Interpreting Arterial Blood Gases

This E-Learning course will provide nursing students and registered nurses with detailed information on Arterial Blood Gas (ABG) analysis, an overview of the physiological mechanisms behind ABG's and its importance in nursing practice. This course will identify the values are measured in an ABG analysis and include a systemic approach on how to interpret ABG results. This course will also cover information on acid-base balance and common acid-base balance disorders. 

This course will not provide information on how to take and ABG analysis sample but will provide information about possible complications that may affect the results of an ABG sample. 

On the completion of this learning module you should be able to:

Understanding Arterial Blood Gases

What is an Arterial Blood Gas Analysis

  • Arterial Blood Gas (ABG) analysis is an important tool to assist nurses in measuring the level of oxygenation and the acid-base balance in the blood ( Roman, Thimothee & Vidal, 2008).
  • ABG analysis is becoming a common investigation in emergency department and intensive care units for monitoring patients with acute respiratory failure ( Mohammed & Abdelatief, 2016). 
  • ABGs are used to assess how effectively the lungs move oxygen into the blood and remove carbon dioxide from the blood (Roman et al,. 2008). 
  • An arterial blood gas analysis is an essential component of nursing care when managing critically ill patients, caring for patients with respiratory conditions and also patients in respiratory failure (Casey, 2013). 

Five components of an ABG analysis

The values that are measured through and ABG analysis have five basic components that are used to assess the patients. These components include:

  •  (Sa02)- The percentage of haemoglobin saturated with oxygen in the arterial blood
  •  (Pa02)- Partial pressure of oxygen dissolved in the arterial blood
  •   (pH)-The Arterial blood acidity or alkalinity 
  •  (PaC02) - Partial pressure of carbon dioxide in arterial blood
  •  (HCO3)- The concentration of bicarbonate ion in arterial blood. This value can be used to determine if the acid-base imbalance is respiratory of metabolic (We'll discuss more about acid-base imbalances later). 

(Pruitt et al,. 2004).

Indications for an ABG analysis

Analysis of arterial blood can assist in the assessment of the patients metabolic and respiratory systems. The indications for ABG analysis include:

  • Altered level of consciousness 
  • Respiratory distress- hypoxia 
  • Collapse on unknown cause
  • Poisons/toxins ingestion 
  • metabolic disorders 
  • Titration of artificial ventilation 
  • Trauma- management of raised inter cranial pressure 
  • Patient in shock- sepsis 
  • Evaluation of intervention- fluid restriction 
  • Inotropic therapy ohammed& Abdelatief 2016).

(Mohammed & Abdelatief, 2016).

Complications that may affect the results of an ABG analysis

There are a number of sampling and environmental factors may affect the result of the analysis: 

  •  Increased processing time of the ABG the sample may lead to a falsely low PaO2, as the delay may allow leukocytes to consume the oxygen. This factor can be avoided by quickly transporting the sample on some ice. 
  • .Air bubbles can also be introduced when performing the arterial puncture which can lead to a false high reading of the PaO2 and a falsely low PaCO2. This can be avoided by gently removing air bubbles within the specimen immediately after collection without agitating the sample.
  • Body temperature can also affect arterial blood gas tensions.This becomes a relevant factor in febrile or hypothermic patients. A patients body temperature should always be recorded at the time of the sample collection. 

(Verma & Roach 2010).

Limitations of an ABG analysis

Although an ABG analysis is a very helpful test in clinical practice an ABG analysis can also present a few limitations in clinical practice, these limitations may include:

  • An ABG analysis can not identify a specific diagnosis. A patients ABG values with pneumonia can sometimes have very similar numbers to a patient with asthma. 
  • The analysis doesn't reflect how much an abnormal value can affect a patient.
  • A low Pa02 does not always mean tissue hypoxia and a normal Pa02 doesn't always mean adequate oxygenation. 
  • An ABG analysis cannot be used as a screening tool cannot be used for early pulmonary disease.

(Verma and Roach, 2010)

Match the ABG component to its definition

  • (pH)
    To test the acidity or alkalinity of the arterial blood
  • (PaC02)
    Partial pressure of the carbon dioxide in arterial blood
  • (Pa02).
    Partial pressure of oxygen dissolved in arterial blood
  • (Sa02)
    Percentage of haemoglobin saturated with oxygen in arterial blood
  • (HC03)
    Concentration of bicarbonate ions in arterial blood

Mechanisms of Acid-Base Balance

Acid-Base Balance

It is essential when interpreting ABG results that you firstly have an understanding of the fundamental knowledge behind acid-base balance. Acid-base balance is a reflection of the pH level of the blood, as for cells to function normally an existing control of the balance between acids and bases should exist, this  is known as maintaining pH homoeostasis. 

