Arterial Blood Gas (ABG) interpretation for medical students, OSCEs and MRCP PACES

This section presents how to interpret arterial blood gases. It explains each component in turn followed by clinical examples to work through.

The most important points when assessing a patient are the history, examination and basic observations. Investigations such as arterial blood gases add to the information you have already gained to guide your management.

Before starting…

Arterial blood gas analysis can be used to assess gas exchange and acid base status as well as to provide immediate information about electrolytes.
It is also useful to have access to any previous gases. This is particularly important if your patient is known to have chronic respiratory disease with existing chronic ABG changes.

Normal values for arterial blood gas (ABG)

Normal values are given below. Note that these may vary slightly between analysers. Be sure to know the normal ranges and units for the analyser you will be using.
pH: 7.35 – 7.45
pO2: 10 – 14kPa*
pCO2: 4.5 – 6kPa*
Base excess (BE): -2 – 2 mmol/l
HCO3: 22 – 26 mmol/l

*1kPa = 7.5mmHg. p stands for the ‘partial pressure of…’

Click here for related pages: ABG examples and ABG exam questions

Components of the ABG

pH

pH is a logarithmic scale of the concentration of hydrogen ions in a solution. It is inversely proportional to the concentration of hydrogen ions.
When a solution becomes more acidic the concentration of hydrogen ions increases and the pH falls.
Normally the body’s pH is closely controlled at between 7.35 – 7.45. This is achieved through buffering and excretion of acids. Buffers include plasma proteins and bicarbonate (extracellular) and proteins, phosphate and haemoglobin (intracellularly).
Hydrogen ions are excreted via the kidney and carbon dioxide is excreted via the lungs.
Changes in ventilation are the primary way in which the concentration of H+ ions is regulated. Ventilation is controlled of the concentration of CO₂ in the blood.
If the buffers and excretion mechanisms are overwhelmed and acid is continually produced, the he pH falls. This creates a metabolic acidosis.
If the ability to excrete CO₂ is compromised this creates a respiratory acidosis.
Note that a normal pH doesn’t rule out respiratory or metabolic pathology. This why you must always look at all the values other than pH as there may be a compensated or mixed disorder.

Partial pressure (PP)

Partial pressure is a way of assessing the number of molecules of a particular gas in a mixture of gases. It is the amount of pressure a particular gas contributes to the total pressure. For example, we normally breathe air which at sea level has a pressure of 100kPa, oxygen contributes 21% of 100kPa, which corresponds to a partial pressure of 21kPa.
When used in blood gases, Henry’s law is used to ascertain the partial pressures of gases in the blood. This law states that when a gas is dissolved in a liquid the partial pressure (i.e. concentration of gas) within the liquid is the same as in the gas in contact with the liquid. Therefore you can measure the partial pressure of gases in the blood.
PaO₂ is the partial pressure of oxygen in arterial blood
PaCO₂ is the partial pressure of carbon dioxide in arterial blood.

Base excess (BE)

This is the amount of strong acid which would need to be added or subtracted from a substance in order to return the pH to normal (7.40).
A value outside of the normal range (-2 to +2 mEq/L) suggests a metabolic cause for the acidosis or alkalosis.
In terms of basic interpretation
A base excess more than +2 mEq/L indicates a metabolic alkalosis.
A base excess less than -2 mEq/L indicates a metabolic acidosis.

Bicarbonate (HCO₃)

Bicarbonate is produced by the kidneys and acts as a buffer to maintain a normal pH. The normal range for bicarbonate is 22 – 26mmol/l.
If there are additional acids in the blood the level of bicarbonate will fall as ions are used to buffer these acids. If there is a chronic acidosis additional bicarbonate is produced by the kidneys to keep the pH in range.
It is for this reason that a raised bicarbonate may be seen in chronic type 2 respiratory failure where the pH remains normal despite a raised CO₂.

Electrolytes

A venous or arterial blood gas is a good way to quickly check potassium and sodium values. This is particularly important in the immediate management of cardiac arrhythmias as it gives an immediate result.

Lactate

Lactate is produced as a by-product of anaerobic respiration. A raised lactate can be caused by any process which causes tissue to use anaerobic respiration. It is a good indicator of poor tissue perfusion.

Haemoglobin (Hb)

Haemoglobin acts as a guide but is notoriously inaccurate in an ABG. Lab samples should be used to verify results.

