In addition to the typical topics & concepts presented to the nursing staff by Respiratory Therapists, our class was presented with a topic completely foreign to me, and I'll bet approximately 99% of my peers had never been 'exposed' to this concept as well. What could be so life-altering for an experienced Intensive Care Unit Registered Nurse going from one tertiary care center/level 1 trauma center to another high-volume/high-acuity facility? Which concept, when presented by one of the most intelligently educated and under-utilized health care professionals has the potential to make so many 'other' seemingly basic concepts suddenly 'make sense' and 'Mind = Blow' at the same time?
How many providers, typically nurses, have been received and transcribed the following order: "Titrate O2 to maintain SpO2 > 93%." I've always thought, "What an odd order? Kind of random percentage? What's so special about 93%" Nursing staff would always joke about which resident or attending was 'reading articles' because they'd write some unusual or non-typical orders, maybe medication orders, or diagnostic orders, etc.
So why should a patient's SpO2 be 93% or greater? It has to do something known at the Oxyhemoglobin Dissociation Curve, or ODC. Anyone that knows me, or has sat through any of my lectures or training, knows that I include the ODC in every topic, to me, it is that prolific! That important. That integral to effectively managing critically ill patients. To sum it up, the ODC represents hemoglobin's affinity for oxygen. Sounds simple, right? It is, but it's implications are far-reaching and clinical significant and applicable on a large-scale. Below is one picture representation of the ODC.
The ODC basically describes how the hemoglobin molecule binds or releases O2 to the lungs and then tissues. Several factors affect how oxygen interacts with hemoglobin, specifically: Temperature, 2,3 DPG, & H+ ions, or CO2. Changes in these factors affect 1) if there is a net change in direction, & 2) which direction the curve moves; either to the Right or Left. Additionally, two effects, the Haldane Effect & Bohr Effect also influence the curve. Both effects can be confusing and are related intrinsically.
The simplest way to differentiate the two effects is to identify which molecule is the cause of the change. The Haldane effect describes how oxygen concentrations determine hemoglobin's affinity for carbon dioxide. For example, high oxygen concentrations enhance the unloading of carbon dioxide. The converse is also true: low oxygen concentrations promote loading of carbon dioxide onto hemoglobin. In both situations, it is oxygen that causes the change in carbon dioxide levels. The Bohr effect, on the other hand, describes how carbon dioxide and H+ affect the affinity of hemoglobin for oxygen. High CO2 and H+ concentrations cause decreases in affinity for oxygen, while low concentrations cause high affinity for oxygen. To further illustrate the difference, it might help to look at specific examples. In the lungs, when hemoglobin loaded with carbon dioxide is exposed to high oxygen levels, hemoglobin's affinity for carbon dioxide decreases. This is an example of the Haldane effect. In active muscles, carbon dioxide and H+ levels are high. Oxygenated blood that flows past is affected by these conditions, and the affinity of hemoglobin for oxygen is decreased, allowing oxygen to be transferred to the tissues. Because we are looking at the situation from the perspective of carbon dioxide changing oxygen affinity, this is an example of the Bohr effect.
End of Part 1
There is also an aspect of solubility difference between CO2 and O2. CO2 is much more soluble than O2 due to being a polar molecule. In high CO2 environments the ability of the blood to not only bind CO2 but carry it in the plasma has an effect.
ReplyDeletebtw that was Matt
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