Pao2 and spo2 relationship trust

Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve

pao2 and spo2 relationship trust

SaO2: Arterial oxygen saturation (measured by arterial blood gas) Body of evidence can be trusted to guide practice in most situations .. difference between PaCO2 and venous PCO2 was mmHg higher for the venous reading, but with. Sir,. Sarkar et al. in , in their recent article,[1] have very nicely elucidated various mechanisms of hypoxemia, and I would like to congratulate them for this . Department of Anaesthetics, Kings College Hospital NHS Trust, Denmark Hill, reliably predict changes in oxygen saturation (SaO2); is not reflective . Relationship between supranormal oxygen tension and outcome after.

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.

pao2 and spo2 relationship trust

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.

Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve

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. 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. 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.

pao2 and spo2 relationship trust

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.

PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation

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.

pao2 and spo2 relationship trust

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. In this context, mismatch between the low saturation versus the PaO2 which will often be elevated suggests a diagnosis of methemoglobinemia 6. Why measuring PaO2 is generally unhelpful. The answer is that clinicians are very good at diagnosing heart failure without BNP 8.