Alkalemia and alkalosis - definitions and examples

Alkalemia is the deviation of blood pH above 7.44.

Alkalosis is defined as a pathophysiological condition characterized by by the buildup of excess base or alkali in the body, which results in alkalemia.

Acid-base normogram

Metabolic alkalosis

Metabolic alkalosis is far more common than respiratory alkalosis. It is caused by a negative balance of strong acids in the ECF.

Causes of metabolic alkalosis:

1) Increased supply of HCO₃⁻

• Absolute - inappropriate administration of bicarbonates by infusion.
  • With preserved kidney function, the bicarbonate load is excreted, however with renal failure, bicarbonate excretion may be impaired.
• Relative – after dehydration, which increases bicarbonate concentration in the ECF.

2) Decreased supply of hydrogen ions to the ECF

• Vomiting
  • increased losses of H⁺ and Cl⁻ → hypochloremic metabolic alkalosis (paradoxical acidification of urine).
  • Therapy must include chloride; without it the kidneys cannot compensate by reducing H⁺ excretion and increasing bicarbonate excretion.

• Potassium depletion
  • Causes H⁺ to shift from ECF into cells in exchange for K⁺.
  • Occurs in hyperaldosteronism.
  • Leads to metabolic acidosis inside cells, but metabolic alkalosis in the ECF.

• Bartter syndrome
  • K⁺ depletion and subsequent H⁺/K⁺ exchange at the cell membrane, combined with urinary H⁺ losses, → hypochloremic metabolic alkalosis.

Compensation:

Respiratory compensation (hypoventilation), but smaller than in acidosis because it is limited by oxygen tension. Kidneys can (if chloride is not deficient) increase bicarbonate excretion.

Symptoms

Respiratory alkalosis

Respiratory alkalosis = depletion of CO₂ → hypokapnia.

Causes

a) Hyperventilation during mechanical ventilation

• Either accidental or intentional (e.g., prevention of cerebral edema or “Robin Hood effect”).

b) Stimulation of the respiratory center

• Psychogenic or neural factors
  • Causes: hysteria, a salicylate overdose, sometimes with tetany.
  • It may lead to loss of consciousness due to cerebral vasoconstriction.

• Hypoxemia
  • drop of PaO₂ below 8 kPa is a strong stimulus for the respiratory center leading to an increase of ventilation
  • Causes: high-altitude stay, congenital heart defects with right-to-left shunt
  • Usually also leads to development of lactic metabolic acidosis

Compensation of respiratory alkalosis

Renal compensation by increased bicarbonate excretion → retention of a small amount of strong acids in the ECF → correction of pH toward normal. Hypokapnia causes vasoconstriction in the cerebral circulation (and vice versa).

References

1. Česko-slovenská pediatrie [online]. Časopis České lékařské společnosti J. E. Purkyně; 2023. Link: https://cspediatrie.cz/pdfs/ped/2023/01/02.pdf
2. Sur M., Hashmi M. F. Alkalosis [online]. StatPearls Publishing, 2025. Link: https://www.ncbi.nlm.nih.gov/books/NBK545269/