Acids and bases are by-products of the bodies natural metabolism and the regulation in the body is controlled by various mechanisms. An acid is a substance that is able to provide hydrogen, and a base (alkali) is able to accept hydrogen. In the human body there has to be an equal balance of the intake, production and removal of hydrogen.The more hydrogen ions in the blood the more acidic it will become. 

Buffering mechanisms together with the respiratory and renal systems play an essential role in maintaining homoeostasis of the body's acid-base balance through the body's negative feedback mechanisms. When an acid base imbalance occurs which will get into more detail about on the next slide the treatment involves stabilisation of the hydrogen ions in the blood. These mechanisms work by removing hydrogen ions from the body by the excretion or exhalation in the urine.

(Lynch, 2009 & Pruitt et al,.2004 &, Rogers and McCutcheon, 2015). 

Acid-Base Imbalance disorders

Acid-base imbalance disorders are characterised by many changes in the following: 

  • pH of the arterial blood 
  • partial pressure of carbon dioxide in the arterial blood  
  • Serum bicarbonate (HCO3) levels in the arterial blood

Acid-Base imbalance disorders:

  • Respiratory acidosis

Respiratory acidosis has a pH less than 7.5 and a PaC02 greater then 45 mm Hg, acidosis is mainly caused by a buildup of carbonic acid. Conditions that result in hypo-ventilation can cause respiratory acidosis as it prevents the exhalation of C02. Some of these conditions can include: impaired respiratory muscle function due to a spinal cord injury. pneumonia, pneumothorax, central nervous system depression or pulmonary oedema.

  • Respiratory alkalosis:

Respiratory alkalosis has a pH greater than 7.45 and a PaC02 less than 35 mm Hg. Conditions that cause hyperventalation can result in respiratory alkalosis. These conditions may include: unresolved pain, sepsis, pregnancy, central nervous system lesions and psychological responses such as severe stress, fear of anxiety 

  • Metabolic acidosis

Metabolic acidosis has a pH of less than 7.35 and a HC03 level of less than 22 mmol/L. This acid-base imbalance can be caused by either an insufficiency of base in the blood stream or an excess amount of acids, other than C02. Common causes of metabolic acidosis includes: anaerobic metabolism, starvation, renal failure or diabetic ketoacidosis & diarrhoea 

  • Metabolic alkalosis

Metabolic alkalosis has a pH greater than 7.45 and a HC03 level higher than 26 mmol/L. A loss of an acid or an excess in base within the body can result in metabolic alkalosis. Some common causes of metabolic alkalosis include: aggressive gastric suctioning, an excess administration of diuretics or prolonged vomiting. 

(Mohammed & Abdelatief, 2016)


  • Respiratory acidosis

  • Respiratory alkalosis 

  • Metabolic acidosis                       

  • Metabolic alkalosis 





- ketoacidosis 

- renal failure 

- diarrhoea 

- vomiting 

- gastric suctioning 

- HC03 retention 

pH      HC03    PaC02 

.low        normal      high 

high       normal      low 

low         low           normal 

high       high         normal

(Rogers & McCutcheon , 2015).

Buffering systems

Buffers are able to stop changes to the pH due to their ability to either absorb or release hydrogen ions.The body has two buffer systems that are in place to keep the pH withing its narrow range:

  • Respiratory buffer 

Carbon dioxide is a normal by-product of cellular metabolism which is carried in the blood to the lungs. The excess carbon dioxide combines with water which it then forms carbonic acid. The amount of carbonic acid in the blood will change the pH of the blood, as the more carbonic acid present in the blood the more acidic the pH will become. The lungs will respond by either increasing or decreasing the rate of breathing until acid-base balance is restored. In a health person this process will take place in 1-3 minutes. 

  • Renal Buffer 

Through the renal systems ability to release or absorb bicarbonate this allows it to act as a buffer. The kidneys provide a delayed but sustained response to acid-base imbalances taking days to occur by either excreting acidic or alkalonic urine. Bicarbonate is known as a powerful buffer and is considered as alkaline. As the pH of the blood becomes more acidic the kidneys will then compensate by retaining bicarbonate and as the pH becomes alkalonic the kidneys will then excrete bicarbonate maintain the pH of the blood between its limits. 

If either or both of these buffering systems were to break down the patient is put in compromised state resulting in abnormal ABG values. The earlier a compromise like this is identified the higher chance there is at restoring an equal balance. 

(Mohammed & Abdelatief, 2016 & Lynch 2009).

