Shunt fraction | PFTBlog
electrode and can be used to assess oxygen exchange through a few relationships. Normal PaO2/FiO2 is > mmHg; Approximate PaO2 by multiplying FiO2 by 5 . This collection of drug, procedures and test information is derived from. A normal PaO2 and alveolar-arterial PO2 difference (A-a gradient) do NOT rule out pulmonary embolism. 4. A given liter flow rate of nasal O2 does not equal any specific FIO2. 6. .. should be measured as part of each arterial blood gas test. medical mixture of oxygen and air (FIO2 ) was tension difference [P(A-a) O2] and PaO2/FIO2 ratio using t-tests or Mann–Whitney tests as appropriate.
If only an arterial sample can be obtained which is usually the case in a PFT Lab an arterial-venous O2 content difference of between 4. The limitations of the shunt fraction calculations have to do in part with some of the assumptions about normal values and in part with the accuracy of blood gas measurements.
The alveolar air equation, for example, assumes that the respiratory exchange ration RER is 0. Strictly speaking, an RER that is different than 0. Using an a-v O2 content difference of 4. Two different studies have shown that the type of syringe used to obtain an ABG glass versus plastic and how it was stored on ice or at room temperature made a significant difference in the calculated shunt fraction even when the ABG samples were analyzed quickly.
When there was a longer wait before analysis the error in PaO2 could cause the calculated shunt fraction to be twice as large as it really was. The reason this happens is partly due to diffusion through the plastic syringes and partily to continued metabolism within a blood sample when kept at room temperature.
The least amount of change was seen when glass syringes kept on ice. The measured PO2 tended to rise, again more in plastic syringes than in glass, and again this likely due to diffusion. Interestingly, PO2 fell in glass syringes kept on ice and the authors, Knowles et al, point out that the solubility of O2 rises as temperature falls and that with more O2 in solution PO2 may decrease.
Finally, blood gas analyzers are usually calibrated using gas concentrations in the normal physiological range. The shunt fraction test is not commonly performed in pulmonary function labs.
Perfusion inhomogeneities exist just as much as ventilation inhomogeneities but this may be overlooked because pulmonary function testing is oriented far more around the ventilation side of respiration than the perfusion side.
Ventilation and perfusion inhomogeneities are core features of many pulmonary diseases. For this reason the shunt fraction and the differences between its physiological and anatomical components need to be part of the education of all pulmonary technologists.
Churchill Livingston Publishing, Lung Function, 6th Edition. Effects of syringe material, sample storage time and temperature on blood gases and oxygen saturation in arterialized human blood samples.
Respir Care ; 51 7: 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. 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.
- PaO2/FiO2 Ratio
- Shunt fraction
- Fraction of inspired oxygen
The A-a gradient is the difference in oxygen tension between arterial blood and alveolar gas. Medical school courses love this.
PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
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. Often, the saturation will bounce back rapidly on its own.
Thus, we are constantly paying attention to oxygenation trends and averaging the oxygen saturation over time. If we obtain an ABG, this sort of trending and averaging is impossible. We have access to only one point in time. It is impossible to know whether the oxygen saturation was transiently low, or if it was continuously low. This assumption is frequently wrong. The oxygenation is worsening, so this indicates that we must intubate the patient.
Please step away from the laryngoscope. Mallat compared back-to-back ABGs drawn via arterial catheters in ICU patients to determine the repeatability of this test. There are large differences between these nearly simultaneous PaO2 values. This is consistent with previous studies 5. 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. The most common situation where ABG is needed to test oxygenation is when pulse oximetry cannot provide a reliable waveform. Poor perfusion may lead to an erratic waveform.