OxyHemoglobin Dissociation CurveShockwave
PaO2 is a major determinant of SaO2, and the relationship is the familiar sigmoid -shaped Unlike either PaO2 or SaO2, the value of CaO2 directly reflects. Introduction: Blood gas analysis is often used to evaluate oxygenation status in critically ill patients. Although arterial blood gas (ABG) remains the gold standard, . 10% reduction in SpO2 from 90% to 80%, decrease PaO2 by mmHg for each saturation varies with the PaO2 in a nonlinear relationship and is affected by.
Correlation between the levels of SpO2 and PaO2
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.
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.
PaO2 vs SaO2 | allnurses
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.
What’s The Difference Between Oxygen Saturation And PaO2?
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.
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. 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. 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 Without getting too far into it, roughly speaking all other things being equal PaO2 and SaO2 relate as follows: Jun 9, '10 Occupation: I thought it would be helpful to post the oxyhemoglobin dissociation curve pic from my slides Enjoy!
Jun 9, '10 Joined: I believe that the SaO2 is an "assumed" value, meaning that it actually measures the hemes that are saturated, but only assumes they are saturated with O2.
CO poisoning will artificially elevate the SaO2 level, because CO has more affinity to the hemes or vice versa, doesn't matter here for the sake of this argument than O2 does, and the measurement is not actually the number of O2 molecules that are bound, but how many "seats on the bus" are taken up. Last edit by wannabHishands on Jun 9, '10 Sep 11, '11 Occupation: Looking for homecare Specialty: Since PaO2 is assumed during routine testing, how would it specifically be tested?