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

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.

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

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