PULSE OXIMETRY – Dr Rajiv Desai
This study evaluates the ORI to PaO2 relationship during surgery. . 0 ± 6%; P oxygen saturation and mixed venous oxygen tension, as well as increased Guarinoni, A.; Ponzo, R.; Basini, L. [ENI Refining and Marketing Div., San Donato . Spo2 is hemoglobin saturation, Pao2 is plasma saturation. SpO2 is the Saturation (peripheral) of Haemoglobin with Oxygen, expressed as a. O2 saturation varies with the PaO2 in a nonlinear relationship and is affected by temperature, pH, 2,3 diphosphoglycerate, and PaCO2 (partial pressure of.
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.
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. 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.
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. 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. The amount of oxygen transported around the body is determined mainly by the degree to which hemoglobin binds to oxygen lung factorhemoglobin concentration anaemic factorand cardiac output cardiac factor. Oxygen saturation is an indicator of oxygen transport in the body, and indicates if sufficient oxygen is being supplied to the body.
Oxygen is inhaled into the lungs, and carbon carbon dioxide is exhaled from the lungs to the air.
What’s The Difference Between Oxygen Saturation And PaO2?
This process is called ventilation. Inhaled air flows into the upper airway, then into the peripheral airways, and is finally distributed into the lungs.
This process is called distribution. The lungs consist of tissues called alveoli. Oxygen is absorbed from the alveoli, then into the lung capillaries via alveolar membranes, while carbon dioxide moves from the lung capillaries to the alveoli. This process is called diffusion. After oxygen is breathed into the lungs, it combines with the haemoglobin in red blood cells as they pass through the pulmonary capillaries.
The heart pumps blood continuously around the body to deliver oxygen to the tissues. There are five important things that must happen in order to deliver enough oxygen to the tissues: This is called alveolar gas exchange. To support oxygen transport, hemoglobin must bind O2 efficiently at the partial pressure of oxygen PO2 of the alveolus, retain it, and release it to tissues at the PO2 of tissue capillary beds. Oxygen acquisition and delivery over a relatively narrow range of oxygen tensions depend on a property inherent in the tetrameric arrangement of heme and globin subunits within the hemoglobin molecule called cooperativity or heme-heme interaction.
At low oxygen tensions, the hemoglobin tetramer is fully deoxygenated. Oxygen binding begins slowly as O2 tension rises. However, as soon as some oxygen has been bound by the tetramer, an abrupt increase occurs in the oxygen binding.
Thus, hemoglobin molecules that have bound some oxygen develop a higher oxygen affinity, greatly accelerating their ability to combine with more oxygen. There are four heme sites, and hence four oxygen binding sites per hemoglobin molecule.
iROCKET Learning Module: Intro to Arterial Blood Gases, Pt. 1
The percentage of all the available heme binding sites saturated with oxygen is the hemoglobin oxygen saturation in arterial blood, the SaO2. However, hemoglobin is stable only when bound to 4 molecules of oxygen or when not bound to any oxygen. It is very unstable when bound to 1 to 3 molecules of oxygen. Therefore, as shown in the above figure, hemoglobin exists in the body in the form of deoxygenated hemoglobin Hb with no oxygen bound, or as oxygenated hemoglobin O2Hb with 4 molecules of oxygen.
This oxygen saturation in percentage is measured by pulse oximetry. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during hypoventilation training or strenuous physical exercise.
Hypoxemia means low oxygen content of arterial blood. Hypoxaemia invariably leads to hypoxia but hypoxia can occur even without hypoxaemia. Ischemia, meaning insufficient blood flow to a tissue, can also result in hypoxia.
This can include an embolic event, a heart attack that decreases overall blood flow, or trauma to a tissue that results in damage. An example of insufficient blood flow causing local hypoxia is gangrene that occurs in diabetes. Diseases such as peripheral vascular disease can also result in local hypoxia. Hypoxemic hypoxia hypoxic hypoxia refers specifically to hypoxic states where the oxygen content of arterial blood is insufficient due to poor oxygenation.
This can be caused by alterations in respiratory drive, such as in respiratory alkalosis, physiological or pathological shunting of blood, diseases interfering in lung function resulting in a ventilation-perfusion mismatch, such as a pulmonary embolus, or alterations in the partial pressure of oxygen in the environment or lung alveoli, such as may occur at altitude or when diving.
A chronic hypoxic state can result from a poorly compensated anaemia.
Histotoxic hypoxia results when the quantity of oxygen reaching the cells is normal, but the cells are unable to use the oxygen effectively, due to disabled oxidative phosphorylation enzymes. This may occur in cyanide poisoning. An another instance of hypoxemic hypoxia would be when carbon monoxide is present in the blood, as hemoglobin has a higher affinity to carbon monoxide than oxygen.
Pulse oximetry can measure hypoxemic hypoxia due to poor oxygenation by measuring oxygen saturation of hemoglobin but cannot measure anemic, circulatory, ischaemic or histotoxic hypoxia. Ordinarily, the clinical picture of patients with hypoxia due to an elevated metabolic rate, as in fever or thyrotoxicosis, is quite different from that in other types of hypoxia: Carbon dioxide is produced by cell metabolism in the mitochondria.
The amount produced depends on the rate of metabolism and the relative amounts of carbohydrate, fat and protein metabolized.
The respiratory quotient RQ is the ratio of CO2 produced to O2 consumed while food is being metabolized. There are 3 ways in which carbon dioxide is transported in the blood: Carbon dioxide is 20 times more soluble than oxygen. Arterial blood contains about 2. A cardiac output of 5 liter per minute will carry ml of dissolved carbon dioxide to the lung, of which 25 ml will be exhaled. Because of this high solubility and diffusing capacity, carbon dioxide partial pressure of alveolar and pulmonary end-capillary blood are virtually the same.
Bound to hemoglobin and plasma proteins: Carbon dioxide combines reversibly with haemoglobin to form carbaminohaemoglobin.Oxygen Saturation Nursing Considerations, Normal Range, Nursing Care, Lab Values Nursing
Carbon dioxide does not bind to iron, as oxygen does, but to amino groups on the polypeptide chains of haemoglobin. Carbon dioxide also binds to amino groups on the polypeptide chains of plasma proteins. The majority of carbon dioxide is transported in this way. Carbon dioxide enters red blood cells in the tissue capillaries where it combines with water to form carbonic acid H2CO3. This reaction is catalysed by the enzyme carbonic anhydrase, which is found in the red blood cells.
The hydrogen ions, formed from the dissociated carbonic acid, combine with the haemoglobin in the red blood cell. CO2 is transported to the lungs where it is released into the alveoli and eliminated in the process of ventilation. So atmospheric pressure of air is sum total of partial pressure of nitrogen and oxygen. Thus the PO2 that we breathe in is mm X 0. After adjusting for dead airway space, elevation, patient temperature, and water vapor, the range of a normal PaO2 should be between mm of Hg.
The arterial PO2 is frequently described as the PaO2 to denote that this is an arterial sample, as opposed to a venous or capillary PO2.
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, 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.
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.
What’s The Difference Between Oxygen Saturation And PaO2?The Airway Jedi
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.
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.