Arterial Blood Gases

• Arterial blood gases (ABG)

  1. Very commonly used for monitoring in the intensive care unit

  2. ABGs give useful information about the acid-base balance, blood gases (O2 and CO2 in the blood), bicarbonate, lactate, haemoglobin, and electrolytes
• Normal Values for ABG markers

  • pH: 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
  • Lactate: 0.5 – 1 mmol/L
• Respiratory Failure


  The first step when analysing an ABG is determining if the patient is hypoxic by looking at the PaO2 (partial pressure of oxygen) in the blood. A low PaO2 indicates hypoxia. Check the FiO2 (fraction of inhaled oxygen), where room air has a FiO2 of 21%
• Items related to ABG analysis

  • Venturi masks
• PaO2


  Partial pressure of oxygen, indicating the amount of oxygen dissolved in the blood
• A low PaO2 indicates hypoxia
• FiO2


  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
• 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: Acidosis
  • 7.35 – 7.45: Normal
  • Above 7.45: Alkalosis
• Identifying acidosis and alkalosis

  Determine whether the cause is respiratory or metabolic
• Respiratory Acidosis: Low pH (acidosis) with a raised PaCO2 indicates a respiratory acidosis
• Bicarbonate is produced by the kidneys to act as a buffer to neutralise the acid in the blood and help maintain a normal pH
• Raised bicarbonate indicates that the patient chronically retains CO2
• Respiratory Alkalosis occurs when a patient has a raised respiratory rate and "blows off" too much CO2
• Metabolic Acidosis: Low pH, low bicarbonate. Causes include raised lactate and raised ketones
• In metabolic acidosis, there is a high PaO2
• Metabolic Acidosis


  In metabolic acidosis, there is a low pH and low bicarbonate
• Causes of metabolic acidosis

  • Raised lactate - 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 and raised bicarbonate
• Ways hydrogen ions can be lost in metabolic alkalosis

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


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


  August 2021