Dr John Gennari had another typically excellent review of metabolic alkalosis in AJKD in October. He suggests an alternative means of classifying a metabolic alkalosis according to the etiology of the alkalosis along with the physiological basis for the maintenance of the alkalosis once it has occurred. He also goes into some detail explaining the key role that chloride depletion has in the development and maintenance of a metabolic alkalosis.
The 3 subtypes that he suggested are:
1. Secondary stimulation of collecting duct ion transport:
This is the commonest type and results largely from a secondary increase in the activity of ENaC in the distal nephron leading to increased sodium reabsorption and hydrogen ion excretion. The commonest causes are chloride depletion syndromes (GI losses, CF) and the use of thiazide and loop diuretics, with congenital disorders such as Bartter and Gitelman syndrome being rarer. Severe potassium depletion can also precipitate a metabolic alkalosis. The mechanism is complicated but it includes increased proximal tubular hydrogen ion secretion, decreased activity of the Na-2K-Cl transporter in the loop of Henle (with increased NH4 transport in this segment also contributing) and subsequent increased activity of ENaC due to the higher distal delivery of sodium. This form of alkalosis is perpetuated by chloride depletion.
2. Primary stimulation of collecting duct ion transport:
This is almost always due to a pathological increase in sodium reabsorption with consequent hypertension and volume expansion. The commonest cause is primary aldosteronism. Other rarer causes include Cushing’s Syndrome, CAH, Liddle’s Syndrome, 11-hydroxysteroid dehydrogenase inhibition (licorice) or deficiency and exogenous mineralocorticoids.
3. Alkali intake or administration:
This is largely an issue of excess alkali administration in patients who are unable to excrete it rapidly – i.e. patients with abnormal renal function
This paper is highly recommended for anyone wanting to understand more about the pathophysiology of metabolic alkalosis