Arterial Blood Gases

• Arterial blood gases (ABGs)

  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 for ABGs

  • 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


  PaO2 is the partial pressure of oxygen, indicating 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
• PaO2


  The amount of oxygen dissolved in the blood
• FiO2


  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
• A low PaO2 indicates respiratory failure
• It is important to distinguish the type of respiratory failure:
• Acid-Base Balance


  pH Under 7.35: Acidosis, 7.35 – 7.45: Normal, Above 7.45: Alkalosis
• Respiratory Acidosis


  Carbon dioxide (CO2) makes blood acidotic by breaking down into carbonic acid (H2CO3). Low pH (acidosis) with a raised PaCO2 indicates respiratory acidosis
• Bicarbonate


  The kidneys produce bicarbonate which acts as a buffer to neutralise the acid in the blood and helps 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 of metabolic acidosis

  • 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 tubula
• Metabolic acidosis


  • Low pH
  • Low bicarbonate
  • Causes: Raised lactate (released during anaerobic respiration), 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, renal failure, or type 2 renal tubular acidosis)
• Metabolic alkalosis


  • Raised pH
  • Raised bicarbonate
  • Results from the loss of hydrogen ions
• Metabolic alkalosis


  Results from the loss of hydrogen ions from the gastrointestinal tract (due to vomiting) or kidneys (due to increased activity of aldosterone)
• Causes of increased activity of aldosterone


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