the right or to the left. Certain conditions cause the hemoglobin to pick up more oxygen from the blood stream, and certain conditions cause the hemoglobin to release oxygen to the blood stream.

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)
  • Hypocarbia
  • CO2 poisoning
  • Cold tissues Alkalotic
  • Decreased 2.3 DPG
  • Fetal Hemoglobin (fetus needs less oxygen and can live off lower PO2s)
Perhaps you're wondering now what 2.3 DPG is. It's a substance in the blood that controls movement of oxygen from the blood (RBCs) to the tissue.

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)
  • COPD
  • Cystic Fibrosis
  • Congenital heart diseases
  • Anything that increases metabolism (HEAT), such as acidosis, exercise, fever, etc.
Decreasing 2.3 DPG: Results from lack of DPG enzymes to make 2.3 DPG. Body responds by increasing RBCs that are weak and burst easy. This moves the curve to the left. When this happens your body will increase 2.3 DPG production to try to move it back to normal. The following conditions cause this:
  • Erythrocytosis
  • Anemia
  • Large blood transfusion

12 comments:

Anonymous said...

love the opening line...explained to a nurse time and again?? hahaha...nice power trip but let's not forget who has more education. No need to put down other professions.

Rick Frea said...

Sorry, but nurses don't have more education than RTs. It was also not a put down of another profession. If I had said, All nurses, it would have been a put down. Thanks for keeping me honest.

Anonymous said...

I'm an RN student and your site helped me understand the oxyhemoglobin dissociation curve better. But one question: Is the picture of the curve correct? It states that the red and green lines are both pH of 7.6. I'm assuming that the shift to the right was meant to be acidotic.

Rick Frea said...

Thanks for your observation. I have replaced the picture with a new one that I think is more accurate.

Anonymous said...

Book says fetal hgb shifts the curve to the left

Rick Frea said...

Good observation. You are correct.

Anonymous said...

Hi Rick

I am a student(RT) and I have a question:
-Which is better, a shift to the right or a shift to the left?
I say: none of them are good...but,which one is more dangerous?

Thanks,
Marina

Rick Frea said...

I would think a gradual shift a little either way your body would compensate, such as in your COPD retainer. Although a shift to the left I'd think would be more life threatening.

A. Jefferson said...

Very informative post, and a nice review. Just an observation -- your point estimates use percent for PO2, but PO2 (partial pressure) is measured in mmHg, not %. Mostly important if you draw an ABG for A-a gradient.

Anonymous said...

Rt have more education we have to go one more semester than them

Anonymous said...

Wow...I am neither an RN or an RT but can see there are some jealous RT's on this comment post. please do us all a favor and keep it professional. By the way BIG deal you have 1semester more. please spare us the snide comments and stick to subject and helping others.

n2searay said...

The comment was in response to a NURSE saying that they have more education than an RT. It was a RESPONSE, not someone lashing out.