PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
O2 Saturation vs. ABG (cont.) The relationship between oxygen saturation and PaO2 is shown by the oxyhemoglobin dissociation curve: The horizontal axis is. fusion capacity may explain the difference between PAO2 and SaO2 ¼ Arterial oxygen saturation (%) .. Foundation, and the Sir Halley Stewart Trust. Trust, London, UK; 3 Southend. University . oxygen saturation measured by pulse oximetry (SpO2) does not always However, the arterial oxygen tension ( PaO2) is less accurate in .. such as comparative studies, correlation studies and .
The use of pulse oximetry reduces the need for arterial blood gas analysis SaO2 as many patients who are not at risk of hypercapnic respiratory failure or metabolic acidosis and have acceptable SpO2 do not necessarily require blood gas analysis.
While arterial sampling remains the gold-standard method of assessing ventilation and oxygenation, in those patients in whom blood gas analysis is indicated, arterialised capillary samples also have a valuable role in patient care. The clinical role of venous blood gases however remains less well defined.
Short abstract Understand the role of oximetry in clinical practice and how oxygen delivery, saturation and partial pressure relate http: Oxygen delivery is dependent on oxygen availability, the ability of arterial blood to transport oxygen and tissue perfusion [ 1 ].
Of the oxygen transported by the blood, a very small proportion is dissolved in simple solution, with the great majority chemically bound to the haemoglobin molecule in red blood cells, a process which is reversible. The content or concentration of oxygen in arterial blood CaO2 is expressed in mL of oxygen per mL or per L of blood, while the arterial oxygen saturation SaO2 is expressed as a percentage which represents the overall percentage of binding sites on haemoglobin which are occupied by oxygen.
The maximum volume of oxygen which the blood can carry when fully saturated is termed the oxygen carrying capacity, which, with a normal haemoglobin concentration, is approximately 20 mL oxygen per mL blood. Oxygen delivery to the tissues Oxygen delivery to the tissues each minute is the product of arterial oxygen content and cardiac output. Hence oxygen delivery can be compromised as much by a low haemoglobin concentration or low cardiac output as by a fall in the SaO2.
The level of oxygenation of peripheral venous blood, however, will vary depending on local metabolism and oxygen consumption. The medicine team was alarmed by this low PaO2, so they titrated up her oxygen while monitoring serial ABGs. My recommendation was to stop obtaining ABGs and to titrate the oxygen based on pulse oximetry.A&P: Never Trust SpO2 and Oxygen Delivery DO2 Video (SaO2 v PaO2 v SpO2)
This case may seem silly, but it highlights some common issues. 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.
PULSE OXIMETRY – Dr Rajiv Desai
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.
Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation 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.
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.
Average barometric pressure at sea level is mm Hg; it has been measured at mm Hg on the top of Mt. Everest 8, metres above sea level. As one ascends rapidly to m 10, ftthe reduction of the O2 content of inspired air FiO2 leads to a decrease in alveolar PO2 to approximately 60 mmHg, and a condition termed high-altitude illness develops.
At higher altitudes, arterial saturation declines rapidly and symptoms become more serious; and at m, unacclimated individuals usually cease to be able to function normally owing to the changes in CNS functions. Normal PaO2 decreases with age. A patient over age 70 may have a normal PaO2 around mm Hg, at sea level. The body does not store oxygen.
Correlation between the levels of SpO2 and PaO2
If a patient needs supplemental oxygen it should be for a specific physiologic need, e. To give more oxygen requires a hyperbaric chamber.
A given liter flow rate of nasal O2 does not equal any specific FIO2. Tissues need a requisite amount of O2 molecules for metabolism.
Neither the PaO2 nor the SaO2 provide information on the number of oxygen molecules, i. Note that neither PaO2 nor SaO2 have units that denote any quantity. This is because CaO2 is the only value that incorporates the hemoglobin content. Oxygen content can be measured directly or calculated by the oxygen content equation: If the haemoglobin level is halved, the oxygen content of arterial blood will be halved. An additional small quantity of O2 is carried dissolved in plasma: Given normal pulmonary gas exchange i.
PaO2 is a measurement of pressure exerted by uncombined oxygen molecules dissolved in plasma; once oxygen molecules chemically bind to hemoglobin they no longer exert any pressure. PaO2 affects oxygen content by determining, along with other factors such as pH and temperature, the oxygen saturation of hemoglobin SaO2. The familiar O2-dissociation curve can be plotted as SaO2 vs.
PaO2 and as PaO2 vs. For the latter plot the hemoglobin concentration must be stipulated. When hemoglobin content is adequate, patients can have a reduced PaO2 defect in gas transfer and still have sufficient oxygen content for the tissues e. Conversely, patients can have a normal PaO2 and be profoundly hypoxemic by virtue of a reduced CaO2. This paradox — normal PaO2 and hypoxemia — generally occurs one of two ways: In the presence of a right to left intrapulmonary shunt anemia can lower PaO2 by lowering the mixed venous oxygen content; when mixed venous blood shunted past the lungs mixes with oxygenated blood leaving the pulmonary capillaries, lowering the resulting PaO2 Obviously, however, the lower the hemoglobin content the lower the oxygen content.
Anemia can also confound the clinical suspicion of hypoxemia since anemic patients do not generally manifest cyanosis even when PaO2 is very low. Altered hemoglobin affinity may occur from shifts of the oxygen dissociation curve e. To know the oxygen content one needs to know the hemoglobin content and the SaO2; both should be measured as part of each arterial blood gas test. A calculated SaO2 may be way off the mark and can be clinically misleading.
This is true even without excess CO in the blood. In a patient who is in good health: In addition, a small quantity of oxygen is dissolved in the blood.
This delivers about ml of oxygen to the tissues per minute. This will increase the amount of dissolved oxygen in the blood and will improve tissue oxygen delivery by a small amount. Blood transfusion may be life-saving. Similarly, the visual presence of cyanosis is dependent upon the concentration of desaturated blue hemoglobin.
In this comparison, the more cyanotic patient is doing better with a higher oxygen content and oxygen delivery. The hematocrit is directly proportional to the hemoglobin concentration. The hematocrit in percent is roughly three times the hemoglobin concentration in gm per dl. Chronically hypoxic patients can survive by raising their hematocrit as a compensation maneuver.
Chronic hypoxia stimulates erythropoietin which stimulates RBC production raising the hematocrit. The former patient looks pink, while the latter patient looks blue. The last factor is the oxygen delivery rate.