Compensation mechanisms

As the body always aims to maintain a normal pH, sometimes causes of an acid-base imbalance cannot be changed without the respiratory or renal systems intervening. The respiratory and metabolic systems will oppose each other as an attempt to create a balance. 

Compensated acid-base balance occurs when a patient has a normal pH value but an abnormal C02 or HC03 value. Compensation is the bodies way of trying the to return the carbon dioxide and bicarbonate ratio to normal. Partial compensation will be represented by an abnormal pH value and the PaC02 and HC03 will also have abnormal values, not enough to correct the imbalance. 

For example: 

If the problem was a patient was in respiratory acidosis, the kidneys would try to compensate through reabsorbing HC03 and excreting hydrogen ions. With respiratory alkalosis the kidneys react by excreting HC03 and absorbing hydrogen ions. 

(Roman et al,. 2008 & Lynch, 2009).

What is the role of the body's compensation mechanism?

Interpreting ABG results

Normal arterial blood gas values

Before you go on to interpreting ABG results,  below there is a summary of the normal values and abnormal values for the five components that are measured in an ABG test.This is a helpful and easy guide for you to remember when you are taking ABG's when you graduate or in you current workplace. 

ABG component 






Normal Values 


80-100 mmHg

35-45 mm Hg

22-26 mmol/L


Abnormal Values 

<7.35=acidosis. >7.45=alkalosis

<80mm Hg= Hypoxemia

<35= acidosis. >45=alkalosis

<22= acidosis. >26 alkalosis

<90% assess patient 

( Pruitt et al,.2004.& Lynch,2009).

Interpreting ABG analysis results

Systemic approach to interpreting ABG results 

Now that you have more of an understanding behind the reasoning for ABG analysis we're now going to present to you six simple steps to help you interpret ABG results. Before interpreting an ABG analysis always ensure you obtain a relevant clinical history of the patient, which gives insight into the possible cause of the patients acid-base disorder. 

These six steps will further enhance your skills to interpret ABG results when you take an ABG sample. Nurses can make a big difference to their patients care when having obtained this skill and may result in quicker medical interventions increasing the chance of a positive patient outcome. 

The six steps are: 

  1.  Examine both the Pa02 and Sa02 levels to determine if hypoxemia exists and intervene if required. If Pa02 and Sa02 is low it indicates inadeqaue supply of oxygen and if the Pa02 and Sa02 is high then there is an adequate supply of oxygen. 
  2. Examine the pH of the patients arterial blood to identify the presence of acidosis or alkalosis
  3. Examine the PaC02 and determine if it indicates acidosis or alkalosis in the respiratory component of the ABG. 
  4. Examine the HC03 and determine whether it indicates acidosis or alkalosis in the metabolic component of the ABG.
  5. Identify the origin of the acid-base disturbance as respiratory or metabolic. This step is completed by matching up the acidosis or alkalosis of the PaC02 or the HC03 to the pH value. 

Example: If the pH was 7.52 the PaC02 was 30mm Hg and the HC03 was 24 mmol/L. The pH is alkalonic which matches with the PaC02 value. The origin of the acid-base disturbance is therefore respiratory.  

6. Now to determine if the patient is compensating. This is identified through examining the pH, if the pH is within normal limits the patient is fully compensated. If not look at the value that didn't match with the pH. If that value is within normal limits then the patient is uncompensated. However if this value resides outside its normal limits then the patient is partially compensating. 

The best way to master your skills at interpreting ABG results is to practice practice practice!

The next slide will present to you some scenarios and test your skills at interpreting ABG results through using this six step method. 

(Pruitt et al,. 2004, Rodgers & McCutcheon, 2015). 

Putting it all together

This is going to be an example of how to use the six step method below using an example of a patients set of ABG values: 


You have taken an ABG sample and your patients ABG results are as follows:

  • pH- 7.32
  • PaC02- 58 mm Hg
  • HC03- 29mmol/L
  • Pa02- 65 mm Hg
  • Sa02- 87%

Six step method: 

  1. The Pa02 and Sa02 indicates mild hypoxemia, so you'll firstly administer oxygen and continue to monitor your patients oxygen status 
  2. Assess the pH values which is identified as acidic
  3. The PaC02 values shows acidosis in the respiratory compartment of the ABG 
  4. HC03 values shows alkalosis in the metabolic compartment of the ABG 
  5. The patient is in an acidosis state due to the pH being below normal. The origin of the acidosis is respiratory as the PaC02 status matches the pH acid-base status. 
  6. The HC03 isn't within normal limits and neither is the pH so the patient is partially compensated 

Therefore the patient is in partially compensated respiratory acidosis with mild hypoxemia 

(Pruitt et al,. 2004). 

