Arterial Blood Gases (ABGs)

• Arterial blood gases (ABGs) are used to monitor acute respiratory conditions, especially type 2 respiratory failure (e.g., COPD).
• ABGs provide information on acid-base balance, blood gases (O2 and CO2), bicarbonate, lactate, hemoglobin, and electrolytes.

Normal Values

• pH: 7.35 – 7.45
• PaO2 (oxygen in blood): 10.7 – 13.3 kPa
• PaCO2 (carbon dioxide in 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 dissolved in the blood, and a low PaO2 indicates hypoxia.
• FiO2 is the fraction of inhaled oxygen, with room air at 21%.
• Venturi masks precisely control FiO2, while other masks approximate it.
• A low PaO2 signifies hypoxia and respiratory failure.
• Type 1 respiratory failure: Normal PaCO2 with low PaO2.
• Type 2 respiratory failure: Raised PaCO2 with low PaO2.

Acid-Base Balance

• pH below 7.35 indicates acidosis.
• pH between 7.35 – 7.45 is normal.
• pH above 7.45 indicates alkalosis.
• Determine if the cause of acidosis or alkalosis is respiratory or metabolic.

Respiratory Acidosis

• CO2 makes blood acidotic by breaking down into carbonic acid (H2CO3).
• Low pH (acidosis) with raised PaCO2 indicates respiratory acidosis.
• Respiratory acidosis suggests acute CO2 retention.

Bicarbonate

• Kidneys produce bicarbonate, which neutralizes acid and maintains normal pH.
• Bicarbonate production is slow, making it insufficient to compensate for acute respiratory acidosis.
• Raised bicarbonate indicates chronic CO2 retention, as kidneys produce more bicarbonate to balance the acidic CO2.
• COPD patients show this adaptation.
• In acute COPD exacerbation, the kidneys cannot keep up, and the patient becomes acidotic despite elevated bicarbonate levels.

Respiratory Alkalosis

• Respiratory alkalosis occurs when a patient "blows off" too much CO2 due to a raised respiratory rate.
• High pH (alkalosis) and low PaCO2 characterize respiratory alkalosis.
• Common scenarios include hyperventilation syndrome (e.g., due to anxiety) and pulmonary embolism (PE).
• Patients with PE will have a low PaO2, while those with hyperventilation syndrome will have a high PaO2.

Metabolic Acidosis

• In metabolic acidosis:
  • Low pH
  • Low bicarbonate

Causes of Metabolic Acidosis

• Raised lactate from anaerobic respiration (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 (bicarbonate loss), renal failure, or type 2 renal tubular acidosis

Metabolic Alkalosis

• In metabolic alkalosis:

  • Raised pH
  • Raised bicarbonate

• Metabolic alkalosis results from the loss of hydrogen (H+) ions.

Hydrogen Ion Loss

• Gastrointestinal tract - due to vomiting (loss of stomach hydrochloric acid)
• Kidneys - usually due to increased aldosterone activity, leading to increased hydrogen ion excretion

Increased Aldosterone Activity

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