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 . PaO2 and SaO2 are both measures of oxygen content in the blood. However, PaO2 and SaO2 differ slightly in how they measure oxygen content in the blood. What is the difference between PAO2 and SAO2? Partial Pressure is defined by Dalton's law of Partial pressures, where it is stated that the.
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. As PaO2 falls, the Hgb saturation also falls as Hgb releases oxygen to the tissues in the areas of lower oxygen supply.
This is because Hgb binding sites become less attracted to oxygen as it is bound to fewer oxygen molecules. This property allows Hgb to rapidly release oxygen to the tissues. Deoxygenated blood returns to the heart to be pumped to the lungs and the cycle repeats. Since a normal PaO2 is between mmHg, some people may think that an O2 saturation of 90 is normal as well — after all 90 was a pretty good grade to get in school.
Correlation between the levels of SpO2 and PaO2
However, this interpretation is very wrong. This is the minimum oxygen concentration providing enough oxygen to prevent ischemia in tissues.
As good as they are they can have problems. Movement can cause inaccurate readings. This is especially common in small children. Another problem is that poor perfusion from extreme vasoconstriction, hypotension, hypovolemia, and septic shock can all decrease peripheral blood flow.
This sometimes makes it impossible for the sensor to measure the concentration correctly, or at all.Understanding Oxygen Saturation Levels
You can often put the sensor on the ear lobe and get a more accurate reading. The presence of CO fools the monitor into reading high. The patient with CO poisoning appears flushed and pink.
Children should have sensors appropriate to their size. The monitor is small, portable for use in the field, operating rooms, and in patient hospital rooms to provide continuous, real time monitoring of the patient.
Click on the title to see the Preface and Table of Contents.
The book is available from the publisher at orand also at the following web sites: In the field of blood gas interpretation, confusion about PaO2, SaO2 and oxygen content is second only to confusion about mixed acid-base disturbances. Arterial PO2 little 'a' gives us valuable information about adequacy of gas exchange within the lungs, when and only when it is subtracted from the calculated alveolar PO2 big A.
The A-a gradient answers the important question: Are the lungs transferring oxygen properly from the atmosphere to the pulmonary circulation? If the A-a gradient is elevated, the answer is NO.
What’s The Difference Between Oxygen Saturation And PaO2?
If the A-a gradient is normal the answer is YES. There is a second, equally important question concerning oxygen and gas exchange, which is the subject of this section: How much oxygen is in the blood, and is it adequate for the patient? The answer must obviously be based on some oxygen value, but which one? After all, blood gases give us three different oxygen values: The other two values --oxygen saturation and oxygen content -- are more useful for this purpose.
I will briefly define these three terms for measuring oxygen and then present a more detailed discussion of each, emphasizing their inter-relationships.
Difference Between PAO2 and SAO2
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.
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.
Difference Between PAO2 and SAO2 | PAO2 vs SAO2
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.