Glucose

Don’t forget to check this. Glucose is especially pertinent in the management of the patient who has decreased consciousness or seizures. It is also important in patients with known or suspected diabetes.
Glucose may also be raised in patients with severe sepsis or other metabolic stress.

Other components of the ABG

These are rarely deranged and often overlooked. However, it is important to notice them if they are abnormal. This is especially true in the case of carbon monoxide as there may be other people at risk.

Carbon monoxide (CO)

Normally CO is 10% indicates poisoning, commonly from poorly ventilated boilers or old heating systems.
At levels of 10 -20% symptoms such as nausea, headache vomiting and dizziness will be predominant. At higher levels patients may experience arrhythmias, cardiac ischaemia, respiratory failure and seizures.

Methaemoglobin (metHb)

MetHb is an oxidized form of haemoglobin. Levels of >2% are abnormal.
Methaemoglobinaemia is a rare condition but again it is important not to miss. It may be caused by errors of metabolism or by exposure to toxins such as nitrates.

Compensation

pH is closely controlled in the human body and there are various mechanisms to maintain it at a constant value. It is important to note that the body will never overcompensate as the drivers for compensation cease as the pH returns to normal. In essence compensation for an acidosis will not cause an alkalosis or visa versa.

Respiratory Compensation

If a metabolic acidosis develops the change is sensed by chemoreceptors centrally in the medulla oblongata and peripherally in the carotid bodies.

The body responds by increasing depth and rate of respiration therefore increasing the excretion of CO2 to try to keep the pH constant.

The classic example of this is ‘Kussmaul breathing’ the deep sighing pattern of respiration seen in severe acidosis including diabetic ketoacidosis. Here you will see a low pH and a low pCO2 which would be described as a metabolic acidosis with partial respiratory compensation (partial as a normal pH has not been reached).

Metabolic Compensation

In response to a respiratory acidosis, for example in CO2 retention secondary to COPD, the kidneys will start to retain more HCO3 in order to correct the pH.
Here you would see a low normal pH with a high CO2 and high bicarbonate.
This process takes place over days.
It is important to ensure that the compensation that you see is appropriate, i.e. as you would expect. If not then you should start to think about mixed acid base disorders.

How to interpret an ABG

A systematic approach to ABG interpretation leads to easy interpretation. Here is one such system:

Look at the patient! Review history and examination findings.
What is the pO2 – how much oxygen was your patient on when the gas was taken?
What is the pH? Is the patient acidaemic or alkalaemic.
What is the pCO2?
What is the HCO3 and base excess?
Is the patient compensating?
What are the other values? Ensure that you look at all other figures on the gas.

How to present an ABG

State that this is an arterial blood gas sample (rather than venous).
State the patients name and outline history/pertinent examination findings.
State the time the sample was taken and how much oxygen the patient was on at the time.
Present your findings: e.g. this showed type one respiratory failure with a p02 of 7
Present any abnormal findings or important negatives from the rest of the values.
A one line summary of your findings.

For example:

“This is an arterial blood gas sample taken from Mrs Smith, a 70 year old lady who presented this morning with shortness of breath. She has a back ground of heart failure and diabetes and on auscultation of her chest she has bibasal crackles.
This gas was taken at 10 a.m. today when Mrs Smith was on 15l per minute of oxygen via a non rebreathe mask.
It showed type one respiratory failure with a p02 of 10.0 and a pCO2 of 4.1.
There is no acid base disturbance although her glucose was noted to be 15.

In summary this lady has type 1 respiratory failure.”

N.B. the pO2 of 10 whilst on 15l/min of oxygen is indicative of severe respiratory disease. This is why including all the information in the presentation is incredibly important as a pO2 of 10 on air would be far less worrying.
In patients with chronic respiratory disease it is very useful to see an old ABG as this may give useful clues as to a patient’s normal respiratory status.

Respiratory failure

Respiratory failure can be split into Type one or Type 2 respiratory failure. These are differentiated by the pCO2.

Type 1 Respiratory failure (T1RF)

Type one respiratory failure is defined as a PaO2 less than 8 and a PaCO2 which is low or normal.
T1RF is caused by pathological processes which reduce the ability of the lungs to exchange oxygen, without changing the ability to excrete CO2.
Examples of T1RF are pulmonary embolus, pneumonia, asthma and pulmonary oedema.

