What’s The Difference Between Oxygen Saturation And PaO2?The Airway Jedi
Pop Quiz .. This PO2/SaO2 relationship is direct but not linear: .. Incorrect results may lead to more invasive, expensive, and unnecessary tests. • Incorrect. o Pulse Oximeter Quiz 1 o Oxygen o Actions to be taken when SpO2 is 94% and below o Pulse Oximeter Quiz 4 . the difference in haemoglobin saturation. Sir,. Sarkar et al. in , in their recent article, have very nicely elucidated various mechanisms of hypoxemia, and I would like to congratulate them for this .
Pulse oximetry is one obvious monitoring tool to identify hypoxemia and hypoxia. Multiple studies have identified this as a knowledge gap.
This is such a key concept that we all must take pains to ensure our staff understands how to use this valuable monitoring tool. Some of the material below is from my book Anyone Can Intubate. What Is Oxygen Saturation? Hemoglobin is a chemical molecule in the red blood cell RBC that carries oxygen on specific binding sites. Each Hgb molecule, if fully saturated, can bind four oxygen molecules. Depending on conditions, Hgb releases some percentage of the oxygen molecules to the tissues when the RBC passes through the capillaries.
We can measure how many of these binding sites are combined, or saturated, with oxygen.
Oxygen Saturation (SaO2)
What Is Arterial PaO2 Pa02, put simply, is a measurement of the actual oxygen content in arterial blood. Partial pressure refers to the pressure exerted on the container walls by a specific gas in a mixture of other gases.
When dealing with gases dissolved in liquids like oxygen in blood, partial pressure is the pressure that the dissolved gas would have if the blood were allowed to equilibrate with a volume of gas in a container. In other words, if a gas like oxygen is present in an air space like the lungs and also dissolved in a liquid like blood, and the air space and liquid are in contact with each other, the two partial pressures will equalize.
The Oxygen-Hemoglobin Dissociation Curve Shows the Difference To see why this is relevant, look at the oxygen-hemoglobin dissociation curve. As the partial pressure of oxygen rises, there are more and more oxygen molecules available to bind with Hgb. As each of the four binding sites on an Hgb molecule binds to an oxygen molecule, its attraction to the next oxygen molecule increases and continues to increase as successive molecules of oxygen bind.
The more oxygen is bound, the easier it is for the next oxygen molecule to bind, so the speed of binding increases and the oxygen saturation percentage rises rapidly on the curve.
As all of the binding sites fill up, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen.
This tendency makes it easy for Hgb to rapidly pick up oxygen in the lungs as it passes through.
Oxygen-Haemoglobin Dissociation Curve
As PaO2 falls, the Hgb saturation also falls as Hgb releases oxygen to the tissues in the areas of lower oxygen supply. Oxygen molecules dissolved in plasma i.
This "impingement" of free O2 molecules is reflected as the partial pressure of oxygen; if the sample being tested is arterial blood, then it is the PaO2. Although the number of O2 molecules dissolved in plasma determines, along with other factors, how many molecules will bind to hemoglobin, once bound the oxygen molecules no longer exert any pressure bound oxygen molecules are no longer free to impinge on the measuring electrode.
Since PaO2 reflects only free oxygen molecules dissolved in plasma and not those bound to hemoglobin, PaO2 cannot tell us "how much" oxygen is in the blood; for that you need to know how much oxygen is also bound to hemoglobin, information given by the SaO2 and hemoglobin content.
- Arterial blood gas analysis and pulse oximetry
- What’s The Difference Between Oxygen Saturation And PaO2?
There are four heme sites, and hence four oxygen binding sites, per hemoglobin molecule. Heme sites occupied by oxygen molecules are said to be "saturated" with oxygen. The percentage of all the available heme binding sites saturated with oxygen is the hemoglobin oxygen saturation in arterial blood, the SaO2.
Note that SaO2 alone doesn't reveal how much oxygen is in the blood; for that we also need to know the hemoglobin content. Tissues need a requisite amount of O2 molecules for metabolism.
Neither the PaO2 nor the SaO2 provide information on the number of oxygen molecules, i. Note that neither PaO2 nor SaO2 have units that denote any quantity. This is because CaO2 is the only value that incorporates the hemoglobin content. Oxygen content can be measured directly or calculated by the oxygen content equation introduced in Chapter 2: I have shown the 3 short paragraphs above to dozens of students, interns, residents; almost all will say they understand the differences, no problem.
But, when given questions to test their understanding, they don't show much understanding.
So more instruction is needed and, yes, a few problems along the way. Understanding will come from closely reviewing this material AND working on all the problems; do that, and you should be able to teach the subject! PaO2, the partial pressure of oxygen in the plasma phase of arterial blood, is registered by an electrode that senses randomly-moving, dissolved oxygen molecules.
The amount of dissolved oxygen in the plasma phase -- and hence the PaO2 -- is determined by alveolar PO2 and lung architecture only, and is unrelated to anything about hemoglobin. In this situation a sufficient amount of blood with low venous O2 content can enter the arterial circulation and lead to a reduced PaO2.
However, given a normal amount of shunting, neither anemia nor abnormal hemoglobin binding will affect PaO2. Oxygen molecules that pass through the thin alveolar-capillary membrane enter the plasma phase as dissolved free molecules; most of these molecules quickly enter the red blood cell and bind with hemoglobin Figure There is a dynamic equilibrium between the freely dissolved and the hemoglobin-bound oxygen molecules. However, the more dissolved molecules there are i. Oxygen pressure, saturation and content.
Schematic shows cross section of lungs and pulmonary circulation.
CO2, nitrogen and other gas molecules are omitted for clarity. PaO2 is always slightly lower than PAO2 because of normal venous admixture, here represented by a connection between the venous and pulmonary circulations. See text for discussion. Thus hemoglobin is like an efficient sponge that soaks up oxygen so more can enter the blood.
Hemoglobin continues to soak up oxygen molecules until it becomes saturated with the maximum amount it can hold - an amount that is largely determined by the PaO2.
Oxygen-Haemoglobin Dissociation Curve
Of course this whole process is near instantaneous and dynamic; at any given moment a given O2 molecule could be bound or dissolved. However, depending on the PaO2 and other factors, a certain percentage of all O2 molecules will be dissolved and a certain percentage will be bound Figure If there is no interference as from carbon monoxide, for examplethe free O2 molecules bind to these sites with great avidity.
The total percentage of sites actually bound with O2 is constant for a given set of conditions, and is the 'saturation of blood with oxygen'. In summary, PaO2 is determined by alveolar PO2 and the state of the alveolar-capillary interface, not by the amount of hemoglobin available to soak them up.
PaO2, in turn, determines the oxygen saturation of hemoglobin along with other factors that affect the position of the O2-dissociation curve, discussed below. Neither the amount of hemoglobin, nor the binding characteristics of hemoglobin, should affect the amount of dissolved oxygen, and hence should not affect the PaO2.
Stated another way, the number of dissolved oxygen molecules is independent of the amount of hemoglobin or what is bound to it. To repeat one more time because it is so importantPaO2 is not a function of hemoglobin content or of its characteristics, but only of the alveolar PO2 and the lung architecture alveolar-capillary interface. This explains why, for example, patients with severe anemia or carbon monoxide poisoning or methemoglobinemia can and often do have a normal PaO2.
The most common physiologic disturbance of lung architecture, and hence of a reduced PaO2, is ventilation-perfusion V-Q imbalance. Less common causes are reduced alveolar ventilation, diffusion block, and anatomic right to left shunting of blood. SaO2 is determined mainly by PaO2. The relationship between the two variables is the familiar oxygen dissociation curve Figure