Arterial Blood Gases

Arterial blood gases (ABG) are very commonly used for monitoring in the intensive care unit. Arterial lines make it easy to obtain an arterial blood sample. ABGs give useful information about the acid-base balance, blood gases (O2 and CO2 in the blood), bicarbonate, lactate, haemoglobin and electrolytes. 


Normal Values


Normal Values


7.35 – 7.45

PaO2 (oxygen in the blood)

10.7 – 13.3 kPa

PaCO2 (carbon dioxide in the blood)

4.7 – 6.0 kPa

HCO3 (bicarbonate)

22 – 26 mmol/L

Base excess

-2 – +2


0.5 – 1 mmol/L


Respiratory Failure

The first step when analysing an ABG is determining if the patient is hypoxic. Look at the PaO2, which is the partial pressure of oxygen, meaning the amount of oxygen dissolved in the blood. A low PaO2 indicates hypoxia.

Check the FiO2. FiO2 is the fraction of inhaled oxygen. Room air has a FiO2 of 21%, meaning the concentration of oxygen in room air is 21%. Venturi masks can be used to control the FiO2. Other masks only give an approximate FiO2.

It is important to be able to distinguish the types of respiratory failure:

  • Low PaO2 indicates hypoxia and respiratory failure
  • Normal pCO2 with low PaO2 indicates type 1 respiratory failure (only one is affected)
  • Raised pCO2 with low PaO2 indicates type 2 respiratory failure (two are affected)


Acid-Base Balance

The next step when analysing an ABG is to check the acid-base balance.


Acid-Base Balance

Under 7.35


7.35 – 7.45


Above 7.45


Once you identify an acidosis and alkalosis, you need to determine whether the cause is respiratory or metabolic. 


Respiratory Acidosis

Remember that carbon dioxide (CO2) makes blood acidotic by breaking down into carbonic acid (H2CO3). 

Low pH (acidosis) with a raised PaCO2 indicates a respiratory acidosis. This suggests the patient is acutely retaining CO2 (unable to get rid of it), and their blood has become acidotic.



Bicarbonate is produced by the kidneys. It acts as a buffer to neutralise the acid in the blood and helps maintain a normal pH. It takes a while for the kidneys to produce bicarbonate. In an acute episode of respiratory acidosis, the bicarbonate cannot be produced fast enough to compensate for the rising carbon dioxide. 

Raised bicarbonate indicates that the patient chronically retains CO2. Their kidneys have responded by producing additional bicarbonate to balance the acidic CO2 and maintain a normal pH. This is usually seen in patients with chronic obstructive pulmonary disease (COPD). In an acute exacerbation of COPD, the kidneys cannot keep up with the rising level of CO2, so the patient becomes acidotic despite having higher bicarbonate than someone without COPD.


Respiratory Alkalosis

Respiratory alkalosis occurs when a patient has a raised respiratory rate and “blows off” too much CO2. There will be a high pH (alkalosis) and a low PaCO2. 

TOM TIP: The most common scenarios where you will see a respiratory alkalosis in exams are hyperventilation syndrome (e.g., due to anxiety) and patients with a pulmonary embolism. Patients with a PE will have a low PaO2, whereas patients with hyperventilation syndrome will have a high PaO2.


Metabolic Acidosis

In metabolic acidosis, there is a:

  • Low pH
  • Low bicarbonate


Causes of metabolic acidosis include:

  • Raised lactate – lactate is released during anaerobic respiration (indicating tissue hypoxia)
  • Raised ketones – typically in diabetic ketoacidosis
  • Increased hydrogen ions – due to renal failure, type 1 renal tubular acidosis or rhabdomyolysis
  • Reduced bicarbonate – due to diarrhoea (stools contain bicarbonate), renal failure or type 2 renal tubular acidosis


Metabolic Alkalosis

In metabolic alkalosis, there is a:

  • Raised pH
  • Raised bicarbonate


Metabolic alkalosis results from the loss of hydrogen (H+) ions. Hydrogen ions can be lost from:

  • Gastrointestinal tract – due to vomiting (the stomach produces hydrochloric acid)
  • Kidneys – usually due to increased activity of aldosterone, which results in hydrogen ion excretion


Increased activity of aldosterone can be due to:

  • Conn’s syndrome (primary hyperaldosteronism)
  • Liver cirrhosis
  • Heart failure
  • Loop diuretics
  • Thiazide diuretics


Last updated August 2021
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