What’s The Difference Between Oxygen Saturation And PaO2?The Airway Jedi
Oxygen saturation can be assessed by SaO2 or SpO2. SaO2 is a disease. The severity of a disease can be determined by clinical symptoms including SpO2. Lung India • Volume 34 • Issue 3 • May - June 10% reduction in SpO2 from 90% to 80%, decrease PaO2 by mmHg for each percent reduction in. Clear discussion of the difference between oxygen saturation and PaO2, why commonly pronounced “Oh Two SAT”. it is also referred to as SPO2. If O2 Sat Can Miss Big Problems Like This, Why Use It Instead Of PaO2?.
This case may seem silly, but it highlights some common issues.Blood Gases (O2, CO2 and ABG)
In a patient with an adequate pulse oximetry waveform, what is the best way to monitor oxygenation? What if the ABG and pulse oximetry seem to disagree? Ten reasons why pulse oximetry is generally the best way to monitor oxygenation. For a patient with a good pulse oximetry waveform, pulse oximetry has numerous advantages compared to ABG monitoring: Pulse oximetry is a better measurement of oxygen delivery to the tissues. PaO2, the oxygen tension in arterial blood, is the best way to determine how well the lungs are working.
However, oxygen saturation is a better measurement of the systemic oxygen delivery to the tissues DO2 7: ABG is an invasive, painful, and expensive procedure.
An ABG is painful for the wrist and the wallet. In contrast, pulse oximetry is noninvasive, painless, and free 2. Occasionally, an arterial catheter might even be placed for the purpose of measuring frequent ABGs. This is generally a terrible idea.
The availability of an easy source of arterial blood encourages frequent ABGs and other labs as well. For example, one study found that the presence of an arterial catheter correlated with a four-fold greater volume of phlebotomy Tarpey Thus, it may not be obvious that the sample was venous. These devices typically measure PaO2 and subsequently use this to calculate the oxygen saturation assuming a normal PaO2 vs.
For patients with abnormal hemoglobin dissociation curves, this calculated saturation will be wrong. ABG measurement may delay critical decisions. Occasionally, physicians may feel obligated to check an ABG before calling for help, to exercise due diligence. Regardless, the practice of delaying treatment to obtain an ABG is usually unnecessary, particularly when oxygenation is concerned 3. PaO2 values are frequently misinterpreted. We are constantly exposed to oxygen saturation values, leading to the development of a good sense about what they mean.
Meanwhile, we are exposed to PaO2 values far less often, so we may struggle to interpret them. The most common error is panicking about a low PaO2 value. PaO2 values are always much lower than oxygen saturation values. This is simply a reflection of the oxygen saturation curve figure above.
Dissociated Oxygen Saturations
The lower number is scarier. This cognitive bias is often seen when ABGs are obtained in patients on mechanical ventilation. For a patient with mild hypoxemia, the PaO2 value will often be surprisingly low. Checking the A-a gradient is over-utilized and potentially misleading. The A-a gradient is the difference in oxygen tension between arterial blood and alveolar gas. Medical school courses love this. However, trying to use the ABG to diagnose the etiology of respiratory failure works poorly in real life: I sometimes see practitioners measure the A-a gradient of a critically ill patient who is requiring moderate to high levels of supplemental oxygen e.
Measuring this is pointless, because such patients will invariably have an elevated A-a gradient if the patient had a normal A-a gradient, then they would require at most a low amount of supplemental oxygen 4. A single ABG only measures a snapshot in time. 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. As PaO2 falls, the Hgb saturation also falls as Hgb releases oxygen to the tissues in the areas of lower oxygen supply.
PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
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. However, this interpretation is very wrong.
- What’s The Difference Between Oxygen Saturation And PaO2?
- PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
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.
Correlation between the levels of SpO2 and PaO2
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. 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.