Arterial Blood Gas Analysis

Previous Topic
Next Topic

Metabolic Acidosis

Arterial Blood Gas Analysis Uncompensated Acid-base Disorders Metabolic Acidosis

CO2 + H2O H2CO3 H+ + HCO3–

In metabolic acidosis, there is either additional acid (H+) production on the right side of the equation, or direct loss of bicarbonate which drives the equation to the right, increasing H+ and lowering pH. [2,3,5]

The consequent reduction in pH stimulates the respiratory centre to increase ventilation and lower pCO2. This in turn drives the reaction to the left, lowering both bicarbonate and H+ to achieve compensation. There is also increased H+ secretion in the kidneys (linked to increased HCO3– reabsorption), further lowering H+. [2,3,5]

Further classification of a metabolic acidosis depends on the anion gap the difference between the major plasma cations (Na+ and K+) and anions (Cl– and HCO3–):

Anion gap = (Na+ + K+) (Cl– + HCO3–)

A normal anion gap is in the range 9-14 mmol/l.

Calculating the anion gap often helps identify the cause of the acidosis.

Causes of a raised anion gap metabolic acidosis [2,3,5]

A raised anion gap can be due to excess acid production or ingestion contributing extra H+:

  • Methanol poisoning with formic acid formation
  • Uraemia from advanced renal failure
  • Diabetes also producing ketoacidosis
  • Paraldehyde poisoning with acetic and chloracetic acid formation
  • Isoniazid / Iron overload
  • Lactate from tissue hypoxia (respiratory compromise, sepsis, ischaemic bowel
  • Ethylene glycol poisoning (with glycolic and lactic acid production) or Ethanol poisoning producing ketoacidosis
  • Rhabdomyolysis
  • Salicylate from aspirin overdose

Remember: MUDPILERS

For example, in a patient with diabetic ketoacidosis, without any compensation:

  • pH 7.22
  • pCO2 4.8 kPa
  • pO2 12.1 kPa
  • Bicarbonate 15 mmol/L
  • Na+ 138 mmol/L
  • K+ 4.6 mmol/L
  • Cl- 104 mmol/L

The pH is low, the pCO2 is normal and the bicarbonate is low indicating a metabolic acidosis. The anion gap is raised at 23.6 due to the ketoacidosis. [2,3,5]

Causes of a normal anion gap metabolic acidosis [2,3,5]

In a normal anion gap acidosis, bicarbonate is lost from the gut or the kidneys and there is a raised chloride, which compensates for the extra cations, thus keeping the gap normal. This occurs as a result of reabsorption of sodium chloride via the kidneys:

  • H+ secretion failure from the kidneys types 1 and 4 renal tubular acidosis
  • Acetazolamide
  • Renal tubular acidosis (type 2) with loss of HCO3– from the kidneys
  • Diarrhoea loss of lower GI secretions including HCO3–
  • Ureteropelvic fistula loss of HCO3– containing secretions
  • Post-hypocapnia
  • Spironolactone

Irrespective of its cause, a metabolic acidosis has a detrimental effect on the cardiovascular system: there is impaired cardiac contractility and a reduced response to catecholamines. There is also increased pulmonary vascular resistance and decreased hepatic and renal perfusion. The threshold for ventricular fibrillation is lowered.

Remember: HARDUPS

Previous Topic
Back to Lesson
Next Topic