Arterial Blood Gases

Arterial blood gases (ABGs) are commonly used to monitor acute respiratory conditions, especially where there is a risk of type 2 respiratory failure (e.g., chronic obstructive pulmonary disease). ABGs give helpful information about the acid-base balance, blood gases (O2 and CO2 in the blood), bicarbonate, lactate, haemoglobin and electrolytes.

Normal Values

Respiratory Failure

PaO₂ is the partial pressure of oxygen, the amount of oxygen dissolved in the blood. A low PaO₂ indicates hypoxia.

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

A low PaO₂ indicates hypoxia and respiratory failure. It is important to distinguish the type of respiratory failure:

• Normal PaCO₂ with low PaO₂ indicates type 1 respiratory failure (only one is affected)
• Raised PaCO₂ with low PaO₂ indicates type 2 respiratory failure (two are affected)

Acid-Base Balance

Where there is acidosis or 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 respiratory acidosis. This suggests the patient is acutely retaining CO₂ (unable to get rid of it), and their blood has become acidotic.

Bicarbonate

The kidneys produce bicarbonate. Bicarbonate 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. During an acute episode of respiratory acidosis, 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 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. They are breathing too fast and getting rid of 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 the:

• Gastrointestinal tract – due to vomiting (the stomach produces hydrochloric acid)
• Kidneys – usually due to increased activity of aldosterone, which results in increased 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 May 2023

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