Test your skills on interpreting ABG's

The patients ABG results are: pH 7.36, PaC02 29 mm Hg, HC03, 20mmol/L, Pa02 108 mm Hg and Sa02 99%:

  1. The patients Pa02 and Sa02 indicates  
  2. The pH indicates  
  3. The PaC02 values shows  in the  compartment of the ABG 
  4. The HC03 value shows  in the  compartment of the ABG 
  5. The patient is in due to the pH being on the low side of the normal range. The origin of the acidosis is  as the HC03 matches the same acid-base status of the pH.
  6. The PaC02  within normal limits, but the pH  making the patient  compensated. 

Therefore the patient is in  compensated  with normal oxygenation. 

Can you match the normal ranges?

  • Sa02
  • PaC02
    35-45 mm Hg
  • Pa02
    80-100 mm Hg
  • HC03
    22-26 mmol/L
  • pH

Time to test you knowledge!

Which of the following is not a component of an ABG analysis?

  • Sa02
  • HCO3
  • PaC02
  • H20

Is respiratory distress an indication for an ABG analysis

  • Yes
  • No

Interpreting ABG's

The patients ABG results are as follows: pH 7.32, PaC02 31 mm Hg, HC03 19 mmol/L, Pa02 78 mm Hg, Sa02, 89% 

  1. The patients Pa02 and Sa02 indicates  administer supplemental oxygen and continue to monitor the patients 02 status 
  2. The pH indicates  
  3. The PaC02 indicates  in the  component of the ABG 
  4. The HC03 level indicates  in the  component of the ABG 
  5. The patient is in  as the pH is below normal. The origin of the  is  as the  matches the acid-base status of the pH. 
  6. The PaC02  within normal limits, and neither is the pH so the patient is  compensated. 

The patient is in  compensated   with 


Which of the following is not an acid-base imbalance disorders

  • Respiratory alkalosis
  • Metabolic acidosis
  • Respiratory pH
  • Respiratory acidosis

What are the 6 steps to interpreting ABG results in order from 1-6

  • Examine both the Pa02 and Sa02 levels to determine if hypoxemia exists and intervene if required.
  • Examine the pH of the patients arterial blood to identify the presence of acidosis or alkalosis.
  • Examine the PaC02 and determine if it indicates acidosis or alkalosis in the respiratory component of the ABG.
  • Examine the HC03 and determine whether it indicates acidosis or alkalosis in the metabolic component of the ABG.
  • Identify the origin of the acid-base disturbance as respiratory or metabolic.
  • Determine if the patient is compensating.

Survey and Feedback


Thank you for completion this e-learning program on interpreting ABG results, I hope you now feel confident in applying this knowledge when you are practising as a nurse or using it when you are a student. I hope this program has provided you with viable knowledge and has helped you understand the significance of the importance of ABG's and you feel more confident in interpreting ABG results. Now there will be some multiple choice questions on the next few slides to provide some feedback about how you found the content of this e-learning program. Please leave some comments at the end if there is anything we can do to improve this course for you or if there was anything you found positive about the program. 

Again, thank you! 

How did you find the information provided in this e-learning program?

  • Very complex
  • Complex
  • Just right
  • Simple
  • Very simple

Do you feel confident in interpreting ABG results?

  • Yes
  • No

Would you recommend the program to other learners?

  • Yes
  • No

Please leave some comments below if you found the information in this e-learning course helpful and relevant and if there is anything we can do to improve this course?


References used

Reference list

  •  Casey, G. (2013). Interpreting arterial blood gases. Kai Tiaki Nursing New Zealand, 19(6), 20-24. 
  • Lynch, F. (2009). Arterial blood gas analysis: implications for nursing. Paediatric Nursing21(1), 41-44
  • Mohammed, H. M., & Abdelatief, D. A. (2016). Easy blood gas analysis: Implications for nursing. Egyptian Journal Of Chest Diseases And Tuberculosis65(1), 369.
  • Pruitt, W. C., & Jacobs, M. (2004). INTERPRETING ARTERIAL BLOOD GASES: EASY AS A B C. Nursing, 34(8), 50-53
  • •Rogers, K. A., & McCutcheon, K. (2015). Four steps to interpreting arterial blood gases. Journal Of Perioperative Practice, 25(3), 46-52
  • Roman, M., Thimothee, S., & Vidal, J. E. (2008). Arterial Blood Gases. MEDSURG Nursing17(4), 268-269
  • Verma, A. K., & Roach, P. (2010). The interpretation of arterial blood gases. Australian Prescriber33(4), 124-129.