Correlation between the levels of SpO2 and PaO2
Whenever there is dissociation between the PO2 and the SpO2, Consider consultation with poison control and/or medical toxicology. 1. 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 . reliably predict changes in oxygen saturation (SaO2); is not reflective of oxygen This kind of patient advice based on 'logical' thinking in . (). Relationship between supranormal oxygen tension and outcome after.
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.
Dissociated Oxygen Saturations
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, with a normal amount of shunting, anemia and hemoglobin variables do not affect PaO2. By administering supplemental oxygen or placing a patient in a hyperbaric chamber, PaO2 can be increased considerably resulting in increase of amount of oxygen that is dissolved in the arterial blood. The higher the partial pressure of oxygen, the more oxygen will be dissolved in blood.
At the same time, blood receives carbon dioxide from the tissues, and brings it back to the lungs. The amount of gas dissolved in a liquid blood, in this case is proportional to the pressure partial pressure of the gas.
In addition, each gas has a different solubility. There are two mechanisms by which oxygen could be coalesced with blood. The first is when oxygen is dissolved in plasma due to the partial pressure difference of oxygen that is present in the surrounding atmosphere and the blood in the lungs.
Partial pressure is the pressure exerted by a single component of a mixture of gases, commonly expressed in mm Hg; for a gas dissolved in a liquid, the partial pressure is that of a gas that would be in equilibrium with the dissolved gas. This causes oxygen to dissolve in the plasma of the blood, for each 1mmHg partial pressure of oxygen 0.
This suggests that a human could not get sufficient oxygen if solubility were the only way to get oxygen in the blood. For this reason, hemoglobin Hb has an important role as a carrier of oxygen.
This is the second mechanism when oxygen binds with hemoglobin that is found in the red blood cells and forms oxyhemoglobin, which thereafter could be transported to all over the body, where the oxygen could be taken up, relieving the hemoglobin back to its original state.
Here for every 1gm of hemoglobin, 1. Since ml of blood contain about 15 g of hemoglobin, the hemoglobin contained in ml of blood can bind to The dissolved fraction is available to tissues first and then, the fraction bound to hemoglobin.
So as tissues metabolize oxygen or if oxygen becomes difficult to pick up through the lungs, the dissolved oxygen and the oxygen bound to hemoglobin will eventually become depleted.
The dissolved oxygen can be measured by arterial blood gas analysis but this is not yet a practical field application. This fraction is not measured by pulse oximeter. The presence of available oxygen in form of oxyhaemoglobin in the blood could be simplified or rather related to what we call the oxygen saturation that is calculated by the pulse oximeter. 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.
There is a dynamic equilibrium between the freely dissolved and the hemoglobin-bound oxygen molecules.
PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
However, the more dissolved molecules there are i. Because there is a virtually unlimited supply of oxygen molecules in the atmosphere, the dissolved O2 molecules that leave the plasma to bind with hemoglobin are quickly replaced by others; once bound, oxygen no longer exerts a gas pressure.
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 about 1. PaO2 measures the oxygen that has passed through the lungs and into the blood. SaO2 measures the oxygen that has saturated the Hemoglobin in red blood cells after oxygen has passed into the blood from the lungs.
In summary, PaO2 is determined by alveolar PO2 and the state of the alveolar-capillary interface, not by the amount of hemoglobin available to soak them up. PaO2, in turn, determines the oxygen saturation of hemoglobin along with other factors that affect the position of the O2-dissociation curve, discussed below. If the air is thin at Mount Everest-low atmospheric pressure or the lungs cannot take in oxygen appropriately due to any number of diseases, then obviously little oxygen gets into the lungs, into circulation, or both, thereby decreasing arterial partial pressure of oxygen.
After oxygen has entered and dissolved within the blood, then, and only then, can oxygen bind to the hemoglobin in our blood. It is SaO2 that measures oxygen saturation of hemoglobin, and it should be clear that it depends on the partial pressure of arterial oxygen. But oxygen saturation is tricky!
