ABG interpretation made easy: Oxygenation

An arterial blood gas can help you determine how well patient is oxygenating. Essentially, all you have to do is memorize the following chart.

	PaO2	SpO2
Normal	80-100	95-99%
Mild Hypoxemia	60-79	90-94%
Moderate Hypoxemia	40-59	75-89%
Severe Hypoxemia	< 40	< 75%

Oxygen Therapy.  Using oxygen therapy to improve oxygenation.  It generally involves inhaling an FiO2 greater than that which is contained in room air.

Fraction of Inspired Oxygen (FiO2).  This is the percent of oxygen in the air inhaled.  Room air contains 21% FiO2.  Oxygen Therapy may supply an FiO2 from 22-100%, depending on the device used. To learn more, check out "Oxygen Therapy Made Easy."

Goal of Oxygenation.  Most protocols now recommend the least amount of supplemental oxygen to maintain an SpO2 of 90% and a PaO2 of 60.  For some patients with lung disease, lower SpO2s may  be acceptable. For instance, with some cases of advanced COPD, an SpO2 of 88% may be acceptable.

Responsive Hypoxemia.  Supplemental oxygen improves oxygenation levels.  Or, increasing FiO2 increases SpO2 and PaO2 to acceptable levels.

Refractory Hypoxemia.  Supplemental oxygen does not improve oxygenation levels.  Or, increasing FiO2 does not result in an increase in SpO2 and PaO2.  It's commonly described as an SpO2 of less than 60 despite receiving 100% FiO2.

Hypoxemic Respiratory Failure.  Failure of the heart and lungs to oxygenate the blood despite the application of supplemental oxygen via oxygen therapy.

• PaO2 less than 60 on 50% or greater FiO2
• PaO2 less than 40 on any FiO2

Desired FiO2.  Calculated:  Desired PaO2 + Known FiO2 divided by known PaO2

How to use ABG results to determine if oxygen therapy is working over time. 

1. Expected PaO2 = FiO2 * 5
• Example.  If a patient is on 100% oxygen, you should expect a PaO2 of 500.  If the PaO2 is only 200, you know the patient is not oxygenating well.  
2. Actual PaO2/ Expected PaO2 = % of patient expected PaO2:
• Should be recorded daily 
• Shows if patient is oxygenating better
• Better indicator than simply looking at actual PaO2 and FiO2
• Normal = zero (patient requiring no supplemental oxygen)

Examples of % expected PaO2: (Despite lower PaO2, patient still oxygenating better)

• January 1 PO2 40 on 100% FiO2 = 80%
• January 5 PO2 60 on 40% FiO2 = 30%
• January 6 PO2 55 on 50% FiO2 = 20%

Another example of % expected PaO2 (PO2 look good, but is patient really oxygenating?)

• January 1 PaO2 200 on 100% FiO2 = 40%
• January 5 PaO2 100 on 100% = 20%
• January 6 PaO2 100 on 90% = 22%
• January 10 PaO2 55 on 80% = 13%

You don't necessarily need to use these formulas to see if patient oxygenating well, yet sometimes they can be useful. Especially for the more complicated cases, it helps to see the numbers and the trends.

Post originally published on 8/11/10 on respiratory therapy cave; updated by Rick Frea for accuracy and simplicity.

Further Reading

• ABG interpretation made easy: interpretation of acid base balance 
• The ABG lexicon
• Learn about the 4-5-6, 7-8-9 rule
• Oxyhemoglobin Dissociation Curve
• How to know if your patient is a CO2 retainer
• Cord Blood Gases made easy
• The argument against ABGs
• What is an ABG
• Respriatory Therapy Formulas

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