ABG measurement may delay critical decisions. PaO2 values are frequently misinterpreted. Checking the A-a gradient is over-utilized and potentially misleading. A single ABG only measures a snapshot in time. Changes in PaO2 are widely misinterpreted. When is ABG useful to investigate oxygenation? There are some situations when it may be helpful to use an ABG to investigate oxygenation.
Pulse oximetry waveform is unreliable. Diagnosis of methemoglobinemia. For further discussion of this problem please see a blog here. Why measuring PaO2 is generally unhelpful. The top ten reasons for this are: Saturation is a more direct measurement of tissue oxygen delivery than PaO2. ABG is a painful and expensive test. ABGs may be contaminated with venous blood.
Point-of-care ABG monitors calculate oxygen saturation, rather than measuring it directly. Obtaining an ABG may delay management. PaO2 values are easily misunderstood. Measuring the A-a gradient is over-utilized and potentially misleading. ABG only measures oxygenation at a single time point. Costs include materials themselves, procedure costs, and analytic costs.
All critically ill patients are monitored with pulse oximetry, so there is no added cost required to obtain this. The only scenario in which calculating the A-a gradient could be useful is in a patient with mild occult hypoventilation and normal lungs.
If the hypoxemia is purely due to hypoventilation, then it should be easily overcome by increasing the FiO2 slightly e. Thus, the use of ABG to determine A-a gradient may occasionally be helpful, but only in selected cases of very mild Thorson and Sasse Investigation for methemoglobinemia may be justified when the patient has been exposed to drugs which may cause methemoglobinemia, and when there are other signs of methemoglobinemia e.
Of course, oxygen saturation is only part of DO2. Nonetheless, oxygen saturation is more closely related to DO2 than PaO2 is. Particularly with the use of lung ultrasonography. Prior to the availability of bedside cardiac and lung ultrasonography, I think BNP might have had more of a role. Author Recent Posts. Social Me. Josh Farkas. Josh is the creator of PulmCrit. Latest posts by Josh Farkas see all. We may delete without a full, true name. Your Job i. Inline Feedbacks. James French. Reply to James French.
Brandon O. Reply to Brandon O. Samuel Anaya. Reply to Josh Farkas. What's Your Job? Justin C. I feel like this article needs to be a poster to hang in our ICUs. Reply to Justin C. This formula, which is not an exact mathematically proven entity, can be of extreme help to ICU residents and consultants. National Center for Biotechnology Information , U.
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This is depicted by the plateau area of the oxygen-hemoglobin dissociation curve. This saturation pattern is the reason for the characteristic sigmoid shaped curve of the O 2 — Hemoglobin curve. PAO 2 and SAO 2 define cardiac efficiency and are considered as markers to assess the metabolic conditions of the lungs and heart in terms of Oxygen levels. PAO 2 is the pressure exerted by O 2 on the arterial wall.
SAO 2 is the percentage of hemoglobin binding sites that are occupied with O 2. Deviations of these levels act as markers and are important in analyzing the abnormalities in hemoglobins and Carbon monoxide poisoning. You can download PDF version of this article and use it for offline purposes as per citation note. Collins, Julie-Ann, et al. 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. 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. This is the minimum oxygen concentration providing enough oxygen to prevent ischemia in tissues. A simple formula to estimate what the arterial oxygen concentration should be is to multiply the inspired oxygen concentration by 5.
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. When peripheral flow is disturbed, the body tends to try to protect central blood flow to the head.
You can often put the sensor on the ear lobe and get a more accurate reading. Disposable sensors can also be placed on the forehead, bridge of the nose, and can also be pinched around the corner of the mouth making sure that the light and the detector are directly opposite each other. 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.
Measurement of PaO2 requires drawing and testing an arterial blood sample —something that requires a trained provider, a lab, and time. Waiting for an ABG can sometimes delay clinical decisions. I have been practicing anesthesia for 35 years and the use of pulse oximetry revolutionized patient safety when we started using it. Oxygen saturation is one of the most valuable tools I have. It can give you early warning about many things, including:.
Our staff needs to know that numbers in the low 90s, while acceptable, indicate significant changes in oxygenation that need to be monitored and addressed.
Thank you for the detailed explanation- much more cohesive and detailed than my advanced pathophysiology text.
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