If all of a sudden someone loses a lot of hemoglobin, as long as PaO2 remains the same, so will oxygen saturation. Therefore both oxygen saturation and the partial pressure of oxygen in arterial blood are independent of the amount of hemoglobin in the blood. It is important to understand the difference between the PaO2, the oxygen saturation SaO2the oxygen content and the oxygen delivery rate. If the patient breathes supplemental oxygen, the inspired PO2 increases to mmHg, mmHg or higher depending on how much oxygen is inhaled.
The higher PaO2 will increase dissolved oxygen in plasma but oxygen carried by hemoglobin will remain same. Red blood cells contain hemoglobin. Oxygen is carried in the blood attached to haemoglobin molecules.
Oxygen saturation is a measure of how much oxygen the blood is carrying as a percentage of the maximum it could carry. One haemoglobin molecule can carry a maximum of four molecules of oxygen.
Most of the haemoglobin in blood combines with oxygen as it passes through the lungs. If the level is below 90 percent, it is considered low resulting in hypoxemia. Blood oxygen levels below 80 percent may compromise organ function, such as the brain and heart, and should be promptly addressed.Oxygen Hemoglobin Dissociation Curve Explained Clearly (Oxyhemoglobin Curve)
Continued low oxygen levels may lead to respiratory or cardiac arrest. Oxygen therapy may be used to assist in raising blood oxygen levels. Oxygenation occurs when oxygen molecules O2 enter the tissues of the body.
For example, blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygenation is commonly used to refer to medical oxygen saturation.
Extremes of altitude will affect these numbers. Arterial blood looks bright red whilst venous blood looks dark red. The difference in colour is due to the difference in haemoglobin saturation. Oxygen saturation is a measurement of the percentage of oxygen binding sites that contain oxygen. Oxygen saturation is defined as the ratio of oxy-hemoglobin to the total concentration of hemoglobin present in the blood i.
When arterial oxy-hemoglobin saturation is measured by an arterial blood gas it is called SaO2. When arterial oxy-hemoglobin saturation is measured non-invasively by a finger pulse oximeter or handheld pulse oximeter, it is called SpO2.
It is important to understand the principle of the pulse oximeter so that a clinician has an understanding of what is actually being measured by the pulse oximeter and what its limitations are. An understanding of fractional oximetry SaO2 versus functional oximetry SpO2 is necessary. Oximeters can measure either functional or fractional oxygen saturations. Functional saturation is the ratio of oxygenated haemoglobin to all haemoglobin capable of carrying oxygen; fractional saturation is the ratio of oxygenated haemoglobin to all haemoglobin including that which does not carry oxygen.
- What’s The Difference Between Oxygen Saturation And PaO2?
- Methemoglobinemia: Not the Usual Blue Man With Low SpO2
The total hemoglobin denominator in the calculation of fractional hemoglobin might include abnormal or variant hemoglobin molecules with limited oxygen-carrying properties. In situations such as dyshemoglobinemias, pulse-oximetry readings do not adequately reflect the oxygen-carrying properties of arterial blood. You multiply above fraction by to get SaO2 in percentage. These values are determined by analysis of arterial blood sample using co-oximetry.
SpO2 is defined as the oxyhemoglobin divided by all the functional hemoglobin in a sample and can be written as: It determines fractional oxygen saturation. A normal range is mm Hg, although 60 or better is usually considered acceptable. It determines functional oxygen saturation. CaO2 is arterial oxygen content. Unlike either PaO2 or SaO2, the value of CaO2 directly reflects the total number of oxygen molecules in arterial blood, both bound and unbound to hemoglobin.
CaO2 depends on the hemoglobin content, SaO2, and the amount of dissolved oxygen. FIO2 is the same at all altitudes. The percentage of individual gases in air oxygen, nitrogen, etc. PaO2 declines with altitude because the inspired oxygen pressure declines with altitude inspired oxygen pressure is fraction of oxygen times the atmospheric pressure. This is such a key concept that we all must take pains to ensure our staff understands how to use this valuable monitoring tool.
Some of the material below is from my book Anyone Can Intubate. What Is Oxygen Saturation? Hemoglobin is a chemical molecule in the red blood cell RBC that carries oxygen on specific binding sites.
PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
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. 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.
What’s The Difference Between Oxygen Saturation And PaO2?The Airway Jedi
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.
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. 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. 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.