Type 2 respiratory failure (T2RF)

This is defined as a PaO2 of less than 8 and a raised PaCO2.
You can think of it as being caused by a problem with the lungs or by a problem with the mechanics or control of respiration.

Pulmonary problems	Mechanical problems	Central problems
COPD	Chest wall trauma	Opiate overdose
Pulmonary oedema	Muscular dystrophies	Acute CNS disease
Pneumonia	Motor neurone disease
Myasthenia Gravis

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33 Responses

October 6, 2020 Sharon Lockett Reply I found this very informative, as a training nurse associate this was simple enough to understand but have the complexity of actual findings that are displayed on an arterial blood gas. Excellent, thank you

October 1, 2020 Anonymous Reply You guys have defined base excess as the amount of strong base that needs to be added or subtracted from a substance in order to get the pH back to normal (~7.4). However, I think it’s the amount of strong acid that needs to be added or subtracted. If it’s to do with strong base, your example of needing to add a strong base to an alkalosis to get to a ‘neutral’ pH would not make sense as it would just get even more alkalotic.

October 9, 2020 Oxford Medical Education Reply Great spot – now corrected, many thanks.

August 16, 2020 Erin Reply If PH is low at 7.299 and high Pc02 of 6.45,low P02 of 1.27, HC03 32.2 and negative base of -3.5. Is that respiratory acidosis partially compensated. Or is it mixed acidosis.

January 24, 2022 Anonymous Reply Partial compensation respiratory acidosis

February 15, 2020 Sara Reply If the patient is having respiratory acidosis and metabolic compensation, and base excess of +4, what does it mean? only metabolic compensation or mixed disorder with seperate metabolic acidosis?

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April 5, 2018 Dr A Latif Soomro Reply Nice and best style of teaching, very well designed and presented.

November 8, 2017 Faheemah Peerbhai Reply Hi
great article .Good training session used with final year students.

November 27, 2017 sunil sah Reply For rajip gandhi university. ..in medical techonology what type of question asked

November 5, 2019 Temmi Reply Very helpful article also so systematic to learn November 6, 2017 Prabakaran Reply really I got what I needed ….. superb explanation November 5, 2017 Jayasreesudheer Chandrakumar Reply Very useful from a nursing head September 24, 2017 Anonymous Reply well done for better information keep it up

September 1, 2017 Annmarie Reply Hi I have brittle asthma but my sats are dipping most nights to 88 sometimes even 70 been going on for months they want to do blood gases but appointment isn’t till afternoon if they have dipped during the night will the co2 reading show this hours later or if it hasn’t dipped that night at all will it show it from the might before
Thanks annmarie

September 1, 2017 Oxford Medical Education Reply I’m afraid we can’t comment on specific patient-related questions. Sorry about that and best of luck.

August 24, 2017 Anonymous Reply sir, what will happen in compensated respiratory alkalosis.. ?

September 1, 2017 Jack Reply Essentially the the compensatory response is a fall in bicarbonate level: Compensation in an ACUTE Respiratory Alkalosis
Mechanism: Changes in the physicochemical equilibrium occur due to the lowered pCO2 and this results in a slight decrease in HCO3-. There is insufficient time for the kidneys to respond so this is the only change in an acute respiratory alkalosis. The buffering is predominantly by protein and occurs intracellularly; this alters the equilibrium position of the bicarbonate system.
Magnitude: There is a drop in HCO3- by 2 mmol/l for every 10mmHg decrease in pCO2 from the reference value of 40mmHg.
Limit: The lower limit of ‘compensation’ for this process is 18mmol/l – so bicarbonate levels below that in an acute respiratory alkalosis indicate a co-existing metabolic acidosis. (Alternatively, their may be some renal compensation if the alkalosis has been present longer than realised.) Compensation in a CHRONIC Respiratory Alkalosis
Mechanism: Renal loss of bicarbonate causes a further fall in plasma bicarbonate (in addition to the acute drop due to the physicochemical effect and protein buffering).
Magnitude: Studies have shown an average 5 mmol/l decrease in [HCO3-] per 10mmHg decrease in pCO2 from the reference value of 40mmHg. This maximal response takes 2 to 3 days to reach.
Limit: The limit of compensation is a [HCO3-] of 12 to 15 mmol/l.

July 17, 2017 Dr Amila Jeewantha Reply very very good explanation. highly understandable.Great work.Thank you very much madam/sir.