Change in anion gap versus change in HCO3

If there is an increased anion gap, it may be helpful to relate this to the fall in plasma HCO3. Fifty per cent of excess H+ is buffered by cells and not by HCO3, but most of the excess anions remain in the extracelluar fluid as they cannot cross the cell membrane. Therefore, the increase in anion gap

Box 3.1 Causes of an increased anion gap metabolic acidosis

Exogenous acids

• salicylate poisoning

• methanol/ethylene glycol poisoning

• tricyclic poisoning

Lactic acidosis - type A

• anaerobic tissue metabolism in states of hypoperfusion - this

is the most common cause of any metabolic acidosis

Lactic acidosis - type B

• reduced hepatic lactate metabolism in liver failure

• metformin (rare)

Ketoacidosis

insulin deficiency (DKA)

• starvation

Renal failure

usually exceeds the fall in plasma HCO3. In lactic acidosis the ratio is around 16 to 10. In ketoacidosis the ratio is nearer 10 to 10 because of the loss of ketoacids in the urine which does not occur in lactic acidosis. Thus, calculating the change in anion gap versus change in HCO3 can help differentiate ketoacidosis from other acidosis.

HCO3 as sodium bicarbonate may be administered intravenously to raise blood pH in severe metabolic acidosis but this has several problems. It increases the formation of CO2, which passes readily into cells (unlike HCO3). This worsens intracellular acidosis. Some compromised patients may need ventilation to counter the increased CO2 production caused by an infusion of sodium bicarbonate. The oxygen-dissociation curve is shifted to the left by alkalosis leading to impaired oxygen delivery to the tissues. Sodium bicarbonate contains a significant sodium load and because 8-4% solution is hypertonic, the increase in plasma osmolality can lead to vasodilatation and hypotension. Tissue necrosis can result from extravasation from the cannula. Many of the causes of metabolic acidosis respond to restoration of intravascular volume, tissue perfusion, and treatment of the underlying cause. For these reasons, intravenous sodium bicarbonate is not often used in metabolic acidosis; 8-4% sodium bicarbonate = 1 mmol ml-1 of sodium or bicarbonate.

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