Send Feedback

Print

Mobile

 Back

CDHB

Acidosis / Alkalosis

Interpretation of blood gas results

When determining the status of metabolic acidosis or alkalosis, allowance must be made for the influence of respiratory abnormalities of pH and HCO₃. The following table may be helpful:

The causes of respiratory acid/base disturbance are usually obvious and reflect the underlying abnormal pulmonary function. This should be investigated and managed if necessary with the assistance of a Respiratory Physician.

The diagnosis of metabolic acidosis may be more difficult. Acidosis may be associated with an increased anion gap which is of some value in diagnosis. The gap may be increased by ketones, lactate, some poisonings e.g., salicylates, and in advanced renal failure.

Causes of Metabolic Acidosis

Calculate Anion Gap

Increased anion gap

• Increased acid production:
  • Ketoacidosis: diabetes, starvation, alcoholism.
  • Lactic acidosis: respiratory/circulatory failure (including anaemia, carbon monoxide, shock); neoplastic disease; liver failure (decreased metabolism of lactate); drugs/toxins (including metformin).
  • Poisoning: salicylate, ethylene glycol, methanol.
• Renal failure.

Normal anion gap (chloride increase matches bicarbonate decrease)

• Renal tubular dysfunction:
  • Renal tubular acidosis.
  • Hypoaldosteronism.
  • Potassium-sparing diuretics.
• Loss of alkali:
  • Diarrhoea.
  • Ureterosigmoidostomy.
  • Carbonic anhydrase inhibitors (acetazolamide).
• Acid intake:
  • Ammonium chloride, cationic amino acids.

Investigations

• Na, K, Ca, PO₄, creatinine, urea, Cl, glucose.
• Arterial blood gases.
• Toxicology - as appropriate.
• Ketones if indicated.
• Lactate.

Treatment

• Treat underlying cause.
• Recent evidence suggests that in most forms of metabolic acidosis, the use of sodium bicarbonate offers no benefit and may even be harmful.
• If sodium bicarbonate is used give 1 mmol/kg and review pH in one hour. Do not overcorrect, aim for pH of 7.1.
• Specific indications for sodium bicarbonate include methanol and tricyclic antidepressant poisoning - maintaining a normal pH probably reduces toxicity.

Causes of Metabolic Alkalosis

• Volume deficit (sodium conservation is coupled to bicarbonate reabsorption and therefore metabolic alkalosis is sustained).
  • Vomiting, gastric suction.
  • Diuretics (not acetazolamide, potassium-sparing).
• Mineralocorticoid excess:
  • Cushing's Syndrome.
  • Primary hyperaldosteronism.
  • Bartter's syndrome (decreased sodium chloride absorption in kidney leading to increased renin/aldosterone).
• Severe potassium depletion.
• Milk-alkali syndrome - chronic excess soluble calcium salts plus alkali cause a nephropathy which impairs bicarbonate excretion.
• Post-hypercapnic (one of the most common in general medical setting).
  • High bicarbonate due to metabolic compensation of respiratory acidosis. When ventilation improves an alkalosis may result which will resolve spontaneously as long as patient is not volume deficient.
• Gastric outlet obstruction.

Investigations

• Arterial blood gas.
• Na, K, Cl, creatinine.
• Urine:
  • Chloride low in volume depletion (<10 mmol/L).

Treatment

• Treat the underlying cause.

Topic Code: 1399