iROCKET Learning Module: Intro to Arterial Blood Gases, Pt. 1
base excess, and alveolar–arterial oxygen difference were calculated. Results. PaO2 fell with increasing altitude, whereas SaO2 was relatively stable. .. and Mervyn Singer for their advice during the preparation of the. The haemoglobin–oxygen dissociation curve, a graphical representation of the relationship between oxygen saturation and oxygen partial pressure helps us to. 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 .
I have shown the 3 short paragraphs above to dozens of students, interns, residents; almost all will say they understand the differences, no problem. But, when given questions to test their understanding, they don't show much understanding.Oxygen Saturation Nursing Considerations, Normal Range, Nursing Care, Lab Values Nursing
So more instruction is needed and, yes, a few problems along the way. Understanding will come from closely reviewing this material AND working on all the problems; do that, and you should be able to teach the subject! 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, given a normal amount of shunting, neither anemia nor abnormal hemoglobin binding will affect PaO2. 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 Figure There is a dynamic equilibrium between the freely dissolved and the hemoglobin-bound oxygen molecules.
However, the more dissolved molecules there are i. Oxygen pressure, saturation and content. Schematic shows cross section of lungs and pulmonary circulation. CO2, nitrogen and other gas molecules are omitted for clarity.
Oxygen content (video) | Khan Academy
PaO2 is always slightly lower than PAO2 because of normal venous admixture, here represented by a connection between the venous and pulmonary circulations. See text for discussion.
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 and a certain percentage will be bound Figure If there is no interference as from carbon monoxide, for examplethe free O2 molecules bind to these sites with great avidity.
The total percentage of sites actually bound with O2 is constant for a given set of conditions, and is the 'saturation of blood with oxygen'. 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. Neither the amount of hemoglobin, nor the binding characteristics of hemoglobin, should affect the amount of dissolved oxygen, and hence should not affect the PaO2.
- Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve
Stated another way, the number of dissolved oxygen molecules is independent of the amount of hemoglobin or what is bound to it. To repeat one more time because it is so importantPaO2 is not a function of hemoglobin content or of its characteristics, but only of the alveolar PO2 and the lung architecture alveolar-capillary interface.
This explains why, for example, patients with severe anemia or carbon monoxide poisoning or methemoglobinemia can and often do have a normal PaO2. The most common physiologic disturbance of lung architecture, and hence of a reduced PaO2, is ventilation-perfusion V-Q imbalance. Less common causes are reduced alveolar ventilation, diffusion block, and anatomic right to left shunting of blood.
SaO2 is determined mainly by PaO2. The relationship between the two variables is the familiar oxygen dissociation curve Figure The dissociation curve is experimentally determined from in vitro titration of blood with increasing partial pressures of oxygen.
Correlation between the levels of SpO2 and PaO2
At low oxygen pressures there is relatively little increase in SaO2 for a given change in PaO2. PaO2 is the most important but not the only determinant of SaO2. Shifts of the O2-dissociation curve are discussed in Chapter 6.
For now, consolidate your understanding of the difference between PaO2 and SaO2. Think of PaO2 as the driving pressure for oxygen molecules entering the red blood cell and chemically binding to hemoglobin; the higher the PaO2, the higher the SaO2. The oxygen dissociation curve, showing PaO2 vs. SaO2 and PaO2 vs. X represents blood gas values of a case presented in Chapter 6. The relationship between SaO2 and CaO2 for any given hemoglobin content is linear excluding the minor influence of dissolved oxygen with normal PO2 values.
The so-called "steep part" of the O2 dissociation curve is between 20 and 60 mm Hg PaO2. Compared with the flatter portions, small increases in PaO2 in this region have a much greater effect on improving SaO2 and therefore O2 content. Note that the shape and position of the curve are the same irrespective of the hemoglobin content. The haemoglobin—oxygen dissociation curve describing the relationship between oxygen partial pressure and saturation can be modelled mathematically and routinely obtained clinical data support the accuracy of a historical equation used to describe this relationship.
Educational Aims To understand how oxygen is delivered to the tissues. To understand the relationships between oxygen saturation, partial pressure, content and tissue delivery. The clinical relevance of the haemoglobin—oxygen dissociation curve will be reviewed and we will show how a mathematical model of the curve, derived in the s from limited laboratory data, accurately describes the relationship between oxygen saturation and partial pressure in a large number of routinely obtained clinical samples.
To understand the role of pulse oximetry in clinical practice. To understand the differences between arterial, capillary and venous blood gas samples and the role of their measurement in clinical practice. The delivery of oxygen by arterial blood to the tissues of the body has a number of critical determinants including blood oxygen concentration contentsaturation SO2 and partial pressure, haemoglobin concentration and cardiac output, including its distribution.
Historically this curve was derived from very limited data based on blood samples from small numbers of healthy subjects which were manipulated in vitro and ultimately determined by equations such as those described by Severinghaus in Oxygen saturation by pulse oximetry SpO2 is nowadays the standard clinical method for assessing arterial oxygen saturation, providing a convenient, pain-free means of continuously assessing oxygenation, provided the interpreting clinician is aware of important limitations.
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.
What’s The Difference Between Oxygen Saturation And PaO2?
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.